ioctl VIDIOC_DBG_G_REGISTER, VIDIOC_DBG_S_REGISTER

&manvol;

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

## Arguments

`fd`

- &fd;

`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 &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.addr 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_BRIDGE`, match.addr selects the nth non-sub-device chip on the TV card. The number zero always selects the host chip, ⪚ 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_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-INFO; 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.

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

### 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-INFO; 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-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 http://linuxtv.org/repo/ for access instructions.

## struct v4l2_dbg_match

&cs-ustr;
__u32 | type | See ident-chip-match-types for a list of possible types.
union | (anonymous)
| __u32 | addr | Match a chip by this number, interpreted according to the type field.
| char | name[32] | Match a chip by this name, interpreted according to the type field.

## struct v4l2_dbg_register

struct v4l2_dbg_match | match | How to match the chip, see v4l2-dbg-match.
__u64 | reg | A register number.
__u64 | val | The value read from, or to be written into the register.

## Chip Match Types

&cs-def;
`V4L2_CHIP_MATCH_BRIDGE` | 0 | Match the nth chip on the card, zero for the bridge chip. Does not match sub-devices.
`V4L2_CHIP_MATCH_I2C_DRIVER` | 1 | Match an &i2c; chip by its driver name.
`V4L2_CHIP_MATCH_I2C_ADDR` | 2 | Match a chip by its 7 bit &i2c; bus address.
`V4L2_CHIP_MATCH_AC97` | 3 | Match the nth anciliary AC97 chip.
`V4L2_CHIP_MATCH_SUBDEV` | 4 | Match the nth sub-device.

&return-value;

EPERM

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