vidioc-g-fmt.rst

  1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
  2.. c:namespace:: V4L
  3
  4.. _VIDIOC_G_FMT:
  5
  6************************************************
  7ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT
  8************************************************
  9
 10Name
 11====
 12
 13VIDIOC_G_FMT - VIDIOC_S_FMT - VIDIOC_TRY_FMT - Get or set the data format, try a format
 14
 15Synopsis
 16========
 17
 18.. c:macro:: VIDIOC_G_FMT
 19
 20``int ioctl(int fd, VIDIOC_G_FMT, struct v4l2_format *argp)``
 21
 22.. c:macro:: VIDIOC_S_FMT
 23
 24``int ioctl(int fd, VIDIOC_S_FMT, struct v4l2_format *argp)``
 25
 26.. c:macro:: VIDIOC_TRY_FMT
 27
 28``int ioctl(int fd, VIDIOC_TRY_FMT, struct v4l2_format *argp)``
 29
 30Arguments
 31=========
 32
 33``fd``
 34    File descriptor returned by :c:func:`open()`.
 35
 36``argp``
 37    Pointer to struct :c:type:`v4l2_format`.
 38
 39Description
 40===========
 41
 42These ioctls are used to negotiate the format of data (typically image
 43format) exchanged between driver and application.
 44
 45To query the current parameters applications set the ``type`` field of a
 46struct :c:type:`v4l2_format` to the respective buffer (stream)
 47type. For example video capture devices use
 48``V4L2_BUF_TYPE_VIDEO_CAPTURE`` or
 49``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE``. When the application calls the
 50:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl with a pointer to this structure the driver fills
 51the respective member of the ``fmt`` union. In case of video capture
 52devices that is either the struct
 53:c:type:`v4l2_pix_format` ``pix`` or the struct
 54:c:type:`v4l2_pix_format_mplane` ``pix_mp``
 55member. When the requested buffer type is not supported drivers return
 56an ``EINVAL`` error code.
 57
 58To change the current format parameters applications initialize the
 59``type`` field and all fields of the respective ``fmt`` union member.
 60For details see the documentation of the various devices types in
 61:ref:`devices`. Good practice is to query the current parameters
 62first, and to modify only those parameters not suitable for the
 63application. When the application calls the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl with
 64a pointer to a struct :c:type:`v4l2_format` structure the driver
 65checks and adjusts the parameters against hardware abilities. Drivers
 66should not return an error code unless the ``type`` field is invalid,
 67this is a mechanism to fathom device capabilities and to approach
 68parameters acceptable for both the application and driver. On success
 69the driver may program the hardware, allocate resources and generally
 70prepare for data exchange. Finally the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl returns
 71the current format parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does. Very simple,
 72inflexible devices may even ignore all input and always return the
 73default parameters. However all V4L2 devices exchanging data with the
 74application must implement the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
 75ioctl. When the requested buffer type is not supported drivers return an
 76EINVAL error code on a :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` attempt. When I/O is already in
 77progress or the resource is not available for other reasons drivers
 78return the ``EBUSY`` error code.
 79
 80The :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl is equivalent to :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` with one
 81exception: it does not change driver state. It can also be called at any
 82time, never returning ``EBUSY``. This function is provided to negotiate
 83parameters, to learn about hardware limitations, without disabling I/O
 84or possibly time consuming hardware preparations. Although strongly
 85recommended drivers are not required to implement this ioctl.
 86
 87The format as returned by :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` must be identical to what
 88:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` returns for the same input or output.
 89
 90.. c:type:: v4l2_format
 91
 92.. tabularcolumns::  |p{7.4cm}|p{4.4cm}|p{5.5cm}|
 93
 94.. flat-table:: struct v4l2_format
 95    :header-rows:  0
 96    :stub-columns: 0
 97
 98    * - __u32
 99      - ``type``
100      - Type of the data stream, see :c:type:`v4l2_buf_type`.
101    * - union {
102      - ``fmt``
103    * - struct :c:type:`v4l2_pix_format`
104      - ``pix``
105      - Definition of an image format, see :ref:`pixfmt`, used by video
106	capture and output devices.
107    * - struct :c:type:`v4l2_pix_format_mplane`
108      - ``pix_mp``
109      - Definition of an image format, see :ref:`pixfmt`, used by video
110	capture and output devices that support the
111	:ref:`multi-planar version of the API <planar-apis>`.
112    * - struct :c:type:`v4l2_window`
113      - ``win``
114      - Definition of an overlaid image, see :ref:`overlay`, used by
115	video overlay devices.
116    * - struct :c:type:`v4l2_vbi_format`
117      - ``vbi``
118      - Raw VBI capture or output parameters. This is discussed in more
119	detail in :ref:`raw-vbi`. Used by raw VBI capture and output
120	devices.
121    * - struct :c:type:`v4l2_sliced_vbi_format`
122      - ``sliced``
123      - Sliced VBI capture or output parameters. See :ref:`sliced` for
124	details. Used by sliced VBI capture and output devices.
125    * - struct :c:type:`v4l2_sdr_format`
126      - ``sdr``
127      - Definition of a data format, see :ref:`pixfmt`, used by SDR
128	capture and output devices.
129    * - struct :c:type:`v4l2_meta_format`
130      - ``meta``
131      - Definition of a metadata format, see :ref:`meta-formats`, used by
132	metadata capture devices.
133    * - __u8
134      - ``raw_data``\ [200]
135      - Place holder for future extensions.
136    * - }
137      -
138
139Return Value
140============
141
142On success 0 is returned, on error -1 and the ``errno`` variable is set
143appropriately. The generic error codes are described at the
144:ref:`Generic Error Codes <gen-errors>` chapter.
145
146EINVAL
147    The struct :c:type:`v4l2_format` ``type`` field is
148    invalid or the requested buffer type not supported.
149
150EBUSY
151    The device is busy and cannot change the format. This could be
152    because or the device is streaming or buffers are allocated or
153    queued to the driver. Relevant for :ref:`VIDIOC_S_FMT
154    <VIDIOC_G_FMT>` only.