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 A.64, “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. If the new control value is inappropriate (e.g. the given menu index is not supported by the menu control), then this will also result in an EINVAL error code error.

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 A.62. 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 A.63. struct v4l2_ext_controls

__u32ctrl_classThe control class to which all controls belong, see Table A.64, “Control classes”. Drivers that use a kernel framework for handling controls will also accept a value of 0 here, meaning that the controls can belong to any control class. Whether drivers support this can be tested by setting ctrl_class to 0 and calling VIDIOC_TRY_EXT_CTRLS with a count of 0. If that succeeds, then the driver supports this feature.
__u32countThe number of controls in the controls array. May also be zero.
__u32error_idx

Set by the driver in case of an error. If the error is associated with a particular control, then error_idx is set to the index of that control. If the error is not related to a specific control, or the validation step failed (see below), then error_idx is set to count. The value is undefined if the ioctl returned 0 (success).

Before controls are read from/written to hardware a validation step takes place: this checks if all controls in the list are valid controls, if no attempt is made to write to a read-only control or read from a write-only control, and any other up-front checks that can be done without accessing the hardware. The exact validations done during this step are driver dependent since some checks might require hardware access for some devices, thus making it impossible to do those checks up-front. However, drivers should make a best-effort to do as many up-front checks as possible.

This check is done to avoid leaving the hardware in an inconsistent state due to easy-to-avoid problems. But it leads to another problem: the application needs to know whether an error came from the validation step (meaning that the hardware was not touched) or from an error during the actual reading from/writing to hardware.

The, in hindsight quite poor, solution for that is to set error_idx to count if the validation failed. This has the unfortunate side-effect that it is not possible to see which control failed the validation. If the validation was successful and the error happened while accessing the hardware, then error_idx is less than count and only the controls up to error_idx-1 were read or written correctly, and the state of the remaining controls is undefined.

Since VIDIOC_TRY_EXT_CTRLS does not access hardware there is also no need to handle the validation step in this special way, so error_idx will just be set to the control that failed the validation step instead of to count. This means that if VIDIOC_S_EXT_CTRLS fails with error_idx set to count, then you can call VIDIOC_TRY_EXT_CTRLS to try to discover the actual control that failed the validation step. Unfortunately, there is no TRY equivalent for VIDIOC_G_EXT_CTRLS.

__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 A.64. Control classes

V4L2_CTRL_CLASS_USER0x980000The class containing user controls. These controls are described in the section called “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 the section called “Codec Control Reference”.
V4L2_CTRL_CLASS_CAMERA0x9a0000The class containing camera controls. These controls are described in the section called “Camera Control Reference”.
V4L2_CTRL_CLASS_FM_TX0x9b0000The class containing FM Transmitter (FM TX) controls. These controls are described in the section called “FM Transmitter Control Reference”.
V4L2_CTRL_CLASS_FLASH0x9c0000The class containing flash device controls. These controls are described in the section called “Flash Control Reference”.
V4L2_CTRL_CLASS_JPEG0x9d0000The class containing JPEG compression controls. These controls are described in the section called “JPEG Control Reference”.
V4L2_CTRL_CLASS_IMAGE_SOURCE0x9e0000The class containing image source controls. These controls are described in the section called “Image Source Control Reference”.
V4L2_CTRL_CLASS_IMAGE_PROC0x9f0000The class containing image processing controls. These controls are described in the section called “Image Process Control Reference”.
V4L2_CTRL_CLASS_FM_RX0xa10000The class containing FM Receiver (FM RX) controls. These controls are described in the section called “FM Receiver Control Reference”.
V4L2_CTRL_CLASS_RF_TUNER0xa20000The class containing RF tuner controls. These controls are described in the section called “RF Tuner Control Reference”.

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.

EINVAL

The struct v4l2_ext_control id is invalid, the struct v4l2_ext_controls ctrl_class is invalid, or the struct v4l2_ext_control value was inappropriate (e.g. the given menu index is not supported by the driver). 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.

EACCES

Attempt to try or set a read-only control or to get a write-only control.