VIDIOC_DBG_G_REGISTER, VIDIOC_DBG_S_REGISTER -- Read or write hardware registers


int ioctl(int fd, int request, struct v4l2_register *argp);

int ioctl(int fd, int request, const struct v4l2_register *argp);



File descriptor returned by open().





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 should 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_register and call VIDIOC_DBG_S_REGISTER with a pointer to this structure. The match_type and match_chip 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 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_chip selects the nth non-I2C chip on the TV card. Drivers may also interpret match_chip as a random ID, but we recommend against that. The number zero always selects the host chip, e. g. the chip connected to the PCI bus. You can find out which chips are present with the VIDIOC_G_CHIP_IDENT ioctl.

When match_type is V4L2_CHIP_MATCH_I2C_DRIVER, match_chip contains a driver ID as defined in the linux/i2c-id.h header file. For instance I2C_DRIVERID_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_G_CHIP_IDENT ioctl you can find out which I2C chips are present.

When match_type is V4L2_CHIP_MATCH_I2C_ADDR, match_chip selects a chip by its 7 bit I2C bus address.

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_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_G_CHIP_IDENT. Conversely it may support VIDIOC_G_CHIP_IDENT but not these ioctls.

VIDIOC_DBG_G_REGISTER and VIDIOC_DBG_S_REGISTER were introduced in Linux 2.6.21.

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 1. struct v4l2_register

__u32match_typeSee Table 2 for a list of possible types. 
__u32match_chipMatch a chip by this number, interpreted according to the match_type field. 
__u64regA register number. 
__u64valThe value read from, or to be written into the register. 

Table 2. 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 ID from the linux/i2c-id.h header file.
V4L2_CHIP_MATCH_I2C_ADDR2Match a chip by its 7 bit I2C bus address.

Return Value

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


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.


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