Anatomy of a V4L driver - Revision history

Jimbley: Added category: Development

2009-05-02T20:35:41Z

Added category: Development

← Older revision		Revision as of 20:35, 2 May 2009
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	Additionally, by checking out the v4l source, there are many files in there which can be used as templates for drivers. It is not easy to put all the knowledge together, but with time it comes.		Additionally, by checking out the v4l source, there are many files in there which can be used as templates for drivers. It is not easy to put all the knowledge together, but with time it comes.
	* [http://linuxtv.org/hg/v4l-dvb/raw-file/4c7466ea8d64/linux/Documentation/video4linux/v4l2-framework.txt V4L2 Framework]		* [http://linuxtv.org/hg/v4l-dvb/raw-file/4c7466ea8d64/linux/Documentation/video4linux/v4l2-framework.txt V4L2 Framework]
		+	[[Category:Development]]

Pete e at 21:14, 2 April 2009

2009-04-02T21:14:31Z

← Older revision		Revision as of 21:14, 2 April 2009
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	Additionally, by checking out the v4l source, there are many files in there which can be used as templates for drivers. It is not easy to put all the knowledge together, but with time it comes.		Additionally, by checking out the v4l source, there are many files in there which can be used as templates for drivers. It is not easy to put all the knowledge together, but with time it comes.
-	* [http://linuxtv.org/hg~~/~hverkuil~~/v4l-dvb-~~media2/~~file/~~6292505ca617~~/linux/Documentation/video4linux/v4l2-framework.txt V4L2 Framework]	+	* [http://linuxtv.org/hg/v4l-dvb/raw-file/4c7466ea8d64/linux/Documentation/video4linux/v4l2-framework.txt V4L2 Framework]

CityK: 10 revision(s)

2008-11-20T02:25:54Z

10 revision(s)

← Older revision

Revision as of 02:25, 20 November 2008

CityK: add link

2008-11-20T02:23:53Z

add link

← Older revision		Revision as of 02:23, 20 November 2008
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	Additionally, by checking out the v4l source, there are many files in there which can be used as templates for drivers. It is not easy to put all the knowledge together, but with time it comes.		Additionally, by checking out the v4l source, there are many files in there which can be used as templates for drivers. It is not easy to put all the knowledge together, but with time it comes.
		+	* [http://linuxtv.org/hg/~hverkuil/v4l-dvb-media2/file/6292505ca617/linux/Documentation/video4linux/v4l2-framework.txt V4L2 Framework]

CityK: Anatomy of a v4l driver moved to Anatomy of a V4L driver: name

2008-11-20T02:22:08Z

Anatomy of a v4l driver moved to Anatomy of a V4L driver: name

← Older revision

Revision as of 02:22, 20 November 2008

Hexa: /* How a driver gets loaded into memory */

2007-06-26T00:27:20Z

How a driver gets loaded into memory

← Older revision		Revision as of 00:27, 26 June 2007
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	Firstly, a v4l driver is a kernel module that hooks into at least the videodev module (v4l) and possibly other modules such as v4l1-compat or videobuf, depending what it uses.		Firstly, a v4l driver is a kernel module that hooks into at least the videodev module (v4l) and possibly other modules such as v4l1-compat or videobuf, depending what it uses.

-	Since a v4l driver is a kernel module, it will be loaded using modprobe or insmod. If using insmod, make sure to load all ~~dependancies~~, or else your module will have unresolved ~~symboles~~ and will fail to load, such as this:	+	Since a v4l driver is a kernel module, it will be loaded using modprobe or insmod. If using insmod, make sure to load all dependencies, or else your module will have unresolved symbols and will fail to load, such as this:


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-	These errors can be frustrating, but a good place to start is determining the ~~dependancies~~. In this case we look at the prefix (videobuf_), and it hints to us that the video-buf module needs to be loaded. Same thing for v4l-compat.	+	These errors can be frustrating, but a good place to start is determining the dependencies. In this case we look at the prefix (videobuf_), and it hints to us that the video-buf module needs to be loaded. Same thing for v4l-compat.
	More information about Unresolved Symbols is available [http://www.tldp.org/HOWTO/Module-HOWTO/basekerncompat.html here].		More information about Unresolved Symbols is available [http://www.tldp.org/HOWTO/Module-HOWTO/basekerncompat.html here].

Lucgallant: added info on writing v4l drivers

2005-11-08T16:07:54Z

added info on writing v4l drivers

← Older revision		Revision as of 16:07, 8 November 2005
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	Usually, when an application (such as mythtv, xawtv) starts up, you need to tell it which device to communicate to. Say the device is on /dev/video5, then the application will bind itself to /dev/video5 and the ioctls will be forwared to the driver in question via videodev module. The driver will then interpret the ioctl command (such as channel up, down, volume up, down, read, open...) and do the appropriate action.		Usually, when an application (such as mythtv, xawtv) starts up, you need to tell it which device to communicate to. Say the device is on /dev/video5, then the application will bind itself to /dev/video5 and the ioctls will be forwared to the driver in question via videodev module. The driver will then interpret the ioctl command (such as channel up, down, volume up, down, read, open...) and do the appropriate action.
		+
		+
		+	== Additional information on writing v4l drivers ==
		+
		+	A great resource for getting started writing drivers (although outdated), is the following [http://www.thedirks.org/v4l2/v4l2dwg.htm link].
		+
		+	Additionally, by checking out the v4l source, there are many files in there which can be used as templates for drivers. It is not easy to put all the knowledge together, but with time it comes.

Lucgallant at 13:19, 31 October 2005

2005-10-31T13:19:53Z

← Older revision		Revision as of 13:19, 31 October 2005
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-		+	vloopback_crc: module not supported by Novell, setting U taint flag.
-	vloopback_crc: module not supported by Novell, setting U taint flag.	+	vloopback_crc: Unknown symbol videobuf_reqbufs, st_info == 0x1
-	vloopback_crc: Unknown symbol videobuf_reqbufs, st_info == 0x1	+	vloopback_crc: Unknown symbol v4l_compat_translate_ioctl, st_info == 0x1
-	vloopback_crc: Unknown symbol v4l_compat_translate_ioctl, st_info == 0x1	+	vloopback_crc: Unknown symbol videobuf_dqbuf, st_info == 0x1
-	vloopback_crc: Unknown symbol videobuf_dqbuf, st_info == 0x1	+	vloopback_crc: Unknown symbol videobuf_qbuf, st_info == 0x1
-	vloopback_crc: Unknown symbol videobuf_qbuf, st_info == 0x1	+	vloopback_crc: Unknown symbol videobuf_read_one, st_info == 0x1
-	vloopback_crc: Unknown symbol videobuf_read_one, st_info == 0x1	+
-		+


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	When a driver, and pretty much any kernel module gets started up, there needs to be a generic way for the module to initialize itself (this must be a way that the kernel will know how to get the module going). In the code of the module, there needs to be code such as this:		When a driver, and pretty much any kernel module gets started up, there needs to be a generic way for the module to initialize itself (this must be a way that the kernel will know how to get the module going). In the code of the module, there needs to be code such as this:

-		+	module_init(vloopback_init);
-	module_init(vloopback_init);	+	module_exit(cleanup_vloopback_module);
-	module_exit(cleanup_vloopback_module);	+
-		+
-		+

	module_init and module_exit being the keywords here. With the appropriate make flags, the compiler will know to bind these functions (vloopback_init, cleanup_vloopback_module) to the initialize and cleanup functions for the kernel to call.		module_init and module_exit being the keywords here. With the appropriate make flags, the compiler will know to bind these functions (vloopback_init, cleanup_vloopback_module) to the initialize and cleanup functions for the kernel to call.

Lucgallant at 13:08, 31 October 2005

2005-10-31T13:08:38Z

← Older revision		Revision as of 13:08, 31 October 2005
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	Since a v4l driver is a kernel module, it will be loaded using modprobe or insmod. If using insmod, make sure to load all dependancies, or else your module will have unresolved symboles and will fail to load, such as this:		Since a v4l driver is a kernel module, it will be loaded using modprobe or insmod. If using insmod, make sure to load all dependancies, or else your module will have unresolved symboles and will fail to load, such as this:

-	~~*******************************************************************~~	+
		+
	vloopback_crc: module not supported by Novell, setting U taint flag.		vloopback_crc: module not supported by Novell, setting U taint flag.
	vloopback_crc: Unknown symbol videobuf_reqbufs, st_info == 0x1		vloopback_crc: Unknown symbol videobuf_reqbufs, st_info == 0x1
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	vloopback_crc: Unknown symbol videobuf_qbuf, st_info == 0x1		vloopback_crc: Unknown symbol videobuf_qbuf, st_info == 0x1
	vloopback_crc: Unknown symbol videobuf_read_one, st_info == 0x1		vloopback_crc: Unknown symbol videobuf_read_one, st_info == 0x1
-	~~*******************************************************************~~	+
		+

	These errors can be frustrating, but a good place to start is determining the dependancies. In this case we look at the prefix (videobuf_), and it hints to us that the video-buf module needs to be loaded. Same thing for v4l-compat.		These errors can be frustrating, but a good place to start is determining the dependancies. In this case we look at the prefix (videobuf_), and it hints to us that the video-buf module needs to be loaded. Same thing for v4l-compat.
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	When a driver, and pretty much any kernel module gets started up, there needs to be a generic way for the module to initialize itself (this must be a way that the kernel will know how to get the module going). In the code of the module, there needs to be code such as this:		When a driver, and pretty much any kernel module gets started up, there needs to be a generic way for the module to initialize itself (this must be a way that the kernel will know how to get the module going). In the code of the module, there needs to be code such as this:

-	~~*****************************************~~	+
	module_init(vloopback_init);		module_init(vloopback_init);
	module_exit(cleanup_vloopback_module);		module_exit(cleanup_vloopback_module);
-	~~*****************************************~~	+
		+
		+
	module_init and module_exit being the keywords here. With the appropriate make flags, the compiler will know to bind these functions (vloopback_init, cleanup_vloopback_module) to the initialize and cleanup functions for the kernel to call.		module_init and module_exit being the keywords here. With the appropriate make flags, the compiler will know to bind these functions (vloopback_init, cleanup_vloopback_module) to the initialize and cleanup functions for the kernel to call.

Lucgallant at 15:54, 28 October 2005

2005-10-28T15:54:10Z

← Older revision		Revision as of 15:54, 28 October 2005
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-	I write this because throughout my time porting vloopback to v4l2, I have found it hard to determine how the v4l driver actually works. I am not an expert but hopefully what I write here can help someone.
-
-
	== How a driver gets loaded into memory ==		== How a driver gets loaded into memory ==