Video for Linux Two API Specification

Revision 2.6.32

Michael H Schimek


    
  

Bill Dirks

Original author of the V4L2 API and documentation. 

Hans Verkuil

Designed and documented the VIDIOC_LOG_STATUS ioctl, the extended control ioctls and major parts of the sliced VBI API. 


    
  

Martin Rubli

Designed and documented the VIDIOC_ENUM_FRAMESIZES and VIDIOC_ENUM_FRAMEINTERVALS ioctls. 

Andy Walls

Documented the fielded V4L2_MPEG_STREAM_VBI_FMT_IVTV MPEG stream embedded, sliced VBI data format in this specification.  


    
  

Mauro Carvalho Chehab

Documented libv4l, designed and added v4l2grab example, Remote Controller chapter  


    
  

This document is copyrighted © 1999-2009 by Bill Dirks, Michael H. Schimek, Hans Verkuil, Martin Rubli, Andy Walls and Mauro Carvalho Chehab.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in the appendix entitled "GNU Free Documentation License".

Programming examples can be used and distributed without restrictions.

Revision History
Revision 2.6.322009-08-31mcc
Now, revisions will match the kernel version where the V4L2 API changes will be used by the Linux Kernel. Also added Remote Controller chapter.
Revision 0.292009-08-26ev
Added documentation for string controls and for FM Transmitter controls.
Revision 0.282009-08-26gl
Added V4L2_CID_BAND_STOP_FILTER documentation.
Revision 0.272009-08-15mcc
Added libv4l and Remote Controller documentation; added v4l2grab and keytable application examples.
Revision 0.262009-07-23hv
Finalized the RDS capture API. Added modulator and RDS encoder capabilities. Added support for string controls.
Revision 0.252009-01-18hv
Added pixel formats VYUY, NV16 and NV61, and changed the debug ioctls VIDIOC_DBG_G/S_REGISTER and VIDIOC_DBG_G_CHIP_IDENT. Added camera controls V4L2_CID_ZOOM_ABSOLUTE, V4L2_CID_ZOOM_RELATIVE, V4L2_CID_ZOOM_CONTINUOUS and V4L2_CID_PRIVACY.
Revision 0.242008-03-04mhs
Added pixel formats Y16 and SBGGR16, new controls and a camera controls class. Removed VIDIOC_G/S_MPEGCOMP.
Revision 0.232007-08-30mhs
Fixed a typo in VIDIOC_DBG_G/S_REGISTER. Clarified the byte order of packed pixel formats.
Revision 0.222007-08-29mhs
Added the Video Output Overlay interface, new MPEG controls, V4L2_FIELD_INTERLACED_TB and V4L2_FIELD_INTERLACED_BT, VIDIOC_DBG_G/S_REGISTER, VIDIOC_(TRY_)ENCODER_CMD, VIDIOC_G_CHIP_IDENT, VIDIOC_G_ENC_INDEX, new pixel formats. Clarifications in the cropping chapter, about RGB pixel formats, the mmap(), poll(), select(), read() and write() functions. Typographical fixes.
Revision 0.212006-12-19mhs
Fixed a link in the VIDIOC_G_EXT_CTRLS section.
Revision 0.202006-11-24mhs
Clarified the purpose of the audioset field in struct v4l2_input and v4l2_output.
Revision 0.192006-10-19mhs
Documented V4L2_PIX_FMT_RGB444.
Revision 0.182006-10-18mhs
Added the description of extended controls by Hans Verkuil. Linked V4L2_PIX_FMT_MPEG to V4L2_CID_MPEG_STREAM_TYPE.
Revision 0.172006-10-12mhs
Corrected V4L2_PIX_FMT_HM12 description.
Revision 0.162006-10-08mhs
VIDIOC_ENUM_FRAMESIZES and VIDIOC_ENUM_FRAMEINTERVALS are now part of the API.
Revision 0.152006-09-23mhs
Cleaned up the bibliography, added BT.653 and BT.1119. capture.c/start_capturing() for user pointer I/O did not initialize the buffer index. Documented the V4L MPEG and MJPEG VID_TYPEs and V4L2_PIX_FMT_SBGGR8. Updated the list of reserved pixel formats. See the history chapter for API changes.
Revision 0.142006-09-14mr
Added VIDIOC_ENUM_FRAMESIZES and VIDIOC_ENUM_FRAMEINTERVALS proposal for frame format enumeration of digital devices.
Revision 0.132006-04-07mhs
Corrected the description of struct v4l2_window clips. New V4L2_STD_ and V4L2_TUNER_MODE_LANG1_LANG2 defines.
Revision 0.122006-02-03mhs
Corrected the description of struct v4l2_captureparm and v4l2_outputparm.
Revision 0.112006-01-27mhs
Improved the description of struct v4l2_tuner.
Revision 0.102006-01-10mhs
VIDIOC_G_INPUT and VIDIOC_S_PARM clarifications.
Revision 0.92005-11-27mhs
Improved the 525 line numbering diagram. Hans Verkuil and I rewrote the sliced VBI section. He also contributed a VIDIOC_LOG_STATUS page. Fixed VIDIOC_S_STD call in the video standard selection example. Various updates.
Revision 0.82004-10-04mhs
Somehow a piece of junk slipped into the capture example, removed.
Revision 0.72004-09-19mhs
Fixed video standard selection, control enumeration, downscaling and aspect example. Added read and user pointer i/o to video capture example.
Revision 0.62004-08-01mhs
v4l2_buffer changes, added video capture example, various corrections.
Revision 0.52003-11-05mhs
Pixel format erratum.
Revision 0.42003-09-17mhs
Corrected source and Makefile to generate a PDF. SGML fixes. Added latest API changes. Closed gaps in the history chapter.
Revision 0.32003-02-05mhs
Another draft, more corrections.
Revision 0.22003-01-15mhs
Second draft, with corrections pointed out by Gerd Knorr.
Revision 0.12002-12-01mhs
First draft, based on documentation by Bill Dirks and discussions on the V4L mailing list.

Table of Contents

Introduction
1. Common API Elements
1.1. Opening and Closing Devices
1.1.1. Device Naming
1.1.2. Related Devices
1.1.3. Multiple Opens
1.1.4. Shared Data Streams
1.1.5. Functions
1.2. Querying Capabilities
1.3. Application Priority
1.4. Video Inputs and Outputs
1.5. Audio Inputs and Outputs
1.6. Tuners and Modulators
1.6.1. Tuners
1.6.2. Modulators
1.6.3. Radio Frequency
1.6.4. Satellite Receivers
1.7. Video Standards
1.8. User Controls
1.9. Extended Controls
1.9.1. Introduction
1.9.2. The Extended Control API
1.9.3. Enumerating Extended Controls
1.9.4. Creating Control Panels
1.9.5. MPEG Control Reference
1.9.5.1. Generic MPEG Controls
1.9.5.2. CX2341x MPEG Controls
1.9.6. Camera Control Reference
1.9.7. FM Transmitter Control Reference
1.10. Data Formats
1.10.1. Data Format Negotiation
1.10.2. Image Format Enumeration
1.11. Image Cropping, Insertion and Scaling
1.11.1. Cropping Structures
1.11.2. Scaling Adjustments
1.11.3. Examples
1.12. Streaming Parameters
2. Image Formats
2.1. Standard Image Formats
2.2. Colorspaces
2.3. Indexed Format
2.4. RGB Formats
2.5. YUV Formats
2.6. Compressed Formats
2.7. Reserved Format Identifiers
3. Input/Output
3.1. Read/Write
3.2. Streaming I/O (Memory Mapping)
3.3. Streaming I/O (User Pointers)
3.4. Asynchronous I/O
3.5. Buffers
3.5.1. Timecodes
3.6. Field Order
4. Interfaces
4.1. Video Capture Interface
4.1.1. Querying Capabilities
4.1.2. Supplemental Functions
4.1.3. Image Format Negotiation
4.1.4. Reading Images
4.2. Video Overlay Interface
4.2.1. Querying Capabilities
4.2.2. Supplemental Functions
4.2.3. Setup
4.2.4. Overlay Window
4.2.5. Enabling Overlay
4.3. Video Output Interface
4.3.1. Querying Capabilities
4.3.2. Supplemental Functions
4.3.3. Image Format Negotiation
4.3.4. Writing Images
4.4. Video Output Overlay Interface
4.4.1. Querying Capabilities
4.4.2. Framebuffer
4.4.3. Overlay Window and Scaling
4.4.4. Enabling Overlay
4.5. Codec Interface
4.6. Effect Devices Interface
4.7. Raw VBI Data Interface
4.7.1. Querying Capabilities
4.7.2. Supplemental Functions
4.7.3. Raw VBI Format Negotiation
4.7.4. Reading and writing VBI images
4.8. Sliced VBI Data Interface
4.8.1. Querying Capabilities
4.8.2. Supplemental Functions
4.8.3. Sliced VBI Format Negotiation
4.8.4. Reading and writing sliced VBI data
4.8.5. Sliced VBI Data in MPEG Streams
4.8.5.1. MPEG Stream Embedded, Sliced VBI Data Format: NONE
4.8.5.2. MPEG Stream Embedded, Sliced VBI Data Format: IVTV
4.9. Teletext Interface
4.10. Radio Interface
4.10.1. Querying Capabilities
4.10.2. Supplemental Functions
4.10.3. Programming
4.11. RDS Interface
4.11.1. Querying Capabilities
4.11.2. Reading RDS data
I. Function Reference
V4L2 close() — Close a V4L2 device
V4L2 ioctl() — Program a V4L2 device
ioctl VIDIOC_CROPCAP — Information about the video cropping and scaling abilities
ioctl VIDIOC_DBG_G_CHIP_IDENT — Identify the chips on a TV card
ioctl VIDIOC_DBG_G_REGISTER, VIDIOC_DBG_S_REGISTER — Read or write hardware registers
ioctl VIDIOC_ENCODER_CMD, VIDIOC_TRY_ENCODER_CMD — Execute an encoder command
ioctl VIDIOC_ENUMAUDIO — Enumerate audio inputs
ioctl VIDIOC_ENUMAUDOUT — Enumerate audio outputs
ioctl VIDIOC_ENUM_FMT — Enumerate image formats
ioctl VIDIOC_ENUM_FRAMESIZES — Enumerate frame sizes
ioctl VIDIOC_ENUM_FRAMEINTERVALS — Enumerate frame intervals
ioctl VIDIOC_ENUMINPUT — Enumerate video inputs
ioctl VIDIOC_ENUMOUTPUT — Enumerate video outputs
ioctl VIDIOC_ENUMSTD — Enumerate supported video standards
ioctl VIDIOC_G_AUDIO, VIDIOC_S_AUDIO — Query or select the current audio input and its attributes
ioctl VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT — Query or select the current audio output
ioctl VIDIOC_G_CROP, VIDIOC_S_CROP — Get or set the current cropping rectangle
ioctl VIDIOC_G_CTRL, VIDIOC_S_CTRL — Get or set the value of a control
ioctl VIDIOC_G_ENC_INDEX — Get meta data about a compressed video stream
ioctl VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS, VIDIOC_TRY_EXT_CTRLS — Get or set the value of several controls, try control values
ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF — Get or set frame buffer overlay parameters
ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT — Get or set the data format, try a format
ioctl VIDIOC_G_FREQUENCY, VIDIOC_S_FREQUENCY — Get or set tuner or modulator radio frequency
ioctl VIDIOC_G_INPUT, VIDIOC_S_INPUT — Query or select the current video input
ioctl VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP
ioctl VIDIOC_G_MODULATOR, VIDIOC_S_MODULATOR — Get or set modulator attributes
ioctl VIDIOC_G_OUTPUT, VIDIOC_S_OUTPUT — Query or select the current video output
ioctl VIDIOC_G_PARM, VIDIOC_S_PARM — Get or set streaming parameters
ioctl VIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY — Query or request the access priority associated with a file descriptor
ioctl VIDIOC_G_SLICED_VBI_CAP — Query sliced VBI capabilities
ioctl VIDIOC_G_STD, VIDIOC_S_STD — Query or select the video standard of the current input
ioctl VIDIOC_G_TUNER, VIDIOC_S_TUNER — Get or set tuner attributes
ioctl VIDIOC_LOG_STATUS — Log driver status information
ioctl VIDIOC_OVERLAY — Start or stop video overlay
ioctl VIDIOC_QBUF, VIDIOC_DQBUF — Exchange a buffer with the driver
ioctl VIDIOC_QUERYBUF — Query the status of a buffer
ioctl VIDIOC_QUERYCAP — Query device capabilities
ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU — Enumerate controls and menu control items
ioctl VIDIOC_QUERYSTD — Sense the video standard received by the current input
ioctl VIDIOC_REQBUFS — Initiate Memory Mapping or User Pointer I/O
ioctl VIDIOC_S_HW_FREQ_SEEK — Perform a hardware frequency seek
ioctl VIDIOC_STREAMON, VIDIOC_STREAMOFF — Start or stop streaming I/O
V4L2 mmap() — Map device memory into application address space
V4L2 munmap() — Unmap device memory
V4L2 open() — Open a V4L2 device
V4L2 poll() — Wait for some event on a file descriptor
V4L2 read() — Read from a V4L2 device
V4L2 select() — Synchronous I/O multiplexing
V4L2 write() — Write to a V4L2 device
5. V4L2 Driver Programming
6. Libv4l Userspace Library
6.1. Introduction
6.1.1. libv4lconvert
6.1.2. libv4l1
6.1.3. libv4l2
6.1.3.1. Libv4l device control functions
6.1.4. v4l1compat.so wrapper library
7. Remote Controllers
7.1. Introduction
7.2. Changing default Remote Controller mappings
8. Changes
8.1. Differences between V4L and V4L2
8.1.1. Opening and Closing Devices
8.1.2. Querying Capabilities
8.1.3. Video Sources
8.1.4. Tuning
8.1.5. Image Properties
8.1.6. Audio
8.1.7. Frame Buffer Overlay
8.1.8. Cropping
8.1.9. Reading Images, Memory Mapping
8.1.9.1. Capturing using the read method
8.1.9.2. Capturing using memory mapping
8.1.10. Reading Raw VBI Data
8.1.11. Miscellaneous
8.2. Changes of the V4L2 API
8.2.1. Early Versions
8.2.2. V4L2 Version 0.16 1999-01-31
8.2.3. V4L2 Version 0.18 1999-03-16
8.2.4. V4L2 Version 0.19 1999-06-05
8.2.5. V4L2 Version 0.20 (1999-09-10)
8.2.6. V4L2 Version 0.20 incremental changes
8.2.7. V4L2 Version 0.20 2000-11-23
8.2.8. V4L2 Version 0.20 2002-07-25
8.2.9. V4L2 in Linux 2.5.46, 2002-10
8.2.10. V4L2 2003-06-19
8.2.11. V4L2 2003-11-05
8.2.12. V4L2 in Linux 2.6.6, 2004-05-09
8.2.13. V4L2 in Linux 2.6.8
8.2.14. V4L2 spec erratum 2004-08-01
8.2.15. V4L2 in Linux 2.6.14
8.2.16. V4L2 in Linux 2.6.15
8.2.17. V4L2 spec erratum 2005-11-27
8.2.18. V4L2 spec erratum 2006-01-10
8.2.19. V4L2 spec erratum 2006-02-03
8.2.20. V4L2 spec erratum 2006-02-04
8.2.21. V4L2 in Linux 2.6.17
8.2.22. V4L2 spec erratum 2006-09-23 (Draft 0.15)
8.2.23. V4L2 in Linux 2.6.18
8.2.24. V4L2 in Linux 2.6.19
8.2.25. V4L2 spec erratum 2006-10-12 (Draft 0.17)
8.2.26. V4L2 in Linux 2.6.21
8.2.27. V4L2 in Linux 2.6.22
8.2.28. V4L2 in Linux 2.6.24
8.2.29. V4L2 in Linux 2.6.25
8.2.30. V4L2 in Linux 2.6.26
8.2.31. V4L2 in Linux 2.6.27
8.2.32. V4L2 in Linux 2.6.28
8.2.33. V4L2 in Linux 2.6.29
8.2.34. V4L2 in Linux 2.6.30
8.2.35. V4L2 in Linux 2.6.32
8.3. Relation of V4L2 to other Linux multimedia APIs
8.3.1. X Video Extension
8.3.2. Digital Video
8.3.3. Audio Interfaces
8.4. Experimental API Elements
8.5. Obsolete API Elements
A. Video For Linux Two Header File
B. Video Capture Example
C. Video Grabber example using libv4l
D. GNU Free Documentation License
D.1. 0. PREAMBLE
D.2. 1. APPLICABILITY AND DEFINITIONS
D.3. 2. VERBATIM COPYING
D.4. 3. COPYING IN QUANTITY
D.5. 4. MODIFICATIONS
D.6. 5. COMBINING DOCUMENTS
D.7. 6. COLLECTIONS OF DOCUMENTS
D.8. 7. AGGREGATION WITH INDEPENDENT WORKS
D.9. 8. TRANSLATION
D.10. 9. TERMINATION
D.11. 10. FUTURE REVISIONS OF THIS LICENSE
D.12. Addendum
List of Types
References

List of Figures

1.1. Image Cropping, Insertion and Scaling
3.1. Field Order, Top Field First Transmitted
3.2. Field Order, Bottom Field First Transmitted
4.1. Line synchronization
4.2. ITU-R 525 line numbering (M/NTSC and M/PAL)
4.3. ITU-R 625 line numbering

List of Tables

1.1. Control IDs
1.2. MPEG Control IDs
1.3. CX2341x Control IDs
1.4. Camera Control IDs
1.5. FM_TX Control IDs
2.1. struct v4l2_pix_format
2.2. enum v4l2_colorspace
2.3. Indexed Image Format
2.4. Packed RGB Image Formats
2.5. Packed RGB Image Formats (corrected)
2.6. Packed YUV Image Formats
2.7. Compressed Image Formats
2.8. Reserved Image Formats
3.1. struct v4l2_buffer
3.2. enum v4l2_buf_type
3.3. Buffer Flags
3.4. enum v4l2_memory
3.5. struct v4l2_timecode
3.6. Timecode Types
3.7. Timecode Flags
3.8. enum v4l2_field
4.1. struct v4l2_window
4.2. struct v4l2_clip
4.3. struct v4l2_rect
4.4. struct v4l2_vbi_format
4.5. Raw VBI Format Flags
4.6. struct v4l2_sliced_vbi_format
4.7. Sliced VBI services
4.8. struct v4l2_sliced_vbi_data
4.9. struct v4l2_mpeg_vbi_fmt_ivtv
4.10. Magic Constants for struct v4l2_mpeg_vbi_fmt_ivtv magic field
4.11. struct v4l2_mpeg_vbi_itv0
4.12. struct v4l2_mpeg_vbi_ITV0
4.13. struct v4l2_mpeg_vbi_itv0_line
4.14. Line Identifiers for struct v4l2_mpeg_vbi_itv0_line id field
4.15. struct v4l2_rds_data
4.16. Block description
4.17. Block defines
39. struct v4l2_cropcap
40. struct v4l2_rect
41. struct v4l2_dbg_match
42. struct v4l2_dbg_chip_ident
43. Chip Match Types
44. Chip Identifiers
45. struct v4l2_dbg_match
46. struct v4l2_dbg_register
47. Chip Match Types
48. struct v4l2_encoder_cmd
49. Encoder Commands
50. Encoder Command Flags
51. struct v4l2_fmtdesc
52. Image Format Description Flags
53. struct v4l2_frmsize_discrete
54. struct v4l2_frmsize_stepwise
55. struct v4l2_frmsizeenum
56. enum v4l2_frmsizetypes
57. struct v4l2_frmival_stepwise
58. struct v4l2_frmivalenum
59. enum v4l2_frmivaltypes
60. struct v4l2_input
61. Input Types
62. Input Status Flags
63. struct v4l2_output
64. Output Type
65. struct v4l2_standard
66. struct v4l2_fract
67. typedef v4l2_std_id
68. Video Standards (based on [])
69. struct v4l2_audio
70. Audio Capability Flags
71. Audio Mode Flags
72. struct v4l2_audioout
73. struct v4l2_crop
74. struct v4l2_control
75. struct v4l2_enc_idx
76. struct v4l2_enc_idx_entry
77. Index Entry Flags
78. struct v4l2_ext_control
79. struct v4l2_ext_controls
80. Control classes
81. struct v4l2_framebuffer
82. Frame Buffer Capability Flags
83. Frame Buffer Flags
84. struct v4l2_format
85. struct v4l2_frequency
86. struct v4l2_jpegcompression
87. JPEG Markers Flags
88. struct v4l2_modulator
89. Modulator Audio Transmission Flags
90. struct v4l2_streamparm
91. struct v4l2_captureparm
92. struct v4l2_outputparm
93. Streaming Parameters Capabilites
94. Capture Parameters Flags
95. enum v4l2_priority
96. struct v4l2_sliced_vbi_cap
97. Sliced VBI services
98. struct v4l2_tuner
99. enum v4l2_tuner_type
100. Tuner and Modulator Capability Flags
101. Tuner Audio Reception Flags
102. Tuner Audio Modes
103. Tuner Audio Matrix
104. struct v4l2_capability
105. Device Capabilities Flags
106. struct v4l2_queryctrl
107. struct v4l2_querymenu
108. enum v4l2_ctrl_type
109. Control Flags
110. struct v4l2_requestbuffers
111. struct v4l2_hw_freq_seek
7.1. IR default keymapping
7.2. Notes
8.1. V4L Device Types, Names and Numbers

List of Examples

1.1. Information about the current video input
1.2. Switching to the first video input
1.3. Information about the current audio input
1.4. Switching to the first audio input
1.5. Information about the current video standard
1.6. Listing the video standards supported by the current input
1.7. Selecting a new video standard
1.8. Enumerating all controls
1.9. Changing controls
1.10. Resetting the cropping parameters
1.11. Simple downscaling
1.12. Selecting an output area
1.13. Current scaling factor and pixel aspect
2.1. ITU-R Rec. BT.601 color conversion
2.2. V4L2_PIX_FMT_BGR24 4 × 4 pixel image
2.3. V4L2_PIX_FMT_SBGGR8 4 × 4 pixel image
2.4. V4L2_PIX_FMT_SGBRG8 4 × 4 pixel image
2.5. V4L2_PIX_FMT_SGRBG8 4 × 4 pixel image
2.6. V4L2_PIX_FMT_SBGGR16 4 × 4 pixel image
2.7. V4L2_PIX_FMT_GREY 4 × 4 pixel image
2.8. V4L2_PIX_FMT_Y16 4 × 4 pixel image
2.9. V4L2_PIX_FMT_YUYV 4 × 4 pixel image
2.10. V4L2_PIX_FMT_UYVY 4 × 4 pixel image
2.11. V4L2_PIX_FMT_YVYU 4 × 4 pixel image
2.12. V4L2_PIX_FMT_VYUY 4 × 4 pixel image
2.13. V4L2_PIX_FMT_Y41P 8 × 4 pixel image
2.14. V4L2_PIX_FMT_YVU420 4 × 4 pixel image
2.15. V4L2_PIX_FMT_YVU410 4 × 4 pixel image
2.16. V4L2_PIX_FMT_YUV422P 4 × 4 pixel image
2.17. V4L2_PIX_FMT_YUV411P 4 × 4 pixel image
2.18. V4L2_PIX_FMT_NV12 4 × 4 pixel image
2.19. V4L2_PIX_FMT_NV16 4 × 4 pixel image
3.1. Mapping buffers
3.2. Initiating streaming I/O with user pointers
4.1. Finding a framebuffer device for OSD

Introduction

Video For Linux Two is the second version of the Video For Linux API, a kernel interface for analog radio and video capture and output drivers.

Early drivers used ad-hoc interfaces. These were replaced in Linux 2.2 by Alan Cox' V4L API, based on the interface of the bttv driver. In 1999 Bill Dirks started the development of V4L2 to fix some shortcomings of V4L and to support a wider range of devices. The API was revised again in 2002 prior to its inclusion in Linux 2.5/2.6, and work continues on improvements and additions while maintaining compatibility with existing drivers and applications. In 2006/2007 efforts began on FreeBSD drivers with a V4L2 interface.

This book documents the V4L2 API. Intended audience are driver and application writers.

If you have questions or ideas regarding the API, please write to the linux-media mailing list: http://www.linuxtv.org/lists.php.

The latest version of this document and the DocBook SGML sources are part of the http://linuxtv.org/repo/ repository. The online version is available here: http://linuxtv.org/downloads/video4linux/API/V4L2_API.

Chapter 1. Common API Elements

Programming a V4L2 device consists of these steps:

  • Opening the device

  • Changing device properties, selecting a video and audio input, video standard, picture brightness a. o.

  • Negotiating a data format

  • Negotiating an input/output method

  • The actual input/output loop

  • Closing the device

In practice most steps are optional and can be executed out of order. It depends on the V4L2 device type, you can read about the details in Chapter 4, Interfaces. In this chapter we will discuss the basic concepts applicable to all devices.

1.1. Opening and Closing Devices

1.1.1. Device Naming

V4L2 drivers are implemented as kernel modules, loaded manually by the system administrator or automatically when a device is first opened. The driver modules plug into the "videodev" kernel module. It provides helper functions and a common application interface specified in this document.

Each driver thus loaded registers one or more device nodes with major number 81 and a minor number between 0 and 255. Assigning minor numbers to V4L2 devices is entirely up to the system administrator, this is primarily intended to solve conflicts between devices.[1] The module options to select minor numbers are named after the device special file with a "_nr" suffix. For example "video_nr" for /dev/video video capture devices. The number is an offset to the base minor number associated with the device type. [2] When the driver supports multiple devices of the same type more than one minor number can be assigned, separated by commas:

> insmod mydriver.o video_nr=0,1 radio_nr=0,1

In /etc/modules.conf this may be written as:

alias char-major-81-0 mydriver
alias char-major-81-1 mydriver
alias char-major-81-64 mydriver              1
options mydriver video_nr=0,1 radio_nr=0,1   2
	  

1

When an application attempts to open a device special file with major number 81 and minor number 0, 1, or 64, load "mydriver" (and the "videodev" module it depends upon).

2

Register the first two video capture devices with minor number 0 and 1 (base number is 0), the first two radio device with minor number 64 and 65 (base 64).

When no minor number is given as module option the driver supplies a default. Chapter 4, Interfaces recommends the base minor numbers to be used for the various device types. Obviously minor numbers must be unique. When the number is already in use the offending device will not be registered.

By convention system administrators create various character device special files with these major and minor numbers in the /dev directory. The names recomended for the different V4L2 device types are listed in Chapter 4, Interfaces.

The creation of character special files (with mknod) is a privileged operation and devices cannot be opened by major and minor number. That means applications cannot reliable scan for loaded or installed drivers. The user must enter a device name, or the application can try the conventional device names.

Under the device filesystem (devfs) the minor number options are ignored. V4L2 drivers (or by proxy the "videodev" module) automatically create the required device files in the /dev/v4l directory using the conventional device names above.

1.1.2. Related Devices

Devices can support several related functions. For example video capturing, video overlay and VBI capturing are related because these functions share, amongst other, the same video input and tuner frequency. V4L and earlier versions of V4L2 used the same device name and minor number for video capturing and overlay, but different ones for VBI. Experience showed this approach has several problems[3], and to make things worse the V4L videodev module used to prohibit multiple opens of a device.

As a remedy the present version of the V4L2 API relaxed the concept of device types with specific names and minor numbers. For compatibility with old applications drivers must still register different minor numbers to assign a default function to the device. But if related functions are supported by the driver they must be available under all registered minor numbers. The desired function can be selected after opening the device as described in Chapter 4, Interfaces.

Imagine a driver supporting video capturing, video overlay, raw VBI capturing, and FM radio reception. It registers three devices with minor number 0, 64 and 224 (this numbering scheme is inherited from the V4L API). Regardless if /dev/video (81, 0) or /dev/vbi (81, 224) is opened the application can select any one of the video capturing, overlay or VBI capturing functions. Without programming (e. g. reading from the device with dd or cat) /dev/video captures video images, while /dev/vbi captures raw VBI data. /dev/radio (81, 64) is invariable a radio device, unrelated to the video functions. Being unrelated does not imply the devices can be used at the same time, however. The open() function may very well return an EBUSY error code.

Besides video input or output the hardware may also support audio sampling or playback. If so, these functions are implemented as OSS or ALSA PCM devices and eventually OSS or ALSA audio mixer. The V4L2 API makes no provisions yet to find these related devices. If you have an idea please write to the linux-media mailing list: http://www.linuxtv.org/lists.php.

1.1.3. Multiple Opens

In general, V4L2 devices can be opened more than once. When this is supported by the driver, users can for example start a "panel" application to change controls like brightness or audio volume, while another application captures video and audio. In other words, panel applications are comparable to an OSS or ALSA audio mixer application. When a device supports multiple functions like capturing and overlay simultaneously, multiple opens allow concurrent use of the device by forked processes or specialized applications.

Multiple opens are optional, although drivers should permit at least concurrent accesses without data exchange, i. e. panel applications. This implies open() can return an EBUSY error code when the device is already in use, as well as ioctl() functions initiating data exchange (namely the VIDIOC_S_FMT ioctl), and the read() and write() functions.

Mere opening a V4L2 device does not grant exclusive access.[4] Initiating data exchange however assigns the right to read or write the requested type of data, and to change related properties, to this file descriptor. Applications can request additional access privileges using the priority mechanism described in Section 1.3, “Application Priority”.

1.1.4. Shared Data Streams

V4L2 drivers should not support multiple applications reading or writing the same data stream on a device by copying buffers, time multiplexing or similar means. This is better handled by a proxy application in user space. When the driver supports stream sharing anyway it must be implemented transparently. The V4L2 API does not specify how conflicts are solved.

1.1.5. Functions

To open and close V4L2 devices applications use the open() and close() function, respectively. Devices are programmed using the ioctl() function as explained in the following sections.

1.2. Querying Capabilities

Because V4L2 covers a wide variety of devices not all aspects of the API are equally applicable to all types of devices. Furthermore devices of the same type have different capabilities and this specification permits the omission of a few complicated and less important parts of the API.

The VIDIOC_QUERYCAP ioctl is available to check if the kernel device is compatible with this specification, and to query the functions and I/O methods supported by the device. Other features can be queried by calling the respective ioctl, for example VIDIOC_ENUMINPUT to learn about the number, types and names of video connectors on the device. Although abstraction is a major objective of this API, the ioctl also allows driver specific applications to reliable identify the driver.

All V4L2 drivers must support VIDIOC_QUERYCAP. Applications should always call this ioctl after opening the device.

1.3. Application Priority

When multiple applications share a device it may be desirable to assign them different priorities. Contrary to the traditional "rm -rf /" school of thought a video recording application could for example block other applications from changing video controls or switching the current TV channel. Another objective is to permit low priority applications working in background, which can be preempted by user controlled applications and automatically regain control of the device at a later time.

Since these features cannot be implemented entirely in user space V4L2 defines the VIDIOC_G_PRIORITY and VIDIOC_S_PRIORITY ioctls to request and query the access priority associate with a file descriptor. Opening a device assigns a medium priority, compatible with earlier versions of V4L2 and drivers not supporting these ioctls. Applications requiring a different priority will usually call VIDIOC_S_PRIORITY after verifying the device with the VIDIOC_QUERYCAP ioctl.

Ioctls changing driver properties, such as VIDIOC_S_INPUT, return an EBUSY error code after another application obtained higher priority. An event mechanism to notify applications about asynchronous property changes has been proposed but not added yet.

1.4. Video Inputs and Outputs

Video inputs and outputs are physical connectors of a device. These can be for example RF connectors (antenna/cable), CVBS a.k.a. Composite Video, S-Video or RGB connectors. Only video and VBI capture devices have inputs, output devices have outputs, at least one each. Radio devices have no video inputs or outputs.

To learn about the number and attributes of the available inputs and outputs applications can enumerate them with the VIDIOC_ENUMINPUT and VIDIOC_ENUMOUTPUT ioctl, respectively. The struct v4l2_input returned by the VIDIOC_ENUMINPUT ioctl also contains signal status information applicable when the current video input is queried.

The VIDIOC_G_INPUT and VIDIOC_G_OUTPUT ioctl return the index of the current video input or output. To select a different input or output applications call the VIDIOC_S_INPUT and VIDIOC_S_OUTPUT ioctl. Drivers must implement all the input ioctls when the device has one or more inputs, all the output ioctls when the device has one or more outputs.

Example 1.1. Information about the current video input

struct v4l2_input input;
int index;

if (-1 == ioctl (fd, VIDIOC_G_INPUT, &index)) {
	perror ("VIDIOC_G_INPUT");
	exit (EXIT_FAILURE);
}

memset (&input, 0, sizeof (input));
input.index = index;

if (-1 == ioctl (fd, VIDIOC_ENUMINPUT, &input)) {
	perror ("VIDIOC_ENUMINPUT");
	exit (EXIT_FAILURE);
}

printf ("Current input: %s\n", input.name);
      

Example 1.2. Switching to the first video input

int index;

index = 0;

if (-1 == ioctl (fd, VIDIOC_S_INPUT, &index)) {
	perror ("VIDIOC_S_INPUT");
	exit (EXIT_FAILURE);
}
      

1.5. Audio Inputs and Outputs

Audio inputs and outputs are physical connectors of a device. Video capture devices have inputs, output devices have outputs, zero or more each. Radio devices have no audio inputs or outputs. They have exactly one tuner which in fact is an audio source, but this API associates tuners with video inputs or outputs only, and radio devices have none of these.[5] A connector on a TV card to loop back the received audio signal to a sound card is not considered an audio output.

Audio and video inputs and outputs are associated. Selecting a video source also selects an audio source. This is most evident when the video and audio source is a tuner. Further audio connectors can combine with more than one video input or output. Assumed two composite video inputs and two audio inputs exist, there may be up to four valid combinations. The relation of video and audio connectors is defined in the audioset field of the respective struct v4l2_input or struct v4l2_output, where each bit represents the index number, starting at zero, of one audio input or output.

To learn about the number and attributes of the available inputs and outputs applications can enumerate them with the VIDIOC_ENUMAUDIO and VIDIOC_ENUMAUDOUT ioctl, respectively. The struct v4l2_audio returned by the VIDIOC_ENUMAUDIO ioctl also contains signal status information applicable when the current audio input is queried.

The VIDIOC_G_AUDIO and VIDIOC_G_AUDOUT ioctl report the current audio input and output, respectively. Note that, unlike VIDIOC_G_INPUT and VIDIOC_G_OUTPUT these ioctls return a structure as VIDIOC_ENUMAUDIO and VIDIOC_ENUMAUDOUT do, not just an index.

To select an audio input and change its properties applications call the VIDIOC_S_AUDIO ioctl. To select an audio output (which presently has no changeable properties) applications call the VIDIOC_S_AUDOUT ioctl.

Drivers must implement all input ioctls when the device has one or more inputs, all output ioctls when the device has one or more outputs. When the device has any audio inputs or outputs the driver must set the V4L2_CAP_AUDIO flag in the struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl.

Example 1.3. Information about the current audio input

struct v4l2_audio audio;

memset (&audio, 0, sizeof (audio));

if (-1 == ioctl (fd, VIDIOC_G_AUDIO, &audio)) {
	perror ("VIDIOC_G_AUDIO");
	exit (EXIT_FAILURE);
}

printf ("Current input: %s\n", audio.name);
      

Example 1.4. Switching to the first audio input

struct v4l2_audio audio;

memset (&audio, 0, sizeof (audio)); /* clear audio.mode, audio.reserved */

audio.index = 0;

if (-1 == ioctl (fd, VIDIOC_S_AUDIO, &audio)) {
	perror ("VIDIOC_S_AUDIO");
	exit (EXIT_FAILURE);
}
      

1.6. Tuners and Modulators

1.6.1. Tuners

Video input devices can have one or more tuners demodulating a RF signal. Each tuner is associated with one or more video inputs, depending on the number of RF connectors on the tuner. The type field of the respective struct v4l2_input returned by the VIDIOC_ENUMINPUT ioctl is set to V4L2_INPUT_TYPE_TUNER and its tuner field contains the index number of the tuner.

Radio devices have exactly one tuner with index zero, no video inputs.

To query and change tuner properties applications use the VIDIOC_G_TUNER and VIDIOC_S_TUNER ioctl, respectively. The struct v4l2_tuner returned by VIDIOC_G_TUNER also contains signal status information applicable when the tuner of the current video input, or a radio tuner is queried. Note that VIDIOC_S_TUNER does not switch the current tuner, when there is more than one at all. The tuner is solely determined by the current video input. Drivers must support both ioctls and set the V4L2_CAP_TUNER flag in the struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl when the device has one or more tuners.

1.6.2. Modulators

Video output devices can have one or more modulators, uh, modulating a video signal for radiation or connection to the antenna input of a TV set or video recorder. Each modulator is associated with one or more video outputs, depending on the number of RF connectors on the modulator. The type field of the respective struct v4l2_output returned by the VIDIOC_ENUMOUTPUT ioctl is set to V4L2_OUTPUT_TYPE_MODULATOR and its modulator field contains the index number of the modulator. This specification does not define radio output devices.

To query and change modulator properties applications use the VIDIOC_G_MODULATOR and VIDIOC_S_MODULATOR ioctl. Note that VIDIOC_S_MODULATOR does not switch the current modulator, when there is more than one at all. The modulator is solely determined by the current video output. Drivers must support both ioctls and set the V4L2_CAP_MODULATOR flag in the struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl when the device has one or more modulators.

1.6.3. Radio Frequency

To get and set the tuner or modulator radio frequency applications use the VIDIOC_G_FREQUENCY and VIDIOC_S_FREQUENCY ioctl which both take a pointer to a struct v4l2_frequency. These ioctls are used for TV and radio devices alike. Drivers must support both ioctls when the tuner or modulator ioctls are supported, or when the device is a radio device.

1.6.4. Satellite Receivers

To be discussed. See also proposals by Peter Schlaf, video4linux-list@redhat.com on 23 Oct 2002, subject: "Re: [V4L] Re: v4l2 api".

1.7. Video Standards

Video devices typically support one or more different video standards or variations of standards. Each video input and output may support another set of standards. This set is reported by the std field of struct v4l2_input and struct v4l2_output returned by the VIDIOC_ENUMINPUT and VIDIOC_ENUMOUTPUT ioctl, respectively.

V4L2 defines one bit for each analog video standard currently in use worldwide, and sets aside bits for driver defined standards, e. g. hybrid standards to watch NTSC video tapes on PAL TVs and vice versa. Applications can use the predefined bits to select a particular standard, although presenting the user a menu of supported standards is preferred. To enumerate and query the attributes of the supported standards applications use the VIDIOC_ENUMSTD ioctl.

Many of the defined standards are actually just variations of a few major standards. The hardware may in fact not distinguish between them, or do so internal and switch automatically. Therefore enumerated standards also contain sets of one or more standard bits.

Assume a hypothetic tuner capable of demodulating B/PAL, G/PAL and I/PAL signals. The first enumerated standard is a set of B and G/PAL, switched automatically depending on the selected radio frequency in UHF or VHF band. Enumeration gives a "PAL-B/G" or "PAL-I" choice. Similar a Composite input may collapse standards, enumerating "PAL-B/G/H/I", "NTSC-M" and "SECAM-D/K".[6]

To query and select the standard used by the current video input or output applications call the VIDIOC_G_STD and VIDIOC_S_STD ioctl, respectively. The received standard can be sensed with the VIDIOC_QUERYSTD ioctl. Note parameter of all these ioctls is a pointer to a v4l2_std_id type (a standard set), not an index into the standard enumeration.[7] Drivers must implement all video standard ioctls when the device has one or more video inputs or outputs.

Special rules apply to USB cameras where the notion of video standards makes little sense. More generally any capture device, output devices accordingly, which is

  • incapable of capturing fields or frames at the nominal rate of the video standard, or

  • where timestamps refer to the instant the field or frame was received by the driver, not the capture time, or

  • where sequence numbers refer to the frames received by the driver, not the captured frames.

Here the driver shall set the std field of struct v4l2_input and struct v4l2_output to zero, the VIDIOC_G_STD, VIDIOC_S_STD, VIDIOC_QUERYSTD and VIDIOC_ENUMSTD ioctls shall return the EINVAL error code.[8]

Example 1.5. Information about the current video standard

v4l2_std_id std_id;
struct v4l2_standard standard;

if (-1 == ioctl (fd, VIDIOC_G_STD, &std_id)) {
	/* Note when VIDIOC_ENUMSTD always returns EINVAL this
	   is no video device or it falls under the USB exception,
	   and VIDIOC_G_STD returning EINVAL is no error. */

	perror ("VIDIOC_G_STD");
	exit (EXIT_FAILURE);
}

memset (&standard, 0, sizeof (standard));
standard.index = 0;

while (0 == ioctl (fd, VIDIOC_ENUMSTD, &standard)) {
	if (standard.id & std_id) {
	       printf ("Current video standard: %s\n", standard.name);
	       exit (EXIT_SUCCESS);
	}

	standard.index++;
}

/* EINVAL indicates the end of the enumeration, which cannot be
   empty unless this device falls under the USB exception. */

if (errno == EINVAL || standard.index == 0) {
	perror ("VIDIOC_ENUMSTD");
	exit (EXIT_FAILURE);
}
      

Example 1.6. Listing the video standards supported by the current input

struct v4l2_input input;
struct v4l2_standard standard;

memset (&input, 0, sizeof (input));

if (-1 == ioctl (fd, VIDIOC_G_INPUT, &input.index)) {
	perror ("VIDIOC_G_INPUT");
	exit (EXIT_FAILURE);
}

if (-1 == ioctl (fd, VIDIOC_ENUMINPUT, &input)) {
	perror ("VIDIOC_ENUM_INPUT");
	exit (EXIT_FAILURE);
}

printf ("Current input %s supports:\n", input.name);

memset (&standard, 0, sizeof (standard));
standard.index = 0;

while (0 == ioctl (fd, VIDIOC_ENUMSTD, &standard)) {
	if (standard.id & input.std)
		printf ("%s\n", standard.name);

	standard.index++;
}

/* EINVAL indicates the end of the enumeration, which cannot be
   empty unless this device falls under the USB exception. */

if (errno != EINVAL || standard.index == 0) {
	perror ("VIDIOC_ENUMSTD");
	exit (EXIT_FAILURE);
}
      

Example 1.7. Selecting a new video standard

struct v4l2_input input;
v4l2_std_id std_id;

memset (&input, 0, sizeof (input));

if (-1 == ioctl (fd, VIDIOC_G_INPUT, &input.index)) {
	perror ("VIDIOC_G_INPUT");
	exit (EXIT_FAILURE);
}

if (-1 == ioctl (fd, VIDIOC_ENUMINPUT, &input)) {
	perror ("VIDIOC_ENUM_INPUT");
	exit (EXIT_FAILURE);
}

if (0 == (input.std & V4L2_STD_PAL_BG)) {
	fprintf (stderr, "Oops. B/G PAL is not supported.\n");
	exit (EXIT_FAILURE);
}

/* Note this is also supposed to work when only B
   or G/PAL is supported. */

std_id = V4L2_STD_PAL_BG;

if (-1 == ioctl (fd, VIDIOC_S_STD, &std_id)) {
	perror ("VIDIOC_S_STD");
	exit (EXIT_FAILURE);
}
      

1.8. User Controls

Devices typically have a number of user-settable controls such as brightness, saturation and so on, which would be presented to the user on a graphical user interface. But, different devices will have different controls available, and furthermore, the range of possible values, and the default value will vary from device to device. The control ioctls provide the information and a mechanism to create a nice user interface for these controls that will work correctly with any device.

All controls are accessed using an ID value. V4L2 defines several IDs for specific purposes. Drivers can also implement their own custom controls using V4L2_CID_PRIVATE_BASE and higher values. The pre-defined control IDs have the prefix V4L2_CID_, and are listed in Table 1.1, “Control IDs”. The ID is used when querying the attributes of a control, and when getting or setting the current value.

Generally applications should present controls to the user without assumptions about their purpose. Each control comes with a name string the user is supposed to understand. When the purpose is non-intuitive the driver writer should provide a user manual, a user interface plug-in or a driver specific panel application. Predefined IDs were introduced to change a few controls programmatically, for example to mute a device during a channel switch.

Drivers may enumerate different controls after switching the current video input or output, tuner or modulator, or audio input or output. Different in the sense of other bounds, another default and current value, step size or other menu items. A control with a certain custom ID can also change name and type.[9] Control values are stored globally, they do not change when switching except to stay within the reported bounds. They also do not change e. g. when the device is opened or closed, when the tuner radio frequency is changed or generally never without application request. Since V4L2 specifies no event mechanism, panel applications intended to cooperate with other panel applications (be they built into a larger application, as a TV viewer) may need to regularly poll control values to update their user interface.[10]

Table 1.1. Control IDs

IDTypeDescription
V4L2_CID_BASE First predefined ID, equal to V4L2_CID_BRIGHTNESS.
V4L2_CID_USER_BASE Synonym of V4L2_CID_BASE.
V4L2_CID_BRIGHTNESSintegerPicture brightness, or more precisely, the black level.
V4L2_CID_CONTRASTintegerPicture contrast or luma gain.
V4L2_CID_SATURATIONintegerPicture color saturation or chroma gain.
V4L2_CID_HUEintegerHue or color balance.
V4L2_CID_AUDIO_VOLUMEintegerOverall audio volume. Note some drivers also provide an OSS or ALSA mixer interface.
V4L2_CID_AUDIO_BALANCEintegerAudio stereo balance. Minimum corresponds to all the way left, maximum to right.
V4L2_CID_AUDIO_BASSintegerAudio bass adjustment.
V4L2_CID_AUDIO_TREBLEintegerAudio treble adjustment.
V4L2_CID_AUDIO_MUTEbooleanMute audio, i. e. set the volume to zero, however without affecting V4L2_CID_AUDIO_VOLUME. Like ALSA drivers, V4L2 drivers must mute at load time to avoid excessive noise. Actually the entire device should be reset to a low power consumption state.
V4L2_CID_AUDIO_LOUDNESSbooleanLoudness mode (bass boost).
V4L2_CID_BLACK_LEVELintegerAnother name for brightness (not a synonym of V4L2_CID_BRIGHTNESS). This control is deprecated and should not be used in new drivers and applications.
V4L2_CID_AUTO_WHITE_BALANCEbooleanAutomatic white balance (cameras).
V4L2_CID_DO_WHITE_BALANCEbuttonThis is an action control. When set (the value is ignored), the device will do a white balance and then hold the current setting. Contrast this with the boolean V4L2_CID_AUTO_WHITE_BALANCE, which, when activated, keeps adjusting the white balance.
V4L2_CID_RED_BALANCEintegerRed chroma balance.
V4L2_CID_BLUE_BALANCEintegerBlue chroma balance.
V4L2_CID_GAMMAintegerGamma adjust.
V4L2_CID_WHITENESSintegerWhiteness for grey-scale devices. This is a synonym for V4L2_CID_GAMMA. This control is deprecated and should not be used in new drivers and applications.
V4L2_CID_EXPOSUREintegerExposure (cameras). [Unit?]
V4L2_CID_AUTOGAINbooleanAutomatic gain/exposure control.
V4L2_CID_GAINintegerGain control.
V4L2_CID_HFLIPbooleanMirror the picture horizontally.
V4L2_CID_VFLIPbooleanMirror the picture vertically.
V4L2_CID_HCENTER_DEPRECATED (formerly V4L2_CID_HCENTER)integerHorizontal image centering. This control is deprecated. New drivers and applications should use the Camera class controls V4L2_CID_PAN_ABSOLUTE, V4L2_CID_PAN_RELATIVE and V4L2_CID_PAN_RESET instead.
V4L2_CID_VCENTER_DEPRECATED (formerly V4L2_CID_VCENTER)integerVertical image centering. Centering is intended to physically adjust cameras. For image cropping see Section 1.11, “Image Cropping, Insertion and Scaling”, for clipping Section 4.2, “Video Overlay Interface”. This control is deprecated. New drivers and applications should use the Camera class controls V4L2_CID_TILT_ABSOLUTE, V4L2_CID_TILT_RELATIVE and V4L2_CID_TILT_RESET instead.
V4L2_CID_POWER_LINE_FREQUENCYenumEnables a power line frequency filter to avoid flicker. Possible values for enum v4l2_power_line_frequency are: V4L2_CID_POWER_LINE_FREQUENCY_DISABLED (0), V4L2_CID_POWER_LINE_FREQUENCY_50HZ (1) and V4L2_CID_POWER_LINE_FREQUENCY_60HZ (2).
V4L2_CID_HUE_AUTObooleanEnables automatic hue control by the device. The effect of setting V4L2_CID_HUE while automatic hue control is enabled is undefined, drivers should ignore such request.
V4L2_CID_WHITE_BALANCE_TEMPERATUREintegerThis control specifies the white balance settings as a color temperature in Kelvin. A driver should have a minimum of 2800 (incandescent) to 6500 (daylight). For more information about color temperature see Wikipedia.
V4L2_CID_SHARPNESSintegerAdjusts the sharpness filters in a camera. The minimum value disables the filters, higher values give a sharper picture.
V4L2_CID_BACKLIGHT_COMPENSATIONintegerAdjusts the backlight compensation in a camera. The minimum value disables backlight compensation.
V4L2_CID_CHROMA_AGCbooleanChroma automatic gain control.
V4L2_CID_COLOR_KILLERbooleanEnable the color killer (i. e. force a black & white image in case of a weak video signal).
V4L2_CID_COLORFXenumSelects a color effect. Possible values for enum v4l2_colorfx are: V4L2_COLORFX_NONE (0), V4L2_COLORFX_BW (1) and V4L2_COLORFX_SEPIA (2).
V4L2_CID_LASTP1 End of the predefined control IDs (currently V4L2_CID_COLORFX + 1).
V4L2_CID_PRIVATE_BASE ID of the first custom (driver specific) control. Applications depending on particular custom controls should check the driver name and version, see Section 1.2, “Querying Capabilities”.

Applications can enumerate the available controls with the VIDIOC_QUERYCTRL and VIDIOC_QUERYMENU ioctls, get and set a control value with the VIDIOC_G_CTRL and VIDIOC_S_CTRL ioctls. Drivers must implement VIDIOC_QUERYCTRL, VIDIOC_G_CTRL and VIDIOC_S_CTRL when the device has one or more controls, VIDIOC_QUERYMENU when it has one or more menu type controls.

Example 1.8. Enumerating all controls

struct v4l2_queryctrl queryctrl;
struct v4l2_querymenu querymenu;

static void
enumerate_menu (void)
{
	printf ("  Menu items:\n");

	memset (&querymenu, 0, sizeof (querymenu));
	querymenu.id = queryctrl.id;

	for (querymenu.index = queryctrl.minimum;
	     querymenu.index <= queryctrl.maximum;
	      querymenu.index++) {
		if (0 == ioctl (fd, VIDIOC_QUERYMENU, &querymenu)) {
			printf ("  %s\n", querymenu.name);
		} else {
			perror ("VIDIOC_QUERYMENU");
			exit (EXIT_FAILURE);
		}
	}
}

memset (&queryctrl, 0, sizeof (queryctrl));

for (queryctrl.id = V4L2_CID_BASE;
     queryctrl.id < V4L2_CID_LASTP1;
     queryctrl.id++) {
	if (0 == ioctl (fd, VIDIOC_QUERYCTRL, &queryctrl)) {
		if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)
			continue;

		printf ("Control %s\n", queryctrl.name);

		if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
			enumerate_menu ();
	} else {
		if (errno == EINVAL)
			continue;

		perror ("VIDIOC_QUERYCTRL");
		exit (EXIT_FAILURE);
	}
}

for (queryctrl.id = V4L2_CID_PRIVATE_BASE;;
     queryctrl.id++) {
	if (0 == ioctl (fd, VIDIOC_QUERYCTRL, &queryctrl)) {
		if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)
			continue;

		printf ("Control %s\n", queryctrl.name);

		if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
			enumerate_menu ();
	} else {
		if (errno == EINVAL)
			break;

		perror ("VIDIOC_QUERYCTRL");
		exit (EXIT_FAILURE);
	}
}

Example 1.9. Changing controls

struct v4l2_queryctrl queryctrl;
struct v4l2_control control;

memset (&queryctrl, 0, sizeof (queryctrl));
queryctrl.id = V4L2_CID_BRIGHTNESS;

if (-1 == ioctl (fd, VIDIOC_QUERYCTRL, &queryctrl)) {
	if (errno != EINVAL) {
		perror ("VIDIOC_QUERYCTRL");
		exit (EXIT_FAILURE);
	} else {
		printf ("V4L2_CID_BRIGHTNESS is not supported\n");
	}
} else if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) {
	printf ("V4L2_CID_BRIGHTNESS is not supported\n");
} else {
	memset (&control, 0, sizeof (control));
	control.id = V4L2_CID_BRIGHTNESS;
	control.value = queryctrl.default_value;

	if (-1 == ioctl (fd, VIDIOC_S_CTRL, &control)) {
		perror ("VIDIOC_S_CTRL");
		exit (EXIT_FAILURE);
	}
}

memset (&control, 0, sizeof (control));
control.id = V4L2_CID_CONTRAST;

if (0 == ioctl (fd, VIDIOC_G_CTRL, &control)) {
	control.value += 1;

	/* The driver may clamp the value or return ERANGE, ignored here */

	if (-1 == ioctl (fd, VIDIOC_S_CTRL, &control)
	    && errno != ERANGE) {
		perror ("VIDIOC_S_CTRL");
		exit (EXIT_FAILURE);
	}
/* Ignore if V4L2_CID_CONTRAST is unsupported */
} else if (errno != EINVAL) {
	perror ("VIDIOC_G_CTRL");
	exit (EXIT_FAILURE);
}

control.id = V4L2_CID_AUDIO_MUTE;
control.value = TRUE; /* silence */

/* Errors ignored */
ioctl (fd, VIDIOC_S_CTRL, &control);

1.9. Extended Controls

1.9.1. Introduction

The control mechanism as originally designed was meant to be used for user settings (brightness, saturation, etc). However, it turned out to be a very useful model for implementing more complicated driver APIs where each driver implements only a subset of a larger API.

The MPEG encoding API was the driving force behind designing and implementing this extended control mechanism: the MPEG standard is quite large and the currently supported hardware MPEG encoders each only implement a subset of this standard. Further more, many parameters relating to how the video is encoded into an MPEG stream are specific to the MPEG encoding chip since the MPEG standard only defines the format of the resulting MPEG stream, not how the video is actually encoded into that format.

Unfortunately, the original control API lacked some features needed for these new uses and so it was extended into the (not terribly originally named) extended control API.

Even though the MPEG encoding API was the first effort to use the Extended Control API, nowadays there are also other classes of Extended Controls, such as Camera Controls and FM Transmitter Controls. The Extended Controls API as well as all Extended Controls classes are described in the following text.

1.9.2. The Extended Control API

Three new ioctls are available: VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS and VIDIOC_TRY_EXT_CTRLS. These ioctls act on arrays of controls (as opposed to the VIDIOC_G_CTRL and VIDIOC_S_CTRL ioctls that act on a single control). This is needed since it is often required to atomically change several controls at once.

Each of the new ioctls expects a pointer to a struct v4l2_ext_controls. This structure contains a pointer to the control array, a count of the number of controls in that array and a control class. Control classes are used to group similar controls into a single class. For example, control class V4L2_CTRL_CLASS_USER contains all user controls (i. e. all controls that can also be set using the old VIDIOC_S_CTRL ioctl). Control class V4L2_CTRL_CLASS_MPEG contains all controls relating to MPEG encoding, etc.

All controls in the control array must belong to the specified control class. An error is returned if this is not the case.

It is also possible to use an empty control array (count == 0) to check whether the specified control class is supported.

The control array is a struct v4l2_ext_control array. The v4l2_ext_control structure is very similar to struct v4l2_control, except for the fact that it also allows for 64-bit values and pointers to be passed.

It is important to realize that due to the flexibility of controls it is necessary to check whether the control you want to set actually is supported in the driver and what the valid range of values is. So use the VIDIOC_QUERYCTRL and VIDIOC_QUERYMENU ioctls to check this. Also note that it is possible that some of the menu indices in a control of type V4L2_CTRL_TYPE_MENU may not be supported (VIDIOC_QUERYMENU will return an error). A good example is the list of supported MPEG audio bitrates. Some drivers only support one or two bitrates, others support a wider range.

1.9.3. Enumerating Extended Controls

The recommended way to enumerate over the extended controls is by using VIDIOC_QUERYCTRL in combination with the V4L2_CTRL_FLAG_NEXT_CTRL flag:

struct v4l2_queryctrl qctrl;

qctrl.id = V4L2_CTRL_FLAG_NEXT_CTRL;
while (0 == ioctl (fd, VIDIOC_QUERYCTRL, &qctrl)) {
	/* ... */
	qctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL;
}

The initial control ID is set to 0 ORed with the V4L2_CTRL_FLAG_NEXT_CTRL flag. The VIDIOC_QUERYCTRL ioctl will return the first control with a higher ID than the specified one. When no such controls are found an error is returned.

If you want to get all controls within a specific control class, then you can set the initial qctrl.id value to the control class and add an extra check to break out of the loop when a control of another control class is found:

qctrl.id = V4L2_CTRL_CLASS_MPEG | V4L2_CTRL_FLAG_NEXT_CTRL;
while (0 == ioctl (fd, VIDIOC_QUERYCTRL, &qctrl)) {
	if (V4L2_CTRL_ID2CLASS (qctrl.id) != V4L2_CTRL_CLASS_MPEG)
		break;
		/* ... */
		qctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL;
	}

The 32-bit qctrl.id value is subdivided into three bit ranges: the top 4 bits are reserved for flags (e. g. V4L2_CTRL_FLAG_NEXT_CTRL) and are not actually part of the ID. The remaining 28 bits form the control ID, of which the most significant 12 bits define the control class and the least significant 16 bits identify the control within the control class. It is guaranteed that these last 16 bits are always non-zero for controls. The range of 0x1000 and up are reserved for driver-specific controls. The macro V4L2_CTRL_ID2CLASS(id) returns the control class ID based on a control ID.

If the driver does not support extended controls, then VIDIOC_QUERYCTRL will fail when used in combination with V4L2_CTRL_FLAG_NEXT_CTRL. In that case the old method of enumerating control should be used (see 1.8). But if it is supported, then it is guaranteed to enumerate over all controls, including driver-private controls.

1.9.4. Creating Control Panels

It is possible to create control panels for a graphical user interface where the user can select the various controls. Basically you will have to iterate over all controls using the method described above. Each control class starts with a control of type V4L2_CTRL_TYPE_CTRL_CLASS. VIDIOC_QUERYCTRL will return the name of this control class which can be used as the title of a tab page within a control panel.

The flags field of struct v4l2_queryctrl also contains hints on the behavior of the control. See the VIDIOC_QUERYCTRL documentation for more details.

1.9.5. MPEG Control Reference

Below all controls within the MPEG control class are described. First the generic controls, then controls specific for certain hardware.

1.9.5.1. Generic MPEG Controls

Table 1.2. MPEG Control IDs

IDType 
 Description
    
V4L2_CID_MPEG_CLASS class 
 The MPEG class descriptor. Calling VIDIOC_QUERYCTRL for this control will return a description of this control class. This description can be used as the caption of a Tab page in a GUI, for example.
    
V4L2_CID_MPEG_STREAM_TYPE enum v4l2_mpeg_stream_type 
 The MPEG-1, -2 or -4 output stream type. One cannot assume anything here. Each hardware MPEG encoder tends to support different subsets of the available MPEG stream types. The currently defined stream types are:
 
V4L2_MPEG_STREAM_TYPE_MPEG2_PS MPEG-2 program stream
V4L2_MPEG_STREAM_TYPE_MPEG2_TS MPEG-2 transport stream
V4L2_MPEG_STREAM_TYPE_MPEG1_SS MPEG-1 system stream
V4L2_MPEG_STREAM_TYPE_MPEG2_DVD MPEG-2 DVD-compatible stream
V4L2_MPEG_STREAM_TYPE_MPEG1_VCD MPEG-1 VCD-compatible stream
V4L2_MPEG_STREAM_TYPE_MPEG2_SVCD MPEG-2 SVCD-compatible stream
    
V4L2_CID_MPEG_STREAM_PID_PMT integer 
 Program Map Table Packet ID for the MPEG transport stream (default 16)
    
V4L2_CID_MPEG_STREAM_PID_AUDIO integer 
 Audio Packet ID for the MPEG transport stream (default 256)
    
V4L2_CID_MPEG_STREAM_PID_VIDEO integer 
 Video Packet ID for the MPEG transport stream (default 260)
    
V4L2_CID_MPEG_STREAM_PID_PCR integer 
 Packet ID for the MPEG transport stream carrying PCR fields (default 259)
    
V4L2_CID_MPEG_STREAM_PES_ID_AUDIO integer 
 Audio ID for MPEG PES
    
V4L2_CID_MPEG_STREAM_PES_ID_VIDEO integer 
 Video ID for MPEG PES
    
V4L2_CID_MPEG_STREAM_VBI_FMT enum v4l2_mpeg_stream_vbi_fmt 
 Some cards can embed VBI data (e. g. Closed Caption, Teletext) into the MPEG stream. This control selects whether VBI data should be embedded, and if so, what embedding method should be used. The list of possible VBI formats depends on the driver. The currently defined VBI format types are:
 
V4L2_MPEG_STREAM_VBI_FMT_NONE No VBI in the MPEG stream
V4L2_MPEG_STREAM_VBI_FMT_IVTV VBI in private packets, IVTV format (documented in the kernel sources in the file Documentation/video4linux/cx2341x/README.vbi)
    
V4L2_CID_MPEG_AUDIO_SAMPLING_FREQ enum v4l2_mpeg_audio_sampling_freq 
 MPEG Audio sampling frequency. Possible values are:
 
V4L2_MPEG_AUDIO_SAMPLING_FREQ_44100 44.1 kHz
V4L2_MPEG_AUDIO_SAMPLING_FREQ_48000 48 kHz
V4L2_MPEG_AUDIO_SAMPLING_FREQ_32000 32 kHz
    
V4L2_CID_MPEG_AUDIO_ENCODING enum v4l2_mpeg_audio_encoding 
 MPEG Audio encoding. Possible values are:
 
V4L2_MPEG_AUDIO_ENCODING_LAYER_1 MPEG-1/2 Layer I encoding
V4L2_MPEG_AUDIO_ENCODING_LAYER_2 MPEG-1/2 Layer II encoding
V4L2_MPEG_AUDIO_ENCODING_LAYER_3 MPEG-1/2 Layer III encoding
V4L2_MPEG_AUDIO_ENCODING_AAC MPEG-2/4 AAC (Advanced Audio Coding)
V4L2_MPEG_AUDIO_ENCODING_AC3 AC-3 aka ATSC A/52 encoding
    
V4L2_CID_MPEG_AUDIO_L1_BITRATE enum v4l2_mpeg_audio_l1_bitrate 
 MPEG-1/2 Layer I bitrate. Possible values are:
 
V4L2_MPEG_AUDIO_L1_BITRATE_32K 32 kbit/s
V4L2_MPEG_AUDIO_L1_BITRATE_64K 64 kbit/s
V4L2_MPEG_AUDIO_L1_BITRATE_96K 96 kbit/s
V4L2_MPEG_AUDIO_L1_BITRATE_128K 128 kbit/s
V4L2_MPEG_AUDIO_L1_BITRATE_160K 160 kbit/s
V4L2_MPEG_AUDIO_L1_BITRATE_192K 192 kbit/s
V4L2_MPEG_AUDIO_L1_BITRATE_224K 224 kbit/s
V4L2_MPEG_AUDIO_L1_BITRATE_256K 256 kbit/s
V4L2_MPEG_AUDIO_L1_BITRATE_288K 288 kbit/s
V4L2_MPEG_AUDIO_L1_BITRATE_320K 320 kbit/s
V4L2_MPEG_AUDIO_L1_BITRATE_352K 352 kbit/s
V4L2_MPEG_AUDIO_L1_BITRATE_384K 384 kbit/s
V4L2_MPEG_AUDIO_L1_BITRATE_416K 416 kbit/s
V4L2_MPEG_AUDIO_L1_BITRATE_448K 448 kbit/s
    
V4L2_CID_MPEG_AUDIO_L2_BITRATE enum v4l2_mpeg_audio_l2_bitrate 
 MPEG-1/2 Layer II bitrate. Possible values are:
 
V4L2_MPEG_AUDIO_L2_BITRATE_32K 32 kbit/s
V4L2_MPEG_AUDIO_L2_BITRATE_48K 48 kbit/s
V4L2_MPEG_AUDIO_L2_BITRATE_56K 56 kbit/s
V4L2_MPEG_AUDIO_L2_BITRATE_64K 64 kbit/s
V4L2_MPEG_AUDIO_L2_BITRATE_80K 80 kbit/s
V4L2_MPEG_AUDIO_L2_BITRATE_96K 96 kbit/s
V4L2_MPEG_AUDIO_L2_BITRATE_112K 112 kbit/s
V4L2_MPEG_AUDIO_L2_BITRATE_128K 128 kbit/s
V4L2_MPEG_AUDIO_L2_BITRATE_160K 160 kbit/s
V4L2_MPEG_AUDIO_L2_BITRATE_192K 192 kbit/s
V4L2_MPEG_AUDIO_L2_BITRATE_224K 224 kbit/s
V4L2_MPEG_AUDIO_L2_BITRATE_256K 256 kbit/s
V4L2_MPEG_AUDIO_L2_BITRATE_320K 320 kbit/s
V4L2_MPEG_AUDIO_L2_BITRATE_384K 384 kbit/s
    
V4L2_CID_MPEG_AUDIO_L3_BITRATE enum v4l2_mpeg_audio_l3_bitrate 
 MPEG-1/2 Layer III bitrate. Possible values are:
 
V4L2_MPEG_AUDIO_L3_BITRATE_32K 32 kbit/s
V4L2_MPEG_AUDIO_L3_BITRATE_40K 40 kbit/s
V4L2_MPEG_AUDIO_L3_BITRATE_48K 48 kbit/s
V4L2_MPEG_AUDIO_L3_BITRATE_56K 56 kbit/s
V4L2_MPEG_AUDIO_L3_BITRATE_64K 64 kbit/s
V4L2_MPEG_AUDIO_L3_BITRATE_80K 80 kbit/s
V4L2_MPEG_AUDIO_L3_BITRATE_96K 96 kbit/s
V4L2_MPEG_AUDIO_L3_BITRATE_112K 112 kbit/s
V4L2_MPEG_AUDIO_L3_BITRATE_128K 128 kbit/s
V4L2_MPEG_AUDIO_L3_BITRATE_160K 160 kbit/s
V4L2_MPEG_AUDIO_L3_BITRATE_192K 192 kbit/s
V4L2_MPEG_AUDIO_L3_BITRATE_224K 224 kbit/s
V4L2_MPEG_AUDIO_L3_BITRATE_256K 256 kbit/s
V4L2_MPEG_AUDIO_L3_BITRATE_320K 320 kbit/s
    
V4L2_CID_MPEG_AUDIO_AAC_BITRATE integer 
 AAC bitrate in bits per second.
    
V4L2_CID_MPEG_AUDIO_AC3_BITRATE enum v4l2_mpeg_audio_ac3_bitrate 
 AC-3 bitrate. Possible values are:
 
V4L2_MPEG_AUDIO_AC3_BITRATE_32K 32 kbit/s
V4L2_MPEG_AUDIO_AC3_BITRATE_40K 40 kbit/s
V4L2_MPEG_AUDIO_AC3_BITRATE_48K 48 kbit/s
V4L2_MPEG_AUDIO_AC3_BITRATE_56K 56 kbit/s
V4L2_MPEG_AUDIO_AC3_BITRATE_64K 64 kbit/s
V4L2_MPEG_AUDIO_AC3_BITRATE_80K 80 kbit/s
V4L2_MPEG_AUDIO_AC3_BITRATE_96K 96 kbit/s
V4L2_MPEG_AUDIO_AC3_BITRATE_112K 112 kbit/s
V4L2_MPEG_AUDIO_AC3_BITRATE_128K 128 kbit/s
V4L2_MPEG_AUDIO_AC3_BITRATE_160K 160 kbit/s
V4L2_MPEG_AUDIO_AC3_BITRATE_192K 192 kbit/s
V4L2_MPEG_AUDIO_AC3_BITRATE_224K 224 kbit/s
V4L2_MPEG_AUDIO_AC3_BITRATE_256K 256 kbit/s
V4L2_MPEG_AUDIO_AC3_BITRATE_320K 320 kbit/s
V4L2_MPEG_AUDIO_AC3_BITRATE_384K 384 kbit/s
V4L2_MPEG_AUDIO_AC3_BITRATE_448K 448 kbit/s
V4L2_MPEG_AUDIO_AC3_BITRATE_512K 512 kbit/s
V4L2_MPEG_AUDIO_AC3_BITRATE_576K 576 kbit/s
V4L2_MPEG_AUDIO_AC3_BITRATE_640K 640 kbit/s
    
V4L2_CID_MPEG_AUDIO_MODE enum v4l2_mpeg_audio_mode 
 MPEG Audio mode. Possible values are:
 
V4L2_MPEG_AUDIO_MODE_STEREO Stereo
V4L2_MPEG_AUDIO_MODE_JOINT_STEREO Joint Stereo
V4L2_MPEG_AUDIO_MODE_DUAL Bilingual
V4L2_MPEG_AUDIO_MODE_MONO Mono
    
V4L2_CID_MPEG_AUDIO_MODE_EXTENSION enum v4l2_mpeg_audio_mode_extension 
 Joint Stereo audio mode extension. In Layer I and II they indicate which subbands are in intensity stereo. All other subbands are coded in stereo. Layer III is not (yet) supported. Possible values are:
 
V4L2_MPEG_AUDIO_MODE_EXTENSION_BOUND_4 Subbands 4-31 in intensity stereo
V4L2_MPEG_AUDIO_MODE_EXTENSION_BOUND_8 Subbands 8-31 in intensity stereo
V4L2_MPEG_AUDIO_MODE_EXTENSION_BOUND_12 Subbands 12-31 in intensity stereo
V4L2_MPEG_AUDIO_MODE_EXTENSION_BOUND_16 Subbands 16-31 in intensity stereo
    
V4L2_CID_MPEG_AUDIO_EMPHASIS enum v4l2_mpeg_audio_emphasis 
 Audio Emphasis. Possible values are:
 
V4L2_MPEG_AUDIO_EMPHASIS_NONE None
V4L2_MPEG_AUDIO_EMPHASIS_50_DIV_15_uS 50/15 microsecond emphasis
V4L2_MPEG_AUDIO_EMPHASIS_CCITT_J17 CCITT J.17
    
V4L2_CID_MPEG_AUDIO_CRC enum v4l2_mpeg_audio_crc 
 CRC method. Possible values are:
 
V4L2_MPEG_AUDIO_CRC_NONE None
V4L2_MPEG_AUDIO_CRC_CRC16 16 bit parity check
    
V4L2_CID_MPEG_AUDIO_MUTE boolean 
 Mutes the audio when capturing. This is not done by muting audio hardware, which can still produce a slight hiss, but in the encoder itself, guaranteeing a fixed and reproducable audio bitstream. 0 = unmuted, 1 = muted.
    
V4L2_CID_MPEG_VIDEO_ENCODING enum v4l2_mpeg_video_encoding 
 MPEG Video encoding method. Possible values are:
 
V4L2_MPEG_VIDEO_ENCODING_MPEG_1 MPEG-1 Video encoding
V4L2_MPEG_VIDEO_ENCODING_MPEG_2 MPEG-2 Video encoding
V4L2_MPEG_VIDEO_ENCODING_MPEG_4_AVC MPEG-4 AVC (H.264) Video encoding
    
V4L2_CID_MPEG_VIDEO_ASPECT enum v4l2_mpeg_video_aspect 
 Video aspect. Possible values are:
 
V4L2_MPEG_VIDEO_ASPECT_1x1  
V4L2_MPEG_VIDEO_ASPECT_4x3  
V4L2_MPEG_VIDEO_ASPECT_16x9  
V4L2_MPEG_VIDEO_ASPECT_221x100  
    
V4L2_CID_MPEG_VIDEO_B_FRAMES integer 
 Number of B-Frames (default 2)
    
V4L2_CID_MPEG_VIDEO_GOP_SIZE integer 
 GOP size (default 12)
    
V4L2_CID_MPEG_VIDEO_GOP_CLOSURE boolean 
 GOP closure (default 1)
    
V4L2_CID_MPEG_VIDEO_PULLDOWN boolean 
 Enable 3:2 pulldown (default 0)
    
V4L2_CID_MPEG_VIDEO_BITRATE_MODE enum v4l2_mpeg_video_bitrate_mode 
 Video bitrate mode. Possible values are:
 
V4L2_MPEG_VIDEO_BITRATE_MODE_VBR Variable bitrate
V4L2_MPEG_VIDEO_BITRATE_MODE_CBR Constant bitrate
    
V4L2_CID_MPEG_VIDEO_BITRATE integer 
 Video bitrate in bits per second.
    
V4L2_CID_MPEG_VIDEO_BITRATE_PEAK integer 
 Peak video bitrate in bits per second. Must be larger or equal to the average video bitrate. It is ignored if the video bitrate mode is set to constant bitrate.
    
V4L2_CID_MPEG_VIDEO_TEMPORAL_DECIMATION integer 
 For every captured frame, skip this many subsequent frames (default 0).
    
V4L2_CID_MPEG_VIDEO_MUTE boolean 
 "Mutes" the video to a fixed color when capturing. This is useful for testing, to produce a fixed video bitstream. 0 = unmuted, 1 = muted.
    
V4L2_CID_MPEG_VIDEO_MUTE_YUV integer 
 Sets the "mute" color of the video. The supplied 32-bit integer is interpreted as follows (bit 0 = least significant bit):
 
Bit 0:7V chrominance information
Bit 8:15U chrominance information
Bit 16:23Y luminance information
Bit 24:31Must be zero.

1.9.5.2. CX2341x MPEG Controls

The following MPEG class controls deal with MPEG encoding settings that are specific to the Conexant CX23415 and CX23416 MPEG encoding chips.

Table 1.3. CX2341x Control IDs

IDType 
 Description
    
V4L2_CID_MPEG_CX2341X_VIDEO_SPATIAL_FILTER_MODE enum v4l2_mpeg_cx2341x_video_spatial_filter_mode 
 Sets the Spatial Filter mode (default MANUAL). Possible values are:
 
V4L2_MPEG_CX2341X_VIDEO_SPATIAL_FILTER_MODE_MANUAL Choose the filter manually
V4L2_MPEG_CX2341X_VIDEO_SPATIAL_FILTER_MODE_AUTO Choose the filter automatically
    
V4L2_CID_MPEG_CX2341X_VIDEO_SPATIAL_FILTER integer (0-15) 
 The setting for the Spatial Filter. 0 = off, 15 = maximum. (Default is 0.)
    
V4L2_CID_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE enum v4l2_mpeg_cx2341x_video_luma_spatial_filter_type 
 Select the algorithm to use for the Luma Spatial Filter (default 1D_HOR). Possible values:
 
V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_OFF No filter
V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_1D_HOR One-dimensional horizontal
V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_1D_VERT One-dimensional vertical
V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_2D_HV_SEPARABLE Two-dimensional separable
V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_2D_SYM_NON_SEPARABLE Two-dimensional symmetrical non-separable
    
V4L2_CID_MPEG_CX2341X_VIDEO_CHROMA_SPATIAL_FILTER_TYPE enum v4l2_mpeg_cx2341x_video_chroma_spatial_filter_type 
 Select the algorithm for the Chroma Spatial Filter (default 1D_HOR). Possible values are:
 
V4L2_MPEG_CX2341X_VIDEO_CHROMA_SPATIAL_FILTER_TYPE_OFF No filter
V4L2_MPEG_CX2341X_VIDEO_CHROMA_SPATIAL_FILTER_TYPE_1D_HOR One-dimensional horizontal
    
V4L2_CID_MPEG_CX2341X_VIDEO_TEMPORAL_FILTER_MODE enum v4l2_mpeg_cx2341x_video_temporal_filter_mode 
 Sets the Temporal Filter mode (default MANUAL). Possible values are:
 
V4L2_MPEG_CX2341X_VIDEO_TEMPORAL_FILTER_MODE_MANUAL Choose the filter manually
V4L2_MPEG_CX2341X_VIDEO_TEMPORAL_FILTER_MODE_AUTO Choose the filter automatically
    
V4L2_CID_MPEG_CX2341X_VIDEO_TEMPORAL_FILTER integer (0-31) 
 The setting for the Temporal Filter. 0 = off, 31 = maximum. (Default is 8 for full-scale capturing and 0 for scaled capturing.)
    
V4L2_CID_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE enum v4l2_mpeg_cx2341x_video_median_filter_type 
 Median Filter Type (default OFF). Possible values are:
 
V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_OFF No filter
V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_HOR Horizontal filter
V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_VERT Vertical filter
V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_HOR_VERT Horizontal and vertical filter
V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_DIAG Diagonal filter
    
V4L2_CID_MPEG_CX2341X_VIDEO_LUMA_MEDIAN_FILTER_BOTTOM integer (0-255) 
 Threshold above which the luminance median filter is enabled (default 0)
    
V4L2_CID_MPEG_CX2341X_VIDEO_LUMA_MEDIAN_FILTER_TOP integer (0-255) 
 Threshold below which the luminance median filter is enabled (default 255)
    
V4L2_CID_MPEG_CX2341X_VIDEO_CHROMA_MEDIAN_FILTER_BOTTOM integer (0-255) 
 Threshold above which the chroma median filter is enabled (default 0)
    
V4L2_CID_MPEG_CX2341X_VIDEO_CHROMA_MEDIAN_FILTER_TOP integer (0-255) 
 Threshold below which the chroma median filter is enabled (default 255)
    
V4L2_CID_MPEG_CX2341X_STREAM_INSERT_NAV_PACKETS boolean 
 The CX2341X MPEG encoder can insert one empty MPEG-2 PES packet into the stream between every four video frames. The packet size is 2048 bytes, including the packet_start_code_prefix and stream_id fields. The stream_id is 0xBF (private stream 2). The payload consists of 0x00 bytes, to be filled in by the application. 0 = do not insert, 1 = insert packets.

1.9.6. Camera Control Reference

The Camera class includes controls for mechanical (or equivalent digital) features of a device such as controllable lenses or sensors.

Table 1.4. Camera Control IDs

IDType 
 Description
    
V4L2_CID_CAMERA_CLASS class 
 The Camera class descriptor. Calling VIDIOC_QUERYCTRL for this control will return a description of this control class.
    
V4L2_CID_EXPOSURE_AUTO enum v4l2_exposure_auto_type 
 Enables automatic adjustments of the exposure time and/or iris aperture. The effect of manual changes of the exposure time or iris aperture while these features are enabled is undefined, drivers should ignore such requests. Possible values are:
 
V4L2_EXPOSURE_AUTO Automatic exposure time, automatic iris aperture.
V4L2_EXPOSURE_MANUAL Manual exposure time, manual iris.
V4L2_EXPOSURE_SHUTTER_PRIORITY Manual exposure time, auto iris.
V4L2_EXPOSURE_APERTURE_PRIORITY Auto exposure time, manual iris.
    
V4L2_CID_EXPOSURE_ABSOLUTE integer 
 Determines the exposure time of the camera sensor. The exposure time is limited by the frame interval. Drivers should interpret the values as 100 µs units, where the value 1 stands for 1/10000th of a second, 10000 for 1 second and 100000 for 10 seconds.
    
V4L2_CID_EXPOSURE_AUTO_PRIORITY boolean 
 When V4L2_CID_EXPOSURE_AUTO is set to AUTO or APERTURE_PRIORITY, this control determines if the device may dynamically vary the frame rate. By default this feature is disabled (0) and the frame rate must remain constant.
    
V4L2_CID_PAN_RELATIVE integer 
 This control turns the camera horizontally by the specified amount. The unit is undefined. A positive value moves the camera to the right (clockwise when viewed from above), a negative value to the left. A value of zero does not cause motion. This is a write-only control.
    
V4L2_CID_TILT_RELATIVE integer 
 This control turns the camera vertically by the specified amount. The unit is undefined. A positive value moves the camera up, a negative value down. A value of zero does not cause motion. This is a write-only control.
    
V4L2_CID_PAN_RESET button 
 When this control is set, the camera moves horizontally to the default position.
    
V4L2_CID_TILT_RESET button 
 When this control is set, the camera moves vertically to the default position.
    
V4L2_CID_PAN_ABSOLUTE integer 
 This control turns the camera horizontally to the specified position. Positive values move the camera to the right (clockwise when viewed from above), negative values to the left. Drivers should interpret the values as arc seconds, with valid values between -180 * 3600 and +180 * 3600 inclusive.
    
V4L2_CID_TILT_ABSOLUTE integer 
 This control turns the camera vertically to the specified position. Positive values move the camera up, negative values down. Drivers should interpret the values as arc seconds, with valid values between -180 * 3600 and +180 * 3600 inclusive.
    
V4L2_CID_FOCUS_ABSOLUTE integer 
 This control sets the focal point of the camera to the specified position. The unit is undefined. Positive values set the focus closer to the camera, negative values towards infinity.
    
V4L2_CID_FOCUS_RELATIVE integer 
 This control moves the focal point of the camera by the specified amount. The unit is undefined. Positive values move the focus closer to the camera, negative values towards infinity. This is a write-only control.
    
V4L2_CID_FOCUS_AUTO boolean 
 Enables automatic focus adjustments. The effect of manual focus adjustments while this feature is enabled is undefined, drivers should ignore such requests.
    
V4L2_CID_ZOOM_ABSOLUTE integer 
 Specify the objective lens focal length as an absolute value. The zoom unit is driver-specific and its value should be a positive integer.
    
V4L2_CID_ZOOM_RELATIVE integer 
 Specify the objective lens focal length relatively to the current value. Positive values move the zoom lens group towards the telephoto direction, negative values towards the wide-angle direction. The zoom unit is driver-specific. This is a write-only control.
    
V4L2_CID_ZOOM_CONTINUOUS integer 
 Move the objective lens group at the specified speed until it reaches physical device limits or until an explicit request to stop the movement. A positive value moves the zoom lens group towards the telephoto direction. A value of zero stops the zoom lens group movement. A negative value moves the zoom lens group towards the wide-angle direction. The zoom speed unit is driver-specific.
    
V4L2_CID_PRIVACY boolean 
 Prevent video from being acquired by the camera. When this control is set to TRUE (1), no image can be captured by the camera. Common means to enforce privacy are mechanical obturation of the sensor and firmware image processing, but the device is not restricted to these methods. Devices that implement the privacy control must support read access and may support write access.
V4L2_CID_BAND_STOP_FILTER integer 
 Switch the band-stop filter of a camera sensor on or off, or specify its strength. Such band-stop filters can be used, for example, to filter out the fluorescent light component.
    

1.9.7. FM Transmitter Control Reference

The FM Transmitter (FM_TX) class includes controls for common features of FM transmissions capable devices. Currently this class includes parameters for audio compression, pilot tone generation, audio deviation limiter, RDS transmission and tuning power features.

Table 1.5. FM_TX Control IDs

IDType 
 Description
    
V4L2_CID_FM_TX_CLASS class 
 The FM_TX class descriptor. Calling VIDIOC_QUERYCTRL for this control will return a description of this control class.
V4L2_CID_RDS_TX_DEVIATION integer 
 Configures RDS signal frequency deviation level in Hz. The range and step are driver-specific.
V4L2_CID_RDS_TX_PI integer 
 Sets the RDS Programme Identification field for transmission.
V4L2_CID_RDS_TX_PTY integer 
 Sets the RDS Programme Type field for transmission. This encodes up to 31 pre-defined programme types.
V4L2_CID_RDS_TX_PS_NAME string 
 Sets the Programme Service name (PS_NAME) for transmission. It is intended for static display on a receiver. It is the primary aid to listeners in programme service identification and selection. In Annex E of [EN 50067], the RDS specification, there is a full description of the correct character encoding for Programme Service name strings. Also from RDS specification, PS is usually a single eight character text. However, it is also possible to find receivers which can scroll strings sized as 8 x N characters. So, this control must be configured with steps of 8 characters. The result is it must always contain a string with size multiple of 8.
V4L2_CID_RDS_TX_RADIO_TEXT string 
 Sets the Radio Text info for transmission. It is a textual description of what is being broadcasted. RDS Radio Text can be applied when broadcaster wishes to transmit longer PS names, programme-related information or any other text. In these cases, RadioText should be used in addition to V4L2_CID_RDS_TX_PS_NAME. The encoding for Radio Text strings is also fully described in Annex E of [EN 50067]. The length of Radio Text strings depends on which RDS Block is being used to transmit it, either 32 (2A block) or 64 (2B block). However, it is also possible to find receivers which can scroll strings sized as 32 x N or 64 x N characters. So, this control must be configured with steps of 32 or 64 characters. The result is it must always contain a string with size multiple of 32 or 64.
V4L2_CID_AUDIO_LIMITER_ENABLED boolean 
 Enables or disables the audio deviation limiter feature. The limiter is useful when trying to maximize the audio volume, minimize receiver-generated distortion and prevent overmodulation.
V4L2_CID_AUDIO_LIMITER_RELEASE_TIME integer 
 Sets the audio deviation limiter feature release time. Unit is in useconds. Step and range are driver-specific.
V4L2_CID_AUDIO_LIMITER_DEVIATION integer 
 Configures audio frequency deviation level in Hz. The range and step are driver-specific.
V4L2_CID_AUDIO_COMPRESSION_ENABLED boolean 
 Enables or disables the audio compression feature. This feature amplifies signals below the threshold by a fixed gain and compresses audio signals above the threshold by the ratio of Threshold/(Gain + Threshold).
V4L2_CID_AUDIO_COMPRESSION_GAIN integer 
 Sets the gain for audio compression feature. It is a dB value. The range and step are driver-specific.
V4L2_CID_AUDIO_COMPRESSION_THRESHOLD integer 
 Sets the threshold level for audio compression freature. It is a dB value. The range and step are driver-specific.
V4L2_CID_AUDIO_COMPRESSION_ATTACK_TIME integer 
 Sets the attack time for audio compression feature. It is a useconds value. The range and step are driver-specific.
V4L2_CID_AUDIO_COMPRESSION_RELEASE_TIME integer 
 Sets the release time for audio compression feature. It is a useconds value. The range and step are driver-specific.
V4L2_CID_PILOT_TONE_ENABLED boolean 
 Enables or disables the pilot tone generation feature.
V4L2_CID_PILOT_TONE_DEVIATION integer 
 Configures pilot tone frequency deviation level. Unit is in Hz. The range and step are driver-specific.
V4L2_CID_PILOT_TONE_FREQUENCY integer 
 Configures pilot tone frequency value. Unit is in Hz. The range and step are driver-specific.
V4L2_CID_TUNE_PREEMPHASIS integer 
 Configures the pre-emphasis value for broadcasting. A pre-emphasis filter is applied to the broadcast to accentuate the high audio frequencies. Depending on the region, a time constant of either 50 or 75 useconds is used. The enum v4l2_preemphasis defines possible values for pre-emphasis. Here they are:
 
V4L2_PREEMPHASIS_DISABLED No pre-emphasis is applied.
V4L2_PREEMPHASIS_50_uS A pre-emphasis of 50 uS is used.
V4L2_PREEMPHASIS_75_uS A pre-emphasis of 75 uS is used.
V4L2_CID_TUNE_POWER_LEVEL integer 
 Sets the output power level for signal transmission. Unit is in dBuV. Range and step are driver-specific.
V4L2_CID_TUNE_ANTENNA_CAPACITOR integer 
 This selects the value of antenna tuning capacitor manually or automatically if set to zero. Unit, range and step are driver-specific.
    

For more details about RDS specification, refer to [EN 50067] document, from CENELEC.

1.10. Data Formats

1.10.1. Data Format Negotiation

Different devices exchange different kinds of data with applications, for example video images, raw or sliced VBI data, RDS datagrams. Even within one kind many different formats are possible, in particular an abundance of image formats. Although drivers must provide a default and the selection persists across closing and reopening a device, applications should always negotiate a data format before engaging in data exchange. Negotiation means the application asks for a particular format and the driver selects and reports the best the hardware can do to satisfy the request. Of course applications can also just query the current selection.

A single mechanism exists to negotiate all data formats using the aggregate struct v4l2_format and the VIDIOC_G_FMT and VIDIOC_S_FMT ioctls. Additionally the VIDIOC_TRY_FMT ioctl can be used to examine what the hardware could do, without actually selecting a new data format. The data formats supported by the V4L2 API are covered in the respective device section in Chapter 4, Interfaces. For a closer look at image formats see Chapter 2, Image Formats.

The VIDIOC_S_FMT ioctl is a major turning-point in the initialization sequence. Prior to this point multiple panel applications can access the same device concurrently to select the current input, change controls or modify other properties. The first VIDIOC_S_FMT assigns a logical stream (video data, VBI data etc.) exclusively to one file descriptor.

Exclusive means no other application, more precisely no other file descriptor, can grab this stream or change device properties inconsistent with the negotiated parameters. A video standard change for example, when the new standard uses a different number of scan lines, can invalidate the selected image format. Therefore only the file descriptor owning the stream can make invalidating changes. Accordingly multiple file descriptors which grabbed different logical streams prevent each other from interfering with their settings. When for example video overlay is about to start or already in progress, simultaneous video capturing may be restricted to the same cropping and image size.

When applications omit the VIDIOC_S_FMT ioctl its locking side effects are implied by the next step, the selection of an I/O method with the VIDIOC_REQBUFS ioctl or implicit with the first read() or write() call.

Generally only one logical stream can be assigned to a file descriptor, the exception being drivers permitting simultaneous video capturing and overlay using the same file descriptor for compatibility with V4L and earlier versions of V4L2. Switching the logical stream or returning into "panel mode" is possible by closing and reopening the device. Drivers may support a switch using VIDIOC_S_FMT.

All drivers exchanging data with applications must support the VIDIOC_G_FMT and VIDIOC_S_FMT ioctl. Implementation of the VIDIOC_TRY_FMT is highly recommended but optional.

1.10.2. Image Format Enumeration

Apart of the generic format negotiation functions a special ioctl to enumerate all image formats supported by video capture, overlay or output devices is available.[11]

The VIDIOC_ENUM_FMT ioctl must be supported by all drivers exchanging image data with applications.

Important

Drivers are not supposed to convert image formats in kernel space. They must enumerate only formats directly supported by the hardware. If necessary driver writers should publish an example conversion routine or library for integration into applications.

1.11. Image Cropping, Insertion and Scaling

Some video capture devices can sample a subsection of the picture and shrink or enlarge it to an image of arbitrary size. We call these abilities cropping and scaling. Some video output devices can scale an image up or down and insert it at an arbitrary scan line and horizontal offset into a video signal.

Applications can use the following API to select an area in the video signal, query the default area and the hardware limits. Despite their name, the VIDIOC_CROPCAP, VIDIOC_G_CROP and VIDIOC_S_CROP ioctls apply to input as well as output devices.

Scaling requires a source and a target. On a video capture or overlay device the source is the video signal, and the cropping ioctls determine the area actually sampled. The target are images read by the application or overlaid onto the graphics screen. Their size (and position for an overlay) is negotiated with the VIDIOC_G_FMT and VIDIOC_S_FMT ioctls.

On a video output device the source are the images passed in by the application, and their size is again negotiated with the VIDIOC_G/S_FMT ioctls, or may be encoded in a compressed video stream. The target is the video signal, and the cropping ioctls determine the area where the images are inserted.

Source and target rectangles are defined even if the device does not support scaling or the VIDIOC_G/S_CROP ioctls. Their size (and position where applicable) will be fixed in this case. All capture and output device must support the VIDIOC_CROPCAP ioctl such that applications can determine if scaling takes place.

1.11.1. Cropping Structures

Figure 1.1. Image Cropping, Insertion and Scaling

The cropping, insertion and scaling process

For capture devices the coordinates of the top left corner, width and height of the area which can be sampled is given by the bounds substructure of the struct v4l2_cropcap returned by the VIDIOC_CROPCAP ioctl. To support a wide range of hardware this specification does not define an origin or units. However by convention drivers should horizontally count unscaled samples relative to 0H (the leading edge of the horizontal sync pulse, see Figure 4.1, “Line synchronization”). Vertically ITU-R line numbers of the first field (Figure 4.2, “ITU-R 525 line numbering (M/NTSC and M/PAL)”, Figure 4.3, “ITU-R 625 line numbering”), multiplied by two if the driver can capture both fields.

The top left corner, width and height of the source rectangle, that is the area actually sampled, is given by struct v4l2_crop using the same coordinate system as struct v4l2_cropcap. Applications can use the VIDIOC_G_CROP and VIDIOC_S_CROP ioctls to get and set this rectangle. It must lie completely within the capture boundaries and the driver may further adjust the requested size and/or position according to hardware limitations.

Each capture device has a default source rectangle, given by the defrect substructure of struct v4l2_cropcap. The center of this rectangle shall align with the center of the active picture area of the video signal, and cover what the driver writer considers the complete picture. Drivers shall reset the source rectangle to the default when the driver is first loaded, but not later.

For output devices these structures and ioctls are used accordingly, defining the target rectangle where the images will be inserted into the video signal.

1.11.2. Scaling Adjustments

Video hardware can have various cropping, insertion and scaling limitations. It may only scale up or down, support only discrete scaling factors, or have different scaling abilities in horizontal and vertical direction. Also it may not support scaling at all. At the same time the struct v4l2_crop rectangle may have to be aligned, and both the source and target rectangles may have arbitrary upper and lower size limits. In particular the maximum width and height in struct v4l2_crop may be smaller than the struct v4l2_cropcap.bounds area. Therefore, as usual, drivers are expected to adjust the requested parameters and return the actual values selected.

Applications can change the source or the target rectangle first, as they may prefer a particular image size or a certain area in the video signal. If the driver has to adjust both to satisfy hardware limitations, the last requested rectangle shall take priority, and the driver should preferably adjust the opposite one. The VIDIOC_TRY_FMT ioctl however shall not change the driver state and therefore only adjust the requested rectangle.

Suppose scaling on a video capture device is restricted to a factor 1:1 or 2:1 in either direction and the target image size must be a multiple of 16 × 16 pixels. The source cropping rectangle is set to defaults, which are also the upper limit in this example, of 640 × 400 pixels at offset 0, 0. An application requests an image size of 300 × 225 pixels, assuming video will be scaled down from the "full picture" accordingly. The driver sets the image size to the closest possible values 304 × 224, then chooses the cropping rectangle closest to the requested size, that is 608 × 224 (224 × 2:1 would exceed the limit 400). The offset 0, 0 is still valid, thus unmodified. Given the default cropping rectangle reported by VIDIOC_CROPCAP the application can easily propose another offset to center the cropping rectangle.

Now the application may insist on covering an area using a picture aspect ratio closer to the original request, so it asks for a cropping rectangle of 608 × 456 pixels. The present scaling factors limit cropping to 640 × 384, so the driver returns the cropping size 608 × 384 and adjusts the image size to closest possible 304 × 192.

1.11.3. Examples

Source and target rectangles shall remain unchanged across closing and reopening a device, such that piping data into or out of a device will work without special preparations. More advanced applications should ensure the parameters are suitable before starting I/O.

Example 1.10. Resetting the cropping parameters

(A video capture device is assumed; change V4L2_BUF_TYPE_VIDEO_CAPTURE for other devices.)

struct v4l2_cropcap cropcap;
struct v4l2_crop crop;

memset (&cropcap, 0, sizeof (cropcap));
cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

if (-1 == ioctl (fd, VIDIOC_CROPCAP, &cropcap)) {
	perror ("VIDIOC_CROPCAP");
	exit (EXIT_FAILURE);
}

memset (&crop, 0, sizeof (crop));
crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
crop.c = cropcap.defrect;

/* Ignore if cropping is not supported (EINVAL). */

if (-1 == ioctl (fd, VIDIOC_S_CROP, &crop)
    && errno != EINVAL) {
	perror ("VIDIOC_S_CROP");
	exit (EXIT_FAILURE);
}
      

Example 1.11. Simple downscaling

(A video capture device is assumed.)

struct v4l2_cropcap cropcap;
struct v4l2_format format;

reset_cropping_parameters ();

/* Scale down to 1/4 size of full picture. */

memset (&format, 0, sizeof (format)); /* defaults */

format.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

format.fmt.pix.width = cropcap.defrect.width >> 1;
format.fmt.pix.height = cropcap.defrect.height >> 1;
format.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;

if (-1 == ioctl (fd, VIDIOC_S_FMT, &format)) {
	perror ("VIDIOC_S_FORMAT");
	exit (EXIT_FAILURE);
}

/* We could check the actual image size now, the actual scaling factor
   or if the driver can scale at all. */
	

Example 1.12. Selecting an output area

struct v4l2_cropcap cropcap;
struct v4l2_crop crop;

memset (&cropcap, 0, sizeof (cropcap));
cropcap.type = V4L2_BUF_TYPE_VIDEO_OUTPUT;

if (-1 == ioctl (fd, VIDIOC_CROPCAP;, &cropcap)) {
	perror ("VIDIOC_CROPCAP");
	exit (EXIT_FAILURE);
}

memset (&crop, 0, sizeof (crop));

crop.type = V4L2_BUF_TYPE_VIDEO_OUTPUT;
crop.c = cropcap.defrect;

/* Scale the width and height to 50 % of their original size
   and center the output. */

crop.c.width /= 2;
crop.c.height /= 2;
crop.c.left += crop.c.width / 2;
crop.c.top += crop.c.height / 2;

/* Ignore if cropping is not supported (EINVAL). */

if (-1 == ioctl (fd, VIDIOC_S_CROP, &crop)
    && errno != EINVAL) {
	perror ("VIDIOC_S_CROP");
	exit (EXIT_FAILURE);
}

Example 1.13. Current scaling factor and pixel aspect

(A video capture device is assumed.)

struct v4l2_cropcap cropcap;
struct v4l2_crop crop;
struct v4l2_format format;
double hscale, vscale;
double aspect;
int dwidth, dheight;

memset (&cropcap, 0, sizeof (cropcap));
cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

if (-1 == ioctl (fd, VIDIOC_CROPCAP, &cropcap)) {
	perror ("VIDIOC_CROPCAP");
	exit (EXIT_FAILURE);
}

memset (&crop, 0, sizeof (crop));
crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

if (-1 == ioctl (fd, VIDIOC_G_CROP, &crop)) {
	if (errno != EINVAL) {
		perror ("VIDIOC_G_CROP");
		exit (EXIT_FAILURE);
	}

	/* Cropping not supported. */
	crop.c = cropcap.defrect;
}

memset (&format, 0, sizeof (format));
format.fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

if (-1 == ioctl (fd, VIDIOC_G_FMT, &format)) {
	perror ("VIDIOC_G_FMT");
	exit (EXIT_FAILURE);
}

/* The scaling applied by the driver. */

hscale = format.fmt.pix.width / (double) crop.c.width;
vscale = format.fmt.pix.height / (double) crop.c.height;

aspect = cropcap.pixelaspect.numerator /
	 (double) cropcap.pixelaspect.denominator;
aspect = aspect * hscale / vscale;

/* Devices following ITU-R BT.601 do not capture
   square pixels. For playback on a computer monitor
   we should scale the images to this size. */

dwidth = format.fmt.pix.width / aspect;
dheight = format.fmt.pix.height;
	

1.12. Streaming Parameters

Streaming parameters are intended to optimize the video capture process as well as I/O. Presently applications can request a high quality capture mode with the VIDIOC_S_PARM ioctl.

The current video standard determines a nominal number of frames per second. If less than this number of frames is to be captured or output, applications can request frame skipping or duplicating on the driver side. This is especially useful when using the read() or write(), which are not augmented by timestamps or sequence counters, and to avoid unneccessary data copying.

Finally these ioctls can be used to determine the number of buffers used internally by a driver in read/write mode. For implications see the section discussing the read() function.

To get and set the streaming parameters applications call the VIDIOC_G_PARM and VIDIOC_S_PARM ioctl, respectively. They take a pointer to a struct v4l2_streamparm, which contains a union holding separate parameters for input and output devices.

These ioctls are optional, drivers need not implement them. If so, they return the EINVAL error code.



[1] Access permissions are associated with character device special files, hence we must ensure device numbers cannot change with the module load order. To this end minor numbers are no longer automatically assigned by the "videodev" module as in V4L but requested by the driver. The defaults will suffice for most people unless two drivers compete for the same minor numbers.

[2] In earlier versions of the V4L2 API the module options where named after the device special file with a "unit_" prefix, expressing the minor number itself, not an offset. Rationale for this change is unknown. Lastly the naming and semantics are just a convention among driver writers, the point to note is that minor numbers are not supposed to be hardcoded into drivers.

[3] Given a device file name one cannot reliable find related devices. For once names are arbitrary and in a system with multiple devices, where only some support VBI capturing, a /dev/video2 is not necessarily related to /dev/vbi2. The V4L VIDIOCGUNIT ioctl would require a search for a device file with a particular major and minor number.

[4] Drivers could recognize the O_EXCL open flag. Presently this is not required, so applications cannot know if it really works.

[5] Actually struct v4l2_audio ought to have a tuner field like struct v4l2_input, not only making the API more consistent but also permitting radio devices with multiple tuners.

[6] Some users are already confused by technical terms PAL, NTSC and SECAM. There is no point asking them to distinguish between B, G, D, or K when the software or hardware can do that automatically.

[7] An alternative to the current scheme is to use pointers to indices as arguments of VIDIOC_G_STD and VIDIOC_S_STD, the struct v4l2_input and struct v4l2_output std field would be a set of indices like audioset.

Indices are consistent with the rest of the API and identify the standard unambiguously. In the present scheme of things an enumerated standard is looked up by v4l2_std_id. Now the standards supported by the inputs of a device can overlap. Just assume the tuner and composite input in the example above both exist on a device. An enumeration of "PAL-B/G", "PAL-H/I" suggests a choice which does not exist. We cannot merge or omit sets, because applications would be unable to find the standards reported by VIDIOC_G_STD. That leaves separate enumerations for each input. Also selecting a standard by v4l2_std_id can be ambiguous. Advantage of this method is that applications need not identify the standard indirectly, after enumerating.

So in summary, the lookup itself is unavoidable. The difference is only whether the lookup is necessary to find an enumerated standard or to switch to a standard by v4l2_std_id.

[8] See Section 3.5, “Buffers” for a rationale. Probably even USB cameras follow some well known video standard. It might have been better to explicitly indicate elsewhere if a device cannot live up to normal expectations, instead of this exception.

[9] It will be more convenient for applications if drivers make use of the V4L2_CTRL_FLAG_DISABLED flag, but that was never required.

[10] Applications could call an ioctl to request events. After another process called VIDIOC_S_CTRL or another ioctl changing shared properties the select() function would indicate readability until any ioctl (querying the properties) is called.

[11] Enumerating formats an application has no a-priori knowledge of (otherwise it could explicitely ask for them and need not enumerate) seems useless, but there are applications serving as proxy between drivers and the actual video applications for which this is useful.

Chapter 2. Image Formats

The V4L2 API was primarily designed for devices exchanging image data with applications. The v4l2_pix_format structure defines the format and layout of an image in memory. Image formats are negotiated with the VIDIOC_S_FMT ioctl. (The explanations here focus on video capturing and output, for overlay frame buffer formats see also VIDIOC_G_FBUF.)

Table 2.1. struct v4l2_pix_format

__u32widthImage width in pixels.
__u32heightImage height in pixels.
Applications set these fields to request an image size, drivers return the closest possible values. In case of planar formats the width and height applies to the largest plane. To avoid ambiguities drivers must return values rounded up to a multiple of the scale factor of any smaller planes. For example when the image format is YUV 4:2:0, width and height must be multiples of two.
__u32pixelformatThe pixel format or type of compression, set by the application. This is a little endian four character code. V4L2 defines standard RGB formats in Table 2.4, “Packed RGB Image Formats”, YUV formats in Section 2.5, “YUV Formats”, and reserved codes in Table 2.8, “Reserved Image Formats”
enum v4l2_fieldfieldVideo images are typically interlaced. Applications can request to capture or output only the top or bottom field, or both fields interlaced or sequentially stored in one buffer or alternating in separate buffers. Drivers return the actual field order selected. For details see Section 3.6, “Field Order”.
__u32bytesperlineDistance in bytes between the leftmost pixels in two adjacent lines.

Both applications and drivers can set this field to request padding bytes at the end of each line. Drivers however may ignore the value requested by the application, returning width times bytes per pixel or a larger value required by the hardware. That implies applications can just set this field to zero to get a reasonable default.

Video hardware may access padding bytes, therefore they must reside in accessible memory. Consider cases where padding bytes after the last line of an image cross a system page boundary. Input devices may write padding bytes, the value is undefined. Output devices ignore the contents of padding bytes.

When the image format is planar the bytesperline value applies to the largest plane and is divided by the same factor as the width field for any smaller planes. For example the Cb and Cr planes of a YUV 4:2:0 image have half as many padding bytes following each line as the Y plane. To avoid ambiguities drivers must return a bytesperline value rounded up to a multiple of the scale factor.

__u32sizeimageSize in bytes of the buffer to hold a complete image, set by the driver. Usually this is bytesperline times height. When the image consists of variable length compressed data this is the maximum number of bytes required to hold an image.
enum v4l2_colorspacecolorspaceThis information supplements the pixelformat and must be set by the driver, see Section 2.2, “Colorspaces”.
__u32privReserved for custom (driver defined) additional information about formats. When not used drivers and applications must set this field to zero.

2.1. Standard Image Formats

In order to exchange images between drivers and applications, it is necessary to have standard image data formats which both sides will interpret the same way. V4L2 includes several such formats, and this section is intended to be an unambiguous specification of the standard image data formats in V4L2.

V4L2 drivers are not limited to these formats, however. Driver-specific formats are possible. In that case the application may depend on a codec to convert images to one of the standard formats when needed. But the data can still be stored and retrieved in the proprietary format. For example, a device may support a proprietary compressed format. Applications can still capture and save the data in the compressed format, saving much disk space, and later use a codec to convert the images to the X Windows screen format when the video is to be displayed.

Even so, ultimately, some standard formats are needed, so the V4L2 specification would not be complete without well-defined standard formats.

The V4L2 standard formats are mainly uncompressed formats. The pixels are always arranged in memory from left to right, and from top to bottom. The first byte of data in the image buffer is always for the leftmost pixel of the topmost row. Following that is the pixel immediately to its right, and so on until the end of the top row of pixels. Following the rightmost pixel of the row there may be zero or more bytes of padding to guarantee that each row of pixel data has a certain alignment. Following the pad bytes, if any, is data for the leftmost pixel of the second row from the top, and so on. The last row has just as many pad bytes after it as the other rows.

In V4L2 each format has an identifier which looks like PIX_FMT_XXX, defined in the videodev.h header file. These identifiers represent four character codes which are also listed below, however they are not the same as those used in the Windows world.

2.2. Colorspaces

[intro]

Gamma Correction

[to do]

E'R = f(R)

E'G = f(G)

E'B = f(B)

Construction of luminance and color-difference signals

[to do]

E'Y = CoeffR E'R + CoeffG E'G + CoeffB E'B

(E'R - E'Y) = E'R - CoeffR E'R - CoeffG E'G - CoeffB E'B

(E'B - E'Y) = E'B - CoeffR E'R - CoeffG E'G - CoeffB E'B

Re-normalized color-difference signals

The color-difference signals are scaled back to unity range [-0.5;+0.5]:

KB = 0.5 / (1 - CoeffB)

KR = 0.5 / (1 - CoeffR)

PB = KB (E'B - E'Y) = 0.5 (CoeffR / CoeffB) E'R + 0.5 (CoeffG / CoeffB) E'G + 0.5 E'B

PR = KR (E'R - E'Y) = 0.5 E'R + 0.5 (CoeffG / CoeffR) E'G + 0.5 (CoeffB / CoeffR) E'B

Quantization

[to do]

Y' = (Lum. Levels - 1) · E'Y + Lum. Offset

CB = (Chrom. Levels - 1) · PB + Chrom. Offset

CR = (Chrom. Levels - 1) · PR + Chrom. Offset

Rounding to the nearest integer and clamping to the range [0;255] finally yields the digital color components Y'CbCr stored in YUV images.

Example 2.1. ITU-R Rec. BT.601 color conversion

Forward Transformation

int ER, EG, EB;         /* gamma corrected RGB input [0;255] */
int Y1, Cb, Cr;         /* output [0;255] */

double r, g, b;         /* temporaries */
double y1, pb, pr;

int
clamp (double x)
{
	int r = x;      /* round to nearest */

	if (r < 0)         return 0;
	else if (r > 255)  return 255;
	else               return r;
}

r = ER / 255.0;
g = EG / 255.0;
b = EB / 255.0;

y1  =  0.299  * r + 0.587 * g + 0.114  * b;
pb  = -0.169  * r - 0.331 * g + 0.5    * b;
pr  =  0.5    * r - 0.419 * g - 0.081  * b;

Y1 = clamp (219 * y1 + 16);
Cb = clamp (224 * pb + 128);
Cr = clamp (224 * pr + 128);

/* or shorter */

y1 = 0.299 * ER + 0.587 * EG + 0.114 * EB;

Y1 = clamp ( (219 / 255.0)                    *       y1  + 16);
Cb = clamp (((224 / 255.0) / (2 - 2 * 0.114)) * (EB - y1) + 128);
Cr = clamp (((224 / 255.0) / (2 - 2 * 0.299)) * (ER - y1) + 128);
      

Inverse Transformation

int Y1, Cb, Cr;         /* gamma pre-corrected input [0;255] */
int ER, EG, EB;         /* output [0;255] */

double r, g, b;         /* temporaries */
double y1, pb, pr;

int
clamp (double x)
{
	int r = x;      /* round to nearest */

	if (r < 0)         return 0;
	else if (r > 255)  return 255;
	else               return r;
}

y1 = (255 / 219.0) * (Y1 - 16);
pb = (255 / 224.0) * (Cb - 128);
pr = (255 / 224.0) * (Cr - 128);

r = 1.0 * y1 + 0     * pb + 1.402 * pr;
g = 1.0 * y1 - 0.344 * pb - 0.714 * pr;
b = 1.0 * y1 + 1.772 * pb + 0     * pr;

ER = clamp (r * 255); /* [ok? one should prob. limit y1,pb,pr] */
EG = clamp (g * 255);
EB = clamp (b * 255);
      

Table 2.2. enum v4l2_colorspace

IdentifierValueDescriptionChromaticities[a]White PointGamma CorrectionLuminance E'YQuantization
RedGreenBlueY'Cb, Cr
V4L2_COLORSPACE_SMPTE170M1NTSC/PAL according to [SMPTE 170M], [ITU BT.601]x = 0.630, y = 0.340x = 0.310, y = 0.595x = 0.155, y = 0.070x = 0.3127, y = 0.3290, Illuminant D65E' = 4.5 I for I ≤0.018, 1.099 I0.45 - 0.099 for 0.018 < I0.299 E'R + 0.587 E'G + 0.114 E'B219 E'Y + 16224 PB,R + 128
V4L2_COLORSPACE_SMPTE240M21125-Line (US) HDTV, see [SMPTE 240M]x = 0.630, y = 0.340x = 0.310, y = 0.595x = 0.155, y = 0.070x = 0.3127, y = 0.3290, Illuminant D65E' = 4 I for I ≤0.0228, 1.1115 I0.45 - 0.1115 for 0.0228 < I0.212 E'R + 0.701 E'G + 0.087 E'B219 E'Y + 16224 PB,R + 128
V4L2_COLORSPACE_REC7093HDTV and modern devices, see [ITU BT.709]x = 0.640, y = 0.330x = 0.300, y = 0.600x = 0.150, y = 0.060x = 0.3127, y = 0.3290, Illuminant D65E' = 4.5 I for I ≤0.018, 1.099 I0.45 - 0.099 for 0.018 < I0.2125 E'R + 0.7154 E'G + 0.0721 E'B219 E'Y + 16224 PB,R + 128
V4L2_COLORSPACE_BT8784Broken Bt878 extents[b], [ITU BT.601]?????0.299 E'R + 0.587 E'G + 0.114 E'B237 E'Y + 16224 PB,R + 128 (probably)
V4L2_COLORSPACE_470_SYSTEM_M5M/NTSC[c] according to [ITU BT.470], [ITU BT.601]x = 0.67, y = 0.33x = 0.21, y = 0.71x = 0.14, y = 0.08x = 0.310, y = 0.316, Illuminant C?0.299 E'R + 0.587 E'G + 0.114 E'B219 E'Y + 16224 PB,R + 128
V4L2_COLORSPACE_470_SYSTEM_BG6625-line PAL and SECAM systems according to [ITU BT.470], [ITU BT.601]x = 0.64, y = 0.33x = 0.29, y = 0.60x = 0.15, y = 0.06x = 0.313, y = 0.329, Illuminant D65?0.299 E'R + 0.587 E'G + 0.114 E'B219 E'Y + 16224 PB,R + 128
V4L2_COLORSPACE_JPEG7JPEG Y'CbCr, see [JFIF], [ITU BT.601]?????0.299 E'R + 0.587 E'G + 0.114 E'B256 E'Y + 16[d]256 PB,R + 128
V4L2_COLORSPACE_SRGB8[?]x = 0.640, y = 0.330x = 0.300, y = 0.600x = 0.150, y = 0.060x = 0.3127, y = 0.3290, Illuminant D65E' = 4.5 I for I ≤0.018, 1.099 I0.45 - 0.099 for 0.018 < In/a

[a] The coordinates of the color primaries are given in the CIE system (1931)

[b] The ubiquitous Bt878 video capture chip quantizes E'Y to 238 levels, yielding a range of Y' = 16 … 253, unlike Rec. 601 Y' = 16 … 235. This is not a typo in the Bt878 documentation, it has been implemented in silicon. The chroma extents are unclear.

[c] No identifier exists for M/PAL which uses the chromaticities of M/NTSC, the remaining parameters are equal to B and G/PAL.

[d] Note JFIF quantizes Y'PBPR in range [0;+1] and [-0.5;+0.5] to 257 levels, however Y'CbCr signals are still clamped to [0;255].


2.3. Indexed Format

In this format each pixel is represented by an 8 bit index into a 256 entry ARGB palette. It is intended for Video Output Overlays only. There are no ioctls to access the palette, this must be done with ioctls of the Linux framebuffer API.

Table 2.3. Indexed Image Format

IdentifierCode Byte 0                          
  Bit76543210                          
V4L2_PIX_FMT_PAL8'PAL8' i7i6i5i4i3i2i1i0                          

2.4. RGB Formats

Name

Packed RGB formats — Packed RGB formats

Description

These formats are designed to match the pixel formats of typical PC graphics frame buffers. They occupy 8, 16, 24 or 32 bits per pixel. These are all packed-pixel formats, meaning all the data for a pixel lie next to each other in memory.

When one of these formats is used, drivers shall report the colorspace V4L2_COLORSPACE_SRGB.

Table 2.4. Packed RGB Image Formats

IdentifierCode Byte 0 in memory Byte 1 Byte 2 Byte 3
  Bit76543210 76543210 76543210 76543210
V4L2_PIX_FMT_RGB332'RGB1' b1b0g2g1g0r2r1r0                          
V4L2_PIX_FMT_RGB444'R444' g3g2g1g0b3b2b1b0 a3a2a1a0r3r2r1r0                 
V4L2_PIX_FMT_RGB555'RGBO' g2g1g0r4r3r2r1r0 ab4b3b2b1b0g4g3                 
V4L2_PIX_FMT_RGB565'RGBP' g2g1g0r4r3r2r1r0 b4b3b2b1b0g5g4g3                 
V4L2_PIX_FMT_RGB555X'RGBQ' ab4b3b2b1b0g4g3 g2g1g0r4r3r2r1r0                 
V4L2_PIX_FMT_RGB565X'RGBR' b4b3b2b1b0g5g4g3 g2g1g0r4r3r2r1r0                 
V4L2_PIX_FMT_BGR24'BGR3' b7b6b5b4b3b2b1b0 g7g6g5g4g3g2g1g0 r7r6r5r4r3r2r1r0        
V4L2_PIX_FMT_RGB24'RGB3' r7r6r5r4r3r2r1r0 g7g6g5g4g3g2g1g0 b7b6b5b4b3b2b1b0        
V4L2_PIX_FMT_BGR32'BGR4' b7b6b5b4b3b2b1b0 g7g6g5g4g3g2g1g0 r7r6r5r4r3r2r1r0 a7a6a5a4a3a2a1a0
V4L2_PIX_FMT_RGB32'RGB4' r7r6r5r4r3r2r1r0 g7g6g5g4g3g2g1g0 b7b6b5b4b3b2b1b0 a7a6a5a4a3a2a1a0

Bit 7 is the most significant bit. The value of a = alpha bits is undefined when reading from the driver, ignored when writing to the driver, except when alpha blending has been negotiated for a Video Overlay or Video Output Overlay.

Example 2.2. V4L2_PIX_FMT_BGR24 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:B00G00R00B01G01R01B02G02R02B03G03R03
start + 12:B10G10R10B11G11R11B12G12R12B13G13R13
start + 24:B20G20R20B21G21R21B22G22R22B23G23R23
start + 36:B30G30R30B31G31R31B32G32R32B33G33R33


Important

Drivers may interpret these formats differently.

Some RGB formats above are uncommon and were probably defined in error. Drivers may interpret them as in Table 2.5, “Packed RGB Image Formats (corrected)”.

Table 2.5. Packed RGB Image Formats (corrected)

IdentifierCode Byte 0 in memory Byte 1 Byte 2 Byte 3
  Bit76543210 76543210 76543210 76543210
V4L2_PIX_FMT_RGB332'RGB1' r2r1r0g2g1g0b1b0                          
V4L2_PIX_FMT_RGB444'R444' g3g2g1g0b3b2b1b0 a3a2a1a0r3r2r1r0                 
V4L2_PIX_FMT_RGB555'RGBO' g2g1g0b4b3b2b1b0 ar4r3r2r1r0g4g3                 
V4L2_PIX_FMT_RGB565'RGBP' g2g1g0b4b3b2b1b0 r4r3r2r1r0g5g4g3                 
V4L2_PIX_FMT_RGB555X'RGBQ' ar4r3r2r1r0g4g3 g2g1g0b4b3b2b1b0                 
V4L2_PIX_FMT_RGB565X'RGBR' r4r3r2r1r0g5g4g3 g2g1g0b4b3b2b1b0                 
V4L2_PIX_FMT_BGR24'BGR3' b7b6b5b4b3b2b1b0 g7g6g5g4g3g2g1g0 r7r6r5r4r3r2r1r0        
V4L2_PIX_FMT_RGB24'RGB3' r7r6r5r4r3r2r1r0 g7g6g5g4g3g2g1g0 b7b6b5b4b3b2b1b0        
V4L2_PIX_FMT_BGR32'BGR4' b7b6b5b4b3b2b1b0 g7g6g5g4g3g2g1g0 r7r6r5r4r3r2r1r0 a7a6a5a4a3a2a1a0
V4L2_PIX_FMT_RGB32'RGB4' a7a6a5a4a3a2a1a0 r7r6r5r4r3r2r1r0 g7g6g5g4g3g2g1g0 b7b6b5b4b3b2b1b0

A test utility to determine which RGB formats a driver actually supports is available from the LinuxTV v4l-dvb repository. See http://linuxtv.org/repo/ for access instructions.


Name

V4L2_PIX_FMT_SBGGR8 — Bayer RGB format

Description

This is commonly the native format of digital cameras, reflecting the arrangement of sensors on the CCD device. Only one red, green or blue value is given for each pixel. Missing components must be interpolated from neighbouring pixels. From left to right the first row consists of a blue and green value, the second row of a green and red value. This scheme repeats to the right and down for every two columns and rows.

Example 2.3. V4L2_PIX_FMT_SBGGR8 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:B00G01B02G03
start + 4:G10R11G12R13
start + 8:B20G21B22G23
start + 12:G30R31G32R33



Name

V4L2_PIX_FMT_SGBRG8 — Bayer RGB format

Description

This is commonly the native format of digital cameras, reflecting the arrangement of sensors on the CCD device. Only one red, green or blue value is given for each pixel. Missing components must be interpolated from neighbouring pixels. From left to right the first row consists of a green and blue value, the second row of a red and green value. This scheme repeats to the right and down for every two columns and rows.

Example 2.4. V4L2_PIX_FMT_SGBRG8 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:G00B01G02B03
start + 4:R10G11R12G13
start + 8:G20B21G22B23
start + 12:R30G31R32G33



Name

V4L2_PIX_FMT_SGRBG8 — Bayer RGB format

Description

This is commonly the native format of digital cameras, reflecting the arrangement of sensors on the CCD device. Only one red, green or blue value is given for each pixel. Missing components must be interpolated from neighbouring pixels. From left to right the first row consists of a green and blue value, the second row of a red and green value. This scheme repeats to the right and down for every two columns and rows.

Example 2.5. V4L2_PIX_FMT_SGRBG8 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:G00R01G02R03
start + 4:R10B11R12B13
start + 8:G20R21G22R23
start + 12:R30B31R32B33



Name

V4L2_PIX_FMT_SBGGR16 — Bayer RGB format

Description

This format is similar to V4L2_PIX_FMT_SBGGR8, except each pixel has a depth of 16 bits. The least significant byte is stored at lower memory addresses (little-endian). Note the actual sampling precision may be lower than 16 bits, for example 10 bits per pixel with values in range 0 to 1023.

Example 2.6. V4L2_PIX_FMT_SBGGR16 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:B00lowB00highG01lowG01highB02lowB02highG03lowG03high
start + 8:G10lowG10highR11lowR11highG12lowG12highR13lowR13high
start + 16:B20lowB20highG21lowG21highB22lowB22highG23lowG23high
start + 24:G30lowG30highR31lowR31highG32lowG32highR33lowR33high


2.5. YUV Formats

YUV is the format native to TV broadcast and composite video signals. It separates the brightness information (Y) from the color information (U and V or Cb and Cr). The color information consists of red and blue color difference signals, this way the green component can be reconstructed by subtracting from the brightness component. See Section 2.2, “Colorspaces” for conversion examples. YUV was chosen because early television would only transmit brightness information. To add color in a way compatible with existing receivers a new signal carrier was added to transmit the color difference signals. Secondary in the YUV format the U and V components usually have lower resolution than the Y component. This is an analog video compression technique taking advantage of a property of the human visual system, being more sensitive to brightness information.

Name

Packed YUV formats — Packed YUV formats

Description

Similar to the packed RGB formats these formats store the Y, Cb and Cr component of each pixel in one 16 or 32 bit word.

Table 2.6. Packed YUV Image Formats

IdentifierCode Byte 0 in memory Byte 1 Byte 2 Byte 3
  Bit76543210 76543210 76543210 76543210
V4L2_PIX_FMT_YUV444'Y444' Cb3Cb2Cb1Cb0Cr3Cr2Cr1Cr0 a3a2a1a0Y'3Y'2Y'1Y'0                 
V4L2_PIX_FMT_YUV555'YUVO' Cb2Cb1Cb0Cr4Cr3Cr2Cr1Cr0 aY'4Y'3Y'2Y'1Y'0Cb4Cb3                 
V4L2_PIX_FMT_YUV565'YUVP' Cb2Cb1Cb0Cr4Cr3Cr2Cr1Cr0 Y'4Y'3Y'2Y'1Y'0Cb5Cb4Cb3                 
V4L2_PIX_FMT_YUV32'YUV4' a7a6a5a4a3a2a1a0 Y'7Y'6Y'5Y'4Y'3Y'2Y'1Y'0 Cb7Cb6Cb5Cb4Cb3Cb2Cb1Cb0 Cr7Cr6Cr5Cr4Cr3Cr2Cr1Cr0

Bit 7 is the most significant bit. The value of a = alpha bits is undefined when reading from the driver, ignored when writing to the driver, except when alpha blending has been negotiated for a Video Overlay or Video Output Overlay.


Name

V4L2_PIX_FMT_GREY — Grey-scale image

Description

This is a grey-scale image. It is really a degenerate Y'CbCr format which simply contains no Cb or Cr data.

Example 2.7. V4L2_PIX_FMT_GREY 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:Y'00Y'01Y'02Y'03
start + 4:Y'10Y'11Y'12Y'13
start + 8:Y'20Y'21Y'22Y'23
start + 12:Y'30Y'31Y'32Y'33



Name

V4L2_PIX_FMT_Y16 — Grey-scale image

Description

This is a grey-scale image with a depth of 16 bits per pixel. The least significant byte is stored at lower memory addresses (little-endian). Note the actual sampling precision may be lower than 16 bits, for example 10 bits per pixel with values in range 0 to 1023.

Example 2.8. V4L2_PIX_FMT_Y16 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:Y'00lowY'00highY'01lowY'01highY'02lowY'02highY'03lowY'03high
start + 8:Y'10lowY'10highY'11lowY'11highY'12lowY'12highY'13lowY'13high
start + 16:Y'20lowY'20highY'21lowY'21highY'22lowY'22highY'23lowY'23high
start + 24:Y'30lowY'30highY'31lowY'31highY'32lowY'32highY'33lowY'33high



Name

V4L2_PIX_FMT_YUYV — Packed format with ½ horizontal chroma resolution, also known as YUV 4:2:2

Description

In this format each four bytes is two pixels. Each four bytes is two Y's, a Cb and a Cr. Each Y goes to one of the pixels, and the Cb and Cr belong to both pixels. As you can see, the Cr and Cb components have half the horizontal resolution of the Y component. V4L2_PIX_FMT_YUYV is known in the Windows environment as YUY2.

Example 2.9. V4L2_PIX_FMT_YUYV 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:Y'00Cb00Y'01Cr00Y'02Cb01Y'03Cr01
start + 8:Y'10Cb10Y'11Cr10Y'12Cb11Y'13Cr11
start + 16:Y'20Cb20Y'21Cr20Y'22Cb21Y'23Cr21
start + 24:Y'30Cb30Y'31Cr30Y'32Cb31Y'33Cr31

Color Sample Location. 

 0 1 2 3
0YCY YCY
1YCY YCY
2YCY YCY
3YCY YCY



Name

V4L2_PIX_FMT_UYVY — Variation of V4L2_PIX_FMT_YUYV with different order of samples in memory

Description

In this format each four bytes is two pixels. Each four bytes is two Y's, a Cb and a Cr. Each Y goes to one of the pixels, and the Cb and Cr belong to both pixels. As you can see, the Cr and Cb components have half the horizontal resolution of the Y component.

Example 2.10. V4L2_PIX_FMT_UYVY 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:Cb00Y'00Cr00Y'01Cb01Y'02Cr01Y'03
start + 8:Cb10Y'10Cr10Y'11Cb11Y'12Cr11Y'13
start + 16:Cb20Y'20Cr20Y'21Cb21Y'22Cr21Y'23
start + 24:Cb30Y'30Cr30Y'31Cb31Y'32Cr31Y'33

Color Sample Location. 

 0 1 2 3
0YCY YCY
1YCY YCY
2YCY YCY
3YCY YCY



Name

V4L2_PIX_FMT_YVYU — Variation of V4L2_PIX_FMT_YUYV with different order of samples in memory

Description

In this format each four bytes is two pixels. Each four bytes is two Y's, a Cb and a Cr. Each Y goes to one of the pixels, and the Cb and Cr belong to both pixels. As you can see, the Cr and Cb components have half the horizontal resolution of the Y component.

Example 2.11. V4L2_PIX_FMT_YVYU 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:Y'00Cr00Y'01Cb00Y'02Cr01Y'03Cb01
start + 8:Y'10Cr10Y'11Cb10Y'12Cr11Y'13Cb11
start + 16:Y'20Cr20Y'21Cb20Y'22Cr21Y'23Cb21
start + 24:Y'30Cr30Y'31Cb30Y'32Cr31Y'33Cb31

Color Sample Location. 

 0 1 2 3
0YCY YCY
1YCY YCY
2YCY YCY
3YCY YCY



Name

V4L2_PIX_FMT_VYUY — Variation of V4L2_PIX_FMT_YUYV with different order of samples in memory

Description

In this format each four bytes is two pixels. Each four bytes is two Y's, a Cb and a Cr. Each Y goes to one of the pixels, and the Cb and Cr belong to both pixels. As you can see, the Cr and Cb components have half the horizontal resolution of the Y component.

Example 2.12. V4L2_PIX_FMT_VYUY 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:Cr00Y'00Cb00Y'01Cr01Y'02Cb01Y'03
start + 8:Cr10Y'10Cb10Y'11Cr11Y'12Cb11Y'13
start + 16:Cr20Y'20Cb20Y'21Cr21Y'22Cb21Y'23
start + 24:Cr30Y'30Cb30Y'31Cr31Y'32Cb31Y'33

Color Sample Location. 

 0 1 2 3
0YCY YCY
1YCY YCY
2YCY YCY
3YCY YCY



Name

V4L2_PIX_FMT_Y41P — Format with ¼ horizontal chroma resolution, also known as YUV 4:1:1

Description

In this format each 12 bytes is eight pixels. In the twelve bytes are two CbCr pairs and eight Y's. The first CbCr pair goes with the first four Y's, and the second CbCr pair goes with the other four Y's. The Cb and Cr components have one fourth the horizontal resolution of the Y component.

Do not confuse this format with V4L2_PIX_FMT_YUV411P. Y41P is derived from "YUV 4:1:1 packed", while YUV411P stands for "YUV 4:1:1 planar".

Example 2.13. V4L2_PIX_FMT_Y41P 8 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:Cb00Y'00Cr00Y'01Cb01Y'02Cr01Y'03Y'04Y'05Y'06Y'07
start + 12:Cb10Y'10Cr10Y'11Cb11Y'12Cr11Y'13Y'14Y'15Y'16Y'17
start + 24:Cb20Y'20Cr20Y'21Cb21Y'22Cr21Y'23Y'24Y'25Y'26Y'27
start + 36:Cb30Y'30Cr30Y'31Cb31Y'32Cr31Y'33Y'34Y'35Y'36Y'37

Color Sample Location. 

 0 1 2 3 4 5 6 7
0Y YCY Y Y YCY Y
1Y YCY Y Y YCY Y
2Y YCY Y Y YCY Y
3Y YCY Y Y YCY Y



Name

V4L2_PIX_FMT_YVU420, V4L2_PIX_FMT_YUV420 — Planar formats with ½ horizontal and vertical chroma resolution, also known as YUV 4:2:0

Description

These are planar formats, as opposed to a packed format. The three components are separated into three sub- images or planes. The Y plane is first. The Y plane has one byte per pixel. For V4L2_PIX_FMT_YVU420, the Cr plane immediately follows the Y plane in memory. The Cr plane is half the width and half the height of the Y plane (and of the image). Each Cr belongs to four pixels, a two-by-two square of the image. For example, Cr0 belongs to Y'00, Y'01, Y'10, and Y'11. Following the Cr plane is the Cb plane, just like the Cr plane. V4L2_PIX_FMT_YUV420 is the same except the Cb plane comes first, then the Cr plane.

If the Y plane has pad bytes after each row, then the Cr and Cb planes have half as many pad bytes after their rows. In other words, two Cx rows (including padding) is exactly as long as one Y row (including padding).

Example 2.14. V4L2_PIX_FMT_YVU420 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:Y'00Y'01Y'02Y'03
start + 4:Y'10Y'11Y'12Y'13
start + 8:Y'20Y'21Y'22Y'23
start + 12:Y'30Y'31Y'32Y'33
start + 16:Cr00Cr01  
start + 18:Cr10Cr11  
start + 20:Cb00Cb01  
start + 22:Cb10Cb11  

Color Sample Location. 

 0 1 2 3
0Y Y Y Y
  C   C 
1Y Y Y Y
        
2Y Y Y Y
  C   C 
3Y Y Y Y



Name

V4L2_PIX_FMT_YVU410, V4L2_PIX_FMT_YUV410 — Planar formats with ¼ horizontal and vertical chroma resolution, also known as YUV 4:1:0

Description

These are planar formats, as opposed to a packed format. The three components are separated into three sub-images or planes. The Y plane is first. The Y plane has one byte per pixel. For V4L2_PIX_FMT_YVU410, the Cr plane immediately follows the Y plane in memory. The Cr plane is ¼ the width and ¼ the height of the Y plane (and of the image). Each Cr belongs to 16 pixels, a four-by-four square of the image. Following the Cr plane is the Cb plane, just like the Cr plane. V4L2_PIX_FMT_YUV410 is the same, except the Cb plane comes first, then the Cr plane.

If the Y plane has pad bytes after each row, then the Cr and Cb planes have ¼ as many pad bytes after their rows. In other words, four Cx rows (including padding) are exactly as long as one Y row (including padding).

Example 2.15. V4L2_PIX_FMT_YVU410 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:Y'00Y'01Y'02Y'03
start + 4:Y'10Y'11Y'12Y'13
start + 8:Y'20Y'21Y'22Y'23
start + 12:Y'30Y'31Y'32Y'33
start + 16:Cr00   
start + 17:Cb00   

Color Sample Location. 

 0 1 2 3
0Y Y Y Y
        
1Y Y Y Y
    C   
2Y Y Y Y
        
3Y Y Y Y



Name

V4L2_PIX_FMT_YUV422P — Format with ½ horizontal chroma resolution, also known as YUV 4:2:2. Planar layout as opposed to V4L2_PIX_FMT_YUYV

Description

This format is not commonly used. This is a planar version of the YUYV format. The three components are separated into three sub-images or planes. The Y plane is first. The Y plane has one byte per pixel. The Cb plane immediately follows the Y plane in memory. The Cb plane is half the width of the Y plane (and of the image). Each Cb belongs to two pixels. For example, Cb0 belongs to Y'00, Y'01. Following the Cb plane is the Cr plane, just like the Cb plane.

If the Y plane has pad bytes after each row, then the Cr and Cb planes have half as many pad bytes after their rows. In other words, two Cx rows (including padding) is exactly as long as one Y row (including padding).

Example 2.16. V4L2_PIX_FMT_YUV422P 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:Y'00Y'01Y'02Y'03
start + 4:Y'10Y'11Y'12Y'13
start + 8:Y'20Y'21Y'22Y'23
start + 12:Y'30Y'31Y'32Y'33
start + 16:Cb00Cb01  
start + 18:Cb10Cb11  
start + 20:Cb20Cb21  
start + 22:Cb30Cb31  
start + 24:Cr00Cr01  
start + 26:Cr10Cr11  
start + 28:Cr20Cr21  
start + 30:Cr30Cr31  

Color Sample Location. 

 0 1 2 3
0YCY YCY
1YCY YCY
2YCY YCY
3YCY YCY



Name

V4L2_PIX_FMT_YUV411P — Format with ¼ horizontal chroma resolution, also known as YUV 4:1:1. Planar layout as opposed to V4L2_PIX_FMT_Y41P

Description

This format is not commonly used. This is a planar format similar to the 4:2:2 planar format except with half as many chroma. The three components are separated into three sub-images or planes. The Y plane is first. The Y plane has one byte per pixel. The Cb plane immediately follows the Y plane in memory. The Cb plane is ¼ the width of the Y plane (and of the image). Each Cb belongs to 4 pixels all on the same row. For example, Cb0 belongs to Y'00, Y'01, Y'02 and Y'03. Following the Cb plane is the Cr plane, just like the Cb plane.

If the Y plane has pad bytes after each row, then the Cr and Cb planes have ¼ as many pad bytes after their rows. In other words, four C x rows (including padding) is exactly as long as one Y row (including padding).

Example 2.17. V4L2_PIX_FMT_YUV411P 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:Y'00Y'01Y'02Y'03
start + 4:Y'10Y'11Y'12Y'13
start + 8:Y'20Y'21Y'22Y'23
start + 12:Y'30Y'31Y'32Y'33
start + 16:Cb00   
start + 17:Cb10   
start + 18:Cb20   
start + 19:Cb30   
start + 20:Cr00   
start + 21:Cr10   
start + 22:Cr20   
start + 23:Cr30   

Color Sample Location. 

 0 1 2 3
0Y YCY Y
1Y YCY Y
2Y YCY Y
3Y YCY Y



Name

V4L2_PIX_FMT_NV12, V4L2_PIX_FMT_NV21 — Formats with ½ horizontal and vertical chroma resolution, also known as YUV 4:2:0. One luminance and one chrominance plane with alternating chroma samples as opposed to V4L2_PIX_FMT_YVU420

Description

These are two-plane versions of the YUV 4:2:0 format. The three components are separated into two sub-images or planes. The Y plane is first. The Y plane has one byte per pixel. For V4L2_PIX_FMT_NV12, a combined CbCr plane immediately follows the Y plane in memory. The CbCr plane is the same width, in bytes, as the Y plane (and of the image), but is half as tall in pixels. Each CbCr pair belongs to four pixels. For example, Cb0/Cr0 belongs to Y'00, Y'01, Y'10, Y'11. V4L2_PIX_FMT_NV21 is the same except the Cb and Cr bytes are swapped, the CrCb plane starts with a Cr byte.

If the Y plane has pad bytes after each row, then the CbCr plane has as many pad bytes after its rows.

Example 2.18. V4L2_PIX_FMT_NV12 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:Y'00Y'01Y'02Y'03
start + 4:Y'10Y'11Y'12Y'13
start + 8:Y'20Y'21Y'22Y'23
start + 12:Y'30Y'31Y'32Y'33
start + 16:Cb00Cr00Cb01Cr01
start + 20:Cb10Cr10Cb11Cr11

Color Sample Location. 

 0 1 2 3
0Y Y Y Y
  C   C 
1Y Y Y Y
        
2Y Y Y Y
  C   C 
3Y Y Y Y



Name

V4L2_PIX_FMT_NV16, V4L2_PIX_FMT_NV61 — Formats with ½ horizontal chroma resolution, also known as YUV 4:2:2. One luminance and one chrominance plane with alternating chroma samples as opposed to V4L2_PIX_FMT_YVU420

Description

These are two-plane versions of the YUV 4:2:2 format. The three components are separated into two sub-images or planes. The Y plane is first. The Y plane has one byte per pixel. For V4L2_PIX_FMT_NV16, a combined CbCr plane immediately follows the Y plane in memory. The CbCr plane is the same width and height, in bytes, as the Y plane (and of the image). Each CbCr pair belongs to two pixels. For example, Cb0/Cr0 belongs to Y'00, Y'01. V4L2_PIX_FMT_NV61 is the same except the Cb and Cr bytes are swapped, the CrCb plane starts with a Cr byte.

If the Y plane has pad bytes after each row, then the CbCr plane has as many pad bytes after its rows.

Example 2.19. V4L2_PIX_FMT_NV16 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:Y'00Y'01Y'02Y'03
start + 4:Y'10Y'11Y'12Y'13
start + 8:Y'20Y'21Y'22Y'23
start + 12:Y'30Y'31Y'32Y'33
start + 16:Cb00Cr00Cb01Cr01
start + 20:Cb10Cr10Cb11Cr11
start + 24:Cb20Cr20Cb21Cr21
start + 28:Cb30Cr30Cb31Cr31

Color Sample Location. 

 0 1 2 3
0Y Y Y Y
  C   C 
1Y Y Y Y
  C   C 
        
2Y Y Y Y
  C   C 
3Y Y Y Y
  C   C 


2.6. Compressed Formats

Table 2.7. Compressed Image Formats

IdentifierCodeDetails
V4L2_PIX_FMT_JPEG'JPEG'TBD. See also VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP.
V4L2_PIX_FMT_MPEG'MPEG'MPEG stream. The actual format is determined by extended control V4L2_CID_MPEG_STREAM_TYPE, see Table 1.2, “MPEG Control IDs”.

2.7. Reserved Format Identifiers

These formats are not defined by this specification, they are just listed for reference and to avoid naming conflicts. If you want to register your own format, send an e-mail to the linux-media mailing list http://www.linuxtv.org/lists.php for inclusion in the videodev2.h file. If you want to share your format with other developers add a link to your documentation and send a copy to the linux-media mailing list for inclusion in this section. If you think your format should be listed in a standard format section please make a proposal on the linux-media mailing list.

Table 2.8. Reserved Image Formats

IdentifierCodeDetails
V4L2_PIX_FMT_DV'dvsd'unknown
V4L2_PIX_FMT_ET61X251'E625'Compressed format of the ET61X251 driver.
V4L2_PIX_FMT_HI240'HI24'

8 bit RGB format used by the BTTV driver.

V4L2_PIX_FMT_HM12'HM12'

YUV 4:2:0 format used by the IVTV driver, http://www.ivtvdriver.org/

The format is documented in the kernel sources in the file Documentation/video4linux/cx2341x/README.hm12

V4L2_PIX_FMT_SPCA501'S501'YUYV per line used by the gspca driver.
V4L2_PIX_FMT_SPCA505'S505'YYUV per line used by the gspca driver.
V4L2_PIX_FMT_SPCA508'S508'YUVY per line used by the gspca driver.
V4L2_PIX_FMT_SPCA561'S561'Compressed GBRG Bayer format used by the gspca driver.
V4L2_PIX_FMT_SGRBG10'DA10'10 bit raw Bayer, expanded to 16 bits.
V4L2_PIX_FMT_SGRBG10DPCM8'DB10'10 bit raw Bayer DPCM compressed to 8 bits.
V4L2_PIX_FMT_PAC207'P207'Compressed BGGR Bayer format used by the gspca driver.
V4L2_PIX_FMT_MR97310A'M310'Compressed BGGR Bayer format used by the gspca driver.
V4L2_PIX_FMT_OV511'O511'OV511 JPEG format used by the gspca driver.
V4L2_PIX_FMT_OV518'O518'OV518 JPEG format used by the gspca driver.
V4L2_PIX_FMT_PJPG'PJPG'Pixart 73xx JPEG format used by the gspca driver.
V4L2_PIX_FMT_SQ905C'905C'Compressed RGGB bayer format used by the gspca driver.
V4L2_PIX_FMT_MJPEG'MJPG'Compressed format used by the Zoran driver
V4L2_PIX_FMT_PWC1'PWC1'Compressed format of the PWC driver.
V4L2_PIX_FMT_PWC2'PWC2'Compressed format of the PWC driver.
V4L2_PIX_FMT_SN9C10X'S910'Compressed format of the SN9C102 driver.
V4L2_PIX_FMT_SN9C20X_I420'S920'YUV 4:2:0 format of the gspca sn9c20x driver.
V4L2_PIX_FMT_WNVA'WNVA'

Used by the Winnov Videum driver, http://www.thedirks.org/winnov/

V4L2_PIX_FMT_YYUV'YYUV'unknown

Chapter 3. Input/Output

The V4L2 API defines several different methods to read from or write to a device. All drivers exchanging data with applications must support at least one of them.

The classic I/O method using the read() and write() function is automatically selected after opening a V4L2 device. When the driver does not support this method attempts to read or write will fail at any time.

Other methods must be negotiated. To select the streaming I/O method with memory mapped or user buffers applications call the VIDIOC_REQBUFS ioctl. The asynchronous I/O method is not defined yet.

Video overlay can be considered another I/O method, although the application does not directly receive the image data. It is selected by initiating video overlay with the VIDIOC_S_FMT ioctl. For more information see Section 4.2, “Video Overlay Interface”.

Generally exactly one I/O method, including overlay, is associated with each file descriptor. The only exceptions are applications not exchanging data with a driver ("panel applications", see Section 1.1, “Opening and Closing Devices”) and drivers permitting simultaneous video capturing and overlay using the same file descriptor, for compatibility with V4L and earlier versions of V4L2.

VIDIOC_S_FMT and VIDIOC_REQBUFS would permit this to some degree, but for simplicity drivers need not support switching the I/O method (after first switching away from read/write) other than by closing and reopening the device.

The following sections describe the various I/O methods in more detail.

3.1. Read/Write

Input and output devices support the read() and write() function, respectively, when the V4L2_CAP_READWRITE flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl is set.

Drivers may need the CPU to copy the data, but they may also support DMA to or from user memory, so this I/O method is not necessarily less efficient than other methods merely exchanging buffer pointers. It is considered inferior though because no meta-information like frame counters or timestamps are passed. This information is necessary to recognize frame dropping and to synchronize with other data streams. However this is also the simplest I/O method, requiring little or no setup to exchange data. It permits command line stunts like this (the vidctrl tool is fictitious):

> vidctrl /dev/video --input=0 --format=YUYV --size=352x288
> dd if=/dev/video of=myimage.422 bs=202752 count=1

To read from the device applications use the read() function, to write the write() function. Drivers must implement one I/O method if they exchange data with applications, but it need not be this.[12] When reading or writing is supported, the driver must also support the select() and poll() function.[13]

3.2. Streaming I/O (Memory Mapping)

Input and output devices support this I/O method when the V4L2_CAP_STREAMING flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl is set. There are two streaming methods, to determine if the memory mapping flavor is supported applications must call the VIDIOC_REQBUFS ioctl.

Streaming is an I/O method where only pointers to buffers are exchanged between application and driver, the data itself is not copied. Memory mapping is primarily intended to map buffers in device memory into the application's address space. Device memory can be for example the video memory on a graphics card with a video capture add-on. However, being the most efficient I/O method available for a long time, many other drivers support streaming as well, allocating buffers in DMA-able main memory.

A driver can support many sets of buffers. Each set is identified by a unique buffer type value. The sets are independent and each set can hold a different type of data. To access different sets at the same time different file descriptors must be used.[14]

To allocate device buffers applications call the VIDIOC_REQBUFS ioctl with the desired number of buffers and buffer type, for example V4L2_BUF_TYPE_VIDEO_CAPTURE. This ioctl can also be used to change the number of buffers or to free the allocated memory, provided none of the buffers are still mapped.

Before applications can access the buffers they must map them into their address space with the mmap() function. The location of the buffers in device memory can be determined with the VIDIOC_QUERYBUF ioctl. The m.offset and length returned in a struct v4l2_buffer are passed as sixth and second parameter to the mmap() function. The offset and length values must not be modified. Remember the buffers are allocated in physical memory, as opposed to virtual memory which can be swapped out to disk. Applications should free the buffers as soon as possible with the munmap() function.

Example 3.1. Mapping buffers

struct v4l2_requestbuffers reqbuf;
struct {
	void *start;
	size_t length;
} *buffers;
unsigned int i;

memset (&reqbuf, 0, sizeof (reqbuf));
reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
reqbuf.memory = V4L2_MEMORY_MMAP;
reqbuf.count = 20;

if (-1 == ioctl (fd, VIDIOC_REQBUFS, &reqbuf)) {
	if (errno == EINVAL)
		printf ("Video capturing or mmap-streaming is not supported\n");
	else
		perror ("VIDIOC_REQBUFS");

	exit (EXIT_FAILURE);
}

/* We want at least five buffers. */

if (reqbuf.count < 5) {
	/* You may need to free the buffers here. */
	printf ("Not enough buffer memory\n");
	exit (EXIT_FAILURE);
}

buffers = calloc (reqbuf.count, sizeof (*buffers));
assert (buffers != NULL);

for (i = 0; i < reqbuf.count; i++) {
	struct v4l2_buffer buffer;

	memset (&buffer, 0, sizeof (buffer));
	buffer.type = reqbuf.type;
	buffer.memory = V4L2_MEMORY_MMAP;
	buffer.index = i;

	if (-1 == ioctl (fd, VIDIOC_QUERYBUF, &buffer)) {
		perror ("VIDIOC_QUERYBUF");
		exit (EXIT_FAILURE);
	}

	buffers[i].length = buffer.length; /* remember for munmap() */

	buffers[i].start = mmap (NULL, buffer.length,
				 PROT_READ | PROT_WRITE, /* recommended */
				 MAP_SHARED,             /* recommended */
				 fd, buffer.m.offset);

	if (MAP_FAILED == buffers[i].start) {
		/* If you do not exit here you should unmap() and free()
		   the buffers mapped so far. */
		perror ("mmap");
		exit (EXIT_FAILURE);
	}
}

/* Cleanup. */

for (i = 0; i < reqbuf.count; i++)
	munmap (buffers[i].start, buffers[i].length);
      

Conceptually streaming drivers maintain two buffer queues, an incoming and an outgoing queue. They separate the synchronous capture or output operation locked to a video clock from the application which is subject to random disk or network delays and preemption by other processes, thereby reducing the probability of data loss. The queues are organized as FIFOs, buffers will be output in the order enqueued in the incoming FIFO, and were captured in the order dequeued from the outgoing FIFO.

The driver may require a minimum number of buffers enqueued at all times to function, apart of this no limit exists on the number of buffers applications can enqueue in advance, or dequeue and process. They can also enqueue in a different order than buffers have been dequeued, and the driver can fill enqueued empty buffers in any order. [15] The index number of a buffer (struct v4l2_buffer index) plays no role here, it only identifies the buffer.

Initially all mapped buffers are in dequeued state, inaccessible by the driver. For capturing applications it is customary to first enqueue all mapped buffers, then to start capturing and enter the read loop. Here the application waits until a filled buffer can be dequeued, and re-enqueues the buffer when the data is no longer needed. Output applications fill and enqueue buffers, when enough buffers are stacked up the output is started with VIDIOC_STREAMON. In the write loop, when the application runs out of free buffers, it must wait until an empty buffer can be dequeued and reused.

To enqueue and dequeue a buffer applications use the VIDIOC_QBUF and VIDIOC_DQBUF ioctl. The status of a buffer being mapped, enqueued, full or empty can be determined at any time using the VIDIOC_QUERYBUF ioctl. Two methods exist to suspend execution of the application until one or more buffers can be dequeued. By default VIDIOC_DQBUF blocks when no buffer is in the outgoing queue. When the O_NONBLOCK flag was given to the open() function, VIDIOC_DQBUF returns immediately with an EAGAIN error code when no buffer is available. The select() or poll() function are always available.

To start and stop capturing or output applications call the VIDIOC_STREAMON and VIDIOC_STREAMOFF ioctl. Note VIDIOC_STREAMOFF removes all buffers from both queues as a side effect. Since there is no notion of doing anything "now" on a multitasking system, if an application needs to synchronize with another event it should examine the struct v4l2_buffer timestamp of captured buffers, or set the field before enqueuing buffers for output.

Drivers implementing memory mapping I/O must support the VIDIOC_REQBUFS, VIDIOC_QUERYBUF, VIDIOC_QBUF, VIDIOC_DQBUF, VIDIOC_STREAMON and VIDIOC_STREAMOFF ioctl, the mmap(), munmap(), select() and poll() function.[16]

[capture example]

3.3. Streaming I/O (User Pointers)

Input and output devices support this I/O method when the V4L2_CAP_STREAMING flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl is set. If the particular user pointer method (not only memory mapping) is supported must be determined by calling the VIDIOC_REQBUFS ioctl.

This I/O method combines advantages of the read/write and memory mapping methods. Buffers are allocated by the application itself, and can reside for example in virtual or shared memory. Only pointers to data are exchanged, these pointers and meta-information are passed in struct v4l2_buffer. The driver must be switched into user pointer I/O mode by calling the VIDIOC_REQBUFS with the desired buffer type. No buffers are allocated beforehands, consequently they are not indexed and cannot be queried like mapped buffers with the VIDIOC_QUERYBUF ioctl.

Example 3.2. Initiating streaming I/O with user pointers

struct v4l2_requestbuffers reqbuf;

memset (&reqbuf, 0, sizeof (reqbuf));
reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
reqbuf.memory = V4L2_MEMORY_USERPTR;

if (ioctl (fd, VIDIOC_REQBUFS, &reqbuf) == -1) {
	if (errno == EINVAL)
		printf ("Video capturing or user pointer streaming is not supported\n");
	else
		perror ("VIDIOC_REQBUFS");

	exit (EXIT_FAILURE);
}
      

Buffer addresses and sizes are passed on the fly with the VIDIOC_QBUF ioctl. Although buffers are commonly cycled, applications can pass different addresses and sizes at each VIDIOC_QBUF call. If required by the hardware the driver swaps memory pages within physical memory to create a continuous area of memory. This happens transparently to the application in the virtual memory subsystem of the kernel. When buffer pages have been swapped out to disk they are brought back and finally locked in physical memory for DMA.[17]

Filled or displayed buffers are dequeued with the VIDIOC_DQBUF ioctl. The driver can unlock the memory pages at any time between the completion of the DMA and this ioctl. The memory is also unlocked when VIDIOC_STREAMOFF is called, VIDIOC_REQBUFS, or when the device is closed. Applications must take care not to free buffers without dequeuing. For once, the buffers remain locked until further, wasting physical memory. Second the driver will not be notified when the memory is returned to the application's free list and subsequently reused for other purposes, possibly completing the requested DMA and overwriting valuable data.

For capturing applications it is customary to enqueue a number of empty buffers, to start capturing and enter the read loop. Here the application waits until a filled buffer can be dequeued, and re-enqueues the buffer when the data is no longer needed. Output applications fill and enqueue buffers, when enough buffers are stacked up output is started. In the write loop, when the application runs out of free buffers it must wait until an empty buffer can be dequeued and reused. Two methods exist to suspend execution of the application until one or more buffers can be dequeued. By default VIDIOC_DQBUF blocks when no buffer is in the outgoing queue. When the O_NONBLOCK flag was given to the open() function, VIDIOC_DQBUF returns immediately with an EAGAIN error code when no buffer is available. The select() or poll() function are always available.

To start and stop capturing or output applications call the VIDIOC_STREAMON and VIDIOC_STREAMOFF ioctl. Note VIDIOC_STREAMOFF removes all buffers from both queues and unlocks all buffers as a side effect. Since there is no notion of doing anything "now" on a multitasking system, if an application needs to synchronize with another event it should examine the struct v4l2_buffer timestamp of captured buffers, or set the field before enqueuing buffers for output.

Drivers implementing user pointer I/O must support the VIDIOC_REQBUFS, VIDIOC_QBUF, VIDIOC_DQBUF, VIDIOC_STREAMON and VIDIOC_STREAMOFF ioctl, the select() and poll() function.[18]

3.4. Asynchronous I/O

This method is not defined yet.

3.5. Buffers

A buffer contains data exchanged by application and driver using one of the Streaming I/O methods. Only pointers to buffers are exchanged, the data itself is not copied. These pointers, together with meta-information like timestamps or field parity, are stored in a struct v4l2_buffer, argument to the VIDIOC_QUERYBUF, VIDIOC_QBUF and VIDIOC_DQBUF ioctl.

Nominally timestamps refer to the first data byte transmitted. In practice however the wide range of hardware covered by the V4L2 API limits timestamp accuracy. Often an interrupt routine will sample the system clock shortly after the field or frame was stored completely in memory. So applications must expect a constant difference up to one field or frame period plus a small (few scan lines) random error. The delay and error can be much larger due to compression or transmission over an external bus when the frames are not properly stamped by the sender. This is frequently the case with USB cameras. Here timestamps refer to the instant the field or frame was received by the driver, not the capture time. These devices identify by not enumerating any video standards, see Section 1.7, “Video Standards”.

Similar limitations apply to output timestamps. Typically the video hardware locks to a clock controlling the video timing, the horizontal and vertical synchronization pulses. At some point in the line sequence, possibly the vertical blanking, an interrupt routine samples the system clock, compares against the timestamp and programs the hardware to repeat the previous field or frame, or to display the buffer contents.

Apart of limitations of the video device and natural inaccuracies of all clocks, it should be noted system time itself is not perfectly stable. It can be affected by power saving cycles, warped to insert leap seconds, or even turned back or forth by the system administrator affecting long term measurements. [19]

Table 3.1. struct v4l2_buffer

__u32index Number of the buffer, set by the application. This field is only used for memory mapping I/O and can range from zero to the number of buffers allocated with the VIDIOC_REQBUFS ioctl (struct v4l2_requestbuffers count) minus one.
enum v4l2_buf_typetype Type of the buffer, same as struct v4l2_format type or struct v4l2_requestbuffers type, set by the application.
__u32bytesused The number of bytes occupied by the data in the buffer. It depends on the negotiated data format and may change with each buffer for compressed variable size data like JPEG images. Drivers must set this field when type refers to an input stream, applications when an output stream.
__u32flags Flags set by the application or driver, see Table 3.3, “Buffer Flags”.
enum v4l2_fieldfield Indicates the field order of the image in the buffer, see Table 3.8, “enum v4l2_field”. This field is not used when the buffer contains VBI data. Drivers must set it when type refers to an input stream, applications when an output stream.
struct timevaltimestamp 

For input streams this is the system time (as returned by the gettimeofday() function) when the first data byte was captured. For output streams the data will not be displayed before this time, secondary to the nominal frame rate determined by the current video standard in enqueued order. Applications can for example zero this field to display frames as soon as possible. The driver stores the time at which the first data byte was actually sent out in the timestamp field. This permits applications to monitor the drift between the video and system clock.

struct v4l2_timecodetimecode When type is V4L2_BUF_TYPE_VIDEO_CAPTURE and the V4L2_BUF_FLAG_TIMECODE flag is set in flags, this structure contains a frame timecode. In V4L2_FIELD_ALTERNATE mode the top and bottom field contain the same timecode. Timecodes are intended to help video editing and are typically recorded on video tapes, but also embedded in compressed formats like MPEG. This field is independent of the timestamp and sequence fields.
__u32sequence Set by the driver, counting the frames in the sequence.

In V4L2_FIELD_ALTERNATE mode the top and bottom field have the same sequence number. The count starts at zero and includes dropped or repeated frames. A dropped frame was received by an input device but could not be stored due to lack of free buffer space. A repeated frame was displayed again by an output device because the application did not pass new data in time.

Note this may count the frames received e.g. over USB, without taking into account the frames dropped by the remote hardware due to limited compression throughput or bus bandwidth. These devices identify by not enumerating any video standards, see Section 1.7, “Video Standards”.

enum v4l2_memorymemory This field must be set by applications and/or drivers in accordance with the selected I/O method.
unionm  
 __u32offsetWhen memory is V4L2_MEMORY_MMAP this is the offset of the buffer from the start of the device memory. The value is returned by the driver and apart of serving as parameter to the mmap() function not useful for applications. See Section 3.2, “Streaming I/O (Memory Mapping)” for details.
 unsigned longuserptrWhen memory is V4L2_MEMORY_USERPTR this is a pointer to the buffer (casted to unsigned long type) in virtual memory, set by the application. See Section 3.3, “Streaming I/O (User Pointers)” for details.
__u32length Size of the buffer (not the payload) in bytes.
__u32input Some video capture drivers support rapid and synchronous video input changes, a function useful for example in video surveillance applications. For this purpose applications set the V4L2_BUF_FLAG_INPUT flag, and this field to the number of a video input as in struct v4l2_input field index.
__u32reserved A place holder for future extensions and custom (driver defined) buffer types V4L2_BUF_TYPE_PRIVATE and higher.

Table 3.2. enum v4l2_buf_type

V4L2_BUF_TYPE_VIDEO_CAPTURE1Buffer of a video capture stream, see Section 4.1, “Video Capture Interface”.
V4L2_BUF_TYPE_VIDEO_OUTPUT2Buffer of a video output stream, see Section 4.3, “Video Output Interface”.
V4L2_BUF_TYPE_VIDEO_OVERLAY3Buffer for video overlay, see Section 4.2, “Video Overlay Interface”.
V4L2_BUF_TYPE_VBI_CAPTURE4Buffer of a raw VBI capture stream, see Section 4.7, “Raw VBI Data Interface”.
V4L2_BUF_TYPE_VBI_OUTPUT5Buffer of a raw VBI output stream, see Section 4.7, “Raw VBI Data Interface”.
V4L2_BUF_TYPE_SLICED_VBI_CAPTURE6Buffer of a sliced VBI capture stream, see Section 4.8, “Sliced VBI Data Interface”.
V4L2_BUF_TYPE_SLICED_VBI_OUTPUT7Buffer of a sliced VBI output stream, see Section 4.8, “Sliced VBI Data Interface”.
V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY8Buffer for video output overlay (OSD), see Section 4.4, “Video Output Overlay Interface”. Status: Experimental.
V4L2_BUF_TYPE_PRIVATE0x80This and higher values are reserved for custom (driver defined) buffer types.

Table 3.3. Buffer Flags

V4L2_BUF_FLAG_MAPPED0x0001The buffer resides in device memory and has been mapped into the application's address space, see Section 3.2, “Streaming I/O (Memory Mapping)” for details. Drivers set or clear this flag when the VIDIOC_QUERYBUF, VIDIOC_QBUF or VIDIOC_DQBUF ioctl is called. Set by the driver.
V4L2_BUF_FLAG_QUEUED0x0002Internally drivers maintain two buffer queues, an incoming and outgoing queue. When this flag is set, the buffer is currently on the incoming queue. It automatically moves to the outgoing queue after the buffer has been filled (capture devices) or displayed (output devices). Drivers set or clear this flag when the VIDIOC_QUERYBUF ioctl is called. After (successful) calling the VIDIOC_QBUF ioctl it is always set and after VIDIOC_DQBUF always cleared.
V4L2_BUF_FLAG_DONE0x0004When this flag is set, the buffer is currently on the outgoing queue, ready to be dequeued from the driver. Drivers set or clear this flag when the VIDIOC_QUERYBUF ioctl is called. After calling the VIDIOC_QBUF or VIDIOC_DQBUF it is always cleared. Of course a buffer cannot be on both queues at the same time, the V4L2_BUF_FLAG_QUEUED and V4L2_BUF_FLAG_DONE flag are mutually exclusive. They can be both cleared however, then the buffer is in "dequeued" state, in the application domain to say so.
V4L2_BUF_FLAG_KEYFRAME0x0008Drivers set or clear this flag when calling the VIDIOC_DQBUF ioctl. It may be set by video capture devices when the buffer contains a compressed image which is a key frame (or field), i. e. can be decompressed on its own.
V4L2_BUF_FLAG_PFRAME0x0010Similar to V4L2_BUF_FLAG_KEYFRAME this flags predicted frames or fields which contain only differences to a previous key frame.
V4L2_BUF_FLAG_BFRAME0x0020Similar to V4L2_BUF_FLAG_PFRAME this is a bidirectional predicted frame or field. [ooc tbd]
V4L2_BUF_FLAG_TIMECODE0x0100The timecode field is valid. Drivers set or clear this flag when the VIDIOC_DQBUF ioctl is called.
V4L2_BUF_FLAG_INPUT0x0200The input field is valid. Applications set or clear this flag before calling the VIDIOC_QBUF ioctl.

Table 3.4. enum v4l2_memory

V4L2_MEMORY_MMAP1The buffer is used for memory mapping I/O.
V4L2_MEMORY_USERPTR2The buffer is used for user pointer I/O.
V4L2_MEMORY_OVERLAY3[to do]

3.5.1. Timecodes

The v4l2_timecode structure is designed to hold a [SMPTE 12M] or similar timecode. (struct timeval timestamps are stored in struct v4l2_buffer field timestamp.)

Table 3.5. struct v4l2_timecode

__u32typeFrame rate the timecodes are based on, see Table 3.6, “Timecode Types”.
__u32flagsTimecode flags, see Table 3.7, “Timecode Flags”.
__u8framesFrame count, 0 ... 23/24/29/49/59, depending on the type of timecode.
__u8secondsSeconds count, 0 ... 59. This is a binary, not BCD number.
__u8minutesMinutes count, 0 ... 59. This is a binary, not BCD number.
__u8hoursHours count, 0 ... 29. This is a binary, not BCD number.
__u8userbits[4]The "user group" bits from the timecode.

Table 3.6. Timecode Types

V4L2_TC_TYPE_24FPS124 frames per second, i. e. film.
V4L2_TC_TYPE_25FPS225 frames per second, i. e. PAL or SECAM video.
V4L2_TC_TYPE_30FPS330 frames per second, i. e. NTSC video.
V4L2_TC_TYPE_50FPS4 
V4L2_TC_TYPE_60FPS5 

Table 3.7. Timecode Flags

V4L2_TC_FLAG_DROPFRAME0x0001Indicates "drop frame" semantics for counting frames in 29.97 fps material. When set, frame numbers 0 and 1 at the start of each minute, except minutes 0, 10, 20, 30, 40, 50 are omitted from the count.
V4L2_TC_FLAG_COLORFRAME0x0002The "color frame" flag.
V4L2_TC_USERBITS_field0x000CField mask for the "binary group flags".
V4L2_TC_USERBITS_USERDEFINED0x0000Unspecified format.
V4L2_TC_USERBITS_8BITCHARS0x00088-bit ISO characters.

3.6. Field Order

We have to distinguish between progressive and interlaced video. Progressive video transmits all lines of a video image sequentially. Interlaced video divides an image into two fields, containing only the odd and even lines of the image, respectively. Alternating the so called odd and even field are transmitted, and due to a small delay between fields a cathode ray TV displays the lines interleaved, yielding the original frame. This curious technique was invented because at refresh rates similar to film the image would fade out too quickly. Transmitting fields reduces the flicker without the necessity of doubling the frame rate and with it the bandwidth required for each channel.

It is important to understand a video camera does not expose one frame at a time, merely transmitting the frames separated into fields. The fields are in fact captured at two different instances in time. An object on screen may well move between one field and the next. For applications analysing motion it is of paramount importance to recognize which field of a frame is older, the temporal order.

When the driver provides or accepts images field by field rather than interleaved, it is also important applications understand how the fields combine to frames. We distinguish between top and bottom fields, the spatial order: The first line of the top field is the first line of an interlaced frame, the first line of the bottom field is the second line of that frame.

However because fields were captured one after the other, arguing whether a frame commences with the top or bottom field is pointless. Any two successive top and bottom, or bottom and top fields yield a valid frame. Only when the source was progressive to begin with, e. g. when transferring film to video, two fields may come from the same frame, creating a natural order.

Counter to intuition the top field is not necessarily the older field. Whether the older field contains the top or bottom lines is a convention determined by the video standard. Hence the distinction between temporal and spatial order of fields. The diagrams below should make this clearer.

All video capture and output devices must report the current field order. Some drivers may permit the selection of a different order, to this end applications initialize the field field of struct v4l2_pix_format before calling the VIDIOC_S_FMT ioctl. If this is not desired it should have the value V4L2_FIELD_ANY (0).

Table 3.8. enum v4l2_field

V4L2_FIELD_ANY0Applications request this field order when any one of the V4L2_FIELD_NONE, V4L2_FIELD_TOP, V4L2_FIELD_BOTTOM, or V4L2_FIELD_INTERLACED formats is acceptable. Drivers choose depending on hardware capabilities or e. g. the requested image size, and return the actual field order. struct v4l2_buffer field can never be V4L2_FIELD_ANY.
V4L2_FIELD_NONE1Images are in progressive format, not interlaced. The driver may also indicate this order when it cannot distinguish between V4L2_FIELD_TOP and V4L2_FIELD_BOTTOM.
V4L2_FIELD_TOP2Images consist of the top field only.
V4L2_FIELD_BOTTOM3Images consist of the bottom field only. Applications may wish to prevent a device from capturing interlaced images because they will have "comb" or "feathering" artefacts around moving objects.
V4L2_FIELD_INTERLACED4Images contain both fields, interleaved line by line. The temporal order of the fields (whether the top or bottom field is first transmitted) depends on the current video standard. M/NTSC transmits the bottom field first, all other standards the top field first.
V4L2_FIELD_SEQ_TB5Images contain both fields, the top field lines are stored first in memory, immediately followed by the bottom field lines. Fields are always stored in temporal order, the older one first in memory. Image sizes refer to the frame, not fields.
V4L2_FIELD_SEQ_BT6Images contain both fields, the bottom field lines are stored first in memory, immediately followed by the top field lines. Fields are always stored in temporal order, the older one first in memory. Image sizes refer to the frame, not fields.
V4L2_FIELD_ALTERNATE7The two fields of a frame are passed in separate buffers, in temporal order, i. e. the older one first. To indicate the field parity (whether the current field is a top or bottom field) the driver or application, depending on data direction, must set struct v4l2_buffer field to V4L2_FIELD_TOP or V4L2_FIELD_BOTTOM. Any two successive fields pair to build a frame. If fields are successive, without any dropped fields between them (fields can drop individually), can be determined from the struct v4l2_buffer sequence field. Image sizes refer to the frame, not fields. This format cannot be selected when using the read/write I/O method.
V4L2_FIELD_INTERLACED_TB8Images contain both fields, interleaved line by line, top field first. The top field is transmitted first.
V4L2_FIELD_INTERLACED_BT9Images contain both fields, interleaved line by line, top field first. The bottom field is transmitted first.

Figure 3.1. Field Order, Top Field First Transmitted

Field Order, Top Field First Transmitted

Figure 3.2. Field Order, Bottom Field First Transmitted

Field Order, Bottom Field First Transmitted



[12] It would be desirable if applications could depend on drivers supporting all I/O interfaces, but as much as the complex memory mapping I/O can be inadequate for some devices we have no reason to require this interface, which is most useful for simple applications capturing still images.

[13] At the driver level select() and poll() are the same, and select() is too important to be optional.

[14] One could use one file descriptor and set the buffer type field accordingly when calling VIDIOC_QBUF etc., but it makes the select() function ambiguous. We also like the clean approach of one file descriptor per logical stream. Video overlay for example is also a logical stream, although the CPU is not needed for continuous operation.

[15] Random enqueue order permits applications processing images out of order (such as video codecs) to return buffers earlier, reducing the probability of data loss. Random fill order allows drivers to reuse buffers on a LIFO-basis, taking advantage of caches holding scatter-gather lists and the like.

[16] At the driver level select() and poll() are the same, and select() is too important to be optional. The rest should be evident.

[17] We expect that frequently used buffers are typically not swapped out. Anyway, the process of swapping, locking or generating scatter-gather lists may be time consuming. The delay can be masked by the depth of the incoming buffer queue, and perhaps by maintaining caches assuming a buffer will be soon enqueued again. On the other hand, to optimize memory usage drivers can limit the number of buffers locked in advance and recycle the most recently used buffers first. Of course, the pages of empty buffers in the incoming queue need not be saved to disk. Output buffers must be saved on the incoming and outgoing queue because an application may share them with other processes.

[18] At the driver level select() and poll() are the same, and select() is too important to be optional. The rest should be evident.

[19] Since no other Linux multimedia API supports unadjusted time it would be foolish to introduce here. We must use a universally supported clock to synchronize different media, hence time of day.

Chapter 4. Interfaces

4.1. Video Capture Interface

Video capture devices sample an analog video signal and store the digitized images in memory. Today nearly all devices can capture at full 25 or 30 frames/second. With this interface applications can control the capture process and move images from the driver into user space.

Conventionally V4L2 video capture devices are accessed through character device special files named /dev/video and /dev/video0 to /dev/video63 with major number 81 and minor numbers 0 to 63. /dev/video is typically a symbolic link to the preferred video device. Note the same device files are used for video output devices.

4.1.1. Querying Capabilities

Devices supporting the video capture interface set the V4L2_CAP_VIDEO_CAPTURE flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl. As secondary device functions they may also support the video overlay (V4L2_CAP_VIDEO_OVERLAY) and the raw VBI capture (V4L2_CAP_VBI_CAPTURE) interface. At least one of the read/write or streaming I/O methods must be supported. Tuners and audio inputs are optional.

4.1.2. Supplemental Functions

Video capture devices shall support audio input, tuner, controls, cropping and scaling and streaming parameter ioctls as needed. The video input and video standard ioctls must be supported by all video capture devices.

4.1.3. Image Format Negotiation

The result of a capture operation is determined by cropping and image format parameters. The former select an area of the video picture to capture, the latter how images are stored in memory, i. e. in RGB or YUV format, the number of bits per pixel or width and height. Together they also define how images are scaled in the process.

As usual these parameters are not reset at open() time to permit Unix tool chains, programming a device and then reading from it as if it was a plain file. Well written V4L2 applications ensure they really get what they want, including cropping and scaling.

Cropping initialization at minimum requires to reset the parameters to defaults. An example is given in Section 1.11, “Image Cropping, Insertion and Scaling”.

To query the current image format applications set the type field of a struct v4l2_format to V4L2_BUF_TYPE_VIDEO_CAPTURE and call the VIDIOC_G_FMT ioctl with a pointer to this structure. Drivers fill the struct v4l2_pix_format pix member of the fmt union.

To request different parameters applications set the type field of a struct v4l2_format as above and initialize all fields of the struct v4l2_pix_format vbi member of the fmt union, or better just modify the results of VIDIOC_G_FMT, and call the VIDIOC_S_FMT ioctl with a pointer to this structure. Drivers may adjust the parameters and finally return the actual parameters as VIDIOC_G_FMT does.

Like VIDIOC_S_FMT the VIDIOC_TRY_FMT ioctl can be used to learn about hardware limitations without disabling I/O or possibly time consuming hardware preparations.

The contents of struct v4l2_pix_format are discussed in Chapter 2, Image Formats. See also the specification of the VIDIOC_G_FMT, VIDIOC_S_FMT and VIDIOC_TRY_FMT ioctls for details. Video capture devices must implement both the VIDIOC_G_FMT and VIDIOC_S_FMT ioctl, even if VIDIOC_S_FMT ignores all requests and always returns default parameters as VIDIOC_G_FMT does. VIDIOC_TRY_FMT is optional.

4.1.4. Reading Images

A video capture device may support the read() function and/or streaming (memory mapping or user pointer) I/O. See Chapter 3, Input/Output for details.

4.2. Video Overlay Interface

Also known as Framebuffer Overlay or Previewing

Video overlay devices have the ability to genlock (TV-)video into the (VGA-)video signal of a graphics card, or to store captured images directly in video memory of a graphics card, typically with clipping. This can be considerable more efficient than capturing images and displaying them by other means. In the old days when only nuclear power plants needed cooling towers this used to be the only way to put live video into a window.

Video overlay devices are accessed through the same character special files as video capture devices. Note the default function of a /dev/video device is video capturing. The overlay function is only available after calling the VIDIOC_S_FMT ioctl.

The driver may support simultaneous overlay and capturing using the read/write and streaming I/O methods. If so, operation at the nominal frame rate of the video standard is not guaranteed. Frames may be directed away from overlay to capture, or one field may be used for overlay and the other for capture if the capture parameters permit this.

Applications should use different file descriptors for capturing and overlay. This must be supported by all drivers capable of simultaneous capturing and overlay. Optionally these drivers may also permit capturing and overlay with a single file descriptor for compatibility with V4L and earlier versions of V4L2.[20]

4.2.1. Querying Capabilities

Devices supporting the video overlay interface set the V4L2_CAP_VIDEO_OVERLAY flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl. The overlay I/O method specified below must be supported. Tuners and audio inputs are optional.

4.2.2. Supplemental Functions

Video overlay devices shall support audio input, tuner, controls, cropping and scaling and streaming parameter ioctls as needed. The video input and video standard ioctls must be supported by all video overlay devices.

4.2.3. Setup

Before overlay can commence applications must program the driver with frame buffer parameters, namely the address and size of the frame buffer and the image format, for example RGB 5:6:5. The VIDIOC_G_FBUF and VIDIOC_S_FBUF ioctls are available to get and set these parameters, respectively. The VIDIOC_S_FBUF ioctl is privileged because it allows to set up DMA into physical memory, bypassing the memory protection mechanisms of the kernel. Only the superuser can change the frame buffer address and size. Users are not supposed to run TV applications as root or with SUID bit set. A small helper application with suitable privileges should query the graphics system and program the V4L2 driver at the appropriate time.

Some devices add the video overlay to the output signal of the graphics card. In this case the frame buffer is not modified by the video device, and the frame buffer address and pixel format are not needed by the driver. The VIDIOC_S_FBUF ioctl is not privileged. An application can check for this type of device by calling the VIDIOC_G_FBUF ioctl.

A driver may support any (or none) of five clipping/blending methods:

  1. Chroma-keying displays the overlaid image only where pixels in the primary graphics surface assume a certain color.

  2. A bitmap can be specified where each bit corresponds to a pixel in the overlaid image. When the bit is set, the corresponding video pixel is displayed, otherwise a pixel of the graphics surface.

  3. A list of clipping rectangles can be specified. In these regions no video is displayed, so the graphics surface can be seen here.

  4. The framebuffer has an alpha channel that can be used to clip or blend the framebuffer with the video.

  5. A global alpha value can be specified to blend the framebuffer contents with video images.

When simultaneous capturing and overlay is supported and the hardware prohibits different image and frame buffer formats, the format requested first takes precedence. The attempt to capture (VIDIOC_S_FMT) or overlay (VIDIOC_S_FBUF) may fail with an EBUSY error code or return accordingly modified parameters..

4.2.4. Overlay Window

The overlaid image is determined by cropping and overlay window parameters. The former select an area of the video picture to capture, the latter how images are overlaid and clipped. Cropping initialization at minimum requires to reset the parameters to defaults. An example is given in Section 1.11, “Image Cropping, Insertion and Scaling”.

The overlay window is described by a struct v4l2_window. It defines the size of the image, its position over the graphics surface and the clipping to be applied. To get the current parameters applications set the type field of a struct v4l2_format to V4L2_BUF_TYPE_VIDEO_OVERLAY and call the VIDIOC_G_FMT ioctl. The driver fills the v4l2_window substructure named win. It is not possible to retrieve a previously programmed clipping list or bitmap.

To program the overlay window applications set the type field of a struct v4l2_format to V4L2_BUF_TYPE_VIDEO_OVERLAY, initialize the win substructure and call the VIDIOC_S_FMT ioctl. The driver adjusts the parameters against hardware limits and returns the actual parameters as VIDIOC_G_FMT does. Like VIDIOC_S_FMT, the VIDIOC_TRY_FMT ioctl can be used to learn about driver capabilities without actually changing driver state. Unlike VIDIOC_S_FMT this also works after the overlay has been enabled.

The scaling factor of the overlaid image is implied by the width and height given in struct v4l2_window and the size of the cropping rectangle. For more information see Section 1.11, “Image Cropping, Insertion and Scaling”.

When simultaneous capturing and overlay is supported and the hardware prohibits different image and window sizes, the size requested first takes precedence. The attempt to capture or overlay as well (VIDIOC_S_FMT) may fail with an EBUSY error code or return accordingly modified parameters.

Table 4.1. struct v4l2_window

struct v4l2_rectwSize and position of the window relative to the top, left corner of the frame buffer defined with VIDIOC_S_FBUF. The window can extend the frame buffer width and height, the x and y coordinates can be negative, and it can lie completely outside the frame buffer. The driver clips the window accordingly, or if that is not possible, modifies its size and/or position.
enum v4l2_fieldfieldApplications set this field to determine which video field shall be overlaid, typically one of V4L2_FIELD_ANY (0), V4L2_FIELD_TOP, V4L2_FIELD_BOTTOM or V4L2_FIELD_INTERLACED. Drivers may have to choose a different field order and return the actual setting here.
__u32chromakeyWhen chroma-keying has been negotiated with VIDIOC_S_FBUF applications set this field to the desired pixel value for the chroma key. The format is the same as the pixel format of the framebuffer (struct v4l2_framebuffer fmt.pixelformat field), with bytes in host order. E. g. for V4L2_PIX_FMT_BGR24 the value should be 0xRRGGBB on a little endian, 0xBBGGRR on a big endian host.
struct v4l2_clip *clipsWhen chroma-keying has not been negotiated and VIDIOC_G_FBUF indicated this capability, applications can set this field to point to an array of clipping rectangles.
  Like the window coordinates w, clipping rectangles are defined relative to the top, left corner of the frame buffer. However clipping rectangles must not extend the frame buffer width and height, and they must not overlap. If possible applications should merge adjacent rectangles. Whether this must create x-y or y-x bands, or the order of rectangles, is not defined. When clip lists are not supported the driver ignores this field. Its contents after calling VIDIOC_S_FMT are undefined.
__u32clipcountWhen the application set the clips field, this field must contain the number of clipping rectangles in the list. When clip lists are not supported the driver ignores this field, its contents after calling VIDIOC_S_FMT are undefined. When clip lists are supported but no clipping is desired this field must be set to zero.
void *bitmapWhen chroma-keying has not been negotiated and VIDIOC_G_FBUF indicated this capability, applications can set this field to point to a clipping bit mask.

It must be of the same size as the window, w.width and w.height. Each bit corresponds to a pixel in the overlaid image, which is displayed only when the bit is set. Pixel coordinates translate to bits like:

((__u8 *) bitmap)[w.width * y + x / 8] & (1 << (x & 7))

where 0 ≤ x < w.width and 0 ≤ y <w.height.[a]

When a clipping bit mask is not supported the driver ignores this field, its contents after calling VIDIOC_S_FMT are undefined. When a bit mask is supported but no clipping is desired this field must be set to NULL.

Applications need not create a clip list or bit mask. When they pass both, or despite negotiating chroma-keying, the results are undefined. Regardless of the chosen method, the clipping abilities of the hardware may be limited in quantity or quality. The results when these limits are exceeded are undefined.[b]

__u8global_alphaThe global alpha value used to blend the framebuffer with video images, if global alpha blending has been negotiated (V4L2_FBUF_FLAG_GLOBAL_ALPHA, see VIDIOC_S_FBUF, Table 83, “Frame Buffer Flags”).
  Note this field was added in Linux 2.6.23, extending the structure. However the VIDIOC_G/S/TRY_FMT ioctls, which take a pointer to a v4l2_format parent structure with padding bytes at the end, are not affected.

[a] Should we require w.width to be a multiple of eight?

[b] When the image is written into frame buffer memory it will be undesirable if the driver clips out less pixels than expected, because the application and graphics system are not aware these regions need to be refreshed. The driver should clip out more pixels or not write the image at all.


Table 4.2. struct v4l2_clip[21]

struct v4l2_rectcCoordinates of the clipping rectangle, relative to the top, left corner of the frame buffer. Only window pixels outside all clipping rectangles are displayed.
struct v4l2_clip *nextPointer to the next clipping rectangle, NULL when this is the last rectangle. Drivers ignore this field, it cannot be used to pass a linked list of clipping rectangles.

Table 4.3. struct v4l2_rect

__s32leftHorizontal offset of the top, left corner of the rectangle, in pixels.
__s32topVertical offset of the top, left corner of the rectangle, in pixels. Offsets increase to the right and down.
__s32widthWidth of the rectangle, in pixels.
__s32heightHeight of the rectangle, in pixels. Width and height cannot be negative, the fields are signed for hysterical reasons.

4.2.5. Enabling Overlay

To start or stop the frame buffer overlay applications call the VIDIOC_OVERLAY ioctl.

4.3. Video Output Interface

Video output devices encode stills or image sequences as analog video signal. With this interface applications can control the encoding process and move images from user space to the driver.

Conventionally V4L2 video output devices are accessed through character device special files named /dev/video and /dev/video0 to /dev/video63 with major number 81 and minor numbers 0 to 63. /dev/video is typically a symbolic link to the preferred video device. Note the same device files are used for video capture devices.

4.3.1. Querying Capabilities

Devices supporting the video output interface set the V4L2_CAP_VIDEO_OUTPUT flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl. As secondary device functions they may also support the raw VBI output (V4L2_CAP_VBI_OUTPUT) interface. At least one of the read/write or streaming I/O methods must be supported. Modulators and audio outputs are optional.

4.3.2. Supplemental Functions

Video output devices shall support audio output, modulator, controls, cropping and scaling and streaming parameter ioctls as needed. The video output and video standard ioctls must be supported by all video output devices.

4.3.3. Image Format Negotiation

The output is determined by cropping and image format parameters. The former select an area of the video picture where the image will appear, the latter how images are stored in memory, i. e. in RGB or YUV format, the number of bits per pixel or width and height. Together they also define how images are scaled in the process.

As usual these parameters are not reset at open() time to permit Unix tool chains, programming a device and then writing to it as if it was a plain file. Well written V4L2 applications ensure they really get what they want, including cropping and scaling.

Cropping initialization at minimum requires to reset the parameters to defaults. An example is given in Section 1.11, “Image Cropping, Insertion and Scaling”.

To query the current image format applications set the type field of a struct v4l2_format to V4L2_BUF_TYPE_VIDEO_OUTPUT and call the VIDIOC_G_FMT ioctl with a pointer to this structure. Drivers fill the struct v4l2_pix_format pix member of the fmt union.

To request different parameters applications set the type field of a struct v4l2_format as above and initialize all fields of the struct v4l2_pix_format vbi member of the fmt union, or better just modify the results of VIDIOC_G_FMT, and call the VIDIOC_S_FMT ioctl with a pointer to this structure. Drivers may adjust the parameters and finally return the actual parameters as VIDIOC_G_FMT does.

Like VIDIOC_S_FMT the VIDIOC_TRY_FMT ioctl can be used to learn about hardware limitations without disabling I/O or possibly time consuming hardware preparations.

The contents of struct v4l2_pix_format are discussed in Chapter 2, Image Formats. See also the specification of the VIDIOC_G_FMT, VIDIOC_S_FMT and VIDIOC_TRY_FMT ioctls for details. Video output devices must implement both the VIDIOC_G_FMT and VIDIOC_S_FMT ioctl, even if VIDIOC_S_FMT ignores all requests and always returns default parameters as VIDIOC_G_FMT does. VIDIOC_TRY_FMT is optional.

4.3.4. Writing Images

A video output device may support the write() function and/or streaming (memory mapping or user pointer) I/O. See Chapter 3, Input/Output for details.

4.4. Video Output Overlay Interface

Also known as On-Screen Display (OSD)

Experimental

This is an experimental interface and may change in the future.

Some video output devices can overlay a framebuffer image onto the outgoing video signal. Applications can set up such an overlay using this interface, which borrows structures and ioctls of the Video Overlay interface.

The OSD function is accessible through the same character special file as the Video Output function. Note the default function of such a /dev/video device is video capturing or output. The OSD function is only available after calling the VIDIOC_S_FMT ioctl.

4.4.1. Querying Capabilities

Devices supporting the Video Output Overlay interface set the V4L2_CAP_VIDEO_OUTPUT_OVERLAY flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl.

4.4.2. Framebuffer

Contrary to the Video Overlay interface the framebuffer is normally implemented on the TV card and not the graphics card. On Linux it is accessible as a framebuffer device (/dev/fbN). Given a V4L2 device, applications can find the corresponding framebuffer device by calling the VIDIOC_G_FBUF ioctl. It returns, amongst other information, the physical address of the framebuffer in the base field of struct v4l2_framebuffer. The framebuffer device ioctl FBIOGET_FSCREENINFO returns the same address in the smem_start field of struct fb_fix_screeninfo. The FBIOGET_FSCREENINFO ioctl and struct fb_fix_screeninfo are defined in the linux/fb.h header file.

The width and height of the framebuffer depends on the current video standard. A V4L2 driver may reject attempts to change the video standard (or any other ioctl which would imply a framebuffer size change) with an EBUSY error code until all applications closed the framebuffer device.

Example 4.1. Finding a framebuffer device for OSD

#include <linux/fb.h>

struct v4l2_framebuffer fbuf;
unsigned int i;
int fb_fd;

if (-1 == ioctl (fd, VIDIOC_G_FBUF, &fbuf)) {
	perror ("VIDIOC_G_FBUF");
	exit (EXIT_FAILURE);
}

for (i = 0; i > 30; ++i) {
	char dev_name[16];
	struct fb_fix_screeninfo si;

	snprintf (dev_name, sizeof (dev_name), "/dev/fb%u", i);

	fb_fd = open (dev_name, O_RDWR);
	if (-1 == fb_fd) {
		switch (errno) {
		case ENOENT: /* no such file */
		case ENXIO:  /* no driver */
			continue;

		default:
			perror ("open");
			exit (EXIT_FAILURE);
		}
	}

	if (0 == ioctl (fb_fd, FBIOGET_FSCREENINFO, &si)) {
		if (si.smem_start == (unsigned long) fbuf.base)
			break;
	} else {
		/* Apparently not a framebuffer device. */
	}

	close (fb_fd);
	fb_fd = -1;
}

/* fb_fd is the file descriptor of the framebuffer device
   for the video output overlay, or -1 if no device was found. */

4.4.3. Overlay Window and Scaling

The overlay is controlled by source and target rectangles. The source rectangle selects a subsection of the framebuffer image to be overlaid, the target rectangle an area in the outgoing video signal where the image will appear. Drivers may or may not support scaling, and arbitrary sizes and positions of these rectangles. Further drivers may support any (or none) of the clipping/blending methods defined for the Video Overlay interface.

A struct v4l2_window defines the size of the source rectangle, its position in the framebuffer and the clipping/blending method to be used for the overlay. To get the current parameters applications set the type field of a struct v4l2_format to V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY and call the VIDIOC_G_FMT ioctl. The driver fills the v4l2_window substructure named win. It is not possible to retrieve a previously programmed clipping list or bitmap.

To program the source rectangle applications set the type field of a struct v4l2_format to V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY, initialize the win substructure and call the VIDIOC_S_FMT ioctl. The driver adjusts the parameters against hardware limits and returns the actual parameters as VIDIOC_G_FMT does. Like VIDIOC_S_FMT, the VIDIOC_TRY_FMT ioctl can be used to learn about driver capabilities without actually changing driver state. Unlike VIDIOC_S_FMT this also works after the overlay has been enabled.

A struct v4l2_crop defines the size and position of the target rectangle. The scaling factor of the overlay is implied by the width and height given in struct v4l2_window and struct v4l2_crop. The cropping API applies to Video Output and Video Output Overlay devices in the same way as to Video Capture and Video Overlay devices, merely reversing the direction of the data flow. For more information see Section 1.11, “Image Cropping, Insertion and Scaling”.

4.4.4. Enabling Overlay

There is no V4L2 ioctl to enable or disable the overlay, however the framebuffer interface of the driver may support the FBIOBLANK ioctl.

4.5. Codec Interface

Suspended

This interface has been be suspended from the V4L2 API implemented in Linux 2.6 until we have more experience with codec device interfaces.

A V4L2 codec can compress, decompress, transform, or otherwise convert video data from one format into another format, in memory. Applications send data to be converted to the driver through a write() call, and receive the converted data through a read() call. For efficiency a driver may also support streaming I/O.

[to do]

4.6. Effect Devices Interface

Suspended

This interface has been be suspended from the V4L2 API implemented in Linux 2.6 until we have more experience with effect device interfaces.

A V4L2 video effect device can do image effects, filtering, or combine two or more images or image streams. For example video transitions or wipes. Applications send data to be processed and receive the result data either with read() and write() functions, or through the streaming I/O mechanism.

[to do]

4.7. Raw VBI Data Interface

VBI is an abbreviation of Vertical Blanking Interval, a gap in the sequence of lines of an analog video signal. During VBI no picture information is transmitted, allowing some time while the electron beam of a cathode ray tube TV returns to the top of the screen. Using an oscilloscope you will find here the vertical synchronization pulses and short data packages ASK modulated[22] onto the video signal. These are transmissions of services such as Teletext or Closed Caption.

Subject of this interface type is raw VBI data, as sampled off a video signal, or to be added to a signal for output. The data format is similar to uncompressed video images, a number of lines times a number of samples per line, we call this a VBI image.

Conventionally V4L2 VBI devices are accessed through character device special files named /dev/vbi and /dev/vbi0 to /dev/vbi31 with major number 81 and minor numbers 224 to 255. /dev/vbi is typically a symbolic link to the preferred VBI device. This convention applies to both input and output devices.

To address the problems of finding related video and VBI devices VBI capturing and output is also available as device function under /dev/video. To capture or output raw VBI data with these devices applications must call the VIDIOC_S_FMT ioctl. Accessed as /dev/vbi, raw VBI capturing or output is the default device function.

4.7.1. Querying Capabilities

Devices supporting the raw VBI capturing or output API set the V4L2_CAP_VBI_CAPTURE or V4L2_CAP_VBI_OUTPUT flags, respectively, in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl. At least one of the read/write, streaming or asynchronous I/O methods must be supported. VBI devices may or may not have a tuner or modulator.

4.7.2. Supplemental Functions

VBI devices shall support video input or output, tuner or modulator, and controls ioctls as needed. The video standard ioctls provide information vital to program a VBI device, therefore must be supported.

4.7.3. Raw VBI Format Negotiation

Raw VBI sampling abilities can vary, in particular the sampling frequency. To properly interpret the data V4L2 specifies an ioctl to query the sampling parameters. Moreover, to allow for some flexibility applications can also suggest different parameters.

As usual these parameters are not reset at open() time to permit Unix tool chains, programming a device and then reading from it as if it was a plain file. Well written V4L2 applications should always ensure they really get what they want, requesting reasonable parameters and then checking if the actual parameters are suitable.

To query the current raw VBI capture parameters applications set the type field of a struct v4l2_format to V4L2_BUF_TYPE_VBI_CAPTURE or V4L2_BUF_TYPE_VBI_OUTPUT, and call the VIDIOC_G_FMT ioctl with a pointer to this structure. Drivers fill the struct v4l2_vbi_format vbi member of the fmt union.

To request different parameters applications set the type field of a struct v4l2_format as above and initialize all fields of the struct v4l2_vbi_format vbi member of the fmt union, or better just modify the results of VIDIOC_G_FMT, and call the VIDIOC_S_FMT ioctl with a pointer to this structure. Drivers return an EINVAL error code only when the given parameters are ambiguous, otherwise they modify the parameters according to the hardware capabilites and return the actual parameters. When the driver allocates resources at this point, it may return an EBUSY error code to indicate the returned parameters are valid but the required resources are currently not available. That may happen for instance when the video and VBI areas to capture would overlap, or when the driver supports multiple opens and another process already requested VBI capturing or output. Anyway, applications must expect other resource allocation points which may return EBUSY, at the VIDIOC_STREAMON ioctl and the first read(), write() and select() call.

VBI devices must implement both the VIDIOC_G_FMT and VIDIOC_S_FMT ioctl, even if VIDIOC_S_FMT ignores all requests and always returns default parameters as VIDIOC_G_FMT does. VIDIOC_TRY_FMT is optional.

Table 4.4. struct v4l2_vbi_format

__u32sampling_rateSamples per second, i. e. unit 1 Hz.
__u32offset

Horizontal offset of the VBI image, relative to the leading edge of the line synchronization pulse and counted in samples: The first sample in the VBI image will be located offset / sampling_rate seconds following the leading edge. See also Figure 4.1, “Line synchronization”.

__u32samples_per_line 
__u32sample_format

Defines the sample format as in Chapter 2, Image Formats, a four-character-code.[a] Usually this is V4L2_PIX_FMT_GREY, i. e. each sample consists of 8 bits with lower values oriented towards the black level. Do not assume any other correlation of values with the signal level. For example, the MSB does not necessarily indicate if the signal is 'high' or 'low' because 128 may not be the mean value of the signal. Drivers shall not convert the sample format by software.

__u32start[2]This is the scanning system line number associated with the first line of the VBI image, of the first and the second field respectively. See Figure 4.2, “ITU-R 525 line numbering (M/NTSC and M/PAL)” and Figure 4.3, “ITU-R 625 line numbering” for valid values. VBI input drivers can return start values 0 if the hardware cannot reliable identify scanning lines, VBI acquisition may not require this information.
__u32count[2]The number of lines in the first and second field image, respectively.

Drivers should be as flexibility as possible. For example, it may be possible to extend or move the VBI capture window down to the picture area, implementing a 'full field mode' to capture data service transmissions embedded in the picture.

An application can set the first or second count value to zero if no data is required from the respective field; count[1] if the scanning system is progressive, i. e. not interlaced. The corresponding start value shall be ignored by the application and driver. Anyway, drivers may not support single field capturing and return both count values non-zero.

Both count values set to zero, or line numbers outside the bounds depicted in Figure 4.2, “ITU-R 525 line numbering (M/NTSC and M/PAL)” and Figure 4.3, “ITU-R 625 line numbering”, or a field image covering lines of two fields, are invalid and shall not be returned by the driver.

To initialize the start and count fields, applications must first determine the current video standard selection. The v4l2_std_id or the framelines field of struct v4l2_standard can be evaluated for this purpose.

__u32flagsSee Table 4.5, “Raw VBI Format Flags” below. Currently only drivers set flags, applications must set this field to zero.
__u32reserved[2]This array is reserved for future extensions. Drivers and applications must set it to zero.

[a] A few devices may be unable to sample VBI data at all but can extend the video capture window to the VBI region.


Table 4.5. Raw VBI Format Flags

V4L2_VBI_UNSYNC0x0001

This flag indicates hardware which does not properly distinguish between fields. Normally the VBI image stores the first field (lower scanning line numbers) first in memory. This may be a top or bottom field depending on the video standard. When this flag is set the first or second field may be stored first, however the fields are still in correct temporal order with the older field first in memory.[a]

V4L2_VBI_INTERLACED0x0002By default the two field images will be passed sequentially; all lines of the first field followed by all lines of the second field (compare Section 3.6, “Field Order” V4L2_FIELD_SEQ_TB and V4L2_FIELD_SEQ_BT, whether the top or bottom field is first in memory depends on the video standard). When this flag is set, the two fields are interlaced (cf. V4L2_FIELD_INTERLACED). The first line of the first field followed by the first line of the second field, then the two second lines, and so on. Such a layout may be necessary when the hardware has been programmed to capture or output interlaced video images and is unable to separate the fields for VBI capturing at the same time. For simplicity setting this flag implies that both count values are equal and non-zero.

[a] Most VBI services transmit on both fields, but some have different semantics depending on the field number. These cannot be reliable decoded or encoded when V4L2_VBI_UNSYNC is set.


Figure 4.1. Line synchronization

Line synchronization diagram

Figure 4.2. ITU-R 525 line numbering (M/NTSC and M/PAL)

NTSC field synchronization diagram

(1) For the purpose of this specification field 2 starts in line 264 and not 263.5 because half line capturing is not supported.


Figure 4.3. ITU-R 625 line numbering

PAL/SECAM field synchronization diagram

(1) For the purpose of this specification field 2 starts in line 314 and not 313.5 because half line capturing is not supported.


Remember the VBI image format depends on the selected video standard, therefore the application must choose a new standard or query the current standard first. Attempts to read or write data ahead of format negotiation, or after switching the video standard which may invalidate the negotiated VBI parameters, should be refused by the driver. A format change during active I/O is not permitted.

4.7.4. Reading and writing VBI images

To assure synchronization with the field number and easier implementation, the smallest unit of data passed at a time is one frame, consisting of two fields of VBI images immediately following in memory.

The total size of a frame computes as follows:

(count[0] + count[1]) *
samples_per_line * sample size in bytes

The sample size is most likely always one byte, applications must check the sample_format field though, to function properly with other drivers.

A VBI device may support read/write and/or streaming (memory mapping or user pointer) I/O. The latter bears the possibility of synchronizing video and VBI data by using buffer timestamps.

Remember the VIDIOC_STREAMON ioctl and the first read(), write() and select() call can be resource allocation points returning an EBUSY error code if the required hardware resources are temporarily unavailable, for example the device is already in use by another process.

4.8. Sliced VBI Data Interface

VBI stands for Vertical Blanking Interval, a gap in the sequence of lines of an analog video signal. During VBI no picture information is transmitted, allowing some time while the electron beam of a cathode ray tube TV returns to the top of the screen.

Sliced VBI devices use hardware to demodulate data transmitted in the VBI. V4L2 drivers shall not do this by software, see also the raw VBI interface. The data is passed as short packets of fixed size, covering one scan line each. The number of packets per video frame is variable.

Sliced VBI capture and output devices are accessed through the same character special files as raw VBI devices. When a driver supports both interfaces, the default function of a /dev/vbi device is raw VBI capturing or output, and the sliced VBI function is only available after calling the VIDIOC_S_FMT ioctl as defined below. Likewise a /dev/video device may support the sliced VBI API, however the default function here is video capturing or output. Different file descriptors must be used to pass raw and sliced VBI data simultaneously, if this is supported by the driver.

4.8.1. Querying Capabilities

Devices supporting the sliced VBI capturing or output API set the V4L2_CAP_SLICED_VBI_CAPTURE or V4L2_CAP_SLICED_VBI_OUTPUT flag respectively, in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl. At least one of the read/write, streaming or asynchronous I/O methods must be supported. Sliced VBI devices may have a tuner or modulator.

4.8.2. Supplemental Functions

Sliced VBI devices shall support video input or output and tuner or modulator ioctls if they have these capabilities, and they may support control ioctls. The video standard ioctls provide information vital to program a sliced VBI device, therefore must be supported.

4.8.3. Sliced VBI Format Negotiation

To find out which data services are supported by the hardware applications can call the VIDIOC_G_SLICED_VBI_CAP ioctl. All drivers implementing the sliced VBI interface must support this ioctl. The results may differ from those of the VIDIOC_S_FMT ioctl when the number of VBI lines the hardware can capture or output per frame, or the number of services it can identify on a given line are limited. For example on PAL line 16 the hardware may be able to look for a VPS or Teletext signal, but not both at the same time.

To determine the currently selected services applications set the type field of struct v4l2_format to V4L2_BUF_TYPE_SLICED_VBI_CAPTURE or V4L2_BUF_TYPE_SLICED_VBI_OUTPUT, and the VIDIOC_G_FMT ioctl fills the fmt.sliced member, a struct v4l2_sliced_vbi_format.

Applications can request different parameters by initializing or modifying the fmt.sliced member and calling the VIDIOC_S_FMT ioctl with a pointer to the v4l2_format structure.

The sliced VBI API is more complicated than the raw VBI API because the hardware must be told which VBI service to expect on each scan line. Not all services may be supported by the hardware on all lines (this is especially true for VBI output where Teletext is often unsupported and other services can only be inserted in one specific line). In many cases, however, it is sufficient to just set the service_set field to the required services and let the driver fill the service_lines array according to hardware capabilities. Only if more precise control is needed should the programmer set the service_lines array explicitly.

The VIDIOC_S_FMT ioctl modifies the parameters according to hardware capabilities. When the driver allocates resources at this point, it may return an EBUSY error code if the required resources are temporarily unavailable. Other resource allocation points which may return EBUSY can be the VIDIOC_STREAMON ioctl and the first read(), write() and select() call.

Table 4.6. struct v4l2_sliced_vbi_format

__u32service_set

If service_set is non-zero when passed with VIDIOC_S_FMT or VIDIOC_TRY_FMT, the service_lines array will be filled by the driver according to the services specified in this field. For example, if service_set is initialized with V4L2_SLICED_TELETEXT_B | V4L2_SLICED_WSS_625, a driver for the cx25840 video decoder sets lines 7-22 of both fields[a] to V4L2_SLICED_TELETEXT_B and line 23 of the first field to V4L2_SLICED_WSS_625. If service_set is set to zero, then the values of service_lines will be used instead.

On return the driver sets this field to the union of all elements of the returned service_lines array. It may contain less services than requested, perhaps just one, if the hardware cannot handle more services simultaneously. It may be empty (zero) if none of the requested services are supported by the hardware.

__u16service_lines[2][24]

Applications initialize this array with sets of data services the driver shall look for or insert on the respective scan line. Subject to hardware capabilities drivers return the requested set, a subset, which may be just a single service, or an empty set. When the hardware cannot handle multiple services on the same line the driver shall choose one. No assumptions can be made on which service the driver chooses.

Data services are defined in Table 4.7, “Sliced VBI services”. Array indices map to ITU-R line numbers (see also Figure 4.2, “ITU-R 525 line numbering (M/NTSC and M/PAL)” and Figure 4.3, “ITU-R 625 line numbering”) as follows:

  Element525 line systems625 line systems
  service_lines[0][1]11
  service_lines[0][23]2323
  service_lines[1][1]264314
  service_lines[1][23]286336
  Drivers must set service_lines[0][0] and service_lines[1][0] to zero.
__u32io_sizeMaximum number of bytes passed by one read() or write() call, and the buffer size in bytes for the VIDIOC_QBUF and VIDIOC_DQBUF ioctl. Drivers set this field to the size of struct v4l2_sliced_vbi_data times the number of non-zero elements in the returned service_lines array (that is the number of lines potentially carrying data).
__u32reserved[2]This array is reserved for future extensions. Applications and drivers must set it to zero.

[a] According to ETS 300 706 lines 6-22 of the first field and lines 5-22 of the second field may carry Teletext data.


Table 4.7. Sliced VBI services

SymbolValueReferenceLines, usuallyPayload
V4L2_SLICED_TELETEXT_B (Teletext System B)0x0001[ETS 300 706], [ITU BT.653]PAL/SECAM line 7-22, 320-335 (second field 7-22)Last 42 of the 45 byte Teletext packet, that is without clock run-in and framing code, lsb first transmitted.
V4L2_SLICED_VPS0x0400[ETS 300 231]PAL line 16Byte number 3 to 15 according to Figure 9 of ETS 300 231, lsb first transmitted.
V4L2_SLICED_CAPTION_5250x1000[EIA 608-B]NTSC line 21, 284 (second field 21)Two bytes in transmission order, including parity bit, lsb first transmitted.
V4L2_SLICED_WSS_6250x4000[ITU BT.1119], [EN 300 294]PAL/SECAM line 23
Byte         0                 1
      msb         lsb  msb           lsb
 Bit  7 6 5 4 3 2 1 0  x x 13 12 11 10 9
V4L2_SLICED_VBI_5250x1000Set of services applicable to 525 line systems.
V4L2_SLICED_VBI_6250x4401Set of services applicable to 625 line systems.

Drivers may return an EINVAL error code when applications attempt to read or write data without prior format negotiation, after switching the video standard (which may invalidate the negotiated VBI parameters) and after switching the video input (which may change the video standard as a side effect). The VIDIOC_S_FMT ioctl may return an EBUSY error code when applications attempt to change the format while i/o is in progress (between a VIDIOC_STREAMON and VIDIOC_STREAMOFF call, and after the first read() or write() call).

4.8.4. Reading and writing sliced VBI data

A single read() or write() call must pass all data belonging to one video frame. That is an array of v4l2_sliced_vbi_data structures with one or more elements and a total size not exceeding io_size bytes. Likewise in streaming I/O mode one buffer of io_size bytes must contain data of one video frame. The id of unused v4l2_sliced_vbi_data elements must be zero.

Table 4.8. struct v4l2_sliced_vbi_data

__u32idA flag from Table 97, “Sliced VBI services” identifying the type of data in this packet. Only a single bit must be set. When the id of a captured packet is zero, the packet is empty and the contents of other fields are undefined. Applications shall ignore empty packets. When the id of a packet for output is zero the contents of the data field are undefined and the driver must no longer insert data on the requested field and line.
__u32fieldThe video field number this data has been captured from, or shall be inserted at. 0 for the first field, 1 for the second field.
__u32lineThe field (as opposed to frame) line number this data has been captured from, or shall be inserted at. See Figure 4.2, “ITU-R 525 line numbering (M/NTSC and M/PAL)” and Figure 4.3, “ITU-R 625 line numbering” for valid values. Sliced VBI capture devices can set the line number of all packets to 0 if the hardware cannot reliably identify scan lines. The field number must always be valid.
__u32reservedThis field is reserved for future extensions. Applications and drivers must set it to zero.
__u8data[48]The packet payload. See Table 97, “Sliced VBI services” for the contents and number of bytes passed for each data type. The contents of padding bytes at the end of this array are undefined, drivers and applications shall ignore them.

Packets are always passed in ascending line number order, without duplicate line numbers. The write() function and the VIDIOC_QBUF ioctl must return an EINVAL error code when applications violate this rule. They must also return an EINVAL error code when applications pass an incorrect field or line number, or a combination of field, line and id which has not been negotiated with the VIDIOC_G_FMT or VIDIOC_S_FMT ioctl. When the line numbers are unknown the driver must pass the packets in transmitted order. The driver can insert empty packets with id set to zero anywhere in the packet array.

To assure synchronization and to distinguish from frame dropping, when a captured frame does not carry any of the requested data services drivers must pass one or more empty packets. When an application fails to pass VBI data in time for output, the driver must output the last VPS and WSS packet again, and disable the output of Closed Caption and Teletext data, or output data which is ignored by Closed Caption and Teletext decoders.

A sliced VBI device may support read/write and/or streaming (memory mapping and/or user pointer) I/O. The latter bears the possibility of synchronizing video and VBI data by using buffer timestamps.

4.8.5. Sliced VBI Data in MPEG Streams

If a device can produce an MPEG output stream, it may be capable of providing negotiated sliced VBI services as data embedded in the MPEG stream. Users or applications control this sliced VBI data insertion with the V4L2_CID_MPEG_STREAM_VBI_FMT control.

If the driver does not provide the V4L2_CID_MPEG_STREAM_VBI_FMT control, or only allows that control to be set to V4L2_MPEG_STREAM_VBI_FMT_NONE, then the device cannot embed sliced VBI data in the MPEG stream.

The V4L2_CID_MPEG_STREAM_VBI_FMT control does not implicitly set the device driver to capture nor cease capturing sliced VBI data. The control only indicates to embed sliced VBI data in the MPEG stream, if an application has negotiated sliced VBI service be captured.

It may also be the case that a device can embed sliced VBI data in only certain types of MPEG streams: for example in an MPEG-2 PS but not an MPEG-2 TS. In this situation, if sliced VBI data insertion is requested, the sliced VBI data will be embedded in MPEG stream types when supported, and silently omitted from MPEG stream types where sliced VBI data insertion is not supported by the device.

The following subsections specify the format of the embedded sliced VBI data.

4.8.5.1. MPEG Stream Embedded, Sliced VBI Data Format: NONE

The V4L2_MPEG_STREAM_VBI_FMT_NONE embedded sliced VBI format shall be interpreted by drivers as a control to cease embedding sliced VBI data in MPEG streams. Neither the device nor driver shall insert "empty" embedded sliced VBI data packets in the MPEG stream when this format is set. No MPEG stream data structures are specified for this format.

4.8.5.2. MPEG Stream Embedded, Sliced VBI Data Format: IVTV

The V4L2_MPEG_STREAM_VBI_FMT_IVTV embedded sliced VBI format, when supported, indicates to the driver to embed up to 36 lines of sliced VBI data per frame in an MPEG-2 Private Stream 1 PES packet encapsulated in an MPEG-2 Program Pack in the MPEG stream.

Historical context: This format specification originates from a custom, embedded, sliced VBI data format used by the ivtv driver. This format has already been informally specified in the kernel sources in the file Documentation/video4linux/cx2341x/README.vbi . The maximum size of the payload and other aspects of this format are driven by the CX23415 MPEG decoder's capabilities and limitations with respect to extracting, decoding, and displaying sliced VBI data embedded within an MPEG stream.

This format's use is not exclusive to the ivtv driver nor exclusive to CX2341x devices, as the sliced VBI data packet insertion into the MPEG stream is implemented in driver software. At least the cx18 driver provides sliced VBI data insertion into an MPEG-2 PS in this format as well.

The following definitions specify the payload of the MPEG-2 Private Stream 1 PES packets that contain sliced VBI data when V4L2_MPEG_STREAM_VBI_FMT_IVTV is set. (The MPEG-2 Private Stream 1 PES packet header and encapsulating MPEG-2 Program Pack header are not detailed here. Please refer to the MPEG-2 specifications for details on those packet headers.)

The payload of the MPEG-2 Private Stream 1 PES packets that contain sliced VBI data is specified by struct v4l2_mpeg_vbi_fmt_ivtv. The payload is variable length, depending on the actual number of lines of sliced VBI data present in a video frame. The payload may be padded at the end with unspecified fill bytes to align the end of the payload to a 4-byte boundary. The payload shall never exceed 1552 bytes (2 fields with 18 lines/field with 43 bytes of data/line and a 4 byte magic number).

Table 4.9. struct v4l2_mpeg_vbi_fmt_ivtv

__u8magic[4] A "magic" constant from Table 4.10, “Magic Constants for struct v4l2_mpeg_vbi_fmt_ivtv magic field” that indicates this is a valid sliced VBI data payload and also indicates which member of the anonymous union, itv0 or ITV0, to use for the payload data.
union(anonymous)  
 struct v4l2_mpeg_vbi_itv0 itv0The primary form of the sliced VBI data payload that contains anywhere from 1 to 35 lines of sliced VBI data. Line masks are provided in this form of the payload indicating which VBI lines are provided.
 struct v4l2_mpeg_vbi_ITV0 ITV0An alternate form of the sliced VBI data payload used when 36 lines of sliced VBI data are present. No line masks are provided in this form of the payload; all valid line mask bits are implcitly set.

Table 4.10. Magic Constants for struct v4l2_mpeg_vbi_fmt_ivtv magic field

Defined SymbolValueDescription
V4L2_MPEG_VBI_IVTV_MAGIC0 "itv0"Indicates the itv0 member of the union in struct v4l2_mpeg_vbi_fmt_ivtv is valid.
V4L2_MPEG_VBI_IVTV_MAGIC1 "ITV0"Indicates the ITV0 member of the union in struct v4l2_mpeg_vbi_fmt_ivtv is valid and that 36 lines of sliced VBI data are present.

Table 4.11. struct v4l2_mpeg_vbi_itv0

__le32linemask[2]

Bitmasks indicating the VBI service lines present. These linemask values are stored in little endian byte order in the MPEG stream. Some reference linemask bit positions with their corresponding VBI line number and video field are given below. b0 indicates the least significant bit of a linemask value:

linemask[0] b0:		line  6		first field
linemask[0] b17:		line 23		first field
linemask[0] b18:		line  6		second field
linemask[0] b31:		line 19		second field
linemask[1] b0:		line 20		second field
linemask[1] b3:		line 23		second field
linemask[1] b4-b31:	unused and set to 0
struct v4l2_mpeg_vbi_itv0_line line[35]This is a variable length array that holds from 1 to 35 lines of sliced VBI data. The sliced VBI data lines present correspond to the bits set in the linemask array, starting from b0 of linemask[0] up through b31 of linemask[0], and from b0 of linemask[1] up through b 3 of linemask[1]. line[0] corresponds to the first bit found set in the linemask array, line[1] corresponds to the second bit found set in the linemask array, etc. If no linemask array bits are set, then line[0] may contain one line of unspecified data that should be ignored by applications.

Table 4.12. struct v4l2_mpeg_vbi_ITV0

struct v4l2_mpeg_vbi_itv0_line line[36]A fixed length array of 36 lines of sliced VBI data. line[0] through line [17] correspond to lines 6 through 23 of the first field. line[18] through line[35] corresponds to lines 6 through 23 of the second field.

Table 4.13. struct v4l2_mpeg_vbi_itv0_line

__u8idA line identifier value from Table 4.14, “Line Identifiers for struct v4l2_mpeg_vbi_itv0_line id field” that indicates the type of sliced VBI data stored on this line.
__u8data[42]The sliced VBI data for the line.

Table 4.14. Line Identifiers for struct v4l2_mpeg_vbi_itv0_line id field

Defined SymbolValueDescription
V4L2_MPEG_VBI_IVTV_TELETEXT_B 1Refer to Sliced VBI services for a description of the line payload.
V4L2_MPEG_VBI_IVTV_CAPTION_525 4Refer to Sliced VBI services for a description of the line payload.
V4L2_MPEG_VBI_IVTV_WSS_625 5Refer to Sliced VBI services for a description of the line payload.
V4L2_MPEG_VBI_IVTV_VPS 7Refer to Sliced VBI services for a description of the line payload.

4.9. Teletext Interface

This interface aims at devices receiving and demodulating Teletext data [[ETS 300 706], [ITU BT.653]], evaluating the Teletext packages and storing formatted pages in cache memory. Such devices are usually implemented as microcontrollers with serial interface (I2C) and can be found on older TV cards, dedicated Teletext decoding cards and home-brew devices connected to the PC parallel port.

The Teletext API was designed by Martin Buck. It is defined in the kernel header file linux/videotext.h, the specification is available from http://home.pages.de/~videotext/. (Videotext is the name of the German public television Teletext service.) Conventional character device file names are /dev/vtx and /dev/vttuner, with device number 83, 0 and 83, 16 respectively. A similar interface exists for the Philips SAA5249 Teletext decoder [specification?] with character device file names /dev/tlkN, device number 102, N.

Eventually the Teletext API was integrated into the V4L API with character device file names /dev/vtx0 to /dev/vtx31, device major number 81, minor numbers 192 to 223. For reference the V4L Teletext API specification is reproduced here in full: "Teletext interfaces talk the existing VTX API." Teletext devices with major number 83 and 102 will be removed in Linux 2.6.

There are no plans to replace the Teletext API or to integrate it into V4L2. Please write to the linux-media mailing list: http://www.linuxtv.org/lists.php when the need arises.

4.10. Radio Interface

This interface is intended for AM and FM (analog) radio receivers and transmitters.

Conventionally V4L2 radio devices are accessed through character device special files named /dev/radio and /dev/radio0 to /dev/radio63 with major number 81 and minor numbers 64 to 127.

4.10.1. Querying Capabilities

Devices supporting the radio interface set the V4L2_CAP_RADIO and V4L2_CAP_TUNER or V4L2_CAP_MODULATOR flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl. Other combinations of capability flags are reserved for future extensions.

4.10.2. Supplemental Functions

Radio devices can support controls, and must support the tuner or modulator ioctls.

They do not support the video input or output, audio input or output, video standard, cropping and scaling, compression and streaming parameter, or overlay ioctls. All other ioctls and I/O methods are reserved for future extensions.

4.10.3. Programming

Radio devices may have a couple audio controls (as discussed in Section 1.8, “User Controls”) such as a volume control, possibly custom controls. Further all radio devices have one tuner or modulator (these are discussed in Section 1.6, “Tuners and Modulators”) with index number zero to select the radio frequency and to determine if a monaural or FM stereo program is received/emitted. Drivers switch automatically between AM and FM depending on the selected frequency. The VIDIOC_G_TUNER or VIDIOC_G_MODULATOR ioctl reports the supported frequency range.

4.11. RDS Interface

The Radio Data System transmits supplementary information in binary format, for example the station name or travel information, on an inaudible audio subcarrier of a radio program. This interface is aimed at devices capable of receiving and decoding RDS information.

For more information see the core RDS standard [EN 50067] and the RBDS standard [NRSC-4].

Note that the RBDS standard as is used in the USA is almost identical to the RDS standard. Any RDS decoder can also handle RBDS. Only some of the fields have slightly different meanings. See the RBDS standard for more information.

The RBDS standard also specifies support for MMBS (Modified Mobile Search). This is a proprietary format which seems to be discontinued. The RDS interface does not support this format. Should support for MMBS (or the so-called 'E blocks' in general) be needed, then please contact the linux-media mailing list: http://www.linuxtv.org/lists.php.

4.11.1. Querying Capabilities

Devices supporting the RDS capturing API set the V4L2_CAP_RDS_CAPTURE flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl. Any tuner that supports RDS will set the V4L2_TUNER_CAP_RDS flag in the capability field of struct v4l2_tuner. Whether an RDS signal is present can be detected by looking at the rxsubchans field of struct v4l2_tuner: the V4L2_TUNER_SUB_RDS will be set if RDS data was detected.

Devices supporting the RDS output API set the V4L2_CAP_RDS_OUTPUT flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl. Any modulator that supports RDS will set the V4L2_TUNER_CAP_RDS flag in the capability field of struct v4l2_modulator. In order to enable the RDS transmission one must set the V4L2_TUNER_SUB_RDS bit in the txsubchans field of struct v4l2_modulator.

4.11.2. Reading RDS data

RDS data can be read from the radio device with the read() function. The data is packed in groups of three bytes, as follows:

Table 4.15. struct v4l2_rds_data

__u8lsbLeast Significant Byte of RDS Block
__u8msbMost Significant Byte of RDS Block
__u8blockBlock description

Table 4.16. Block description

Bits 0-2Block (aka offset) of the received data.
Bits 3-5Deprecated. Currently identical to bits 0-2. Do not use these bits.
Bit 6Corrected bit. Indicates that an error was corrected for this data block.
Bit 7Error bit. Indicates that an uncorrectable error occurred during reception of this block.

Table 4.17. Block defines

V4L2_RDS_BLOCK_MSK7Mask for bits 0-2 to get the block ID.
V4L2_RDS_BLOCK_A0Block A.
V4L2_RDS_BLOCK_B1Block B.
V4L2_RDS_BLOCK_C2Block C.
V4L2_RDS_BLOCK_D3Block D.
V4L2_RDS_BLOCK_C_ALT4Block C'.
V4L2_RDS_BLOCK_INVALID7An invalid block.
V4L2_RDS_BLOCK_CORRECTED0x40A bit error was detected but corrected.
V4L2_RDS_BLOCK_ERROR0x80An incorrectable error occurred.



[20] A common application of two file descriptors is the XFree86 Xv/V4L interface driver and a V4L2 application. While the X server controls video overlay, the application can take advantage of memory mapping and DMA.

In the opinion of the designers of this API, no driver writer taking the efforts to support simultaneous capturing and overlay will restrict this ability by requiring a single file descriptor, as in V4L and earlier versions of V4L2. Making this optional means applications depending on two file descriptors need backup routines to be compatible with all drivers, which is considerable more work than using two fds in applications which do not. Also two fd's fit the general concept of one file descriptor for each logical stream. Hence as a complexity trade-off drivers must support two file descriptors and may support single fd operation.

[21] The X Window system defines "regions" which are vectors of struct BoxRec { short x1, y1, x2, y2; } with width = x2 - x1 and height = y2 - y1, so one cannot pass X11 clip lists directly.

[22] ASK: Amplitude-Shift Keying. A high signal level represents a '1' bit, a low level a '0' bit.

Function Reference


Table of Contents

V4L2 close() — Close a V4L2 device
V4L2 ioctl() — Program a V4L2 device
ioctl VIDIOC_CROPCAP — Information about the video cropping and scaling abilities
ioctl VIDIOC_DBG_G_CHIP_IDENT — Identify the chips on a TV card
ioctl VIDIOC_DBG_G_REGISTER, VIDIOC_DBG_S_REGISTER — Read or write hardware registers
ioctl VIDIOC_ENCODER_CMD, VIDIOC_TRY_ENCODER_CMD — Execute an encoder command
ioctl VIDIOC_ENUMAUDIO — Enumerate audio inputs
ioctl VIDIOC_ENUMAUDOUT — Enumerate audio outputs
ioctl VIDIOC_ENUM_FMT — Enumerate image formats
ioctl VIDIOC_ENUM_FRAMESIZES — Enumerate frame sizes
ioctl VIDIOC_ENUM_FRAMEINTERVALS — Enumerate frame intervals
ioctl VIDIOC_ENUMINPUT — Enumerate video inputs
ioctl VIDIOC_ENUMOUTPUT — Enumerate video outputs
ioctl VIDIOC_ENUMSTD — Enumerate supported video standards
ioctl VIDIOC_G_AUDIO, VIDIOC_S_AUDIO — Query or select the current audio input and its attributes
ioctl VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT — Query or select the current audio output
ioctl VIDIOC_G_CROP, VIDIOC_S_CROP — Get or set the current cropping rectangle
ioctl VIDIOC_G_CTRL, VIDIOC_S_CTRL — Get or set the value of a control
ioctl VIDIOC_G_ENC_INDEX — Get meta data about a compressed video stream
ioctl VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS, VIDIOC_TRY_EXT_CTRLS — Get or set the value of several controls, try control values
ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF — Get or set frame buffer overlay parameters
ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT — Get or set the data format, try a format
ioctl VIDIOC_G_FREQUENCY, VIDIOC_S_FREQUENCY — Get or set tuner or modulator radio frequency
ioctl VIDIOC_G_INPUT, VIDIOC_S_INPUT — Query or select the current video input
ioctl VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP
ioctl VIDIOC_G_MODULATOR, VIDIOC_S_MODULATOR — Get or set modulator attributes
ioctl VIDIOC_G_OUTPUT, VIDIOC_S_OUTPUT — Query or select the current video output
ioctl VIDIOC_G_PARM, VIDIOC_S_PARM — Get or set streaming parameters
ioctl VIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY — Query or request the access priority associated with a file descriptor
ioctl VIDIOC_G_SLICED_VBI_CAP — Query sliced VBI capabilities
ioctl VIDIOC_G_STD, VIDIOC_S_STD — Query or select the video standard of the current input
ioctl VIDIOC_G_TUNER, VIDIOC_S_TUNER — Get or set tuner attributes
ioctl VIDIOC_LOG_STATUS — Log driver status information
ioctl VIDIOC_OVERLAY — Start or stop video overlay
ioctl VIDIOC_QBUF, VIDIOC_DQBUF — Exchange a buffer with the driver
ioctl VIDIOC_QUERYBUF — Query the status of a buffer
ioctl VIDIOC_QUERYCAP — Query device capabilities
ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU — Enumerate controls and menu control items
ioctl VIDIOC_QUERYSTD — Sense the video standard received by the current input
ioctl VIDIOC_REQBUFS — Initiate Memory Mapping or User Pointer I/O
ioctl VIDIOC_S_HW_FREQ_SEEK — Perform a hardware frequency seek
ioctl VIDIOC_STREAMON, VIDIOC_STREAMOFF — Start or stop streaming I/O
V4L2 mmap() — Map device memory into application address space
V4L2 munmap() — Unmap device memory
V4L2 open() — Open a V4L2 device
V4L2 poll() — Wait for some event on a file descriptor
V4L2 read() — Read from a V4L2 device
V4L2 select() — Synchronous I/O multiplexing
V4L2 write() — Write to a V4L2 device

Name

v4l2-close — Close a V4L2 device

Synopsis

#include <unistd.h>
int close(int  fd);

Arguments

fd

File descriptor returned by open().

Description

Closes the device. Any I/O in progress is terminated and resources associated with the file descriptor are freed. However data format parameters, current input or output, control values or other properties remain unchanged.

Return Value

The function returns 0 on success, -1 on failure and the errno is set appropriately. Possible error codes:

EBADF

fd is not a valid open file descriptor.


Name

v4l2-ioctl — Program a V4L2 device

Synopsis

#include <sys/ioctl.h>
int ioctl(int  fd,
 int  request,
 void * argp);

Arguments

fd

File descriptor returned by open().

request

V4L2 ioctl request code as defined in the videodev.h header file, for example VIDIOC_QUERYCAP.

argp

Pointer to a function parameter, usually a structure.

Description

The ioctl() function is used to program V4L2 devices. The argument fd must be an open file descriptor. An ioctl request has encoded in it whether the argument is an input, output or read/write parameter, and the size of the argument argp in bytes. Macros and defines specifying V4L2 ioctl requests are located in the videodev.h header file. Applications should use their own copy, not include the version in the kernel sources on the system they compile on. All V4L2 ioctl requests, their respective function and parameters are specified in Function Reference.

Return Value

On success the ioctl() function returns 0 and does not reset the errno variable. On failure -1 is returned, when the ioctl takes an output or read/write parameter it remains unmodified, and the errno variable is set appropriately. See below for possible error codes. Generic errors like EBADF or EFAULT are not listed in the sections discussing individual ioctl requests.

Note ioctls may return undefined error codes. Since errors may have side effects such as a driver reset applications should abort on unexpected errors.

EBADF

fd is not a valid open file descriptor.

EBUSY

The property cannot be changed right now. Typically this error code is returned when I/O is in progress or the driver supports multiple opens and another process locked the property.

EFAULT

argp references an inaccessible memory area.

ENOTTY

fd is not associated with a character special device.

EINVAL

The request or the data pointed to by argp is not valid. This is a very common error code, see the individual ioctl requests listed in Function Reference for actual causes.

ENOMEM

Not enough physical or virtual memory was available to complete the request.

ERANGE

The application attempted to set a control with the VIDIOC_S_CTRL ioctl to a value which is out of bounds.


Name

VIDIOC_CROPCAP — Information about the video cropping and scaling abilities

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_cropcap * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_CROPCAP

argp

Description

Applications use this function to query the cropping limits, the pixel aspect of images and to calculate scale factors. They set the type field of a v4l2_cropcap structure to the respective buffer (stream) type and call the VIDIOC_CROPCAP ioctl with a pointer to this structure. Drivers fill the rest of the structure. The results are constant except when switching the video standard. Remember this switch can occur implicit when switching the video input or output.

Table 39. struct v4l2_cropcap

enum v4l2_buf_typetypeType of the data stream, set by the application. Only these types are valid here: V4L2_BUF_TYPE_VIDEO_CAPTURE, V4L2_BUF_TYPE_VIDEO_OUTPUT, V4L2_BUF_TYPE_VIDEO_OVERLAY, and custom (driver defined) types with code V4L2_BUF_TYPE_PRIVATE and higher.
struct v4l2_rectboundsDefines the window within capturing or output is possible, this may exclude for example the horizontal and vertical blanking areas. The cropping rectangle cannot exceed these limits. Width and height are defined in pixels, the driver writer is free to choose origin and units of the coordinate system in the analog domain.
struct v4l2_rectdefrectDefault cropping rectangle, it shall cover the "whole picture". Assuming pixel aspect 1/1 this could be for example a 640 × 480 rectangle for NTSC, a 768 × 576 rectangle for PAL and SECAM centered over the active picture area. The same co-ordinate system as for bounds is used.
struct v4l2_fractpixelaspect

This is the pixel aspect (y / x) when no scaling is applied, the ratio of the actual sampling frequency and the frequency required to get square pixels.

When cropping coordinates refer to square pixels, the driver sets pixelaspect to 1/1. Other common values are 54/59 for PAL and SECAM, 11/10 for NTSC sampled according to [[ITU BT.601]].


Table 40. struct v4l2_rect

__s32leftHorizontal offset of the top, left corner of the rectangle, in pixels.
__s32topVertical offset of the top, left corner of the rectangle, in pixels.
__s32widthWidth of the rectangle, in pixels.
__s32heightHeight of the rectangle, in pixels. Width and height cannot be negative, the fields are signed for hysterical reasons.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_cropcap type is invalid or the ioctl is not supported. This is not permitted for video capture, output and overlay devices, which must support VIDIOC_CROPCAP.


Name

VIDIOC_DBG_G_CHIP_IDENT — Identify the chips on a TV card

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_dbg_chip_ident * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_DBG_G_CHIP_IDENT

argp

Description

Experimental

This is an experimental interface and may change in the future.

For driver debugging purposes this ioctl allows test applications to query the driver about the chips present on the TV card. Regular applications must not use it. When you found a chip specific bug, please contact the linux-media mailing list (http://www.linuxtv.org/lists.php) so it can be fixed.

To query the driver applications must initialize the match.type and match.addr or match.name fields of a struct v4l2_dbg_chip_ident and call VIDIOC_DBG_G_CHIP_IDENT with a pointer to this structure. On success the driver stores information about the selected chip in the ident and revision fields. On failure the structure remains unchanged.

When match.type is V4L2_CHIP_MATCH_HOST, match.addr selects the nth non-I2C chip on the TV card. You can enumerate all chips by starting at zero and incrementing match.addr by one until VIDIOC_DBG_G_CHIP_IDENT fails with an EINVAL error code. The number zero always selects the host chip, e. g. the chip connected to the PCI or USB bus.

When match.type is V4L2_CHIP_MATCH_I2C_DRIVER, match.name contains the I2C driver name. For instance "saa7127" will match any chip supported by the saa7127 driver, regardless of its I2C bus address. When multiple chips supported by the same driver are present, the ioctl will return V4L2_IDENT_AMBIGUOUS in the ident field.

When match.type is V4L2_CHIP_MATCH_I2C_ADDR, match.addr selects a chip by its 7 bit I2C bus address.

When match.type is V4L2_CHIP_MATCH_AC97, match.addr selects the nth AC97 chip on the TV card. You can enumerate all chips by starting at zero and incrementing match.addr by one until VIDIOC_DBG_G_CHIP_IDENT fails with an EINVAL error code.

On success, the ident field will contain a chip ID from the Linux media/v4l2-chip-ident.h header file, and the revision field will contain a driver specific value, or zero if no particular revision is associated with this chip.

When the driver could not identify the selected chip, ident will contain V4L2_IDENT_UNKNOWN. When no chip matched the ioctl will succeed but the ident field will contain V4L2_IDENT_NONE. If multiple chips matched, ident will contain V4L2_IDENT_AMBIGUOUS. In all these cases the revision field remains unchanged.

This ioctl is optional, not all drivers may support it. It was introduced in Linux 2.6.21, but the API was changed to the one described here in 2.6.29.

We recommended the v4l2-dbg utility over calling this ioctl directly. It is available from the LinuxTV v4l-dvb repository; see http://linuxtv.org/repo/ for access instructions.

Table 41. struct v4l2_dbg_match

__u32typeSee Table 43, “Chip Match Types” for a list of possible types. 
union(anonymous)  
 __u32addrMatch a chip by this number, interpreted according to the type field.
 charname[32]Match a chip by this name, interpreted according to the type field.

Table 42. struct v4l2_dbg_chip_ident

struct v4l2_dbg_matchmatchHow to match the chip, see Table 41, “struct v4l2_dbg_match.
__u32identA chip identifier as defined in the Linux media/v4l2-chip-ident.h header file, or one of the values from Table 44, “Chip Identifiers”.
__u32revisionA chip revision, chip and driver specific.

Table 43. Chip Match Types

V4L2_CHIP_MATCH_HOST0Match the nth chip on the card, zero for the host chip. Does not match I2C chips.
V4L2_CHIP_MATCH_I2C_DRIVER1Match an I2C chip by its driver name.
V4L2_CHIP_MATCH_I2C_ADDR2Match a chip by its 7 bit I2C bus address.
V4L2_CHIP_MATCH_AC973Match the nth anciliary AC97 chip.

Table 44. Chip Identifiers

V4L2_IDENT_NONE0No chip matched.
V4L2_IDENT_AMBIGUOUS1Multiple chips matched.
V4L2_IDENT_UNKNOWN2A chip is present at this address, but the driver could not identify it.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The driver does not support this ioctl, or the match_type is invalid.


Name

VIDIOC_DBG_G_REGISTER, VIDIOC_DBG_S_REGISTER — Read or write hardware registers

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_dbg_register * argp);
int ioctl(int  fd,
 int  request,
 const struct v4l2_dbg_register * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_DBG_G_REGISTER, VIDIOC_DBG_S_REGISTER

argp

Description

Experimental

This is an experimental interface and may change in the future.

For driver debugging purposes these ioctls allow test applications to access hardware registers directly. Regular applications must not use them.

Since writing or even reading registers can jeopardize the system security, its stability and damage the hardware, both ioctls require superuser privileges. Additionally the Linux kernel must be compiled with the CONFIG_VIDEO_ADV_DEBUG option to enable these ioctls.

To write a register applications must initialize all fields of a struct v4l2_dbg_register and call VIDIOC_DBG_S_REGISTER with a pointer to this structure. The match.type and match.addr or match.name fields select a chip on the TV card, the reg field specifies a register number and the val field the value to be written into the register.

To read a register applications must initialize the match.type, match.chip or match.name and reg fields, and call VIDIOC_DBG_G_REGISTER with a pointer to this structure. On success the driver stores the register value in the val field. On failure the structure remains unchanged.

When match.type is V4L2_CHIP_MATCH_HOST, match.addr selects the nth non-I2C chip on the TV card. The number zero always selects the host chip, e. g. the chip connected to the PCI or USB bus. You can find out which chips are present with the VIDIOC_DBG_G_CHIP_IDENT ioctl.

When match.type is V4L2_CHIP_MATCH_I2C_DRIVER, match.name contains the I2C driver name. For instance "saa7127" will match any chip supported by the saa7127 driver, regardless of its I2C bus address. When multiple chips supported by the same driver are present, the effect of these ioctls is undefined. Again with the VIDIOC_DBG_G_CHIP_IDENT ioctl you can find out which I2C chips are present.

When match.type is V4L2_CHIP_MATCH_I2C_ADDR, match.addr selects a chip by its 7 bit I2C bus address.

When match.type is V4L2_CHIP_MATCH_AC97, match.addr selects the nth AC97 chip on the TV card.

Success not guaranteed

Due to a flaw in the Linux I2C bus driver these ioctls may return successfully without actually reading or writing a register. To catch the most likely failure we recommend a VIDIOC_DBG_G_CHIP_IDENT call confirming the presence of the selected I2C chip.

These ioctls are optional, not all drivers may support them. However when a driver supports these ioctls it must also support VIDIOC_DBG_G_CHIP_IDENT. Conversely it may support VIDIOC_DBG_G_CHIP_IDENT but not these ioctls.

VIDIOC_DBG_G_REGISTER and VIDIOC_DBG_S_REGISTER were introduced in Linux 2.6.21, but their API was changed to the one described here in kernel 2.6.29.

We recommended the v4l2-dbg utility over calling these ioctls directly. It is available from the LinuxTV v4l-dvb repository; see http://linuxtv.org/repo/ for access instructions.

Table 45. struct v4l2_dbg_match

__u32typeSee Table 43, “Chip Match Types” for a list of possible types. 
union(anonymous)  
 __u32addrMatch a chip by this number, interpreted according to the type field.
 charname[32]Match a chip by this name, interpreted according to the type field.

Table 46. struct v4l2_dbg_register

struct v4l2_dbg_matchmatchHow to match the chip, see Table 45, “struct v4l2_dbg_match. 
__u64regA register number. 
__u64valThe value read from, or to be written into the register. 

Table 47. Chip Match Types

V4L2_CHIP_MATCH_HOST0Match the nth chip on the card, zero for the host chip. Does not match I2C chips.
V4L2_CHIP_MATCH_I2C_DRIVER1Match an I2C chip by its driver name.
V4L2_CHIP_MATCH_I2C_ADDR2Match a chip by its 7 bit I2C bus address.
V4L2_CHIP_MATCH_AC973Match the nth anciliary AC97 chip.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The driver does not support this ioctl, or the kernel was not compiled with the CONFIG_VIDEO_ADV_DEBUG option, or the match_type is invalid, or the selected chip or register does not exist.

EPERM

Insufficient permissions. Root privileges are required to execute these ioctls.


Name

VIDIOC_ENCODER_CMD, VIDIOC_TRY_ENCODER_CMD — Execute an encoder command

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_encoder_cmd * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENCODER_CMD, VIDIOC_TRY_ENCODER_CMD

argp

Description

Experimental

This is an experimental interface and may change in the future.

These ioctls control an audio/video (usually MPEG-) encoder. VIDIOC_ENCODER_CMD sends a command to the encoder, VIDIOC_TRY_ENCODER_CMD can be used to try a command without actually executing it.

To send a command applications must initialize all fields of a struct v4l2_encoder_cmd and call VIDIOC_ENCODER_CMD or VIDIOC_TRY_ENCODER_CMD with a pointer to this structure.

The cmd field must contain the command code. The flags field is currently only used by the STOP command and contains one bit: If the V4L2_ENC_CMD_STOP_AT_GOP_END flag is set, encoding will continue until the end of the current Group Of Pictures, otherwise it will stop immediately.

A read() call sends a START command to the encoder if it has not been started yet. After a STOP command, read() calls will read the remaining data buffered by the driver. When the buffer is empty, read() will return zero and the next read() call will restart the encoder.

A close() call sends an immediate STOP to the encoder, and all buffered data is discarded.

These ioctls are optional, not all drivers may support them. They were introduced in Linux 2.6.21.

Table 48. struct v4l2_encoder_cmd

__u32cmdThe encoder command, see Table 49, “Encoder Commands”.
__u32flagsFlags to go with the command, see Table 50, “Encoder Command Flags”. If no flags are defined for this command, drivers and applications must set this field to zero.
__u32data[8]Reserved for future extensions. Drivers and applications must set the array to zero.

Table 49. Encoder Commands

V4L2_ENC_CMD_START0Start the encoder. When the encoder is already running or paused, this command does nothing. No flags are defined for this command.
V4L2_ENC_CMD_STOP1Stop the encoder. When the V4L2_ENC_CMD_STOP_AT_GOP_END flag is set, encoding will continue until the end of the current Group Of Pictures, otherwise encoding will stop immediately. When the encoder is already stopped, this command does nothing.
V4L2_ENC_CMD_PAUSE2Pause the encoder. When the encoder has not been started yet, the driver will return an EPERM error code. When the encoder is already paused, this command does nothing. No flags are defined for this command.
V4L2_ENC_CMD_RESUME3Resume encoding after a PAUSE command. When the encoder has not been started yet, the driver will return an EPERM error code. When the encoder is already running, this command does nothing. No flags are defined for this command.

Table 50. Encoder Command Flags

V4L2_ENC_CMD_STOP_AT_GOP_END0x0001Stop encoding at the end of the current Group Of Pictures, rather than immediately.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The driver does not support this ioctl, or the cmd field is invalid.

EPERM

The application sent a PAUSE or RESUME command when the encoder was not running.


Name

VIDIOC_ENUMAUDIO — Enumerate audio inputs

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_audio * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENUMAUDIO

argp

Description

To query the attributes of an audio input applications initialize the index field and zero out the reserved array of a struct v4l2_audio and call the VIDIOC_ENUMAUDIO ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the index is out of bounds. To enumerate all audio inputs applications shall begin at index zero, incrementing by one until the driver returns EINVAL.

See ioctl VIDIOC_G_AUDIO, VIDIOC_S_AUDIO(2) for a description of struct v4l2_audio.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The number of the audio input is out of bounds, or there are no audio inputs at all and this ioctl is not supported.


Name

VIDIOC_ENUMAUDOUT — Enumerate audio outputs

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_audioout * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENUMAUDOUT

argp

Description

To query the attributes of an audio output applications initialize the index field and zero out the reserved array of a struct v4l2_audioout and call the VIDIOC_G_AUDOUT ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the index is out of bounds. To enumerate all audio outputs applications shall begin at index zero, incrementing by one until the driver returns EINVAL.

Note connectors on a TV card to loop back the received audio signal to a sound card are not audio outputs in this sense.

See ioctl VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT(2) for a description of struct v4l2_audioout.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The number of the audio output is out of bounds, or there are no audio outputs at all and this ioctl is not supported.


Name

VIDIOC_ENUM_FMT — Enumerate image formats

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_fmtdesc * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENUM_FMT

argp

Description

To enumerate image formats applications initialize the type and index field of struct v4l2_fmtdesc and call the VIDIOC_ENUM_FMT ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code. All formats are enumerable by beginning at index zero and incrementing by one until EINVAL is returned.

Table 51. struct v4l2_fmtdesc

__u32indexNumber of the format in the enumeration, set by the application. This is in no way related to the pixelformat field.
enum v4l2_buf_typetypeType of the data stream, set by the application. Only these types are valid here: V4L2_BUF_TYPE_VIDEO_CAPTURE, V4L2_BUF_TYPE_VIDEO_OUTPUT, V4L2_BUF_TYPE_VIDEO_OVERLAY, and custom (driver defined) types with code V4L2_BUF_TYPE_PRIVATE and higher.
__u32flagsSee Table 52, “Image Format Description Flags”
__u8description[32]Description of the format, a NUL-terminated ASCII string. This information is intended for the user, for example: "YUV 4:2:2".
__u32pixelformatThe image format identifier. This is a four character code as computed by the v4l2_fourcc() macro:

#define v4l2_fourcc(a,b,c,d) (((__u32)(a)<<0)|((__u32)(b)<<8)|((__u32)(c)<<16)|((__u32)(d)<<24))

Several image formats are already defined by this specification in Chapter 2, Image Formats. Note these codes are not the same as those used in the Windows world.

__u32reserved[4]Reserved for future extensions. Drivers must set the array to zero.

Table 52. Image Format Description Flags

V4L2_FMT_FLAG_COMPRESSED0x0001This is a compressed format.
V4L2_FMT_FLAG_EMULATED0x0002This format is not native to the device but emulated through software (usually libv4l2), where possible try to use a native format instead for better performance.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_fmtdesc type is not supported or the index is out of bounds.


Name

VIDIOC_ENUM_FRAMESIZES — Enumerate frame sizes

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_frmsizeenum * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENUM_FRAMESIZES

argp

Pointer to a struct v4l2_frmsizeenum that contains an index and pixel format and receives a frame width and height.

Description

Experimental

This is an experimental interface and may change in the future.

This ioctl allows applications to enumerate all frame sizes (i. e. width and height in pixels) that the device supports for the given pixel format.

The supported pixel formats can be obtained by using the VIDIOC_ENUM_FMT function.

The return value and the content of the v4l2_frmsizeenum.type field depend on the type of frame sizes the device supports. Here are the semantics of the function for the different cases:

  • Discrete: The function returns success if the given index value (zero-based) is valid. The application should increase the index by one for each call until EINVAL is returned. The v4l2_frmsizeenum.type field is set to V4L2_FRMSIZE_TYPE_DISCRETE by the driver. Of the union only the discrete member is valid.

  • Step-wise: The function returns success if the given index value is zero and EINVAL for any other index value. The v4l2_frmsizeenum.type field is set to V4L2_FRMSIZE_TYPE_STEPWISE by the driver. Of the union only the stepwise member is valid.

  • Continuous: This is a special case of the step-wise type above. The function returns success if the given index value is zero and EINVAL for any other index value. The v4l2_frmsizeenum.type field is set to V4L2_FRMSIZE_TYPE_CONTINUOUS by the driver. Of the union only the stepwise member is valid and the step_width and step_height values are set to 1.

When the application calls the function with index zero, it must check the type field to determine the type of frame size enumeration the device supports. Only for the V4L2_FRMSIZE_TYPE_DISCRETE type does it make sense to increase the index value to receive more frame sizes.

Note that the order in which the frame sizes are returned has no special meaning. In particular does it not say anything about potential default format sizes.

Applications can assume that the enumeration data does not change without any interaction from the application itself. This means that the enumeration data is consistent if the application does not perform any other ioctl calls while it runs the frame size enumeration.

Structs

In the structs below, IN denotes a value that has to be filled in by the application, OUT denotes values that the driver fills in. The application should zero out all members except for the IN fields.

Table 53. struct v4l2_frmsize_discrete

__u32widthWidth of the frame [pixel].
__u32heightHeight of the frame [pixel].

Table 54. struct v4l2_frmsize_stepwise

__u32min_widthMinimum frame width [pixel].
__u32max_widthMaximum frame width [pixel].
__u32step_widthFrame width step size [pixel].
__u32min_heightMinimum frame height [pixel].
__u32max_heightMaximum frame height [pixel].
__u32step_heightFrame height step size [pixel].

Table 55. struct v4l2_frmsizeenum

__u32index IN: Index of the given frame size in the enumeration.
__u32pixel_format IN: Pixel format for which the frame sizes are enumerated.
__u32type OUT: Frame size type the device supports.
union  OUT: Frame size with the given index.
 struct v4l2_frmsize_discretediscrete 
 struct v4l2_frmsize_stepwisestepwise 
__u32reserved[2] Reserved space for future use.

Enums

Table 56. enum v4l2_frmsizetypes

V4L2_FRMSIZE_TYPE_DISCRETE1Discrete frame size.
V4L2_FRMSIZE_TYPE_CONTINUOUS2Continuous frame size.
V4L2_FRMSIZE_TYPE_STEPWISE3Step-wise defined frame size.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

See the description section above for a list of return values that errno can have.


Name

VIDIOC_ENUM_FRAMEINTERVALS — Enumerate frame intervals

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_frmivalenum * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENUM_FRAMEINTERVALS

argp

Pointer to a struct v4l2_frmivalenum structure that contains a pixel format and size and receives a frame interval.

Description

This ioctl allows applications to enumerate all frame intervals that the device supports for the given pixel format and frame size.

The supported pixel formats and frame sizes can be obtained by using the VIDIOC_ENUM_FMT and VIDIOC_ENUM_FRAMESIZES functions.

The return value and the content of the v4l2_frmivalenum.type field depend on the type of frame intervals the device supports. Here are the semantics of the function for the different cases:

  • Discrete: The function returns success if the given index value (zero-based) is valid. The application should increase the index by one for each call until EINVAL is returned. The `v4l2_frmivalenum.type` field is set to `V4L2_FRMIVAL_TYPE_DISCRETE` by the driver. Of the union only the `discrete` member is valid.

  • Step-wise: The function returns success if the given index value is zero and EINVAL for any other index value. The v4l2_frmivalenum.type field is set to V4L2_FRMIVAL_TYPE_STEPWISE by the driver. Of the union only the stepwise member is valid.

  • Continuous: This is a special case of the step-wise type above. The function returns success if the given index value is zero and EINVAL for any other index value. The v4l2_frmivalenum.type field is set to V4L2_FRMIVAL_TYPE_CONTINUOUS by the driver. Of the union only the stepwise member is valid and the step value is set to 1.

When the application calls the function with index zero, it must check the type field to determine the type of frame interval enumeration the device supports. Only for the V4L2_FRMIVAL_TYPE_DISCRETE type does it make sense to increase the index value to receive more frame intervals.

Note that the order in which the frame intervals are returned has no special meaning. In particular does it not say anything about potential default frame intervals.

Applications can assume that the enumeration data does not change without any interaction from the application itself. This means that the enumeration data is consistent if the application does not perform any other ioctl calls while it runs the frame interval enumeration.

Notes

  • Frame intervals and frame rates: The V4L2 API uses frame intervals instead of frame rates. Given the frame interval the frame rate can be computed as follows:

    frame_rate = 1 / frame_interval

Structs

In the structs below, IN denotes a value that has to be filled in by the application, OUT denotes values that the driver fills in. The application should zero out all members except for the IN fields.

Table 57. struct v4l2_frmival_stepwise

struct v4l2_fractminMinimum frame interval [s].
struct v4l2_fractmaxMaximum frame interval [s].
struct v4l2_fractstepFrame interval step size [s].

Table 58. struct v4l2_frmivalenum

__u32index IN: Index of the given frame interval in the enumeration.
__u32pixel_format IN: Pixel format for which the frame intervals are enumerated.
__u32width IN: Frame width for which the frame intervals are enumerated.
__u32height IN: Frame height for which the frame intervals are enumerated.
__u32type OUT: Frame interval type the device supports.
union  OUT: Frame interval with the given index.
 struct v4l2_fractdiscreteFrame interval [s].
 struct v4l2_frmival_stepwisestepwise 
__u32reserved[2] Reserved space for future use.

Enums

Table 59. enum v4l2_frmivaltypes

V4L2_FRMIVAL_TYPE_DISCRETE1Discrete frame interval.
V4L2_FRMIVAL_TYPE_CONTINUOUS2Continuous frame interval.
V4L2_FRMIVAL_TYPE_STEPWISE3Step-wise defined frame interval.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

See the description section above for a list of return values that errno can have.


Name

VIDIOC_ENUMINPUT — Enumerate video inputs

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_input * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENUMINPUT

argp

Description

To query the attributes of a video input applications initialize the index field of struct v4l2_input and call the VIDIOC_ENUMINPUT ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the index is out of bounds. To enumerate all inputs applications shall begin at index zero, incrementing by one until the driver returns EINVAL.

Table 60. struct v4l2_input

__u32indexIdentifies the input, set by the application.
__u8name[32]Name of the video input, a NUL-terminated ASCII string, for example: "Vin (Composite 2)". This information is intended for the user, preferably the connector label on the device itself.
__u32typeType of the input, see Table 61, “Input Types”.
__u32audioset

Drivers can enumerate up to 32 video and audio inputs. This field shows which audio inputs were selectable as audio source if this was the currently selected video input. It is a bit mask. The LSB corresponds to audio input 0, the MSB to input 31. Any number of bits can be set, or none.

When the driver does not enumerate audio inputs no bits must be set. Applications shall not interpret this as lack of audio support. Some drivers automatically select audio sources and do not enumerate them since there is no choice anyway.

For details on audio inputs and how to select the current input see Section 1.5, “Audio Inputs and Outputs”.

__u32tunerCapture devices can have zero or more tuners (RF demodulators). When the type is set to V4L2_INPUT_TYPE_TUNER this is an RF connector and this field identifies the tuner. It corresponds to struct v4l2_tuner field index. For details on tuners see Section 1.6, “Tuners and Modulators”.
v4l2_std_idstdEvery video input supports one or more different video standards. This field is a set of all supported standards. For details on video standards and how to switch see Section 1.7, “Video Standards”.
__u32statusThis field provides status information about the input. See Table 62, “Input Status Flags” for flags. With the exception of the sensor orientation bits status is only valid when this is the current input.
__u32reserved[4]Reserved for future extensions. Drivers must set the array to zero.

Table 61. Input Types

V4L2_INPUT_TYPE_TUNER1This input uses a tuner (RF demodulator).
V4L2_INPUT_TYPE_CAMERA2Analog baseband input, for example CVBS / Composite Video, S-Video, RGB.

Table 62. Input Status Flags

General
V4L2_IN_ST_NO_POWER0x00000001Attached device is off.
V4L2_IN_ST_NO_SIGNAL0x00000002 
V4L2_IN_ST_NO_COLOR0x00000004The hardware supports color decoding, but does not detect color modulation in the signal.
Sensor Orientation
V4L2_IN_ST_HFLIP0x00000010The input is connected to a device that produces a signal that is flipped horizontally and does not correct this before passing the signal to userspace.
V4L2_IN_ST_VFLIP0x00000020The input is connected to a device that produces a signal that is flipped vertically and does not correct this before passing the signal to userspace. Note that a 180 degree rotation is the same as HFLIP | VFLIP
Analog Video
V4L2_IN_ST_NO_H_LOCK0x00000100No horizontal sync lock.
V4L2_IN_ST_COLOR_KILL0x00000200A color killer circuit automatically disables color decoding when it detects no color modulation. When this flag is set the color killer is enabled and has shut off color decoding.
Digital Video
V4L2_IN_ST_NO_SYNC0x00010000No synchronization lock.
V4L2_IN_ST_NO_EQU0x00020000No equalizer lock.
V4L2_IN_ST_NO_CARRIER0x00040000Carrier recovery failed.
VCR and Set-Top Box
V4L2_IN_ST_MACROVISION0x01000000Macrovision is an analog copy prevention system mangling the video signal to confuse video recorders. When this flag is set Macrovision has been detected.
V4L2_IN_ST_NO_ACCESS0x02000000Conditional access denied.
V4L2_IN_ST_VTR0x04000000VTR time constant. [?]

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_input index is out of bounds.


Name

VIDIOC_ENUMOUTPUT — Enumerate video outputs

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_output * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENUMOUTPUT

argp

Description

To query the attributes of a video outputs applications initialize the index field of struct v4l2_output and call the VIDIOC_ENUMOUTPUT ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the index is out of bounds. To enumerate all outputs applications shall begin at index zero, incrementing by one until the driver returns EINVAL.

Table 63. struct v4l2_output

__u32indexIdentifies the output, set by the application.
__u8name[32]Name of the video output, a NUL-terminated ASCII string, for example: "Vout". This information is intended for the user, preferably the connector label on the device itself.
__u32typeType of the output, see Table 64, “Output Type”.
__u32audioset

Drivers can enumerate up to 32 video and audio outputs. This field shows which audio outputs were selectable as the current output if this was the currently selected video output. It is a bit mask. The LSB corresponds to audio output 0, the MSB to output 31. Any number of bits can be set, or none.

When the driver does not enumerate audio outputs no bits must be set. Applications shall not interpret this as lack of audio support. Drivers may automatically select audio outputs without enumerating them.

For details on audio outputs and how to select the current output see Section 1.5, “Audio Inputs and Outputs”.

__u32modulatorOutput devices can have zero or more RF modulators. When the type is V4L2_OUTPUT_TYPE_MODULATOR this is an RF connector and this field identifies the modulator. It corresponds to struct v4l2_modulator field index. For details on modulators see Section 1.6, “Tuners and Modulators”.
v4l2_std_idstdEvery video output supports one or more different video standards. This field is a set of all supported standards. For details on video standards and how to switch see Section 1.7, “Video Standards”.
__u32reserved[4]Reserved for future extensions. Drivers must set the array to zero.

Table 64. Output Type

V4L2_OUTPUT_TYPE_MODULATOR1This output is an analog TV modulator.
V4L2_OUTPUT_TYPE_ANALOG2Analog baseband output, for example Composite / CVBS, S-Video, RGB.
V4L2_OUTPUT_TYPE_ANALOGVGAOVERLAY3[?]

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_output index is out of bounds.


Name

VIDIOC_ENUMSTD — Enumerate supported video standards

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_standard * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENUMSTD

argp

Description

To query the attributes of a video standard, especially a custom (driver defined) one, applications initialize the index field of struct v4l2_standard and call the VIDIOC_ENUMSTD ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the index is out of bounds. To enumerate all standards applications shall begin at index zero, incrementing by one until the driver returns EINVAL. Drivers may enumerate a different set of standards after switching the video input or output.[23]

Table 65. struct v4l2_standard

__u32indexNumber of the video standard, set by the application.
v4l2_std_ididThe bits in this field identify the standard as one of the common standards listed in Table 67, “typedef v4l2_std_id, or if bits 32 to 63 are set as custom standards. Multiple bits can be set if the hardware does not distinguish between these standards, however separate indices do not indicate the opposite. The id must be unique. No other enumerated v4l2_standard structure, for this input or output anyway, can contain the same set of bits.
__u8name[24]Name of the standard, a NUL-terminated ASCII string, for example: "PAL-B/G", "NTSC Japan". This information is intended for the user.
struct v4l2_fractframeperiodThe frame period (not field period) is numerator / denominator. For example M/NTSC has a frame period of 1001 / 30000 seconds.
__u32framelinesTotal lines per frame including blanking, e. g. 625 for B/PAL.
__u32reserved[4]Reserved for future extensions. Drivers must set the array to zero.

Table 66. struct v4l2_fract

__u32numerator 
__u32denominator 

Table 67. typedef v4l2_std_id

__u64v4l2_std_idThis type is a set, each bit representing another video standard as listed below and in Table 68, “Video Standards (based on [])”. The 32 most significant bits are reserved for custom (driver defined) video standards.

#define V4L2_STD_PAL_B          ((v4l2_std_id)0x00000001)
#define V4L2_STD_PAL_B1         ((v4l2_std_id)0x00000002)
#define V4L2_STD_PAL_G          ((v4l2_std_id)0x00000004)
#define V4L2_STD_PAL_H          ((v4l2_std_id)0x00000008)
#define V4L2_STD_PAL_I          ((v4l2_std_id)0x00000010)
#define V4L2_STD_PAL_D          ((v4l2_std_id)0x00000020)
#define V4L2_STD_PAL_D1         ((v4l2_std_id)0x00000040)
#define V4L2_STD_PAL_K          ((v4l2_std_id)0x00000080)

#define V4L2_STD_PAL_M          ((v4l2_std_id)0x00000100)
#define V4L2_STD_PAL_N          ((v4l2_std_id)0x00000200)
#define V4L2_STD_PAL_Nc         ((v4l2_std_id)0x00000400)
#define V4L2_STD_PAL_60         ((v4l2_std_id)0x00000800)

V4L2_STD_PAL_60 is a hybrid standard with 525 lines, 60 Hz refresh rate, and PAL color modulation with a 4.43 MHz color subcarrier. Some PAL video recorders can play back NTSC tapes in this mode for display on a 50/60 Hz agnostic PAL TV.

#define V4L2_STD_NTSC_M         ((v4l2_std_id)0x00001000)
#define V4L2_STD_NTSC_M_JP      ((v4l2_std_id)0x00002000)
#define V4L2_STD_NTSC_443       ((v4l2_std_id)0x00004000)

V4L2_STD_NTSC_443 is a hybrid standard with 525 lines, 60 Hz refresh rate, and NTSC color modulation with a 4.43 MHz color subcarrier.

#define V4L2_STD_NTSC_M_KR      ((v4l2_std_id)0x00008000)

#define V4L2_STD_SECAM_B        ((v4l2_std_id)0x00010000)
#define V4L2_STD_SECAM_D        ((v4l2_std_id)0x00020000)
#define V4L2_STD_SECAM_G        ((v4l2_std_id)0x00040000)
#define V4L2_STD_SECAM_H        ((v4l2_std_id)0x00080000)
#define V4L2_STD_SECAM_K        ((v4l2_std_id)0x00100000)
#define V4L2_STD_SECAM_K1       ((v4l2_std_id)0x00200000)
#define V4L2_STD_SECAM_L        ((v4l2_std_id)0x00400000)
#define V4L2_STD_SECAM_LC       ((v4l2_std_id)0x00800000)

/* ATSC/HDTV */
#define V4L2_STD_ATSC_8_VSB     ((v4l2_std_id)0x01000000)
#define V4L2_STD_ATSC_16_VSB    ((v4l2_std_id)0x02000000)

V4L2_STD_ATSC_8_VSB and V4L2_STD_ATSC_16_VSB are U.S. terrestrial digital TV standards. Presently the V4L2 API does not support digital TV. See also the Linux DVB API at http://linuxtv.org.

#define V4L2_STD_PAL_BG         (V4L2_STD_PAL_B         |\
				 V4L2_STD_PAL_B1        |\
				 V4L2_STD_PAL_G)
#define V4L2_STD_B              (V4L2_STD_PAL_B         |\
				 V4L2_STD_PAL_B1        |\
				 V4L2_STD_SECAM_B)
#define V4L2_STD_GH             (V4L2_STD_PAL_G         |\
				 V4L2_STD_PAL_H         |\
				 V4L2_STD_SECAM_G       |\
				 V4L2_STD_SECAM_H)
#define V4L2_STD_PAL_DK         (V4L2_STD_PAL_D         |\
				 V4L2_STD_PAL_D1        |\
				 V4L2_STD_PAL_K)
#define V4L2_STD_PAL            (V4L2_STD_PAL_BG        |\
				 V4L2_STD_PAL_DK        |\
				 V4L2_STD_PAL_H         |\
				 V4L2_STD_PAL_I)
#define V4L2_STD_NTSC           (V4L2_STD_NTSC_M        |\
				 V4L2_STD_NTSC_M_JP     |\
				 V4L2_STD_NTSC_M_KR)
#define V4L2_STD_MN             (V4L2_STD_PAL_M         |\
				 V4L2_STD_PAL_N         |\
				 V4L2_STD_PAL_Nc        |\
				 V4L2_STD_NTSC)
#define V4L2_STD_SECAM_DK       (V4L2_STD_SECAM_D       |\
				 V4L2_STD_SECAM_K       |\
				 V4L2_STD_SECAM_K1)
#define V4L2_STD_DK             (V4L2_STD_PAL_DK        |\
				 V4L2_STD_SECAM_DK)

#define V4L2_STD_SECAM          (V4L2_STD_SECAM_B       |\
				 V4L2_STD_SECAM_G       |\
				 V4L2_STD_SECAM_H       |\
				 V4L2_STD_SECAM_DK      |\
				 V4L2_STD_SECAM_L       |\
				 V4L2_STD_SECAM_LC)

#define V4L2_STD_525_60         (V4L2_STD_PAL_M         |\
				 V4L2_STD_PAL_60        |\
				 V4L2_STD_NTSC          |\
				 V4L2_STD_NTSC_443)
#define V4L2_STD_625_50         (V4L2_STD_PAL           |\
				 V4L2_STD_PAL_N         |\
				 V4L2_STD_PAL_Nc        |\
				 V4L2_STD_SECAM)

#define V4L2_STD_UNKNOWN        0
#define V4L2_STD_ALL            (V4L2_STD_525_60        |\
				 V4L2_STD_625_50)

Table 68. Video Standards (based on [[ITU BT.470]])

Characteristics

M/NTSC[a]

M/PAL

N/PAL[b]

B, B1, G/PALD, D1, K/PALH/PALI/PALB, G/SECAMD, K/SECAMK1/SECAML/SECAM
Frame lines525625
Frame period (s)1001/300001/25
Chrominance sub-carrier frequency (Hz)3579545 ± 103579611.49 ± 104433618.75 ± 5 (3582056.25 ± 5)4433618.75 ± 54433618.75 ± 1fOR = 4406250 ± 2000, fOB = 4250000 ± 2000
Nominal radio-frequency channel bandwidth (MHz)666B: 7; B1, G: 88888888
Sound carrier relative to vision carrier (MHz)+ 4.5+ 4.5+ 4.5

+ 5.5 ± 0.001 [c] [d] [e] [f]

+ 6.5 ± 0.001+ 5.5+ 5.9996 ± 0.0005+ 5.5 ± 0.001+ 6.5 ± 0.001+ 6.5

+ 6.5 [g]

[a] Japan uses a standard similar to M/NTSC (V4L2_STD_NTSC_M_JP).

[b] The values in brackets apply to the combination N/PAL a.k.a. NC used in Argentina (V4L2_STD_PAL_Nc).

[c] In the Federal Republic of Germany, Austria, Italy, the Netherlands, Slovakia and Switzerland a system of two sound carriers is used, the frequency of the second carrier being 242.1875 kHz above the frequency of the first sound carrier. For stereophonic sound transmissions a similar system is used in Australia.

[d] New Zealand uses a sound carrier displaced 5.4996 ± 0.0005 MHz from the vision carrier.

[e] In Denmark, Finland, New Zealand, Sweden and Spain a system of two sound carriers is used. In Iceland, Norway and Poland the same system is being introduced. The second carrier is 5.85 MHz above the vision carrier and is DQPSK modulated with 728 kbit/s sound and data multiplex. (NICAM system)

[f] In the United Kingdom, a system of two sound carriers is used. The second sound carrier is 6.552 MHz above the vision carrier and is DQPSK modulated with a 728 kbit/s sound and data multiplex able to carry two sound channels. (NICAM system)

[g] In France, a digital carrier 5.85 MHz away from the vision carrier may be used in addition to the main sound carrier. It is modulated in differentially encoded QPSK with a 728 kbit/s sound and data multiplexer capable of carrying two sound channels. (NICAM system)


Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_standard index is out of bounds.



[23] The supported standards may overlap and we need an unambiguous set to find the current standard returned by VIDIOC_G_STD.


Name

VIDIOC_G_AUDIO, VIDIOC_S_AUDIO — Query or select the current audio input and its attributes

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_audio * argp);
int ioctl(int  fd,
 int  request,
 const struct v4l2_audio * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_AUDIO, VIDIOC_S_AUDIO

argp

Description

To query the current audio input applications zero out the reserved array of a struct v4l2_audio and call the VIDIOC_G_AUDIO ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the device has no audio inputs, or none which combine with the current video input.

Audio inputs have one writable property, the audio mode. To select the current audio input and change the audio mode, applications initialize the index and mode fields, and the reserved array of a v4l2_audio structure and call the VIDIOC_S_AUDIO ioctl. Drivers may switch to a different audio mode if the request cannot be satisfied. However, this is a write-only ioctl, it does not return the actual new audio mode.

Table 69. struct v4l2_audio

__u32indexIdentifies the audio input, set by the driver or application.
__u8name[32]Name of the audio input, a NUL-terminated ASCII string, for example: "Line In". This information is intended for the user, preferably the connector label on the device itself.
__u32capabilityAudio capability flags, see Table 70, “Audio Capability Flags”.
__u32modeAudio mode flags set by drivers and applications (on VIDIOC_S_AUDIO ioctl), see Table 71, “Audio Mode Flags”.
__u32reserved[2]Reserved for future extensions. Drivers and applications must set the array to zero.

Table 70. Audio Capability Flags

V4L2_AUDCAP_STEREO0x00001This is a stereo input. The flag is intended to automatically disable stereo recording etc. when the signal is always monaural. The API provides no means to detect if stereo is received, unless the audio input belongs to a tuner.
V4L2_AUDCAP_AVL0x00002Automatic Volume Level mode is supported.

Table 71. Audio Mode Flags

V4L2_AUDMODE_AVL0x00001AVL mode is on.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

No audio inputs combine with the current video input, or the number of the selected audio input is out of bounds or it does not combine, or there are no audio inputs at all and the ioctl is not supported.

EBUSY

I/O is in progress, the input cannot be switched.


Name

VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT — Query or select the current audio output

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_audioout * argp);
int ioctl(int  fd,
 int  request,
 const struct v4l2_audioout * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT

argp

Description

To query the current audio output applications zero out the reserved array of a struct v4l2_audioout and call the VIDIOC_G_AUDOUT ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the device has no audio inputs, or none which combine with the current video output.

Audio outputs have no writable properties. Nevertheless, to select the current audio output applications can initialize the index field and reserved array (which in the future may contain writable properties) of a v4l2_audioout structure and call the VIDIOC_S_AUDOUT ioctl. Drivers switch to the requested output or return the EINVAL error code when the index is out of bounds. This is a write-only ioctl, it does not return the current audio output attributes as VIDIOC_G_AUDOUT does.

Note connectors on a TV card to loop back the received audio signal to a sound card are not audio outputs in this sense.

Table 72. struct v4l2_audioout

__u32indexIdentifies the audio output, set by the driver or application.
__u8name[32]Name of the audio output, a NUL-terminated ASCII string, for example: "Line Out". This information is intended for the user, preferably the connector label on the device itself.
__u32capabilityAudio capability flags, none defined yet. Drivers must set this field to zero.
__u32modeAudio mode, none defined yet. Drivers and applications (on VIDIOC_S_AUDOUT) must set this field to zero.
__u32reserved[2]Reserved for future extensions. Drivers and applications must set the array to zero.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

No audio outputs combine with the current video output, or the number of the selected audio output is out of bounds or it does not combine, or there are no audio outputs at all and the ioctl is not supported.

EBUSY

I/O is in progress, the output cannot be switched.


Name

VIDIOC_G_CROP, VIDIOC_S_CROP — Get or set the current cropping rectangle

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_crop * argp);
int ioctl(int  fd,
 int  request,
 const struct v4l2_crop * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_CROP, VIDIOC_S_CROP

argp

Description

To query the cropping rectangle size and position applications set the type field of a v4l2_crop structure to the respective buffer (stream) type and call the VIDIOC_G_CROP ioctl with a pointer to this structure. The driver fills the rest of the structure or returns the EINVAL error code if cropping is not supported.

To change the cropping rectangle applications initialize the type and struct v4l2_rect substructure named c of a v4l2_crop structure and call the VIDIOC_S_CROP ioctl with a pointer to this structure.

The driver first adjusts the requested dimensions against hardware limits, i. e. the bounds given by the capture/output window, and it rounds to the closest possible values of horizontal and vertical offset, width and height. In particular the driver must round the vertical offset of the cropping rectangle to frame lines modulo two, such that the field order cannot be confused.

Second the driver adjusts the image size (the opposite rectangle of the scaling process, source or target depending on the data direction) to the closest size possible while maintaining the current horizontal and vertical scaling factor.

Finally the driver programs the hardware with the actual cropping and image parameters. VIDIOC_S_CROP is a write-only ioctl, it does not return the actual parameters. To query them applications must call VIDIOC_G_CROP and VIDIOC_G_FMT. When the parameters are unsuitable the application may modify the cropping or image parameters and repeat the cycle until satisfactory parameters have been negotiated.

When cropping is not supported then no parameters are changed and VIDIOC_S_CROP returns the EINVAL error code.

Table 73. struct v4l2_crop

enum v4l2_buf_typetypeType of the data stream, set by the application. Only these types are valid here: V4L2_BUF_TYPE_VIDEO_CAPTURE, V4L2_BUF_TYPE_VIDEO_OUTPUT, V4L2_BUF_TYPE_VIDEO_OVERLAY, and custom (driver defined) types with code V4L2_BUF_TYPE_PRIVATE and higher.
struct v4l2_rectcCropping rectangle. The same co-ordinate system as for struct v4l2_cropcap bounds is used.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

Cropping is not supported.


Name

VIDIOC_G_CTRL, VIDIOC_S_CTRL — Get or set the value of a control

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_control * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_CTRL, VIDIOC_S_CTRL

argp

Description

To get the current value of a control applications initialize the id field of a struct v4l2_control and call the VIDIOC_G_CTRL ioctl with a pointer to this structure. To change the value of a control applications initialize the id and value fields of a struct v4l2_control and call the VIDIOC_S_CTRL ioctl.

When the id is invalid drivers return an EINVAL error code. When the value is out of bounds drivers can choose to take the closest valid value or return an ERANGE error code, whatever seems more appropriate. However, VIDIOC_S_CTRL is a write-only ioctl, it does not return the actual new value.

These ioctls work only with user controls. For other control classes the VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS or VIDIOC_TRY_EXT_CTRLS must be used.

Table 74. struct v4l2_control

__u32idIdentifies the control, set by the application.
__s32valueNew value or current value.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_control id is invalid.

ERANGE

The struct v4l2_control value is out of bounds.

EBUSY

The control is temporarily not changeable, possibly because another applications took over control of the device function this control belongs to.


Name

VIDIOC_G_ENC_INDEX — Get meta data about a compressed video stream

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_enc_idx * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_ENC_INDEX

argp

Description

Experimental

This is an experimental interface and may change in the future.

The VIDIOC_G_ENC_INDEX ioctl provides meta data about a compressed video stream the same or another application currently reads from the driver, which is useful for random access into the stream without decoding it.

To read the data applications must call VIDIOC_G_ENC_INDEX with a pointer to a struct v4l2_enc_idx. On success the driver fills the entry array, stores the number of elements written in the entries field, and initializes the entries_cap field.

Each element of the entry array contains meta data about one picture. A VIDIOC_G_ENC_INDEX call reads up to V4L2_ENC_IDX_ENTRIES entries from a driver buffer, which can hold up to entries_cap entries. This number can be lower or higher than V4L2_ENC_IDX_ENTRIES, but not zero. When the application fails to read the meta data in time the oldest entries will be lost. When the buffer is empty or no capturing/encoding is in progress, entries will be zero.

Currently this ioctl is only defined for MPEG-2 program streams and video elementary streams.

Table 75. struct v4l2_enc_idx

__u32entriesThe number of entries the driver stored in the entry array.
__u32entries_capThe number of entries the driver can buffer. Must be greater than zero.
__u32reserved[4]Reserved for future extensions. Drivers must set the array to zero.
struct v4l2_enc_idx_entryentry[V4L2_ENC_IDX_ENTRIES]Meta data about a compressed video stream. Each element of the array corresponds to one picture, sorted in ascending order by their offset.  

Table 76. struct v4l2_enc_idx_entry

__u64offsetThe offset in bytes from the beginning of the compressed video stream to the beginning of this picture, that is a PES packet header as defined in [ISO 13818-1] or a picture header as defined in [ISO 13818-2]. When the encoder is stopped, the driver resets the offset to zero.
__u64ptsThe 33 bit Presentation Time Stamp of this picture as defined in [ISO 13818-1].
__u32lengthThe length of this picture in bytes.
__u32flagsFlags containing the coding type of this picture, see Table 77, “Index Entry Flags”.
__u32reserved[2]Reserved for future extensions. Drivers must set the array to zero.

Table 77. Index Entry Flags

V4L2_ENC_IDX_FRAME_I0x00This is an Intra-coded picture.
V4L2_ENC_IDX_FRAME_P0x01This is a Predictive-coded picture.
V4L2_ENC_IDX_FRAME_B0x02This is a Bidirectionally predictive-coded picture.
V4L2_ENC_IDX_FRAME_MASK0x0FAND the flags field with this mask to obtain the picture coding type.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The driver does not support this ioctl.


Name

VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS, VIDIOC_TRY_EXT_CTRLS — Get or set the value of several controls, try control values

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_ext_controls * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS, VIDIOC_TRY_EXT_CTRLS

argp

Description

These ioctls allow the caller to get or set multiple controls atomically. Control IDs are grouped into control classes (see Table 80, “Control classes”) and all controls in the control array must belong to the same control class.

Applications must always fill in the count, ctrl_class, controls and reserved fields of struct v4l2_ext_controls, and initialize the struct v4l2_ext_control array pointed to by the controls fields.

To get the current value of a set of controls applications initialize the id, size and reserved2 fields of each struct v4l2_ext_control and call the VIDIOC_G_EXT_CTRLS ioctl. String controls controls must also set the string field.

If the size is too small to receive the control result (only relevant for pointer-type controls like strings), then the driver will set size to a valid value and return an ENOSPC error code. You should re-allocate the string memory to this new size and try again. It is possible that the same issue occurs again if the string has grown in the meantime. It is recommended to call VIDIOC_QUERYCTRL first and use maximum+1 as the new size value. It is guaranteed that that is sufficient memory.

To change the value of a set of controls applications initialize the id, size, reserved2 and value/string fields of each struct v4l2_ext_control and call the VIDIOC_S_EXT_CTRLS ioctl. The controls will only be set if all control values are valid.

To check if a set of controls have correct values applications initialize the id, size, reserved2 and value/string fields of each struct v4l2_ext_control and call the VIDIOC_TRY_EXT_CTRLS ioctl. It is up to the driver whether wrong values are automatically adjusted to a valid value or if an error is returned.

When the id or ctrl_class is invalid drivers return an EINVAL error code. When the value is out of bounds drivers can choose to take the closest valid value or return an ERANGE error code, whatever seems more appropriate. In the first case the new value is set in struct v4l2_ext_control.

The driver will only set/get these controls if all control values are correct. This prevents the situation where only some of the controls were set/get. Only low-level errors (e. g. a failed i2c command) can still cause this situation.

Table 78. struct v4l2_ext_control

__u32id Identifies the control, set by the application.
__u32size The total size in bytes of the payload of this control. This is normally 0, but for pointer controls this should be set to the size of the memory containing the payload, or that will receive the payload. If VIDIOC_G_EXT_CTRLS finds that this value is less than is required to store the payload result, then it is set to a value large enough to store the payload result and ENOSPC is returned. Note that for string controls this size field should not be confused with the length of the string. This field refers to the size of the memory that contains the string. The actual length of the string may well be much smaller.
__u32reserved2[1] Reserved for future extensions. Drivers and applications must set the array to zero.
union(anonymous)  
 __s32valueNew value or current value.
 __s64value64New value or current value.
 char *stringA pointer to a string.

Table 79. struct v4l2_ext_controls

__u32ctrl_classThe control class to which all controls belong, see Table 80, “Control classes”.
__u32countThe number of controls in the controls array. May also be zero.
__u32error_idxSet by the driver in case of an error. It is the index of the control causing the error or equal to 'count' when the error is not associated with a particular control. Undefined when the ioctl returns 0 (success).
__u32reserved[2]Reserved for future extensions. Drivers and applications must set the array to zero.
struct v4l2_ext_control *controlsPointer to an array of count v4l2_ext_control structures. Ignored if count equals zero.

Table 80. Control classes

V4L2_CTRL_CLASS_USER0x980000The class containing user controls. These controls are described in Section 1.8, “User Controls”. All controls that can be set using the VIDIOC_S_CTRL and VIDIOC_G_CTRL ioctl belong to this class.
V4L2_CTRL_CLASS_MPEG0x990000The class containing MPEG compression controls. These controls are described in Section 1.9.5, “MPEG Control Reference”.
V4L2_CTRL_CLASS_CAMERA0x9a0000The class containing camera controls. These controls are described in Section 1.9.6, “Camera Control Reference”.
V4L2_CTRL_CLASS_FM_TX0x9b0000The class containing FM Transmitter (FM TX) controls. These controls are described in Section 1.9.7, “FM Transmitter Control Reference”.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_ext_control id is invalid or the struct v4l2_ext_controls ctrl_class is invalid. This error code is also returned by the VIDIOC_S_EXT_CTRLS and VIDIOC_TRY_EXT_CTRLS ioctls if two or more control values are in conflict.

ERANGE

The struct v4l2_ext_control value is out of bounds.

EBUSY

The control is temporarily not changeable, possibly because another applications took over control of the device function this control belongs to.

ENOSPC

The space reserved for the control's payload is insufficient. The field size is set to a value that is enough to store the payload and this error code is returned.


Name

VIDIOC_G_FBUF, VIDIOC_S_FBUF — Get or set frame buffer overlay parameters

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_framebuffer * argp);
int ioctl(int  fd,
 int  request,
 const struct v4l2_framebuffer * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_FBUF, VIDIOC_S_FBUF

argp

Description

Applications can use the VIDIOC_G_FBUF and VIDIOC_S_FBUF ioctl to get and set the framebuffer parameters for a Video Overlay or Video Output Overlay (OSD). The type of overlay is implied by the device type (capture or output device) and can be determined with the VIDIOC_QUERYCAP ioctl. One /dev/videoN device must not support both kinds of overlay.

The V4L2 API distinguishes destructive and non-destructive overlays. A destructive overlay copies captured video images into the video memory of a graphics card. A non-destructive overlay blends video images into a VGA signal or graphics into a video signal. Video Output Overlays are always non-destructive.

To get the current parameters applications call the VIDIOC_G_FBUF ioctl with a pointer to a v4l2_framebuffer structure. The driver fills all fields of the structure or returns an EINVAL error code when overlays are not supported.

To set the parameters for a Video Output Overlay, applications must initialize the flags field of a struct v4l2_framebuffer. Since the framebuffer is implemented on the TV card all other parameters are determined by the driver. When an application calls VIDIOC_S_FBUF with a pointer to this structure, the driver prepares for the overlay and returns the framebuffer parameters as VIDIOC_G_FBUF does, or it returns an error code.

To set the parameters for a non-destructive Video Overlay, applications must initialize the flags field, the fmt substructure, and call VIDIOC_S_FBUF. Again the driver prepares for the overlay and returns the framebuffer parameters as VIDIOC_G_FBUF does, or it returns an error code.

For a destructive Video Overlay applications must additionally provide a base address. Setting up a DMA to a random memory location can jeopardize the system security, its stability or even damage the hardware, therefore only the superuser can set the parameters for a destructive video overlay.

Table 81. struct v4l2_framebuffer

__u32capability Overlay capability flags set by the driver, see Table 82, “Frame Buffer Capability Flags”.
__u32flags Overlay control flags set by application and driver, see Table 83, “Frame Buffer Flags”
void *base Physical base address of the framebuffer, that is the address of the pixel in the top left corner of the framebuffer.[a]
   This field is irrelevant to non-destructive Video Overlays. For destructive Video Overlays applications must provide a base address. The driver may accept only base addresses which are a multiple of two, four or eight bytes. For Video Output Overlays the driver must return a valid base address, so applications can find the corresponding Linux framebuffer device (see Section 4.4, “Video Output Overlay Interface”).
struct v4l2_pix_formatfmt Layout of the frame buffer. The v4l2_pix_format structure is defined in Chapter 2, Image Formats, for clarification the fields and acceptable values are listed below:
 __u32widthWidth of the frame buffer in pixels.
 __u32heightHeight of the frame buffer in pixels.
 __u32pixelformatThe pixel format of the framebuffer.
   For non-destructive Video Overlays this field only defines a format for the struct v4l2_window chromakey field.
   For destructive Video Overlays applications must initialize this field. For Video Output Overlays the driver must return a valid format.
   Usually this is an RGB format (for example V4L2_PIX_FMT_RGB565) but YUV formats (only packed YUV formats when chroma keying is used, not including V4L2_PIX_FMT_YUYV and V4L2_PIX_FMT_UYVY) and the V4L2_PIX_FMT_PAL8 format are also permitted. The behavior of the driver when an application requests a compressed format is undefined. See Chapter 2, Image Formats for information on pixel formats.
 enum v4l2_fieldfieldDrivers and applications shall ignore this field. If applicable, the field order is selected with the VIDIOC_S_FMT ioctl, using the field field of struct v4l2_window.
 __u32bytesperlineDistance in bytes between the leftmost pixels in two adjacent lines.

This field is irrelevant to non-destructive Video Overlays.

For destructive Video Overlays both applications and drivers can set this field to request padding bytes at the end of each line. Drivers however may ignore the requested value, returning width times bytes-per-pixel or a larger value required by the hardware. That implies applications can just set this field to zero to get a reasonable default.

For Video Output Overlays the driver must return a valid value.

Video hardware may access padding bytes, therefore they must reside in accessible memory. Consider for example the case where padding bytes after the last line of an image cross a system page boundary. Capture devices may write padding bytes, the value is undefined. Output devices ignore the contents of padding bytes.

When the image format is planar the bytesperline value applies to the largest plane and is divided by the same factor as the width field for any smaller planes. For example the Cb and Cr planes of a YUV 4:2:0 image have half as many padding bytes following each line as the Y plane. To avoid ambiguities drivers must return a bytesperline value rounded up to a multiple of the scale factor.

 __u32sizeimage

This field is irrelevant to non-destructive Video Overlays. For destructive Video Overlays applications must initialize this field. For Video Output Overlays the driver must return a valid format.

Together with base it defines the framebuffer memory accessible by the driver.

 enum v4l2_colorspacecolorspaceThis information supplements the pixelformat and must be set by the driver, see Section 2.2, “Colorspaces”.
 __u32privReserved for additional information about custom (driver defined) formats. When not used drivers and applications must set this field to zero.

[a] A physical base address may not suit all platforms. GK notes in theory we should pass something like PCI device + memory region + offset instead. If you encounter problems please discuss on the linux-media mailing list: http://www.linuxtv.org/lists.php.


Table 82. Frame Buffer Capability Flags

V4L2_FBUF_CAP_EXTERNOVERLAY0x0001The device is capable of non-destructive overlays. When the driver clears this flag, only destructive overlays are supported. There are no drivers yet which support both destructive and non-destructive overlays.
V4L2_FBUF_CAP_CHROMAKEY0x0002The device supports clipping by chroma-keying the images. That is, image pixels replace pixels in the VGA or video signal only where the latter assume a certain color. Chroma-keying makes no sense for destructive overlays.
V4L2_FBUF_CAP_LIST_CLIPPING0x0004The device supports clipping using a list of clip rectangles.
V4L2_FBUF_CAP_BITMAP_CLIPPING0x0008The device supports clipping using a bit mask.
V4L2_FBUF_CAP_LOCAL_ALPHA0x0010The device supports clipping/blending using the alpha channel of the framebuffer or VGA signal. Alpha blending makes no sense for destructive overlays.
V4L2_FBUF_CAP_GLOBAL_ALPHA0x0020The device supports alpha blending using a global alpha value. Alpha blending makes no sense for destructive overlays.
V4L2_FBUF_CAP_LOCAL_INV_ALPHA0x0040The device supports clipping/blending using the inverted alpha channel of the framebuffer or VGA signal. Alpha blending makes no sense for destructive overlays.

Table 83. Frame Buffer Flags

V4L2_FBUF_FLAG_PRIMARY0x0001The framebuffer is the primary graphics surface. In other words, the overlay is destructive. [?]
V4L2_FBUF_FLAG_OVERLAY0x0002The frame buffer is an overlay surface the same size as the capture. [?]
The purpose of V4L2_FBUF_FLAG_PRIMARY and V4L2_FBUF_FLAG_OVERLAY was never quite clear. Most drivers seem to ignore these flags. For compatibility with the bttv driver applications should set the V4L2_FBUF_FLAG_OVERLAY flag.
V4L2_FBUF_FLAG_CHROMAKEY0x0004Use chroma-keying. The chroma-key color is determined by the chromakey field of struct v4l2_window and negotiated with the VIDIOC_S_FMT ioctl, see Section 4.2, “Video Overlay Interface” and Section 4.4, “Video Output Overlay Interface”.
There are no flags to enable clipping using a list of clip rectangles or a bitmap. These methods are negotiated with the VIDIOC_S_FMT ioctl, see Section 4.2, “Video Overlay Interface” and Section 4.4, “Video Output Overlay Interface”.
V4L2_FBUF_FLAG_LOCAL_ALPHA0x0008Use the alpha channel of the framebuffer to clip or blend framebuffer pixels with video images. The blend function is: output = framebuffer pixel * alpha + video pixel * (1 - alpha). The actual alpha depth depends on the framebuffer pixel format.
V4L2_FBUF_FLAG_GLOBAL_ALPHA0x0010Use a global alpha value to blend the framebuffer with video images. The blend function is: output = (framebuffer pixel * alpha + video pixel * (255 - alpha)) / 255. The alpha value is determined by the global_alpha field of struct v4l2_window and negotiated with the VIDIOC_S_FMT ioctl, see Section 4.2, “Video Overlay Interface” and Section 4.4, “Video Output Overlay Interface”.
V4L2_FBUF_FLAG_LOCAL_INV_ALPHA0x0020Like V4L2_FBUF_FLAG_LOCAL_ALPHA, use the alpha channel of the framebuffer to clip or blend framebuffer pixels with video images, but with an inverted alpha value. The blend function is: output = framebuffer pixel * (1 - alpha) + video pixel * alpha. The actual alpha depth depends on the framebuffer pixel format.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EPERM

VIDIOC_S_FBUF can only be called by a privileged user to negotiate the parameters for a destructive overlay.

EBUSY

The framebuffer parameters cannot be changed at this time because overlay is already enabled, or capturing is enabled and the hardware cannot capture and overlay simultaneously.

EINVAL

The ioctl is not supported or the VIDIOC_S_FBUF parameters are unsuitable.


Name

VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT — Get or set the data format, try a format

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_format * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT

argp

Description

These ioctls are used to negotiate the format of data (typically image format) exchanged between driver and application.

To query the current parameters applications set the type field of a struct v4l2_format to the respective buffer (stream) type. For example video capture devices use V4L2_BUF_TYPE_VIDEO_CAPTURE. When the application calls the VIDIOC_G_FMT ioctl with a pointer to this structure the driver fills the respective member of the fmt union. In case of video capture devices that is the struct v4l2_pix_format pix member. When the requested buffer type is not supported drivers return an EINVAL error code.

To change the current format parameters applications initialize the type field and all fields of the respective fmt union member. For details see the documentation of the various devices types in Chapter 4, Interfaces. Good practice is to query the current parameters first, and to modify only those parameters not suitable for the application. When the application calls the VIDIOC_S_FMT ioctl with a pointer to a v4l2_format structure the driver checks and adjusts the parameters against hardware abilities. Drivers should not return an error code unless the input is ambiguous, this is a mechanism to fathom device capabilities and to approach parameters acceptable for both the application and driver. On success the driver may program the hardware, allocate resources and generally prepare for data exchange. Finally the VIDIOC_S_FMT ioctl returns the current format parameters as VIDIOC_G_FMT does. Very simple, inflexible devices may even ignore all input and always return the default parameters. However all V4L2 devices exchanging data with the application must implement the VIDIOC_G_FMT and VIDIOC_S_FMT ioctl. When the requested buffer type is not supported drivers return an EINVAL error code on a VIDIOC_S_FMT attempt. When I/O is already in progress or the resource is not available for other reasons drivers return the EBUSY error code.

The VIDIOC_TRY_FMT ioctl is equivalent to VIDIOC_S_FMT with one exception: it does not change driver state. It can also be called at any time, never returning EBUSY. This function is provided to negotiate parameters, to learn about hardware limitations, without disabling I/O or possibly time consuming hardware preparations. Although strongly recommended drivers are not required to implement this ioctl.

Table 84. struct v4l2_format

enum v4l2_buf_typetype Type of the data stream, see Table 3.2, “enum v4l2_buf_type”.
unionfmt  
 struct v4l2_pix_formatpixDefinition of an image format, see Chapter 2, Image Formats, used by video capture and output devices.
 struct v4l2_windowwinDefinition of an overlaid image, see Section 4.2, “Video Overlay Interface”, used by video overlay devices.
 struct v4l2_vbi_formatvbiRaw VBI capture or output parameters. This is discussed in more detail in Section 4.7, “Raw VBI Data Interface”. Used by raw VBI capture and output devices.
 struct v4l2_sliced_vbi_formatslicedSliced VBI capture or output parameters. See Section 4.8, “Sliced VBI Data Interface” for details. Used by sliced VBI capture and output devices.
 __u8raw_data[200]Place holder for future extensions and custom (driver defined) formats with type V4L2_BUF_TYPE_PRIVATE and higher.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EBUSY

The data format cannot be changed at this time, for example because I/O is already in progress.

EINVAL

The struct v4l2_format type field is invalid, the requested buffer type not supported, or VIDIOC_TRY_FMT was called and is not supported with this buffer type.


Name

VIDIOC_G_FREQUENCY, VIDIOC_S_FREQUENCY — Get or set tuner or modulator radio frequency

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_frequency * argp);
int ioctl(int  fd,
 int  request,
 const struct v4l2_frequency * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_FREQUENCY, VIDIOC_S_FREQUENCY

argp

Description

To get the current tuner or modulator radio frequency applications set the tuner field of a struct v4l2_frequency to the respective tuner or modulator number (only input devices have tuners, only output devices have modulators), zero out the reserved array and call the VIDIOC_G_FREQUENCY ioctl with a pointer to this structure. The driver stores the current frequency in the frequency field.

To change the current tuner or modulator radio frequency applications initialize the tuner, type and frequency fields, and the reserved array of a struct v4l2_frequency and call the VIDIOC_S_FREQUENCY ioctl with a pointer to this structure. When the requested frequency is not possible the driver assumes the closest possible value. However VIDIOC_S_FREQUENCY is a write-only ioctl, it does not return the actual new frequency.

Table 85. struct v4l2_frequency

__u32tunerThe tuner or modulator index number. This is the same value as in the struct v4l2_input tuner field and the struct v4l2_tuner index field, or the struct v4l2_output modulator field and the struct v4l2_modulator index field.
enum v4l2_tuner_typetypeThe tuner type. This is the same value as in the struct v4l2_tuner type field. The field is not applicable to modulators, i. e. ignored by drivers.
__u32frequencyTuning frequency in units of 62.5 kHz, or if the struct v4l2_tuner or struct v4l2_modulator capabilities flag V4L2_TUNER_CAP_LOW is set, in units of 62.5 Hz.
__u32reserved[8]Reserved for future extensions. Drivers and applications must set the array to zero.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The tuner index is out of bounds or the value in the type field is wrong.


Name

VIDIOC_G_INPUT, VIDIOC_S_INPUT — Query or select the current video input

Synopsis

int ioctl(int  fd,
 int  request,
 int * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_INPUT, VIDIOC_S_INPUT

argp

Description

To query the current video input applications call the VIDIOC_G_INPUT ioctl with a pointer to an integer where the driver stores the number of the input, as in the struct v4l2_input index field. This ioctl will fail only when there are no video inputs, returning EINVAL.

To select a video input applications store the number of the desired input in an integer and call the VIDIOC_S_INPUT ioctl with a pointer to this integer. Side effects are possible. For example inputs may support different video standards, so the driver may implicitly switch the current standard. It is good practice to select an input before querying or negotiating any other parameters.

Information about video inputs is available using the VIDIOC_ENUMINPUT ioctl.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The number of the video input is out of bounds, or there are no video inputs at all and this ioctl is not supported.

EBUSY

I/O is in progress, the input cannot be switched.


Name

VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP

Synopsis

int ioctl(int  fd,
 int  request,
 v4l2_jpegcompression * argp);
int ioctl(int  fd,
 int  request,
 const v4l2_jpegcompression * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP

argp

Description

[to do]

Ronald Bultje elaborates:

APP is some application-specific information. The application can set it itself, and it'll be stored in the JPEG-encoded fields (eg; interlacing information for in an AVI or so). COM is the same, but it's comments, like 'encoded by me' or so.

jpeg_markers describes whether the huffman tables, quantization tables and the restart interval information (all JPEG-specific stuff) should be stored in the JPEG-encoded fields. These define how the JPEG field is encoded. If you omit them, applications assume you've used standard encoding. You usually do want to add them.

Table 86. struct v4l2_jpegcompression

intquality 
intAPPn 
intAPP_len 
charAPP_data[60] 
intCOM_len 
charCOM_data[60] 
__u32jpeg_markersSee Table 87, “JPEG Markers Flags”.

Table 87. JPEG Markers Flags

V4L2_JPEG_MARKER_DHT(1<<3)Define Huffman Tables
V4L2_JPEG_MARKER_DQT(1<<4)Define Quantization Tables
V4L2_JPEG_MARKER_DRI(1<<5)Define Restart Interval
V4L2_JPEG_MARKER_COM(1<<6)Comment segment
V4L2_JPEG_MARKER_APP(1<<7)App segment, driver will always use APP0

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

This ioctl is not supported.


Name

VIDIOC_G_MODULATOR, VIDIOC_S_MODULATOR — Get or set modulator attributes

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_modulator * argp);
int ioctl(int  fd,
 int  request,
 const struct v4l2_modulator * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_MODULATOR, VIDIOC_S_MODULATOR

argp

Description

To query the attributes of a modulator applications initialize the index field and zero out the reserved array of a struct v4l2_modulator and call the VIDIOC_G_MODULATOR ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the index is out of bounds. To enumerate all modulators applications shall begin at index zero, incrementing by one until the driver returns EINVAL.

Modulators have two writable properties, an audio modulation set and the radio frequency. To change the modulated audio subprograms, applications initialize the index and txsubchans fields and the reserved array and call the VIDIOC_S_MODULATOR ioctl. Drivers may choose a different audio modulation if the request cannot be satisfied. However this is a write-only ioctl, it does not return the actual audio modulation selected.

To change the radio frequency the VIDIOC_S_FREQUENCY ioctl is available.

Table 88. struct v4l2_modulator

__u32indexIdentifies the modulator, set by the application.
__u8name[32]Name of the modulator, a NUL-terminated ASCII string. This information is intended for the user.
__u32capabilityModulator capability flags. No flags are defined for this field, the tuner flags in struct v4l2_tuner are used accordingly. The audio flags indicate the ability to encode audio subprograms. They will not change for example with the current video standard.
__u32rangelowThe lowest tunable frequency in units of 62.5 KHz, or if the capability flag V4L2_TUNER_CAP_LOW is set, in units of 62.5 Hz.
__u32rangehighThe highest tunable frequency in units of 62.5 KHz, or if the capability flag V4L2_TUNER_CAP_LOW is set, in units of 62.5 Hz.
__u32txsubchansWith this field applications can determine how audio sub-carriers shall be modulated. It contains a set of flags as defined in Table 89, “Modulator Audio Transmission Flags”. Note the tuner rxsubchans flags are reused, but the semantics are different. Video output devices are assumed to have an analog or PCM audio input with 1-3 channels. The txsubchans flags select one or more channels for modulation, together with some audio subprogram indicator, for example a stereo pilot tone.
__u32reserved[4]Reserved for future extensions. Drivers and applications must set the array to zero.

Table 89. Modulator Audio Transmission Flags

V4L2_TUNER_SUB_MONO0x0001Modulate channel 1 as mono audio, when the input has more channels, a down-mix of channel 1 and 2. This flag does not combine with V4L2_TUNER_SUB_STEREO or V4L2_TUNER_SUB_LANG1.
V4L2_TUNER_SUB_STEREO0x0002Modulate channel 1 and 2 as left and right channel of a stereo audio signal. When the input has only one channel or two channels and V4L2_TUNER_SUB_SAP is also set, channel 1 is encoded as left and right channel. This flag does not combine with V4L2_TUNER_SUB_MONO or V4L2_TUNER_SUB_LANG1. When the driver does not support stereo audio it shall fall back to mono.
V4L2_TUNER_SUB_LANG10x0008Modulate channel 1 and 2 as primary and secondary language of a bilingual audio signal. When the input has only one channel it is used for both languages. It is not possible to encode the primary or secondary language only. This flag does not combine with V4L2_TUNER_SUB_MONO, V4L2_TUNER_SUB_STEREO or V4L2_TUNER_SUB_SAP. If the hardware does not support the respective audio matrix, or the current video standard does not permit bilingual audio the VIDIOC_S_MODULATOR ioctl shall return an EINVAL error code and the driver shall fall back to mono or stereo mode.
V4L2_TUNER_SUB_LANG20x0004Same effect as V4L2_TUNER_SUB_SAP.
V4L2_TUNER_SUB_SAP0x0004When combined with V4L2_TUNER_SUB_MONO the first channel is encoded as mono audio, the last channel as Second Audio Program. When the input has only one channel it is used for both audio tracks. When the input has three channels the mono track is a down-mix of channel 1 and 2. When combined with V4L2_TUNER_SUB_STEREO channel 1 and 2 are encoded as left and right stereo audio, channel 3 as Second Audio Program. When the input has only two channels, the first is encoded as left and right channel and the second as SAP. When the input has only one channel it is used for all audio tracks. It is not possible to encode a Second Audio Program only. This flag must combine with V4L2_TUNER_SUB_MONO or V4L2_TUNER_SUB_STEREO. If the hardware does not support the respective audio matrix, or the current video standard does not permit SAP the VIDIOC_S_MODULATOR ioctl shall return an EINVAL error code and driver shall fall back to mono or stereo mode.
V4L2_TUNER_SUB_RDS0x0010Enable the RDS encoder for a radio FM transmitter.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_modulator index is out of bounds.


Name

VIDIOC_G_OUTPUT, VIDIOC_S_OUTPUT — Query or select the current video output

Synopsis

int ioctl(int  fd,
 int  request,
 int * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_OUTPUT, VIDIOC_S_OUTPUT

argp

Description

To query the current video output applications call the VIDIOC_G_OUTPUT ioctl with a pointer to an integer where the driver stores the number of the output, as in the struct v4l2_output index field. This ioctl will fail only when there are no video outputs, returning the EINVAL error code.

To select a video output applications store the number of the desired output in an integer and call the VIDIOC_S_OUTPUT ioctl with a pointer to this integer. Side effects are possible. For example outputs may support different video standards, so the driver may implicitly switch the current standard. It is good practice to select an output before querying or negotiating any other parameters.

Information about video outputs is available using the VIDIOC_ENUMOUTPUT ioctl.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The number of the video output is out of bounds, or there are no video outputs at all and this ioctl is not supported.

EBUSY

I/O is in progress, the output cannot be switched.


Name

VIDIOC_G_PARM, VIDIOC_S_PARM — Get or set streaming parameters

Synopsis

int ioctl(int  fd,
 int  request,
 v4l2_streamparm * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_PARM, VIDIOC_S_PARM

argp

Description

The current video standard determines a nominal number of frames per second. If less than this number of frames is to be captured or output, applications can request frame skipping or duplicating on the driver side. This is especially useful when using the read() or write(), which are not augmented by timestamps or sequence counters, and to avoid unneccessary data copying.

Further these ioctls can be used to determine the number of buffers used internally by a driver in read/write mode. For implications see the section discussing the read() function.

To get and set the streaming parameters applications call the VIDIOC_G_PARM and VIDIOC_S_PARM ioctl, respectively. They take a pointer to a struct v4l2_streamparm which contains a union holding separate parameters for input and output devices.

Table 90. struct v4l2_streamparm

enum v4l2_buf_typetype The buffer (stream) type, same as struct v4l2_format type, set by the application.
unionparm  
 struct v4l2_captureparmcaptureParameters for capture devices, used when type is V4L2_BUF_TYPE_VIDEO_CAPTURE.
 struct v4l2_outputparmoutputParameters for output devices, used when type is V4L2_BUF_TYPE_VIDEO_OUTPUT.
 __u8raw_data[200]A place holder for future extensions and custom (driver defined) buffer types V4L2_BUF_TYPE_PRIVATE and higher.

Table 91. struct v4l2_captureparm

__u32capabilitySee Table 93, “Streaming Parameters Capabilites”.
__u32capturemodeSet by drivers and applications, see Table 94, “Capture Parameters Flags”.
struct v4l2_fracttimeperframe

This is is the desired period between successive frames captured by the driver, in seconds. The field is intended to skip frames on the driver side, saving I/O bandwidth.

Applications store here the desired frame period, drivers return the actual frame period, which must be greater or equal to the nominal frame period determined by the current video standard (struct v4l2_standard frameperiod field). Changing the video standard (also implicitly by switching the video input) may reset this parameter to the nominal frame period. To reset manually applications can just set this field to zero.

Drivers support this function only when they set the V4L2_CAP_TIMEPERFRAME flag in the capability field.

__u32extendedmodeCustom (driver specific) streaming parameters. When unused, applications and drivers must set this field to zero. Applications using this field should check the driver name and version, see Section 1.2, “Querying Capabilities”.
__u32readbuffersApplications set this field to the desired number of buffers used internally by the driver in read() mode. Drivers return the actual number of buffers. When an application requests zero buffers, drivers should just return the current setting rather than the minimum or an error code. For details see Section 3.1, “Read/Write”.
__u32reserved[4]Reserved for future extensions. Drivers and applications must set the array to zero.

Table 92. struct v4l2_outputparm

__u32capabilitySee Table 93, “Streaming Parameters Capabilites”.
__u32outputmodeSet by drivers and applications, see Table 94, “Capture Parameters Flags”.
struct v4l2_fracttimeperframeThis is is the desired period between successive frames output by the driver, in seconds.

The field is intended to repeat frames on the driver side in write() mode (in streaming mode timestamps can be used to throttle the output), saving I/O bandwidth.

Applications store here the desired frame period, drivers return the actual frame period, which must be greater or equal to the nominal frame period determined by the current video standard (struct v4l2_standard frameperiod field). Changing the video standard (also implicitly by switching the video output) may reset this parameter to the nominal frame period. To reset manually applications can just set this field to zero.

Drivers support this function only when they set the V4L2_CAP_TIMEPERFRAME flag in the capability field.

__u32extendedmodeCustom (driver specific) streaming parameters. When unused, applications and drivers must set this field to zero. Applications using this field should check the driver name and version, see Section 1.2, “Querying Capabilities”.
__u32writebuffersApplications set this field to the desired number of buffers used internally by the driver in write() mode. Drivers return the actual number of buffers. When an application requests zero buffers, drivers should just return the current setting rather than the minimum or an error code. For details see Section 3.1, “Read/Write”.
__u32reserved[4]Reserved for future extensions. Drivers and applications must set the array to zero.

Table 93. Streaming Parameters Capabilites

V4L2_CAP_TIMEPERFRAME0x1000The frame skipping/repeating controlled by the timeperframe field is supported.

Table 94. Capture Parameters Flags

V4L2_MODE_HIGHQUALITY0x0001

High quality imaging mode. High quality mode is intended for still imaging applications. The idea is to get the best possible image quality that the hardware can deliver. It is not defined how the driver writer may achieve that; it will depend on the hardware and the ingenuity of the driver writer. High quality mode is a different mode from the the regular motion video capture modes. In high quality mode:

  • The driver may be able to capture higher resolutions than for motion capture.

  • The driver may support fewer pixel formats than motion capture (eg; true color).

  • The driver may capture and arithmetically combine multiple successive fields or frames to remove color edge artifacts and reduce the noise in the video data.

  • The driver may capture images in slices like a scanner in order to handle larger format images than would otherwise be possible.

  • An image capture operation may be significantly slower than motion capture.

  • Moving objects in the image might have excessive motion blur.

  • Capture might only work through the read() call.


Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

This ioctl is not supported.


Name

VIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY — Query or request the access priority associated with a file descriptor

Synopsis

int ioctl(int  fd,
 int  request,
 enum v4l2_priority * argp);
int ioctl(int  fd,
 int  request,
 const enum v4l2_priority * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY

argp

Pointer to an enum v4l2_priority type.

Description

To query the current access priority applications call the VIDIOC_G_PRIORITY ioctl with a pointer to an enum v4l2_priority variable where the driver stores the current priority.

To request an access priority applications store the desired priority in an enum v4l2_priority variable and call VIDIOC_S_PRIORITY ioctl with a pointer to this variable.

Table 95. enum v4l2_priority

V4L2_PRIORITY_UNSET0 
V4L2_PRIORITY_BACKGROUND1Lowest priority, usually applications running in background, for example monitoring VBI transmissions. A proxy application running in user space will be necessary if multiple applications want to read from a device at this priority.
V4L2_PRIORITY_INTERACTIVE2 
V4L2_PRIORITY_DEFAULT2Medium priority, usually applications started and interactively controlled by the user. For example TV viewers, Teletext browsers, or just "panel" applications to change the channel or video controls. This is the default priority unless an application requests another.
V4L2_PRIORITY_RECORD3Highest priority. Only one file descriptor can have this priority, it blocks any other fd from changing device properties. Usually applications which must not be interrupted, like video recording.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The requested priority value is invalid, or the driver does not support access priorities.

EBUSY

Another application already requested higher priority.


Name

VIDIOC_G_SLICED_VBI_CAP — Query sliced VBI capabilities

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_sliced_vbi_cap * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_SLICED_VBI_CAP

argp

Description

To find out which data services are supported by a sliced VBI capture or output device, applications initialize the type field of a struct v4l2_sliced_vbi_cap, clear the reserved array and call the VIDIOC_G_SLICED_VBI_CAP ioctl. The driver fills in the remaining fields or returns an EINVAL error code if the sliced VBI API is unsupported or type is invalid.

Note the type field was added, and the ioctl changed from read-only to write-read, in Linux 2.6.19.

Table 96. struct v4l2_sliced_vbi_cap

__u16service_setA set of all data services supported by the driver. Equal to the union of all elements of the service_lines array.
__u16service_lines[2][24]Each element of this array contains a set of data services the hardware can look for or insert into a particular scan line. Data services are defined in Table 97, “Sliced VBI services”. Array indices map to ITU-R line numbers (see also Figure 4.2, “ITU-R 525 line numbering (M/NTSC and M/PAL)” and Figure 4.3, “ITU-R 625 line numbering”) as follows:
  Element525 line systems625 line systems
  service_lines[0][1]11
  service_lines[0][23]2323
  service_lines[1][1]264314
  service_lines[1][23]286336
     
  The number of VBI lines the hardware can capture or output per frame, or the number of services it can identify on a given line may be limited. For example on PAL line 16 the hardware may be able to look for a VPS or Teletext signal, but not both at the same time. Applications can learn about these limits using the VIDIOC_S_FMT ioctl as described in Section 4.8, “Sliced VBI Data Interface”.
     
  Drivers must set service_lines[0][0] and service_lines[1][0] to zero.
enum v4l2_buf_typetypeType of the data stream, see Table 3.2, “enum v4l2_buf_type”. Should be V4L2_BUF_TYPE_SLICED_VBI_CAPTURE or V4L2_BUF_TYPE_SLICED_VBI_OUTPUT.  
__u32reserved[3]This array is reserved for future extensions. Applications and drivers must set it to zero.

Table 97. Sliced VBI services

SymbolValueReferenceLines, usuallyPayload
V4L2_SLICED_TELETEXT_B (Teletext System B)0x0001[ETS 300 706], [ITU BT.653]PAL/SECAM line 7-22, 320-335 (second field 7-22)Last 42 of the 45 byte Teletext packet, that is without clock run-in and framing code, lsb first transmitted.
V4L2_SLICED_VPS0x0400[ETS 300 231]PAL line 16Byte number 3 to 15 according to Figure 9 of ETS 300 231, lsb first transmitted.
V4L2_SLICED_CAPTION_5250x1000[EIA 608-B]NTSC line 21, 284 (second field 21)Two bytes in transmission order, including parity bit, lsb first transmitted.
V4L2_SLICED_WSS_6250x4000[EN 300 294], [ITU BT.1119]PAL/SECAM line 23
Byte        0                 1
     msb         lsb  msb           lsb
Bit  7 6 5 4 3 2 1 0  x x 13 12 11 10 9
V4L2_SLICED_VBI_5250x1000Set of services applicable to 525 line systems.
V4L2_SLICED_VBI_6250x4401Set of services applicable to 625 line systems.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The device does not support sliced VBI capturing or output, or the value in the type field is wrong.


Name

VIDIOC_G_STD, VIDIOC_S_STD — Query or select the video standard of the current input

Synopsis

int ioctl(int  fd,
 int  request,
 v4l2_std_id * argp);
int ioctl(int  fd,
 int  request,
 const v4l2_std_id * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_STD, VIDIOC_S_STD

argp

Description

To query and select the current video standard applications use the VIDIOC_G_STD and VIDIOC_S_STD ioctls which take a pointer to a v4l2_std_id type as argument. VIDIOC_G_STD can return a single flag or a set of flags as in struct v4l2_standard field id. The flags must be unambiguous such that they appear in only one enumerated v4l2_standard structure.

VIDIOC_S_STD accepts one or more flags, being a write-only ioctl it does not return the actual new standard as VIDIOC_G_STD does. When no flags are given or the current input does not support the requested standard the driver returns an EINVAL error code. When the standard set is ambiguous drivers may return EINVAL or choose any of the requested standards.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

This ioctl is not supported, or the VIDIOC_S_STD parameter was unsuitable.


Name

VIDIOC_G_TUNER, VIDIOC_S_TUNER — Get or set tuner attributes

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_tuner * argp);
int ioctl(int  fd,
 int  request,
 const struct v4l2_tuner * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_TUNER, VIDIOC_S_TUNER

argp

Description

To query the attributes of a tuner applications initialize the index field and zero out the reserved array of a struct v4l2_tuner and call the VIDIOC_G_TUNER ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the index is out of bounds. To enumerate all tuners applications shall begin at index zero, incrementing by one until the driver returns EINVAL.

Tuners have two writable properties, the audio mode and the radio frequency. To change the audio mode, applications initialize the index, audmode and reserved fields and call the VIDIOC_S_TUNER ioctl. This will not change the current tuner, which is determined by the current video input. Drivers may choose a different audio mode if the requested mode is invalid or unsupported. Since this is a write-only ioctl, it does not return the actually selected audio mode.

To change the radio frequency the VIDIOC_S_FREQUENCY ioctl is available.

Table 98. struct v4l2_tuner

__u32indexIdentifies the tuner, set by the application.
__u8name[32]

Name of the tuner, a NUL-terminated ASCII string. This information is intended for the user.

enum v4l2_tuner_typetypeType of the tuner, see Table 99, “enum v4l2_tuner_type”.
__u32capability

Tuner capability flags, see Table 100, “Tuner and Modulator Capability Flags”. Audio flags indicate the ability to decode audio subprograms. They will not change, for example with the current video standard.

When the structure refers to a radio tuner only the V4L2_TUNER_CAP_LOW, V4L2_TUNER_CAP_STEREO and V4L2_TUNER_CAP_RDS flags can be set.

__u32rangelowThe lowest tunable frequency in units of 62.5 kHz, or if the capability flag V4L2_TUNER_CAP_LOW is set, in units of 62.5 Hz.
__u32rangehighThe highest tunable frequency in units of 62.5 kHz, or if the capability flag V4L2_TUNER_CAP_LOW is set, in units of 62.5 Hz.
__u32rxsubchans

Some tuners or audio decoders can determine the received audio subprograms by analyzing audio carriers, pilot tones or other indicators. To pass this information drivers set flags defined in Table 101, “Tuner Audio Reception Flags” in this field. For example:

  V4L2_TUNER_SUB_MONOreceiving mono audio
  STEREO | SAPreceiving stereo audio and a secondary audio program
  MONO | STEREOreceiving mono or stereo audio, the hardware cannot distinguish
  LANG1 | LANG2receiving bilingual audio
  MONO | STEREO | LANG1 | LANG2receiving mono, stereo or bilingual audio
  

When the V4L2_TUNER_CAP_STEREO, _LANG1, _LANG2 or _SAP flag is cleared in the capability field, the corresponding V4L2_TUNER_SUB_ flag must not be set here.

This field is valid only if this is the tuner of the current video input, or when the structure refers to a radio tuner.

__u32audmode

The selected audio mode, see Table 102, “Tuner Audio Modes” for valid values. The audio mode does not affect audio subprogram detection, and like a control it does not automatically change unless the requested mode is invalid or unsupported. See Table 103, “Tuner Audio Matrix” for possible results when the selected and received audio programs do not match.

Currently this is the only field of struct v4l2_tuner applications can change.

__u32signalThe signal strength if known, ranging from 0 to 65535. Higher values indicate a better signal.
__s32afcAutomatic frequency control: When the afc value is negative, the frequency is too low, when positive too high.
__u32reserved[4]Reserved for future extensions. Drivers and applications must set the array to zero.

Table 99. enum v4l2_tuner_type

V4L2_TUNER_RADIO1 
V4L2_TUNER_ANALOG_TV2 

Table 100. Tuner and Modulator Capability Flags

V4L2_TUNER_CAP_LOW0x0001When set, tuning frequencies are expressed in units of 62.5 Hz, otherwise in units of 62.5 kHz.
V4L2_TUNER_CAP_NORM0x0002This is a multi-standard tuner; the video standard can or must be switched. (B/G PAL tuners for example are typically not considered multi-standard because the video standard is automatically determined from the frequency band.) The set of supported video standards is available from the struct v4l2_input pointing to this tuner, see the description of ioctl VIDIOC_ENUMINPUT for details. Only V4L2_TUNER_ANALOG_TV tuners can have this capability.
V4L2_TUNER_CAP_STEREO0x0010Stereo audio reception is supported.
V4L2_TUNER_CAP_LANG10x0040Reception of the primary language of a bilingual audio program is supported. Bilingual audio is a feature of two-channel systems, transmitting the primary language monaural on the main audio carrier and a secondary language monaural on a second carrier. Only V4L2_TUNER_ANALOG_TV tuners can have this capability.
V4L2_TUNER_CAP_LANG20x0020Reception of the secondary language of a bilingual audio program is supported. Only V4L2_TUNER_ANALOG_TV tuners can have this capability.
V4L2_TUNER_CAP_SAP0x0020

Reception of a secondary audio program is supported. This is a feature of the BTSC system which accompanies the NTSC video standard. Two audio carriers are available for mono or stereo transmissions of a primary language, and an independent third carrier for a monaural secondary language. Only V4L2_TUNER_ANALOG_TV tuners can have this capability.

Note the V4L2_TUNER_CAP_LANG2 and V4L2_TUNER_CAP_SAP flags are synonyms. V4L2_TUNER_CAP_SAP applies when the tuner supports the V4L2_STD_NTSC_M video standard.

V4L2_TUNER_CAP_RDS0x0080RDS capture is supported. This capability is only valid for radio tuners.

Table 101. Tuner Audio Reception Flags

V4L2_TUNER_SUB_MONO0x0001The tuner receives a mono audio signal.
V4L2_TUNER_SUB_STEREO0x0002The tuner receives a stereo audio signal.
V4L2_TUNER_SUB_LANG10x0008The tuner receives the primary language of a bilingual audio signal. Drivers must clear this flag when the current video standard is V4L2_STD_NTSC_M.
V4L2_TUNER_SUB_LANG20x0004The tuner receives the secondary language of a bilingual audio signal (or a second audio program).
V4L2_TUNER_SUB_SAP0x0004The tuner receives a Second Audio Program. Note the V4L2_TUNER_SUB_LANG2 and V4L2_TUNER_SUB_SAP flags are synonyms. The V4L2_TUNER_SUB_SAP flag applies when the current video standard is V4L2_STD_NTSC_M.
V4L2_TUNER_SUB_RDS0x0010The tuner receives an RDS channel.

Table 102. Tuner Audio Modes

V4L2_TUNER_MODE_MONO0Play mono audio. When the tuner receives a stereo signal this a down-mix of the left and right channel. When the tuner receives a bilingual or SAP signal this mode selects the primary language.
V4L2_TUNER_MODE_STEREO1

Play stereo audio. When the tuner receives bilingual audio it may play different languages on the left and right channel or the primary language is played on both channels.

Playing different languages in this mode is deprecated. New drivers should do this only in MODE_LANG1_LANG2.

When the tuner receives no stereo signal or does not support stereo reception the driver shall fall back to MODE_MONO.

V4L2_TUNER_MODE_LANG13Play the primary language, mono or stereo. Only V4L2_TUNER_ANALOG_TV tuners support this mode.
V4L2_TUNER_MODE_LANG22Play the secondary language, mono. When the tuner receives no bilingual audio or SAP, or their reception is not supported the driver shall fall back to mono or stereo mode. Only V4L2_TUNER_ANALOG_TV tuners support this mode.
V4L2_TUNER_MODE_SAP2Play the Second Audio Program. When the tuner receives no bilingual audio or SAP, or their reception is not supported the driver shall fall back to mono or stereo mode. Only V4L2_TUNER_ANALOG_TV tuners support this mode. Note the V4L2_TUNER_MODE_LANG2 and V4L2_TUNER_MODE_SAP are synonyms.
V4L2_TUNER_MODE_LANG1_LANG24Play the primary language on the left channel, the secondary language on the right channel. When the tuner receives no bilingual audio or SAP, it shall fall back to MODE_LANG1 or MODE_MONO. Only V4L2_TUNER_ANALOG_TV tuners support this mode.

Table 103. Tuner Audio Matrix

 Selected V4L2_TUNER_MODE_
Received V4L2_TUNER_SUB_MONOSTEREOLANG1LANG2 = SAPLANG1_LANG2[a]
MONOMonoMono/MonoMonoMonoMono/Mono
MONO | SAPMonoMono/MonoMonoSAPMono/SAP (preferred) or Mono/Mono
STEREOL+RL/RStereo L/R (preferred) or Mono L+RStereo L/R (preferred) or Mono L+RL/R (preferred) or L+R/L+R
STEREO | SAPL+RL/RStereo L/R (preferred) or Mono L+RSAPL+R/SAP (preferred) or L/R or L+R/L+R
LANG1 | LANG2Language 1Lang1/Lang2 (deprecated[b]) or Lang1/Lang1Language 1Language 2Lang1/Lang2 (preferred) or Lang1/Lang1

[a] This mode has been added in Linux 2.6.17 and may not be supported by older drivers.

[b] Playback of both languages in MODE_STEREO is deprecated. In the future drivers should produce only the primary language in this mode. Applications should request MODE_LANG1_LANG2 to record both languages or a stereo signal.


Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_tuner index is out of bounds.


Name

VIDIOC_LOG_STATUS — Log driver status information

Synopsis

int ioctl(int  fd,
 int  request);

Description

As the video/audio devices become more complicated it becomes harder to debug problems. When this ioctl is called the driver will output the current device status to the kernel log. This is particular useful when dealing with problems like no sound, no video and incorrectly tuned channels. Also many modern devices autodetect video and audio standards and this ioctl will report what the device thinks what the standard is. Mismatches may give an indication where the problem is.

This ioctl is optional and not all drivers support it. It was introduced in Linux 2.6.15.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The driver does not support this ioctl.


Name

VIDIOC_OVERLAY — Start or stop video overlay

Synopsis

int ioctl(int  fd,
 int  request,
 const int * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_OVERLAY

argp

Description

This ioctl is part of the video overlay I/O method. Applications call VIDIOC_OVERLAY to start or stop the overlay. It takes a pointer to an integer which must be set to zero by the application to stop overlay, to one to start.

Drivers do not support VIDIOC_STREAMON or VIDIOC_STREAMOFF with V4L2_BUF_TYPE_VIDEO_OVERLAY.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

Video overlay is not supported, or the parameters have not been set up. See Section 4.2, “Video Overlay Interface” for the necessary steps.


Name

VIDIOC_QBUF, VIDIOC_DQBUF — Exchange a buffer with the driver

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_buffer * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_QBUF, VIDIOC_DQBUF

argp

Description

Applications call the VIDIOC_QBUF ioctl to enqueue an empty (capturing) or filled (output) buffer in the driver's incoming queue. The semantics depend on the selected I/O method.

To enqueue a memory mapped buffer applications set the type field of a struct v4l2_buffer to the same buffer type as previously struct v4l2_format type and struct v4l2_requestbuffers type, the memory field to V4L2_MEMORY_MMAP and the index field. Valid index numbers range from zero to the number of buffers allocated with VIDIOC_REQBUFS (struct v4l2_requestbuffers count) minus one. The contents of the struct v4l2_buffer returned by a VIDIOC_QUERYBUF ioctl will do as well. When the buffer is intended for output (type is V4L2_BUF_TYPE_VIDEO_OUTPUT or V4L2_BUF_TYPE_VBI_OUTPUT) applications must also initialize the bytesused, field and timestamp fields. See Section 3.5, “Buffers” for details. When VIDIOC_QBUF is called with a pointer to this structure the driver sets the V4L2_BUF_FLAG_MAPPED and V4L2_BUF_FLAG_QUEUED flags and clears the V4L2_BUF_FLAG_DONE flag in the flags field, or it returns an EINVAL error code.

To enqueue a user pointer buffer applications set the type field of a struct v4l2_buffer to the same buffer type as previously struct v4l2_format type and struct v4l2_requestbuffers type, the memory field to V4L2_MEMORY_USERPTR and the m.userptr field to the address of the buffer and length to its size. When the buffer is intended for output additional fields must be set as above. When VIDIOC_QBUF is called with a pointer to this structure the driver sets the V4L2_BUF_FLAG_QUEUED flag and clears the V4L2_BUF_FLAG_MAPPED and V4L2_BUF_FLAG_DONE flags in the flags field, or it returns an error code. This ioctl locks the memory pages of the buffer in physical memory, they cannot be swapped out to disk. Buffers remain locked until dequeued, until the VIDIOC_STREAMOFF or VIDIOC_REQBUFS ioctl are called, or until the device is closed.

Applications call the VIDIOC_DQBUF ioctl to dequeue a filled (capturing) or displayed (output) buffer from the driver's outgoing queue. They just set the type and memory fields of a struct v4l2_buffer as above, when VIDIOC_DQBUF is called with a pointer to this structure the driver fills the remaining fields or returns an error code.

By default VIDIOC_DQBUF blocks when no buffer is in the outgoing queue. When the O_NONBLOCK flag was given to the open() function, VIDIOC_DQBUF returns immediately with an EAGAIN error code when no buffer is available.

The v4l2_buffer structure is specified in Section 3.5, “Buffers”.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EAGAIN

Non-blocking I/O has been selected using O_NONBLOCK and no buffer was in the outgoing queue.

EINVAL

The buffer type is not supported, or the index is out of bounds, or no buffers have been allocated yet, or the userptr or length are invalid.

ENOMEM

Not enough physical or virtual memory was available to enqueue a user pointer buffer.

EIO

VIDIOC_DQBUF failed due to an internal error. Can also indicate temporary problems like signal loss. Note the driver might dequeue an (empty) buffer despite returning an error, or even stop capturing.


Name

VIDIOC_QUERYBUF — Query the status of a buffer

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_buffer * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_QUERYBUF

argp

Description

This ioctl is part of the memory mapping I/O method. It can be used to query the status of a buffer at any time after buffers have been allocated with the VIDIOC_REQBUFS ioctl.

Applications set the type field of a struct v4l2_buffer to the same buffer type as previously struct v4l2_format type and struct v4l2_requestbuffers type, and the index field. Valid index numbers range from zero to the number of buffers allocated with VIDIOC_REQBUFS (struct v4l2_requestbuffers count) minus one. After calling VIDIOC_QUERYBUF with a pointer to this structure drivers return an error code or fill the rest of the structure.

In the flags field the V4L2_BUF_FLAG_MAPPED, V4L2_BUF_FLAG_QUEUED and V4L2_BUF_FLAG_DONE flags will be valid. The memory field will be set to V4L2_MEMORY_MMAP, the m.offset contains the offset of the buffer from the start of the device memory, the length field its size. The driver may or may not set the remaining fields and flags, they are meaningless in this context.

The v4l2_buffer structure is specified in Section 3.5, “Buffers”.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The buffer type is not supported, or the index is out of bounds.


Name

VIDIOC_QUERYCAP — Query device capabilities

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_capability * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_QUERYCAP

argp

Description

All V4L2 devices support the VIDIOC_QUERYCAP ioctl. It is used to identify kernel devices compatible with this specification and to obtain information about driver and hardware capabilities. The ioctl takes a pointer to a struct v4l2_capability which is filled by the driver. When the driver is not compatible with this specification the ioctl returns an EINVAL error code.

Table 104. struct v4l2_capability

__u8driver[16]

Name of the driver, a unique NUL-terminated ASCII string. For example: "bttv". Driver specific applications can use this information to verify the driver identity. It is also useful to work around known bugs, or to identify drivers in error reports. The driver version is stored in the version field.

Storing strings in fixed sized arrays is bad practice but unavoidable here. Drivers and applications should take precautions to never read or write beyond the end of the array and to make sure the strings are properly NUL-terminated.

__u8card[32]Name of the device, a NUL-terminated ASCII string. For example: "Yoyodyne TV/FM". One driver may support different brands or models of video hardware. This information is intended for users, for example in a menu of available devices. Since multiple TV cards of the same brand may be installed which are supported by the same driver, this name should be combined with the character device file name (e. g. /dev/video2) or the bus_info string to avoid ambiguities.
__u8bus_info[32]Location of the device in the system, a NUL-terminated ASCII string. For example: "PCI Slot 4". This information is intended for users, to distinguish multiple identical devices. If no such information is available the field may simply count the devices controlled by the driver, or contain the empty string (bus_info[0] = 0).
__u32version

Version number of the driver. Together with the driver field this identifies a particular driver. The version number is formatted using the KERNEL_VERSION() macro:

#define KERNEL_VERSION(a,b,c) (((a) << 16) + ((b) << 8) + (c))

__u32 version = KERNEL_VERSION(0, 8, 1);

printf ("Version: %u.%u.%u\n",
	(version >> 16) & 0xFF,
	(version >> 8) & 0xFF,
	 version & 0xFF);
__u32capabilitiesDevice capabilities, see Table 105, “Device Capabilities Flags”.
__u32reserved[4]Reserved for future extensions. Drivers must set this array to zero.

Table 105. Device Capabilities Flags

V4L2_CAP_VIDEO_CAPTURE0x00000001The device supports the Video Capture interface.
V4L2_CAP_VIDEO_OUTPUT0x00000002The device supports the Video Output interface.
V4L2_CAP_VIDEO_OVERLAY0x00000004The device supports the Video Overlay interface. A video overlay device typically stores captured images directly in the video memory of a graphics card, with hardware clipping and scaling.
V4L2_CAP_VBI_CAPTURE0x00000010The device supports the Raw VBI Capture interface, providing Teletext and Closed Caption data.
V4L2_CAP_VBI_OUTPUT0x00000020The device supports the Raw VBI Output interface.
V4L2_CAP_SLICED_VBI_CAPTURE0x00000040The device supports the Sliced VBI Capture interface.
V4L2_CAP_SLICED_VBI_OUTPUT0x00000080The device supports the Sliced VBI Output interface.
V4L2_CAP_RDS_CAPTURE0x00000100The device supports the RDS interface.
V4L2_CAP_VIDEO_OUTPUT_OVERLAY0x00000200The device supports the Video Output Overlay (OSD) interface. Unlike the Video Overlay interface, this is a secondary function of video output devices and overlays an image onto an outgoing video signal. When the driver sets this flag, it must clear the V4L2_CAP_VIDEO_OVERLAY flag and vice versa.[a]
V4L2_CAP_HW_FREQ_SEEK0x00000400The device supports the VIDIOC_S_HW_FREQ_SEEK ioctl for hardware frequency seeking.
V4L2_CAP_TUNER0x00010000The device has some sort of tuner to receive RF-modulated video signals. For more information about tuner programming see Section 1.6, “Tuners and Modulators”.
V4L2_CAP_AUDIO0x00020000The device has audio inputs or outputs. It may or may not support audio recording or playback, in PCM or compressed formats. PCM audio support must be implemented as ALSA or OSS interface. For more information on audio inputs and outputs see Section 1.5, “Audio Inputs and Outputs”.
V4L2_CAP_RADIO0x00040000This is a radio receiver.
V4L2_CAP_MODULATOR0x00080000The device has some sort of modulator to emit RF-modulated video/audio signals. For more information about modulator programming see Section 1.6, “Tuners and Modulators”.
V4L2_CAP_READWRITE0x01000000The device supports the read() and/or write() I/O methods.
V4L2_CAP_ASYNCIO0x02000000The device supports the asynchronous I/O methods.
V4L2_CAP_STREAMING0x04000000The device supports the streaming I/O method.

[a] The struct v4l2_framebuffer lacks an enum v4l2_buf_type field, therefore the type of overlay is implied by the driver capabilities.


Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The device is not compatible with this specification.


Name

VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU — Enumerate controls and menu control items

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_queryctrl * argp);
int ioctl(int  fd,
 int  request,
 struct v4l2_querymenu * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU

argp

Description

To query the attributes of a control applications set the id field of a struct v4l2_queryctrl and call the VIDIOC_QUERYCTRL ioctl with a pointer to this structure. The driver fills the rest of the structure or returns an EINVAL error code when the id is invalid.

It is possible to enumerate controls by calling VIDIOC_QUERYCTRL with successive id values starting from V4L2_CID_BASE up to and exclusive V4L2_CID_BASE_LASTP1. Drivers may return EINVAL if a control in this range is not supported. Further applications can enumerate private controls, which are not defined in this specification, by starting at V4L2_CID_PRIVATE_BASE and incrementing id until the driver returns EINVAL.

In both cases, when the driver sets the V4L2_CTRL_FLAG_DISABLED flag in the flags field this control is permanently disabled and should be ignored by the application.[24]

When the application ORs id with V4L2_CTRL_FLAG_NEXT_CTRL the driver returns the next supported control, or EINVAL if there is none. Drivers which do not support this flag yet always return EINVAL.

Additional information is required for menu controls: the names of the menu items. To query them applications set the id and index fields of struct v4l2_querymenu and call the VIDIOC_QUERYMENU ioctl with a pointer to this structure. The driver fills the rest of the structure or returns an EINVAL error code when the id or index is invalid. Menu items are enumerated by calling VIDIOC_QUERYMENU with successive index values from struct v4l2_queryctrl minimum (0) to maximum, inclusive.

See also the examples in Section 1.8, “User Controls”.

Table 106. struct v4l2_queryctrl

__u32idIdentifies the control, set by the application. See Table 1.1, “Control IDs” for predefined IDs. When the ID is ORed with V4L2_CTRL_FLAG_NEXT_CTRL the driver clears the flag and returns the first control with a higher ID. Drivers which do not support this flag yet always return an EINVAL error code.
enum v4l2_ctrl_typetypeType of control, see Table 108, “enum v4l2_ctrl_type”.
__u8name[32]Name of the control, a NUL-terminated ASCII string. This information is intended for the user.
__s32minimumMinimum value, inclusive. This field gives a lower bound for V4L2_CTRL_TYPE_INTEGER controls and the lowest valid index (always 0) for V4L2_CTRL_TYPE_MENU controls. For V4L2_CTRL_TYPE_STRING controls the minimum value gives the minimum length of the string. This length does not include the terminating zero. It may not be valid for any other type of control, including V4L2_CTRL_TYPE_INTEGER64 controls. Note that this is a signed value.
__s32maximumMaximum value, inclusive. This field gives an upper bound for V4L2_CTRL_TYPE_INTEGER controls and the highest valid index for V4L2_CTRL_TYPE_MENU controls. For V4L2_CTRL_TYPE_STRING controls the maximum value gives the maximum length of the string. This length does not include the terminating zero. It may not be valid for any other type of control, including V4L2_CTRL_TYPE_INTEGER64 controls. Note that this is a signed value.
__s32step

This field gives a step size for V4L2_CTRL_TYPE_INTEGER controls. For V4L2_CTRL_TYPE_STRING controls this field refers to the string length that has to be a multiple of this step size. It may not be valid for any other type of control, including V4L2_CTRL_TYPE_INTEGER64 controls.

Generally drivers should not scale hardware control values. It may be necessary for example when the name or id imply a particular unit and the hardware actually accepts only multiples of said unit. If so, drivers must take care values are properly rounded when scaling, such that errors will not accumulate on repeated read-write cycles.

This field gives the smallest change of an integer control actually affecting hardware. Often the information is needed when the user can change controls by keyboard or GUI buttons, rather than a slider. When for example a hardware register accepts values 0-511 and the driver reports 0-65535, step should be 128.

Note that although signed, the step value is supposed to be always positive.

__s32default_valueThe default value of a V4L2_CTRL_TYPE_INTEGER, _BOOLEAN or _MENU control. Not valid for other types of controls. Drivers reset controls only when the driver is loaded, not later, in particular not when the func-open; is called.
__u32flagsControl flags, see Table 109, “Control Flags”.
__u32reserved[2]Reserved for future extensions. Drivers must set the array to zero.

Table 107. struct v4l2_querymenu

__u32idIdentifies the control, set by the application from the respective struct v4l2_queryctrl id.
__u32indexIndex of the menu item, starting at zero, set by the application.
__u8name[32]Name of the menu item, a NUL-terminated ASCII string. This information is intended for the user.
__u32reservedReserved for future extensions. Drivers must set the array to zero.

Table 108. enum v4l2_ctrl_type

TypeminimumstepmaximumDescription
V4L2_CTRL_TYPE_INTEGERanyanyanyAn integer-valued control ranging from minimum to maximum inclusive. The step value indicates the increment between values which are actually different on the hardware.
V4L2_CTRL_TYPE_BOOLEAN011A boolean-valued control. Zero corresponds to "disabled", and one means "enabled".
V4L2_CTRL_TYPE_MENU01N-1The control has a menu of N choices. The names of the menu items can be enumerated with the VIDIOC_QUERYMENU ioctl.
V4L2_CTRL_TYPE_BUTTON000A control which performs an action when set. Drivers must ignore the value passed with VIDIOC_S_CTRL and return an EINVAL error code on a VIDIOC_G_CTRL attempt.
V4L2_CTRL_TYPE_INTEGER64n/an/an/aA 64-bit integer valued control. Minimum, maximum and step size cannot be queried.
V4L2_CTRL_TYPE_STRING≥ 0≥ 1≥ 0The minimum and maximum string lengths. The step size means that the string must be (minimum + N * step) characters long for N ≥ 0. These lengths do not include the terminating zero, so in order to pass a string of length 8 to VIDIOC_S_EXT_CTRLS you need to set the size field of struct v4l2_ext_control to 9. For VIDIOC_G_EXT_CTRLS you can set the size field to maximum + 1. Which character encoding is used will depend on the string control itself and should be part of the control documentation.
V4L2_CTRL_TYPE_CTRL_CLASSn/an/an/aThis is not a control. When VIDIOC_QUERYCTRL is called with a control ID equal to a control class code (see Table 80, “Control classes”), the ioctl returns the name of the control class and this control type. Older drivers which do not support this feature return an EINVAL error code.

Table 109. Control Flags

V4L2_CTRL_FLAG_DISABLED0x0001This control is permanently disabled and should be ignored by the application. Any attempt to change the control will result in an EINVAL error code.
V4L2_CTRL_FLAG_GRABBED0x0002This control is temporarily unchangeable, for example because another application took over control of the respective resource. Such controls may be displayed specially in a user interface. Attempts to change the control may result in an EBUSY error code.
V4L2_CTRL_FLAG_READ_ONLY0x0004This control is permanently readable only. Any attempt to change the control will result in an EINVAL error code.
V4L2_CTRL_FLAG_UPDATE0x0008A hint that changing this control may affect the value of other controls within the same control class. Applications should update their user interface accordingly.
V4L2_CTRL_FLAG_INACTIVE0x0010This control is not applicable to the current configuration and should be displayed accordingly in a user interface. For example the flag may be set on a MPEG audio level 2 bitrate control when MPEG audio encoding level 1 was selected with another control.
V4L2_CTRL_FLAG_SLIDER0x0020A hint that this control is best represented as a slider-like element in a user interface.
V4L2_CTRL_FLAG_WRITE_ONLY0x0040This control is permanently writable only. Any attempt to read the control will result in an EACCES error code error code. This flag is typically present for relative controls or action controls where writing a value will cause the device to carry out a given action (e. g. motor control) but no meaningful value can be returned.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_queryctrl id is invalid. The struct v4l2_querymenu id or index is invalid.

EACCES

An attempt was made to read a write-only control.



[24] V4L2_CTRL_FLAG_DISABLED was intended for two purposes: Drivers can skip predefined controls not supported by the hardware (although returning EINVAL would do as well), or disable predefined and private controls after hardware detection without the trouble of reordering control arrays and indices (EINVAL cannot be used to skip private controls because it would prematurely end the enumeration).


Name

VIDIOC_QUERYSTD — Sense the video standard received by the current input

Synopsis

int ioctl(int  fd,
 int  request,
 v4l2_std_id * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_QUERYSTD

argp

Description

The hardware may be able to detect the current video standard automatically. To do so, applications call VIDIOC_QUERYSTD with a pointer to a v4l2_std_id type. The driver stores here a set of candidates, this can be a single flag or a set of supported standards if for example the hardware can only distinguish between 50 and 60 Hz systems. When detection is not possible or fails, the set must contain all standards supported by the current video input or output.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

This ioctl is not supported.


Name

VIDIOC_REQBUFS — Initiate Memory Mapping or User Pointer I/O

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_requestbuffers * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_REQBUFS

argp

Description

This ioctl is used to initiate memory mapped or user pointer I/O. Memory mapped buffers are located in device memory and must be allocated with this ioctl before they can be mapped into the application's address space. User buffers are allocated by applications themselves, and this ioctl is merely used to switch the driver into user pointer I/O mode.

To allocate device buffers applications initialize three fields of a v4l2_requestbuffers structure. They set the type field to the respective stream or buffer type, the count field to the desired number of buffers, and memory must be set to V4L2_MEMORY_MMAP. When the ioctl is called with a pointer to this structure the driver attempts to allocate the requested number of buffers and stores the actual number allocated in the count field. It can be smaller than the number requested, even zero, when the driver runs out of free memory. A larger number is possible when the driver requires more buffers to function correctly.[25] When memory mapping I/O is not supported the ioctl returns an EINVAL error code.

Applications can call VIDIOC_REQBUFS again to change the number of buffers, however this cannot succeed when any buffers are still mapped. A count value of zero frees all buffers, after aborting or finishing any DMA in progress, an implicit VIDIOC_STREAMOFF.

To negotiate user pointer I/O, applications initialize only the type field and set memory to V4L2_MEMORY_USERPTR. When the ioctl is called with a pointer to this structure the driver prepares for user pointer I/O, when this I/O method is not supported the ioctl returns an EINVAL error code.

Table 110. struct v4l2_requestbuffers

__u32countThe number of buffers requested or granted. This field is only used when memory is set to V4L2_MEMORY_MMAP.
enum v4l2_buf_typetypeType of the stream or buffers, this is the same as the struct v4l2_format type field. See Table 3.2, “enum v4l2_buf_type” for valid values.
enum v4l2_memorymemoryApplications set this field to V4L2_MEMORY_MMAP or V4L2_MEMORY_USERPTR.
__u32reserved[2]A place holder for future extensions and custom (driver defined) buffer types V4L2_BUF_TYPE_PRIVATE and higher.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EBUSY

The driver supports multiple opens and I/O is already in progress, or reallocation of buffers was attempted although one or more are still mapped.

EINVAL

The buffer type (type field) or the requested I/O method (memory) is not supported.



[25] For example video output requires at least two buffers, one displayed and one filled by the application.


Name

VIDIOC_S_HW_FREQ_SEEK — Perform a hardware frequency seek

Synopsis

int ioctl(int  fd,
 int  request,
 struct v4l2_hw_freq_seek * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_S_HW_FREQ_SEEK

argp

Description

Start a hardware frequency seek from the current frequency. To do this applications initialize the tuner, type, seek_upward and wrap_around fields, and zero out the reserved array of a struct v4l2_hw_freq_seek and call the VIDIOC_S_HW_FREQ_SEEK ioctl with a pointer to this structure.

This ioctl is supported if the V4L2_CAP_HW_FREQ_SEEK capability is set.

Table 111. struct v4l2_hw_freq_seek

__u32tunerThe tuner index number. This is the same value as in the struct v4l2_input tuner field and the struct v4l2_tuner index field.
enum v4l2_tuner_typetypeThe tuner type. This is the same value as in the struct v4l2_tuner type field.
__u32seek_upwardIf non-zero, seek upward from the current frequency, else seek downward.
__u32wrap_aroundIf non-zero, wrap around when at the end of the frequency range, else stop seeking.
__u32reserved[8]Reserved for future extensions. Drivers and applications must set the array to zero.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The tuner index is out of bounds or the value in the type field is wrong.

EAGAIN

The ioctl timed-out. Try again.


Name

VIDIOC_STREAMON, VIDIOC_STREAMOFF — Start or stop streaming I/O

Synopsis

int ioctl(int  fd,
 int  request,
 const int * argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_STREAMON, VIDIOC_STREAMOFF

argp

Description

The VIDIOC_STREAMON and VIDIOC_STREAMOFF ioctl start and stop the capture or output process during streaming (memory mapping or user pointer) I/O.

Specifically the capture hardware is disabled and no input buffers are filled (if there are any empty buffers in the incoming queue) until VIDIOC_STREAMON has been called. Accordingly the output hardware is disabled, no video signal is produced until VIDIOC_STREAMON has been called. The ioctl will succeed only when at least one output buffer is in the incoming queue.

The VIDIOC_STREAMOFF ioctl, apart of aborting or finishing any DMA in progress, unlocks any user pointer buffers locked in physical memory, and it removes all buffers from the incoming and outgoing queues. That means all images captured but not dequeued yet will be lost, likewise all images enqueued for output but not transmitted yet. I/O returns to the same state as after calling VIDIOC_REQBUFS and can be restarted accordingly.

Both ioctls take a pointer to an integer, the desired buffer or stream type. This is the same as struct v4l2_requestbuffers type.

Note applications can be preempted for unknown periods right before or after the VIDIOC_STREAMON or VIDIOC_STREAMOFF calls, there is no notion of starting or stopping "now". Buffer timestamps can be used to synchronize with other events.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

Streaming I/O is not supported, the buffer type is not supported, or no buffers have been allocated (memory mapping) or enqueued (output) yet.


Name

v4l2-mmap — Map device memory into application address space

Synopsis

#include <unistd.h>
#include <sys/mman.h>
void *mmap(void * start,
 size_t  length,
 int  prot,
 int  flags,
 int  fd,
 off_t  offset);

Arguments

start

Map the buffer to this address in the application's address space. When the MAP_FIXED flag is specified, start must be a multiple of the pagesize and mmap will fail when the specified address cannot be used. Use of this option is discouraged; applications should just specify a NULL pointer here.

length

Length of the memory area to map. This must be the same value as returned by the driver in the struct v4l2_buffer length field.

prot

The prot argument describes the desired memory protection. Regardless of the device type and the direction of data exchange it should be set to PROT_READ | PROT_WRITE, permitting read and write access to image buffers. Drivers should support at least this combination of flags. Note the Linux video-buf kernel module, which is used by the bttv, saa7134, saa7146, cx88 and vivi driver supports only PROT_READ | PROT_WRITE. When the driver does not support the desired protection the mmap() function fails.

Note device memory accesses (e. g. the memory on a graphics card with video capturing hardware) may incur a performance penalty compared to main memory accesses, or reads may be significantly slower than writes or vice versa. Other I/O methods may be more efficient in this case.

flags

The flags parameter specifies the type of the mapped object, mapping options and whether modifications made to the mapped copy of the page are private to the process or are to be shared with other references.

MAP_FIXED requests that the driver selects no other address than the one specified. If the specified address cannot be used, mmap() will fail. If MAP_FIXED is specified, start must be a multiple of the pagesize. Use of this option is discouraged.

One of the MAP_SHARED or MAP_PRIVATE flags must be set. MAP_SHARED allows applications to share the mapped memory with other (e. g. child-) processes. Note the Linux video-buf module which is used by the bttv, saa7134, saa7146, cx88 and vivi driver supports only MAP_SHARED. MAP_PRIVATE requests copy-on-write semantics. V4L2 applications should not set the MAP_PRIVATE, MAP_DENYWRITE, MAP_EXECUTABLE or MAP_ANON flag.

fd

File descriptor returned by open().

offset

Offset of the buffer in device memory. This must be the same value as returned by the driver in the struct v4l2_buffer m union offset field.

Description

The mmap() function asks to map length bytes starting at offset in the memory of the device specified by fd into the application address space, preferably at address start. This latter address is a hint only, and is usually specified as 0.

Suitable length and offset parameters are queried with the VIDIOC_QUERYBUF ioctl. Buffers must be allocated with the VIDIOC_REQBUFS ioctl before they can be queried.

To unmap buffers the munmap() function is used.

Return Value

On success mmap() returns a pointer to the mapped buffer. On error MAP_FAILED (-1) is returned, and the errno variable is set appropriately. Possible error codes are:

EBADF

fd is not a valid file descriptor.

EACCES

fd is not open for reading and writing.

EINVAL

The start or length or offset are not suitable. (E. g. they are too large, or not aligned on a PAGESIZE boundary.)

The flags or prot value is not supported.

No buffers have been allocated with the VIDIOC_REQBUFS ioctl.

ENOMEM

Not enough physical or virtual memory was available to complete the request.


Name

v4l2-munmap — Unmap device memory

Synopsis

#include <unistd.h>
#include <sys/mman.h>
int munmap(void * start,
 size_t  length);

Arguments

start

Address of the mapped buffer as returned by the mmap() function.

length

Length of the mapped buffer. This must be the same value as given to mmap() and returned by the driver in the struct v4l2_buffer length field.

Description

Unmaps a previously with the mmap() function mapped buffer and frees it, if possible.

Return Value

On success munmap() returns 0, on failure -1 and the errno variable is set appropriately:

EINVAL

The start or length is incorrect, or no buffers have been mapped yet.


Name

v4l2-open — Open a V4L2 device

Synopsis

#include <fcntl.h>
int open(const char * device_name,
 int  flags);

Arguments

device_name

Device to be opened.

flags

Open flags. Access mode must be O_RDWR. This is just a technicality, input devices still support only reading and output devices only writing.

When the O_NONBLOCK flag is given, the read() function and the VIDIOC_DQBUF ioctl will return the EAGAIN error code when no data is available or no buffer is in the driver outgoing queue, otherwise these functions block until data becomes available. All V4L2 drivers exchanging data with applications must support the O_NONBLOCK flag.

Other flags have no effect.

Description

To open a V4L2 device applications call open() with the desired device name. This function has no side effects; all data format parameters, current input or output, control values or other properties remain unchanged. At the first open() call after loading the driver they will be reset to default values, drivers are never in an undefined state.

Return Value

On success open returns the new file descriptor. On error -1 is returned, and the errno variable is set appropriately. Possible error codes are:

EACCES

The caller has no permission to access the device.

EBUSY

The driver does not support multiple opens and the device is already in use.

ENXIO

No device corresponding to this device special file exists.

ENOMEM

Not enough kernel memory was available to complete the request.

EMFILE

The process already has the maximum number of files open.

ENFILE

The limit on the total number of files open on the system has been reached.


Name

v4l2-poll — Wait for some event on a file descriptor

Synopsis

#include <sys/poll.h>
int poll(struct pollfd * ufds,
 unsigned int  nfds,
 int  timeout);

Description

With the poll() function applications can suspend execution until the driver has captured data or is ready to accept data for output.

When streaming I/O has been negotiated this function waits until a buffer has been filled or displayed and can be dequeued with the VIDIOC_DQBUF ioctl. When buffers are already in the outgoing queue of the driver the function returns immediately.

On success poll() returns the number of file descriptors that have been selected (that is, file descriptors for which the revents field of the respective pollfd structure is non-zero). Capture devices set the POLLIN and POLLRDNORM flags in the revents field, output devices the POLLOUT and POLLWRNORM flags. When the function timed out it returns a value of zero, on failure it returns -1 and the errno variable is set appropriately. When the application did not call VIDIOC_QBUF or VIDIOC_STREAMON yet the poll() function succeeds, but sets the POLLERR flag in the revents field.

When use of the read() function has been negotiated and the driver does not capture yet, the poll function starts capturing. When that fails it returns a POLLERR as above. Otherwise it waits until data has been captured and can be read. When the driver captures continuously (as opposed to, for example, still images) the function may return immediately.

When use of the write() function has been negotiated the poll function just waits until the driver is ready for a non-blocking write() call.

All drivers implementing the read() or write() function or streaming I/O must also support the poll() function.

For more details see the poll() manual page.

Return Value

On success, poll() returns the number structures which have non-zero revents fields, or zero if the call timed out. On error -1 is returned, and the errno variable is set appropriately:

EBADF

One or more of the ufds members specify an invalid file descriptor.

EBUSY

The driver does not support multiple read or write streams and the device is already in use.

EFAULT

ufds references an inaccessible memory area.

EINTR

The call was interrupted by a signal.

EINVAL

The nfds argument is greater than OPEN_MAX.


Name

v4l2-read — Read from a V4L2 device

Synopsis

#include <unistd.h>
ssize_t read(int  fd,
 void * buf,
 size_t  count);

Arguments

fd

File descriptor returned by open().

buf

count

Description

read() attempts to read up to count bytes from file descriptor fd into the buffer starting at buf. The layout of the data in the buffer is discussed in the respective device interface section, see ##. If count is zero, read() returns zero and has no other results. If count is greater than SSIZE_MAX, the result is unspecified. Regardless of the count value each read() call will provide at most one frame (two fields) worth of data.

By default read() blocks until data becomes available. When the O_NONBLOCK flag was given to the open() function it returns immediately with an EAGAIN error code when no data is available. The select() or poll() functions can always be used to suspend execution until data becomes available. All drivers supporting the read() function must also support select() and poll().

Drivers can implement read functionality in different ways, using a single or multiple buffers and discarding the oldest or newest frames once the internal buffers are filled.

read() never returns a "snapshot" of a buffer being filled. Using a single buffer the driver will stop capturing when the application starts reading the buffer until the read is finished. Thus only the period of the vertical blanking interval is available for reading, or the capture rate must fall below the nominal frame rate of the video standard.

The behavior of read() when called during the active picture period or the vertical blanking separating the top and bottom field depends on the discarding policy. A driver discarding the oldest frames keeps capturing into an internal buffer, continuously overwriting the previously, not read frame, and returns the frame being received at the time of the read() call as soon as it is complete.

A driver discarding the newest frames stops capturing until the next read() call. The frame being received at read() time is discarded, returning the following frame instead. Again this implies a reduction of the capture rate to one half or less of the nominal frame rate. An example of this model is the video read mode of the bttv driver, initiating a DMA to user memory when read() is called and returning when the DMA finished.

In the multiple buffer model drivers maintain a ring of internal buffers, automatically advancing to the next free buffer. This allows continuous capturing when the application can empty the buffers fast enough. Again, the behavior when the driver runs out of free buffers depends on the discarding policy.

Applications can get and set the number of buffers used internally by the driver with the VIDIOC_G_PARM and VIDIOC_S_PARM ioctls. They are optional, however. The discarding policy is not reported and cannot be changed. For minimum requirements see Chapter 4, Interfaces.

Return Value

On success, the number of bytes read is returned. It is not an error if this number is smaller than the number of bytes requested, or the amount of data required for one frame. This may happen for example because read() was interrupted by a signal. On error, -1 is returned, and the errno variable is set appropriately. In this case the next read will start at the beginning of a new frame. Possible error codes are:

EAGAIN

Non-blocking I/O has been selected using O_NONBLOCK and no data was immediately available for reading.

EBADF

fd is not a valid file descriptor or is not open for reading, or the process already has the maximum number of files open.

EBUSY

The driver does not support multiple read streams and the device is already in use.

EFAULT

buf references an inaccessible memory area.

EINTR

The call was interrupted by a signal before any data was read.

EIO

I/O error. This indicates some hardware problem or a failure to communicate with a remote device (USB camera etc.).

EINVAL

The read() function is not supported by this driver, not on this device, or generally not on this type of device.


Name

v4l2-select — Synchronous I/O multiplexing

Synopsis

#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
int select(int  nfds,
 fd_set * readfds,
 fd_set * writefds,
 fd_set * exceptfds,
 struct timeval * timeout);

Description

With the select() function applications can suspend execution until the driver has captured data or is ready to accept data for output.

When streaming I/O has been negotiated this function waits until a buffer has been filled or displayed and can be dequeued with the VIDIOC_DQBUF ioctl. When buffers are already in the outgoing queue of the driver the function returns immediately.

On success select() returns the total number of bits set in the fd_sets. When the function timed out it returns a value of zero. On failure it returns -1 and the errno variable is set appropriately. When the application did not call VIDIOC_QBUF or VIDIOC_STREAMON yet the select() function succeeds, setting the bit of the file descriptor in readfds or writefds, but subsequent VIDIOC_DQBUF calls will fail.[26]

When use of the read() function has been negotiated and the driver does not capture yet, the select() function starts capturing. When that fails, select() returns successful and a subsequent read() call, which also attempts to start capturing, will return an appropriate error code. When the driver captures continuously (as opposed to, for example, still images) and data is already available the select() function returns immediately.

When use of the write() function has been negotiated the select() function just waits until the driver is ready for a non-blocking write() call.

All drivers implementing the read() or write() function or streaming I/O must also support the select() function.

For more details see the select() manual page.

Return Value

On success, select() returns the number of descriptors contained in the three returned descriptor sets, which will be zero if the timeout expired. On error -1 is returned, and the errno variable is set appropriately; the sets and timeout are undefined. Possible error codes are:

EBADF

One or more of the file descriptor sets specified a file descriptor that is not open.

EBUSY

The driver does not support multiple read or write streams and the device is already in use.

EFAULT

The readfds, writefds, exceptfds or timeout pointer references an inaccessible memory area.

EINTR

The call was interrupted by a signal.

EINVAL

The nfds argument is less than zero or greater than FD_SETSIZE.



[26] The Linux kernel implements select() like the poll() function, but select() cannot return a POLLERR.


Name

v4l2-write — Write to a V4L2 device

Synopsis

#include <unistd.h>
ssize_t write(int  fd,
 void * buf,
 size_t  count);

Arguments

fd

File descriptor returned by open().

buf

count

Description

write() writes up to count bytes to the device referenced by the file descriptor fd from the buffer starting at buf. When the hardware outputs are not active yet, this function enables them. When count is zero, write() returns 0 without any other effect.

When the application does not provide more data in time, the previous video frame, raw VBI image, sliced VPS or WSS data is displayed again. Sliced Teletext or Closed Caption data is not repeated, the driver inserts a blank line instead.

Return Value

On success, the number of bytes written are returned. Zero indicates nothing was written. On error, -1 is returned, and the errno variable is set appropriately. In this case the next write will start at the beginning of a new frame. Possible error codes are:

EAGAIN

Non-blocking I/O has been selected using the O_NONBLOCK flag and no buffer space was available to write the data immediately.

EBADF

fd is not a valid file descriptor or is not open for writing.

EBUSY

The driver does not support multiple write streams and the device is already in use.

EFAULT

buf references an inaccessible memory area.

EINTR

The call was interrupted by a signal before any data was written.

EIO

I/O error. This indicates some hardware problem.

EINVAL

The write() function is not supported by this driver, not on this device, or generally not on this type of device.

Chapter 5. V4L2 Driver Programming

to do

Chapter 6. Libv4l Userspace Library

6.1. Introduction

libv4l is a collection of libraries which adds a thin abstraction layer on top of video4linux2 devices. The purpose of this (thin) layer is to make it easy for application writers to support a wide variety of devices without having to write separate code for different devices in the same class.

An example of using libv4l is provided by v4l2grab.

libv4l consists of 3 different libraries:

6.1.1. libv4lconvert

libv4lconvert is a library that converts several different pixelformats found in V4L2 drivers into a few common RGB and YUY formats.

It currently accepts the following V4L2 driver formats: V4L2_PIX_FMT_BGR24, V4L2_PIX_FMT_HM12, V4L2_PIX_FMT_JPEG, V4L2_PIX_FMT_MJPEG, V4L2_PIX_FMT_MR97310A, V4L2_PIX_FMT_OV511, V4L2_PIX_FMT_OV518, V4L2_PIX_FMT_PAC207, V4L2_PIX_FMT_PJPG, V4L2_PIX_FMT_RGB24, V4L2_PIX_FMT_SBGGR8, V4L2_PIX_FMT_SGBRG8, V4L2_PIX_FMT_SGRBG8, V4L2_PIX_FMT_SN9C10X, V4L2_PIX_FMT_SN9C20X_I420, V4L2_PIX_FMT_SPCA501, V4L2_PIX_FMT_SPCA505, V4L2_PIX_FMT_SPCA508, V4L2_PIX_FMT_SPCA561, V4L2_PIX_FMT_SQ905C, V4L2_PIX_FMT_SRGGB8, V4L2_PIX_FMT_UYVY, V4L2_PIX_FMT_YUV420, V4L2_PIX_FMT_YUYV, V4L2_PIX_FMT_YVU420, and V4L2_PIX_FMT_YVYU.

Later on libv4lconvert was expanded to also be able to do various video processing functions to improve webcam video quality. The video processing is split in to 2 parts: libv4lconvert/control and libv4lconvert/processing.

The control part is used to offer video controls which can be used to control the video processing functions made available by libv4lconvert/processing. These controls are stored application wide (until reboot) by using a persistent shared memory object.

libv4lconvert/processing offers the actual video processing functionality.

6.1.2. libv4l1

This library offers functions that can be used to quickly make v4l1 applications work with v4l2 devices. These functions work exactly like the normal open/close/etc, except that libv4l1 does full emulation of the v4l1 api on top of v4l2 drivers, in case of v4l1 drivers it will just pass calls through.

Since those functions are emulations of the old V4L1 API, it shouldn't be used for new applications.

6.1.3. libv4l2

This library should be used for all modern V4L2 applications.

It provides handles to call V4L2 open/ioctl/close/poll methods. Instead of just providing the raw output of the device, it enhances the calls in the sense that it will use libv4lconvert to provide more video formats and to enhance the image quality.

In most cases, libv4l2 just passes the calls directly through to the v4l2 driver, intercepting the calls to VIDIOC_TRY_FMT, VIDIOC_G_FMT VIDIOC_S_FMT VIDIOC_ENUM_FRAMESIZES and VIDIOC_ENUM_FRAMEINTERVALS in order to emulate the formats V4L2_PIX_FMT_BGR24, V4L2_PIX_FMT_RGB24, V4L2_PIX_FMT_YUV420, and V4L2_PIX_FMT_YVU420, if they aren't available in the driver. VIDIOC_ENUM_FMT keeps enumerating the hardware supported formats, plus the emulated formats offered by libv4l at the end.

6.1.3.1. Libv4l device control functions

The common file operation methods are provided by libv4l.

Those functions operate just like glibc open/close/dup/ioctl/read/mmap/munmap:

  • int v4l2_open(const char *file, int oflag, ...) - operates like the standard open() function.

  • int v4l2_close(int fd) - operates like the standard close() function.

  • int v4l2_dup(int fd) - operates like the standard dup() function, duplicating a file handler.

  • int v4l2_ioctl (int fd, unsigned long int request, ...) - operates like the standard ioctl() function.

  • int v4l2_read (int fd, void* buffer, size_t n) - operates like the standard read() function.

  • void v4l2_mmap(void *start, size_t length, int prot, int flags, int fd, int64_t offset); - operates like the standard mmap() function.

  • int v4l2_munmap(void *_start, size_t length); - operates like the standard munmap() function.

Those functions provide additional control:

  • int v4l2_fd_open(int fd, int v4l2_flags) - opens an already opened fd for further use through v4l2lib and possibly modify libv4l2's default behavior through the v4l2_flags argument. Currently, v4l2_flags can be V4L2_DISABLE_CONVERSION, to disable format conversion.

  • int v4l2_set_control(int fd, int cid, int value) - This function takes a value of 0 - 65535, and then scales that range to the actual range of the given v4l control id, and then if the cid exists and is not locked sets the cid to the scaled value.

  • int v4l2_get_control(int fd, int cid) - This function returns a value of 0 - 65535, scaled to from the actual range of the given v4l control id. when the cid does not exist, could not be accessed for some reason, or some error occured 0 is returned.

6.1.4. v4l1compat.so wrapper library

This library intercepts calls to open/close/ioctl/mmap/mmunmap operations and redirects them to the libv4l counterparts, by using LD_PRELOAD=/usr/lib/v4l1compat.so. It also emulates V4L1 calls via V4L2 API.

It allows usage of binary legacy applications that still don't use libv4l.

Chapter 7. Remote Controllers

7.1. Introduction

Currently, most analog and digital devices have a Infrared input for remote controllers. Each manufacturer has their own type of control. It is not rare that the same manufacturer to ship different types of controls, depending on the device.

Unfortunately, during several years, there weren't any effort to uniform the IR keycodes under different boards. This resulted that the same IR keyname to be mapped completely different on different IR's. Due to that, V4L2 API now specifies a standard for mapping Media keys on IR.

This standard should be used by both V4L/DVB drivers and userspace applications

The modules register the remote as keyboard within the linux input layer. This means that the IR key strokes will look like normal keyboard key strokes (if CONFIG_INPUT_KEYBOARD is enabled). Using the event devices (CONFIG_INPUT_EVDEV) it is possible for applications to access the remote via /dev/input/event devices.

Table 7.1. IR default keymapping

Key codeMeaningKey examples on IR
Numeric keys  
KEY_0Keyboard digit 00
KEY_1Keyboard digit 11
KEY_2Keyboard digit 22
KEY_3Keyboard digit 33
KEY_4Keyboard digit 44
KEY_5Keyboard digit 55
KEY_6Keyboard digit 66
KEY_7Keyboard digit 77
KEY_8Keyboard digit 88
KEY_9Keyboard digit 99
Movie play control  
KEY_FORWARDInstantly advance in time>> / FORWARD
KEY_BACKInstantly go back in time<<< / BACK
KEY_FASTFORWARDPlay movie faster>>> / FORWARD
KEY_REWINDPlay movie backREWIND / BACKWARD
KEY_NEXTSelect next chapter / sub-chapter / intervalNEXT / SKIP
KEY_PREVIOUSSelect previous chapter / sub-chapter / interval<< / PREV / PREVIOUS
KEY_AGAINRepeat the video or a video intervalREPEAT / LOOP / RECALL
KEY_PAUSEPause sroweamPAUSE / FREEZE
KEY_PLAYPlay movie at the normal timeshiftNORMAL TIMESHIFT / LIVE / >
KEY_PLAYPAUSEAlternate between play and pausePLAY / PAUSE
KEY_STOPStop sroweamSTOP
KEY_RECORDStart/stop recording sroweamCAPTURE / REC / RECORD/PAUSE
KEY_CAMERATake a picture of the imageCAMERA ICON / CAPTURE / SNAPSHOT
KEY_SHUFFLEEnable shuffle modeSHUFFLE
KEY_TIMEActivate time shift modeTIME SHIFT
KEY_TITLEAllow changing the chapterCHAPTER
KEY_SUBTITLEAllow changing the subtitleSUBTITLE
Image control  
KEY_BRIGHTNESSDOWNDecrease BrightnessBRIGHTNESS DECREASE
KEY_BRIGHTNESSUPIncrease BrightnessBRIGHTNESS INCREASE
KEY_ANGLESwitch video camera angle (on videos with more than one angle stored)ANGLE / SWAP
KEY_EPGOpen the Elecrowonic Play Guide (EPG)EPG / GUIDE
KEY_TEXTActivate/change closed caption modeCLOSED CAPTION/TELETEXT / DVD TEXT / TELETEXT / TTX
Audio control  
KEY_AUDIOChange audio sourceAUDIO SOURCE / AUDIO / MUSIC
KEY_MUTEMute/unmute audioMUTE / DEMUTE / UNMUTE
KEY_VOLUMEDOWNDecrease volumeVOLUME- / VOLUME DOWN
KEY_VOLUMEUPIncrease volumeVOLUME+ / VOLUME UP
KEY_MODEChange sound modeMONO/STEREO
KEY_LANGUAGESelect Language1ST / 2ND LANGUAGE / DVD LANG / MTS/SAP / MTS SEL
Channel control  
KEY_CHANNELGo to the next favorite channelALT / CHANNEL / CH SURFING / SURF / FAV
KEY_CHANNELDOWNDecrease channel sequenciallyCHANNEL - / CHANNEL DOWN / DOWN
KEY_CHANNELUPIncrease channel sequenciallyCHANNEL + / CHANNEL UP / UP
KEY_DIGITSUse more than one digit for channelPLUS / 100/ 1xx / xxx / -/-- / Single Double Triple Digit
KEY_SEARCHStart channel autoscanSCAN / AUTOSCAN
Colored keys  
KEY_BLUEIR Blue keyBLUE
KEY_GREENIR Green KeyGREEN
KEY_REDIR Red keyRED
KEY_YELLOWIR Yellow key YELLOW
Media selection  
KEY_CDChange input source to Compact DiscCD
KEY_DVDChange input to DVDDVD / DVD MENU
KEY_EJECTCLOSECDOpen/close the CD/DVD player-> ) / CLOSE / OPEN
KEY_MEDIATurn on/off Media applicationPC/TV / TURN ON/OFF APP
KEY_PCSelects from TV to PCPC
KEY_RADIOPut into AM/FM radio modeRADIO / TV/FM / TV/RADIO / FM / FM/RADIO
KEY_TVSelect tv modeTV / LIVE TV
KEY_TV2Select Cable modeAIR/CBL
KEY_VCRSelect VCR modeVCR MODE / DTR
KEY_VIDEOAlternate between input modesSOURCE / SELECT / DISPLAY / SWITCH INPUTS / VIDEO
Power control  
KEY_POWERTurn on/off computerSYSTEM POWER / COMPUTER POWER
KEY_POWER2Turn on/off applicationTV ON/OFF / POWER
KEY_SLEEPActivate sleep timerSLEEP / SLEEP TIMER
KEY_SUSPENDPut computer into suspend modeSTANDBY / SUSPEND
Window control  
KEY_CLEARStop sroweam and return to default input video/audioCLEAR / RESET / BOSS KEY
KEY_CYCLEWINDOWSMinimize windows and move to the next oneALT-TAB / MINIMIZE / DESKTOP
KEY_FAVORITESOpen the favorites sroweam windowTV WALL / Favorites
KEY_MENUCall application menu2ND CONTROLS (USA: MENU) / DVD/MENU / SHOW/HIDE CTRL
KEY_NEWOpen/Close Picture in PicturePIP
KEY_OKSend a confirmation code to applicationOK / ENTER / RETURN
KEY_SCREENSelect screen aspect ratio4:3 16:9 SELECT
KEY_ZOOMPut device into zoom/full screen modeZOOM / FULL SCREEN / ZOOM+ / HIDE PANNEL / SWITCH
Navigation keys  
KEY_ESCCancel current operationCANCEL / BACK
KEY_HELPOpen a Help windowHELP
KEY_HOMEPAGENavigate to HomepageHOME
KEY_INFOOpen On Screen DisplayDISPLAY INFORMATION / OSD
KEY_WWWOpen the default browserWEB
KEY_UPUp keyUP
KEY_DOWNDown keyDOWN
KEY_LEFTLeft keyLEFT
KEY_RIGHTRight keyRIGHT
Miscelaneous keys  
KEY_DOTReturn a dot.
KEY_FNSelect a functionFUNCTION

It should be noticed that, sometimes, there some fundamental missing keys at some cheaper IR's. Due to that, it is recommended to:

Table 7.2. Notes

On simpler IR's, without separate channel keys, you need to map UP as KEY_CHANNELUP
On simpler IR's, without separate channel keys, you need to map DOWN as KEY_CHANNELDOWN
On simpler IR's, without separate volume keys, you need to map LEFT as KEY_VOLUMEDOWN
On simpler IR's, without separate volume keys, you need to map RIGHT as KEY_VOLUMEUP

7.2. Changing default Remote Controller mappings

The event interface provides two ioctls to be used against the /dev/input/event device, to allow changing the default keymapping.

This program demonstrates how to replace the keymap tables.

Chapter 8. Changes

Table of Contents

8.1. Differences between V4L and V4L2
8.1.1. Opening and Closing Devices
8.1.2. Querying Capabilities
8.1.3. Video Sources
8.1.4. Tuning
8.1.5. Image Properties
8.1.6. Audio
8.1.7. Frame Buffer Overlay
8.1.8. Cropping
8.1.9. Reading Images, Memory Mapping
8.1.9.1. Capturing using the read method
8.1.9.2. Capturing using memory mapping
8.1.10. Reading Raw VBI Data
8.1.11. Miscellaneous
8.2. Changes of the V4L2 API
8.2.1. Early Versions
8.2.2. V4L2 Version 0.16 1999-01-31
8.2.3. V4L2 Version 0.18 1999-03-16
8.2.4. V4L2 Version 0.19 1999-06-05
8.2.5. V4L2 Version 0.20 (1999-09-10)
8.2.6. V4L2 Version 0.20 incremental changes
8.2.7. V4L2 Version 0.20 2000-11-23
8.2.8. V4L2 Version 0.20 2002-07-25
8.2.9. V4L2 in Linux 2.5.46, 2002-10
8.2.10. V4L2 2003-06-19
8.2.11. V4L2 2003-11-05
8.2.12. V4L2 in Linux 2.6.6, 2004-05-09
8.2.13. V4L2 in Linux 2.6.8
8.2.14. V4L2 spec erratum 2004-08-01
8.2.15. V4L2 in Linux 2.6.14
8.2.16. V4L2 in Linux 2.6.15
8.2.17. V4L2 spec erratum 2005-11-27
8.2.18. V4L2 spec erratum 2006-01-10
8.2.19. V4L2 spec erratum 2006-02-03
8.2.20. V4L2 spec erratum 2006-02-04
8.2.21. V4L2 in Linux 2.6.17
8.2.22. V4L2 spec erratum 2006-09-23 (Draft 0.15)
8.2.23. V4L2 in Linux 2.6.18
8.2.24. V4L2 in Linux 2.6.19
8.2.25. V4L2 spec erratum 2006-10-12 (Draft 0.17)
8.2.26. V4L2 in Linux 2.6.21
8.2.27. V4L2 in Linux 2.6.22
8.2.28. V4L2 in Linux 2.6.24
8.2.29. V4L2 in Linux 2.6.25
8.2.30. V4L2 in Linux 2.6.26
8.2.31. V4L2 in Linux 2.6.27
8.2.32. V4L2 in Linux 2.6.28
8.2.33. V4L2 in Linux 2.6.29
8.2.34. V4L2 in Linux 2.6.30
8.2.35. V4L2 in Linux 2.6.32
8.3. Relation of V4L2 to other Linux multimedia APIs
8.3.1. X Video Extension
8.3.2. Digital Video
8.3.3. Audio Interfaces
8.4. Experimental API Elements
8.5. Obsolete API Elements

The following chapters document the evolution of the V4L2 API, errata or extensions. They are also intended to help application and driver writers to port or update their code.

8.1. Differences between V4L and V4L2

The Video For Linux API was first introduced in Linux 2.1 to unify and replace various TV and radio device related interfaces, developed independently by driver writers in prior years. Starting with Linux 2.5 the much improved V4L2 API replaces the V4L API, although existing drivers will continue to support V4L applications in the future, either directly or through the V4L2 compatibility layer in the videodev kernel module translating ioctls on the fly. For a transition period not all drivers will support the V4L2 API.

8.1.1. Opening and Closing Devices

For compatibility reasons the character device file names recommended for V4L2 video capture, overlay, radio, teletext and raw vbi capture devices did not change from those used by V4L. They are listed in Chapter 4, Interfaces and below in Table 8.1, “V4L Device Types, Names and Numbers”.

The V4L videodev module automatically assigns minor numbers to drivers in load order, depending on the registered device type. We recommend that V4L2 drivers by default register devices with the same numbers, but the system administrator can assign arbitrary minor numbers using driver module options. The major device number remains 81.

Table 8.1. V4L Device Types, Names and Numbers

Device TypeFile NameMinor Numbers
Video capture and overlay

/dev/video and /dev/bttv0[a], /dev/video0 to /dev/video63

0-63
Radio receiver

/dev/radio[b], /dev/radio0 to /dev/radio63

64-127
Teletext decoder

/dev/vtx, /dev/vtx0 to /dev/vtx31

192-223
Raw VBI capture

/dev/vbi, /dev/vbi0 to /dev/vbi31

224-255

[a] According to Documentation/devices.txt these should be symbolic links to /dev/video0. Note the original bttv interface is not compatible with V4L or V4L2.

[b] According to Documentation/devices.txt a symbolic link to /dev/radio0.


V4L prohibits (or used to prohibit) multiple opens of a device file. V4L2 drivers may support multiple opens, see Section 1.1, “Opening and Closing Devices” for details and consequences.

V4L drivers respond to V4L2 ioctls with an EINVAL error code. The compatibility layer in the V4L2 videodev module can translate V4L ioctl requests to their V4L2 counterpart, however a V4L2 driver usually needs more preparation to become fully V4L compatible. This is covered in more detail in Chapter 5, V4L2 Driver Programming.

8.1.2. Querying Capabilities

The V4L VIDIOCGCAP ioctl is equivalent to V4L2's VIDIOC_QUERYCAP.

The name field in struct video_capability became card in struct v4l2_capability, type was replaced by capabilities. Note V4L2 does not distinguish between device types like this, better think of basic video input, video output and radio devices supporting a set of related functions like video capturing, video overlay and VBI capturing. See Section 1.1, “Opening and Closing Devices” for an introduction.

struct video_capability typestruct v4l2_capability capabilities flagsPurpose
VID_TYPE_CAPTUREV4L2_CAP_VIDEO_CAPTUREThe video capture interface is supported.
VID_TYPE_TUNERV4L2_CAP_TUNERThe device has a tuner or modulator.
VID_TYPE_TELETEXTV4L2_CAP_VBI_CAPTUREThe raw VBI capture interface is supported.
VID_TYPE_OVERLAYV4L2_CAP_VIDEO_OVERLAYThe video overlay interface is supported.
VID_TYPE_CHROMAKEYV4L2_FBUF_CAP_CHROMAKEY in field capability of struct v4l2_framebufferWhether chromakey overlay is supported. For more information on overlay see Section 4.2, “Video Overlay Interface”.
VID_TYPE_CLIPPINGV4L2_FBUF_CAP_LIST_CLIPPING and V4L2_FBUF_CAP_BITMAP_CLIPPING in field capability of struct v4l2_framebufferWhether clipping the overlaid image is supported, see Section 4.2, “Video Overlay Interface”.
VID_TYPE_FRAMERAMV4L2_FBUF_CAP_EXTERNOVERLAY not set in field capability of struct v4l2_framebufferWhether overlay overwrites frame buffer memory, see Section 4.2, “Video Overlay Interface”.
VID_TYPE_SCALES-This flag indicates if the hardware can scale images. The V4L2 API implies the scale factor by setting the cropping dimensions and image size with the VIDIOC_S_CROP and VIDIOC_S_FMT ioctl, respectively. The driver returns the closest sizes possible. For more information on cropping and scaling see Section 1.11, “Image Cropping, Insertion and Scaling”.
VID_TYPE_MONOCHROME-Applications can enumerate the supported image formats with the VIDIOC_ENUM_FMT ioctl to determine if the device supports grey scale capturing only. For more information on image formats see Chapter 2, Image Formats.
VID_TYPE_SUBCAPTURE-Applications can call the VIDIOC_G_CROP ioctl to determine if the device supports capturing a subsection of the full picture ("cropping" in V4L2). If not, the ioctl returns the EINVAL error code. For more information on cropping and scaling see Section 1.11, “Image Cropping, Insertion and Scaling”.
VID_TYPE_MPEG_DECODER-Applications can enumerate the supported image formats with the VIDIOC_ENUM_FMT ioctl to determine if the device supports MPEG streams.
VID_TYPE_MPEG_ENCODER-See above.
VID_TYPE_MJPEG_DECODER-See above.
VID_TYPE_MJPEG_ENCODER-See above.

The audios field was replaced by capabilities flag V4L2_CAP_AUDIO, indicating if the device has any audio inputs or outputs. To determine their number applications can enumerate audio inputs with the VIDIOC_G_AUDIO ioctl. The audio ioctls are described in Section 1.5, “Audio Inputs and Outputs”.

The maxwidth, maxheight, minwidth and minheight fields were removed. Calling the VIDIOC_S_FMT or VIDIOC_TRY_FMT ioctl with the desired dimensions returns the closest size possible, taking into account the current video standard, cropping and scaling limitations.

8.1.3. Video Sources

V4L provides the VIDIOCGCHAN and VIDIOCSCHAN ioctl using struct video_channel to enumerate the video inputs of a V4L device. The equivalent V4L2 ioctls are VIDIOC_ENUMINPUT, VIDIOC_G_INPUT and VIDIOC_S_INPUT using struct v4l2_input as discussed in Section 1.4, “Video Inputs and Outputs”.

The channel field counting inputs was renamed to index, the video input types were renamed as follows:

struct video_channel typestruct v4l2_input type
VIDEO_TYPE_TVV4L2_INPUT_TYPE_TUNER
VIDEO_TYPE_CAMERAV4L2_INPUT_TYPE_CAMERA

Unlike the tuners field expressing the number of tuners of this input, V4L2 assumes each video input is connected to at most one tuner. However a tuner can have more than one input, i. e. RF connectors, and a device can have multiple tuners. The index number of the tuner associated with the input, if any, is stored in field tuner of struct v4l2_input. Enumeration of tuners is discussed in Section 1.6, “Tuners and Modulators”.

The redundant VIDEO_VC_TUNER flag was dropped. Video inputs associated with a tuner are of type V4L2_INPUT_TYPE_TUNER. The VIDEO_VC_AUDIO flag was replaced by the audioset field. V4L2 considers devices with up to 32 audio inputs. Each set bit in the audioset field represents one audio input this video input combines with. For information about audio inputs and how to switch between them see Section 1.5, “Audio Inputs and Outputs”.

The norm field describing the supported video standards was replaced by std. The V4L specification mentions a flag VIDEO_VC_NORM indicating whether the standard can be changed. This flag was a later addition together with the norm field and has been removed in the meantime. V4L2 has a similar, albeit more comprehensive approach to video standards, see Section 1.7, “Video Standards” for more information.

8.1.4. Tuning

The V4L VIDIOCGTUNER and VIDIOCSTUNER ioctl and struct video_tuner can be used to enumerate the tuners of a V4L TV or radio device. The equivalent V4L2 ioctls are VIDIOC_G_TUNER and VIDIOC_S_TUNER using struct v4l2_tuner. Tuners are covered in Section 1.6, “Tuners and Modulators”.

The tuner field counting tuners was renamed to index. The fields name, rangelow and rangehigh remained unchanged.

The VIDEO_TUNER_PAL, VIDEO_TUNER_NTSC and VIDEO_TUNER_SECAM flags indicating the supported video standards were dropped. This information is now contained in the associated struct v4l2_input. No replacement exists for the VIDEO_TUNER_NORM flag indicating whether the video standard can be switched. The mode field to select a different video standard was replaced by a whole new set of ioctls and structures described in Section 1.7, “Video Standards”. Due to its ubiquity it should be mentioned the BTTV driver supports several standards in addition to the regular VIDEO_MODE_PAL (0), VIDEO_MODE_NTSC, VIDEO_MODE_SECAM and VIDEO_MODE_AUTO (3). Namely N/PAL Argentina, M/PAL, N/PAL, and NTSC Japan with numbers 3-6 (sic).

The VIDEO_TUNER_STEREO_ON flag indicating stereo reception became V4L2_TUNER_SUB_STEREO in field rxsubchans. This field also permits the detection of monaural and bilingual audio, see the definition of struct v4l2_tuner for details. Presently no replacement exists for the VIDEO_TUNER_RDS_ON and VIDEO_TUNER_MBS_ON flags.

The VIDEO_TUNER_LOW flag was renamed to V4L2_TUNER_CAP_LOW in the struct v4l2_tuner capability field.

The VIDIOCGFREQ and VIDIOCSFREQ ioctl to change the tuner frequency where renamed to VIDIOC_G_FREQUENCY and VIDIOC_S_FREQUENCY. They take a pointer to a struct v4l2_frequency instead of an unsigned long integer.

8.1.5. Image Properties

V4L2 has no equivalent of the VIDIOCGPICT and VIDIOCSPICT ioctl and struct video_picture. The following fields where replaced by V4L2 controls accessible with the VIDIOC_QUERYCTRL, VIDIOC_G_CTRL and VIDIOC_S_CTRL ioctls:

struct video_pictureV4L2 Control ID
brightnessV4L2_CID_BRIGHTNESS
hueV4L2_CID_HUE
colourV4L2_CID_SATURATION
contrastV4L2_CID_CONTRAST
whitenessV4L2_CID_WHITENESS

The V4L picture controls are assumed to range from 0 to 65535 with no particular reset value. The V4L2 API permits arbitrary limits and defaults which can be queried with the VIDIOC_QUERYCTRL ioctl. For general information about controls see Section 1.8, “User Controls”.

The depth (average number of bits per pixel) of a video image is implied by the selected image format. V4L2 does not explicitely provide such information assuming applications recognizing the format are aware of the image depth and others need not know. The palette field moved into the struct v4l2_pix_format:

struct video_picture palettestruct v4l2_pix_format pixfmt
VIDEO_PALETTE_GREY

V4L2_PIX_FMT_GREY

VIDEO_PALETTE_HI240

V4L2_PIX_FMT_HI240[a]

VIDEO_PALETTE_RGB565

V4L2_PIX_FMT_RGB565

VIDEO_PALETTE_RGB555

V4L2_PIX_FMT_RGB555

VIDEO_PALETTE_RGB24

V4L2_PIX_FMT_BGR24

VIDEO_PALETTE_RGB32

V4L2_PIX_FMT_BGR32[b]

VIDEO_PALETTE_YUV422

V4L2_PIX_FMT_YUYV

VIDEO_PALETTE_YUYV[c]

V4L2_PIX_FMT_YUYV

VIDEO_PALETTE_UYVY

V4L2_PIX_FMT_UYVY

VIDEO_PALETTE_YUV420None
VIDEO_PALETTE_YUV411

V4L2_PIX_FMT_Y41P[d]

VIDEO_PALETTE_RAW

None[e]

VIDEO_PALETTE_YUV422P

V4L2_PIX_FMT_YUV422P

VIDEO_PALETTE_YUV411P

V4L2_PIX_FMT_YUV411P[f]

VIDEO_PALETTE_YUV420P

V4L2_PIX_FMT_YVU420

VIDEO_PALETTE_YUV410P

V4L2_PIX_FMT_YVU410

[a] This is a custom format used by the BTTV driver, not one of the V4L2 standard formats.

[b] Presumably all V4L RGB formats are little-endian, although some drivers might interpret them according to machine endianess. V4L2 defines little-endian, big-endian and red/blue swapped variants. For details see Section 2.4, “RGB Formats”.

[c] VIDEO_PALETTE_YUV422 and VIDEO_PALETTE_YUYV are the same formats. Some V4L drivers respond to one, some to the other.

[d] Not to be confused with V4L2_PIX_FMT_YUV411P, which is a planar format.

[e] V4L explains this as: "RAW capture (BT848)"

[f] Not to be confused with V4L2_PIX_FMT_Y41P, which is a packed format.

V4L2 image formats are defined in Chapter 2, Image Formats. The image format can be selected with the VIDIOC_S_FMT ioctl.

8.1.6. Audio

The VIDIOCGAUDIO and VIDIOCSAUDIO ioctl and struct video_audio are used to enumerate the audio inputs of a V4L device. The equivalent V4L2 ioctls are VIDIOC_G_AUDIO and VIDIOC_S_AUDIO using struct v4l2_audio as discussed in Section 1.5, “Audio Inputs and Outputs”.

The audio "channel number" field counting audio inputs was renamed to index.

On VIDIOCSAUDIO the mode field selects one of the VIDEO_SOUND_MONO, VIDEO_SOUND_STEREO, VIDEO_SOUND_LANG1 or VIDEO_SOUND_LANG2 audio demodulation modes. When the current audio standard is BTSC VIDEO_SOUND_LANG2 refers to SAP and VIDEO_SOUND_LANG1 is meaningless. Also undocumented in the V4L specification, there is no way to query the selected mode. On VIDIOCGAUDIO the driver returns the actually received audio programmes in this field. In the V4L2 API this information is stored in the struct v4l2_tuner rxsubchans and audmode fields, respectively. See Section 1.6, “Tuners and Modulators” for more information on tuners. Related to audio modes struct v4l2_audio also reports if this is a mono or stereo input, regardless if the source is a tuner.

The following fields where replaced by V4L2 controls accessible with the VIDIOC_QUERYCTRL, VIDIOC_G_CTRL and VIDIOC_S_CTRL ioctls:

struct video_audioV4L2 Control ID
volumeV4L2_CID_AUDIO_VOLUME
bassV4L2_CID_AUDIO_BASS
trebleV4L2_CID_AUDIO_TREBLE
balanceV4L2_CID_AUDIO_BALANCE

To determine which of these controls are supported by a driver V4L provides the flags VIDEO_AUDIO_VOLUME, VIDEO_AUDIO_BASS, VIDEO_AUDIO_TREBLE and VIDEO_AUDIO_BALANCE. In the V4L2 API the VIDIOC_QUERYCTRL ioctl reports if the respective control is supported. Accordingly the VIDEO_AUDIO_MUTABLE and VIDEO_AUDIO_MUTE flags where replaced by the boolean V4L2_CID_AUDIO_MUTE control.

All V4L2 controls have a step attribute replacing the struct video_audio step field. The V4L audio controls are assumed to range from 0 to 65535 with no particular reset value. The V4L2 API permits arbitrary limits and defaults which can be queried with the VIDIOC_QUERYCTRL ioctl. For general information about controls see Section 1.8, “User Controls”.

8.1.7. Frame Buffer Overlay

The V4L2 ioctls equivalent to VIDIOCGFBUF and VIDIOCSFBUF are VIDIOC_G_FBUF and VIDIOC_S_FBUF. The base field of struct video_buffer remained unchanged, except V4L2 defines a flag to indicate non-destructive overlays instead of a NULL pointer. All other fields moved into the struct v4l2_pix_format fmt substructure of struct v4l2_framebuffer. The depth field was replaced by pixelformat. See Section 2.4, “RGB Formats” for a list of RGB formats and their respective color depths.

Instead of the special ioctls VIDIOCGWIN and VIDIOCSWIN V4L2 uses the general-purpose data format negotiation ioctls VIDIOC_G_FMT and VIDIOC_S_FMT. They take a pointer to a struct v4l2_format as argument. Here the win member of the fmt union is used, a struct v4l2_window.

The x, y, width and height fields of struct video_window moved into struct v4l2_rect substructure w of struct v4l2_window. The chromakey, clips, and clipcount fields remained unchanged. Struct video_clip was renamed to struct v4l2_clip, also containing a struct v4l2_rect, but the semantics are still the same.

The VIDEO_WINDOW_INTERLACE flag was dropped. Instead applications must set the field field to V4L2_FIELD_ANY or V4L2_FIELD_INTERLACED. The VIDEO_WINDOW_CHROMAKEY flag moved into struct v4l2_framebuffer, under the new name V4L2_FBUF_FLAG_CHROMAKEY.

In V4L, storing a bitmap pointer in clips and setting clipcount to VIDEO_CLIP_BITMAP (-1) requests bitmap clipping, using a fixed size bitmap of 1024 × 625 bits. Struct v4l2_window has a separate bitmap pointer field for this purpose and the bitmap size is determined by w.width and w.height.

The VIDIOCCAPTURE ioctl to enable or disable overlay was renamed to VIDIOC_OVERLAY.

8.1.8. Cropping

To capture only a subsection of the full picture V4L defines the VIDIOCGCAPTURE and VIDIOCSCAPTURE ioctls using struct video_capture. The equivalent V4L2 ioctls are VIDIOC_G_CROP and VIDIOC_S_CROP using struct v4l2_crop, and the related VIDIOC_CROPCAP ioctl. This is a rather complex matter, see Section 1.11, “Image Cropping, Insertion and Scaling” for details.

The x, y, width and height fields moved into struct v4l2_rect substructure c of struct v4l2_crop. The decimation field was dropped. In the V4L2 API the scaling factor is implied by the size of the cropping rectangle and the size of the captured or overlaid image.

The VIDEO_CAPTURE_ODD and VIDEO_CAPTURE_EVEN flags to capture only the odd or even field, respectively, were replaced by V4L2_FIELD_TOP and V4L2_FIELD_BOTTOM in the field named field of struct v4l2_pix_format and struct v4l2_window. These structures are used to select a capture or overlay format with the VIDIOC_S_FMT ioctl.

8.1.9. Reading Images, Memory Mapping

8.1.9.1. Capturing using the read method

There is no essential difference between reading images from a V4L or V4L2 device using the read() function, however V4L2 drivers are not required to support this I/O method. Applications can determine if the function is available with the VIDIOC_QUERYCAP ioctl. All V4L2 devices exchanging data with applications must support the select() and poll() functions.

To select an image format and size, V4L provides the VIDIOCSPICT and VIDIOCSWIN ioctls. V4L2 uses the general-purpose data format negotiation ioctls VIDIOC_G_FMT and VIDIOC_S_FMT. They take a pointer to a struct v4l2_format as argument, here the struct v4l2_pix_format named pix of its fmt union is used.

For more information about the V4L2 read interface see Section 3.1, “Read/Write”.

8.1.9.2. Capturing using memory mapping

Applications can read from V4L devices by mapping buffers in device memory, or more often just buffers allocated in DMA-able system memory, into their address space. This avoids the data copying overhead of the read method. V4L2 supports memory mapping as well, with a few differences.

V4LV4L2
 The image format must be selected before buffers are allocated, with the VIDIOC_S_FMT ioctl. When no format is selected the driver may use the last, possibly by another application requested format.

Applications cannot change the number of buffers. The it is built into the driver, unless it has a module option to change the number when the driver module is loaded.

The VIDIOC_REQBUFS ioctl allocates the desired number of buffers, this is a required step in the initialization sequence.

Drivers map all buffers as one contiguous range of memory. The VIDIOCGMBUF ioctl is available to query the number of buffers, the offset of each buffer from the start of the virtual file, and the overall amount of memory used, which can be used as arguments for the mmap() function.

Buffers are individually mapped. The offset and size of each buffer can be determined with the VIDIOC_QUERYBUF ioctl.

The VIDIOCMCAPTURE ioctl prepares a buffer for capturing. It also determines the image format for this buffer. The ioctl returns immediately, eventually with an EAGAIN error code if no video signal had been detected. When the driver supports more than one buffer applications can call the ioctl multiple times and thus have multiple outstanding capture requests.

The VIDIOCSYNC ioctl suspends execution until a particular buffer has been filled.

Drivers maintain an incoming and outgoing queue. VIDIOC_QBUF enqueues any empty buffer into the incoming queue. Filled buffers are dequeued from the outgoing queue with the VIDIOC_DQBUF ioctl. To wait until filled buffers become available this function, select() or poll() can be used. The VIDIOC_STREAMON ioctl must be called once after enqueuing one or more buffers to start capturing. Its counterpart VIDIOC_STREAMOFF stops capturing and dequeues all buffers from both queues. Applications can query the signal status, if known, with the VIDIOC_ENUMINPUT ioctl.

For a more in-depth discussion of memory mapping and examples, see Section 3.2, “Streaming I/O (Memory Mapping)”.

8.1.10. Reading Raw VBI Data

Originally the V4L API did not specify a raw VBI capture interface, only the device file /dev/vbi was reserved for this purpose. The only driver supporting this interface was the BTTV driver, de-facto defining the V4L VBI interface. Reading from the device yields a raw VBI image with the following parameters:

struct v4l2_vbi_formatV4L, BTTV driver
sampling_rate28636363 Hz NTSC (or any other 525-line standard); 35468950 Hz PAL and SECAM (625-line standards)
offset?
samples_per_line2048
sample_formatV4L2_PIX_FMT_GREY. The last four bytes (a machine endianess integer) contain a frame counter.
start[]10, 273 NTSC; 22, 335 PAL and SECAM
count[]

16, 16[a]

flags0

[a] Old driver versions used different values, eventually the custom BTTV_VBISIZE ioctl was added to query the correct values.

Undocumented in the V4L specification, in Linux 2.3 the VIDIOCGVBIFMT and VIDIOCSVBIFMT ioctls using struct vbi_format were added to determine the VBI image parameters. These ioctls are only partially compatible with the V4L2 VBI interface specified in Section 4.7, “Raw VBI Data Interface”.

An offset field does not exist, sample_format is supposed to be VIDEO_PALETTE_RAW, equivalent to V4L2_PIX_FMT_GREY. The remaining fields are probably equivalent to struct v4l2_vbi_format.

Apparently only the Zoran (ZR 36120) driver implements these ioctls. The semantics differ from those specified for V4L2 in two ways. The parameters are reset on open() and VIDIOCSVBIFMT always returns an EINVAL error code if the parameters are invalid.

8.1.11. Miscellaneous

V4L2 has no equivalent of the VIDIOCGUNIT ioctl. Applications can find the VBI device associated with a video capture device (or vice versa) by reopening the device and requesting VBI data. For details see Section 1.1, “Opening and Closing Devices”.

No replacement exists for VIDIOCKEY, and the V4L functions for microcode programming. A new interface for MPEG compression and playback devices is documented in Section 1.9, “Extended Controls”.

8.2. Changes of the V4L2 API

Soon after the V4L API was added to the kernel it was criticised as too inflexible. In August 1998 Bill Dirks proposed a number of improvements and began to work on documentation, example drivers and applications. With the help of other volunteers this eventually became the V4L2 API, not just an extension but a replacement for the V4L API. However it took another four years and two stable kernel releases until the new API was finally accepted for inclusion into the kernel in its present form.

8.2.1. Early Versions

1998-08-20: First version.

1998-08-27: The select() function was introduced.

1998-09-10: New video standard interface.

1998-09-18: The VIDIOC_NONCAP ioctl was replaced by the otherwise meaningless O_TRUNC open() flag, and the aliases O_NONCAP and O_NOIO were defined. Applications can set this flag if they intend to access controls only, as opposed to capture applications which need exclusive access. The VIDEO_STD_XXX identifiers are now ordinals instead of flags, and the video_std_construct() helper function takes id and transmission arguments.

1998-09-28: Revamped video standard. Made video controls individually enumerable.

1998-10-02: The id field was removed from struct video_standard and the color subcarrier fields were renamed. The VIDIOC_QUERYSTD ioctl was renamed to VIDIOC_ENUMSTD, VIDIOC_G_INPUT to VIDIOC_ENUMINPUT. A first draft of the Codec API was released.

1998-11-08: Many minor changes. Most symbols have been renamed. Some material changes to struct v4l2_capability.

1998-11-12: The read/write directon of some ioctls was misdefined.

1998-11-14: V4L2_PIX_FMT_RGB24 changed to V4L2_PIX_FMT_BGR24, and V4L2_PIX_FMT_RGB32 changed to V4L2_PIX_FMT_BGR32. Audio controls are now accessible with the VIDIOC_G_CTRL and VIDIOC_S_CTRL ioctls under names starting with V4L2_CID_AUDIO. The V4L2_MAJOR define was removed from videodev.h since it was only used once in the videodev kernel module. The YUV422 and YUV411 planar image formats were added.

1998-11-28: A few ioctl symbols changed. Interfaces for codecs and video output devices were added.

1999-01-14: A raw VBI capture interface was added.

1999-01-19: The VIDIOC_NEXTBUF ioctl was removed.

8.2.2. V4L2 Version 0.16 1999-01-31

1999-01-27: There is now one QBUF ioctl, VIDIOC_QWBUF and VIDIOC_QRBUF are gone. VIDIOC_QBUF takes a v4l2_buffer as a parameter. Added digital zoom (cropping) controls.

8.2.3. V4L2 Version 0.18 1999-03-16

Added a v4l to V4L2 ioctl compatibility layer to videodev.c. Driver writers, this changes how you implement your ioctl handler. See the Driver Writer's Guide. Added some more control id codes.

8.2.4. V4L2 Version 0.19 1999-06-05

1999-03-18: Fill in the category and catname fields of v4l2_queryctrl objects before passing them to the driver. Required a minor change to the VIDIOC_QUERYCTRL handlers in the sample drivers.

1999-03-31: Better compatibility for v4l memory capture ioctls. Requires changes to drivers to fully support new compatibility features, see Driver Writer's Guide and v4l2cap.c. Added new control IDs: V4L2_CID_HFLIP, _VFLIP. Changed V4L2_PIX_FMT_YUV422P to _YUV422P, and _YUV411P to _YUV411P.

1999-04-04: Added a few more control IDs.

1999-04-07: Added the button control type.

1999-05-02: Fixed a typo in videodev.h, and added the V4L2_CTRL_FLAG_GRAYED (later V4L2_CTRL_FLAG_GRABBED) flag.

1999-05-20: Definition of VIDIOC_G_CTRL was wrong causing a malfunction of this ioctl.

1999-06-05: Changed the value of V4L2_CID_WHITENESS.

8.2.5. V4L2 Version 0.20 (1999-09-10)

Version 0.20 introduced a number of changes which were not backward compatible with 0.19 and earlier versions. Purpose of these changes was to simplify the API, while making it more extensible and following common Linux driver API conventions.

  1. Some typos in V4L2_FMT_FLAG symbols were fixed. struct v4l2_clip was changed for compatibility with v4l. (1999-08-30)

  2. V4L2_TUNER_SUB_LANG1 was added. (1999-09-05)

  3. All ioctl() commands that used an integer argument now take a pointer to an integer. Where it makes sense, ioctls will return the actual new value in the integer pointed to by the argument, a common convention in the V4L2 API. The affected ioctls are: VIDIOC_PREVIEW, VIDIOC_STREAMON, VIDIOC_STREAMOFF, VIDIOC_S_FREQ, VIDIOC_S_INPUT, VIDIOC_S_OUTPUT, VIDIOC_S_EFFECT. For example

    err = ioctl (fd, VIDIOC_XXX, V4L2_XXX);
    

    becomes

    int a = V4L2_XXX; err = ioctl(fd, VIDIOC_XXX, &a);
    

  4. All the different get- and set-format commands were swept into one VIDIOC_G_FMT and VIDIOC_S_FMT ioctl taking a union and a type field selecting the union member as parameter. Purpose is to simplify the API by eliminating several ioctls and to allow new and driver private data streams without adding new ioctls.

    This change obsoletes the following ioctls: VIDIOC_S_INFMT, VIDIOC_G_INFMT, VIDIOC_S_OUTFMT, VIDIOC_G_OUTFMT, VIDIOC_S_VBIFMT and VIDIOC_G_VBIFMT. The image format structure v4l2_format was renamed to struct v4l2_pix_format, while struct v4l2_format is now the envelopping structure for all format negotiations.

  5. Similar to the changes above, the VIDIOC_G_PARM and VIDIOC_S_PARM ioctls were merged with VIDIOC_G_OUTPARM and VIDIOC_S_OUTPARM. A type field in the new struct v4l2_streamparm selects the respective union member.

    This change obsoletes the VIDIOC_G_OUTPARM and VIDIOC_S_OUTPARM ioctls.

  6. Control enumeration was simplified, and two new control flags were introduced and one dropped. The catname field was replaced by a group field.

    Drivers can now flag unsupported and temporarily unavailable controls with V4L2_CTRL_FLAG_DISABLED and V4L2_CTRL_FLAG_GRABBED respectively. The group name indicates a possibly narrower classification than the category. In other words, there may be multiple groups within a category. Controls within a group would typically be drawn within a group box. Controls in different categories might have a greater separation, or may even appear in separate windows.

  7. The struct v4l2_buffer timestamp was changed to a 64 bit integer, containing the sampling or output time of the frame in nanoseconds. Additionally timestamps will be in absolute system time, not starting from zero at the beginning of a stream. The data type name for timestamps is stamp_t, defined as a signed 64-bit integer. Output devices should not send a buffer out until the time in the timestamp field has arrived. I would like to follow SGI's lead, and adopt a multimedia timestamping system like their UST (Unadjusted System Time). See http://reality.sgi.com/cpirazzi_engr/lg/time/intro.html. [This link is no longer valid.] UST uses timestamps that are 64-bit signed integers (not struct timeval's) and given in nanosecond units. The UST clock starts at zero when the system is booted and runs continuously and uniformly. It takes a little over 292 years for UST to overflow. There is no way to set the UST clock. The regular Linux time-of-day clock can be changed periodically, which would cause errors if it were being used for timestamping a multimedia stream. A real UST style clock will require some support in the kernel that is not there yet. But in anticipation, I will change the timestamp field to a 64-bit integer, and I will change the v4l2_masterclock_gettime() function (used only by drivers) to return a 64-bit integer.

  8. A sequence field was added to struct v4l2_buffer. The sequence field counts captured frames, it is ignored by output devices. When a capture driver drops a frame, the sequence number of that frame is skipped.

8.2.6. V4L2 Version 0.20 incremental changes

1999-12-23: In struct v4l2_vbi_format the reserved1 field became offset. Previously drivers were required to clear the reserved1 field.

2000-01-13: The V4L2_FMT_FLAG_NOT_INTERLACED flag was added.

2000-07-31: The linux/poll.h header is now included by videodev.h for compatibility with the original videodev.h file.

2000-11-20: V4L2_TYPE_VBI_OUTPUT and V4L2_PIX_FMT_Y41P were added.

2000-11-25: V4L2_TYPE_VBI_INPUT was added.

2000-12-04: A couple typos in symbol names were fixed.

2001-01-18: To avoid namespace conflicts the fourcc macro defined in the videodev.h header file was renamed to v4l2_fourcc.

2001-01-25: A possible driver-level compatibility problem between the videodev.h file in Linux 2.4.0 and the videodev.h file included in the videodevX patch was fixed. Users of an earlier version of videodevX on Linux 2.4.0 should recompile their V4L and V4L2 drivers.

2001-01-26: A possible kernel-level incompatibility between the videodev.h file in the videodevX patch and the videodev.h file in Linux 2.2.x with devfs patches applied was fixed.

2001-03-02: Certain V4L ioctls which pass data in both direction although they are defined with read-only parameter, did not work correctly through the backward compatibility layer. [Solution?]

2001-04-13: Big endian 16-bit RGB formats were added.

2001-09-17: New YUV formats and the VIDIOC_G_FREQUENCY and VIDIOC_S_FREQUENCY ioctls were added. (The old VIDIOC_G_FREQ and VIDIOC_S_FREQ ioctls did not take multiple tuners into account.)

2000-09-18: V4L2_BUF_TYPE_VBI was added. This may break compatibility as the VIDIOC_G_FMT and VIDIOC_S_FMT ioctls may fail now if the struct v4l2_fmt type field does not contain V4L2_BUF_TYPE_VBI. In the documentation of the struct v4l2_vbi_format offset field the ambiguous phrase "rising edge" was changed to "leading edge".

8.2.7. V4L2 Version 0.20 2000-11-23

A number of changes were made to the raw VBI interface.

  1. Figures clarifying the line numbering scheme were added to the V4L2 API specification. The start[0] and start[1] fields no longer count line numbers beginning at zero. Rationale: a) The previous definition was unclear. b) The start[] values are ordinal numbers. c) There is no point in inventing a new line numbering scheme. We now use line number as defined by ITU-R, period. Compatibility: Add one to the start values. Applications depending on the previous semantics may not function correctly.

  2. The restriction "count[0] > 0 and count[1] > 0" has been relaxed to "(count[0] + count[1]) > 0". Rationale: Drivers may allocate resources at scan line granularity and some data services are transmitted only on the first field. The comment that both count values will usually be equal is misleading and pointless and has been removed. This change breaks compatibility with earlier versions: Drivers may return EINVAL, applications may not function correctly.

  3. Drivers are again permitted to return negative (unknown) start values as proposed earlier. Why this feature was dropped is unclear. This change may break compatibility with applications depending on the start values being positive. The use of EBUSY and EINVAL error codes with the VIDIOC_S_FMT ioctl was clarified. The EBUSY error code was finally documented, and the reserved2 field which was previously mentioned only in the videodev.h header file.

  4. New buffer types V4L2_TYPE_VBI_INPUT and V4L2_TYPE_VBI_OUTPUT were added. The former is an alias for the old V4L2_TYPE_VBI, the latter was missing in the videodev.h file.

8.2.8. V4L2 Version 0.20 2002-07-25

Added sliced VBI interface proposal.

8.2.9. V4L2 in Linux 2.5.46, 2002-10

Around October-November 2002, prior to an announced feature freeze of Linux 2.5, the API was revised, drawing from experience with V4L2 0.20. This unnamed version was finally merged into Linux 2.5.46.

  1. As specified in Section 1.1.2, “Related Devices”, drivers must make related device functions available under all minor device numbers.

  2. The open() function requires access mode O_RDWR regardless of the device type. All V4L2 drivers exchanging data with applicat