VIDIOC_DBG_G_REGISTER, VIDIOC_DBG_S_REGISTER — Read or write hardware registers


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



File descriptor returned by open().






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 except for size and call VIDIOC_DBG_S_REGISTER with a pointer to this structure. The match.type and match.addr or 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.addr or 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 and the size (in bytes) of the value in size.

When match.type is V4L2_CHIP_MATCH_BRIDGE, match.addr selects the nth non-sub-device 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_INFO ioctl.

When match.type is V4L2_CHIP_MATCH_SUBDEV, match.addr selects the nth sub-device.

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_INFO. Conversely it may support VIDIOC_DBG_G_CHIP_INFO 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 for access instructions.

Table A.7. struct v4l2_dbg_match

__u32typeSee Table A.9, “Chip Match Types” for a list of possible types. 
 __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. Currently unused.

Table A.8. struct v4l2_dbg_register

struct v4l2_dbg_matchmatchHow to match the chip, see Table A.7, “struct v4l2_dbg_match. 
__u32sizeThe register size in bytes. 
__u64regA register number. 
__u64valThe value read from, or to be written into the register. 

Table A.9. Chip Match Types

V4L2_CHIP_MATCH_BRIDGE0Match the nth chip on the card, zero for the bridge chip. Does not match sub-devices.
V4L2_CHIP_MATCH_SUBDEV4Match the nth sub-device.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately. The generic error codes are described at the Generic Error Codes chapter.


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