7.59. ioctl VIDIOC_SUBDEV_G_FMT, VIDIOC_SUBDEV_S_FMT¶

7.59.1. Name¶

VIDIOC_SUBDEV_G_FMT - VIDIOC_SUBDEV_S_FMT - Get or set the data format on a subdev pad

7.59.2. Synopsis¶

VIDIOC_SUBDEV_G_FMT¶

int ioctl(int fd, VIDIOC_SUBDEV_G_FMT, struct v4l2_subdev_format *argp)

VIDIOC_SUBDEV_S_FMT¶

int ioctl(int fd, VIDIOC_SUBDEV_S_FMT, struct v4l2_subdev_format *argp)

7.59.3. Arguments¶

fd: File descriptor returned by open().
argp: Pointer to struct v4l2_subdev_format.

7.59.4. Description¶

These ioctls are used to negotiate the frame format at specific subdev pads in the image pipeline.

To retrieve the current format applications set the pad field of a struct v4l2_subdev_format to the desired pad number as reported by the media API and the which field to V4L2_SUBDEV_FORMAT_ACTIVE. When they call the VIDIOC_SUBDEV_G_FMT ioctl with a pointer to this structure the driver fills the members of the format field.

To change the current format applications set both the pad and which fields and all members of the format field. When they call the VIDIOC_SUBDEV_S_FMT ioctl with a pointer to this structure the driver verifies the requested format, adjusts it based on the hardware capabilities and configures the device. Upon return the struct v4l2_subdev_format contains the current format as would be returned by a VIDIOC_SUBDEV_G_FMT call.

Applications can query the device capabilities by setting the which to V4L2_SUBDEV_FORMAT_TRY. When set, ‘try’ formats are not applied to the device by the driver, but are changed exactly as active formats and stored in the sub-device file handle. Two applications querying the same sub-device would thus not interact with each other.

For instance, to try a format at the output pad of a sub-device, applications would first set the try format at the sub-device input with the VIDIOC_SUBDEV_S_FMT ioctl. They would then either retrieve the default format at the output pad with the VIDIOC_SUBDEV_G_FMT ioctl, or set the desired output pad format with the VIDIOC_SUBDEV_S_FMT ioctl and check the returned value.

Try formats do not depend on active formats, but can depend on the current links configuration or sub-device controls value. For instance, a low-pass noise filter might crop pixels at the frame boundaries, modifying its output frame size.

If the subdev device node has been registered in read-only mode, calls to VIDIOC_SUBDEV_S_FMT are only valid if the which field is set to V4L2_SUBDEV_FORMAT_TRY, otherwise an error is returned and the errno variable is set to -EPERM.

Drivers must not return an error solely because the requested format doesn’t match the device capabilities. They must instead modify the format to match what the hardware can provide. The modified format should be as close as possible to the original request.

type v4l2_subdev_format¶

struct v4l2_subdev_format¶
__u32	`pad`	Pad number as reported by the media controller API.
__u32	`which`	Format to modified, from enum v4l2_subdev_format_whence.
struct `v4l2_mbus_framefmt`	`format`	Definition of an image format, see `v4l2_mbus_framefmt` for details.
__u32	`stream`	Stream identifier.
__u32	`reserved`[7]	Reserved for future extensions. Applications and drivers must set the array to zero.

enum v4l2_subdev_format_whence¶
V4L2_SUBDEV_FORMAT_TRY	0	Try formats, used for querying device capabilities.
V4L2_SUBDEV_FORMAT_ACTIVE	1	Active formats, applied to the hardware.

7.59.5. 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.

EBUSY: The format can’t be changed because the pad is currently busy. This can be caused, for instance, by an active video stream on the pad. The ioctl must not be retried without performing another action to fix the problem first. Only returned by VIDIOC_SUBDEV_S_FMT
EINVAL: The struct v4l2_subdev_format pad references a non-existing pad, or the which field has an unsupported value.
EPERM: The VIDIOC_SUBDEV_S_FMT ioctl has been called on a read-only subdevice and the which field is set to V4L2_SUBDEV_FORMAT_ACTIVE.