xref: /kernel/linux/linux-5.10/Documentation/userspace-api/media/v4l/vidioc-subdev-g-fmt.rst

.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
.. c:namespace:: V4L

.. _VIDIOC_SUBDEV_G_FMT:

**********************************************
ioctl VIDIOC_SUBDEV_G_FMT, VIDIOC_SUBDEV_S_FMT
**********************************************

Name
====

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

Synopsis
========

.. c:macro:: VIDIOC_SUBDEV_G_FMT

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

.. c:macro:: VIDIOC_SUBDEV_S_FMT

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

Arguments
=========

``fd``
    File descriptor returned by :c:func:`open()`.

``argp``
    Pointer to struct :c:type:`v4l2_subdev_format`.

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 :c:type:`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
:c:type:`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.

.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|

.. c:type:: v4l2_subdev_format

.. flat-table:: struct v4l2_subdev_format
    :header-rows:  0
    :stub-columns: 0
    :widths:       1 1 2

    * - __u32
      - ``pad``
      - Pad number as reported by the media controller API.
    * - __u32
      - ``which``
      - Format to modified, from enum
	:ref:`v4l2_subdev_format_whence <v4l2-subdev-format-whence>`.
    * - struct :c:type:`v4l2_mbus_framefmt`
      - ``format``
      - Definition of an image format, see :c:type:`v4l2_mbus_framefmt` for
	details.
    * - __u32
      - ``reserved``\ [8]
      - Reserved for future extensions. Applications and drivers must set
	the array to zero.


.. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|

.. _v4l2-subdev-format-whence:

.. flat-table:: enum v4l2_subdev_format_whence
    :header-rows:  0
    :stub-columns: 0
    :widths:       3 1 4

    * - V4L2_SUBDEV_FORMAT_TRY
      - 0
      - Try formats, used for querying device capabilities.
    * - V4L2_SUBDEV_FORMAT_ACTIVE
      - 1
      - Active formats, applied to the hardware.

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
:ref:`Generic Error Codes <gen-errors>` 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 :c:type:`v4l2_subdev_format`
    ``pad`` references a non-existing pad, or the ``which`` field
    references a non-existing format.

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``.

============

On success 0 is returned, on error -1 and the ``errno`` variable is set
appropriately. The generic error codes are described at the
:ref:`Generic Error Codes <gen-errors>` chapter.