Documentation/media/uapi/v4l/buffer.rst

   1 .. Permission is granted to copy, distribute and/or modify this
   2 .. document under the terms of the GNU Free Documentation License,
   3 .. Version 1.1 or any later version published by the Free Software
   4 .. Foundation, with no Invariant Sections, no Front-Cover Texts
   5 .. and no Back-Cover Texts. A copy of the license is included at
   6 .. Documentation/media/uapi/fdl-appendix.rst.
   7 ..
   8 .. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
   9
  10 .. _buffer:
  11
  12 *******
  13 Buffers
  14 *******
  15
  16 A buffer contains data exchanged by application and driver using one of
  17 the Streaming I/O methods. In the multi-planar API, the data is held in
  18 planes, while the buffer structure acts as a container for the planes.
  19 Only pointers to buffers (planes) are exchanged, the data itself is not
  20 copied. These pointers, together with meta-information like timestamps
  21 or field parity, are stored in a struct :c:type:`v4l2_buffer`,
  22 argument to the :ref:`VIDIOC_QUERYBUF`,
  23 :ref:`VIDIOC_QBUF <VIDIOC_QBUF>` and
  24 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl. In the multi-planar API,
  25 some plane-specific members of struct :c:type:`v4l2_buffer`,
  26 such as pointers and sizes for each plane, are stored in struct
  27 struct :c:type:`v4l2_plane` instead. In that case, struct
  28 struct :c:type:`v4l2_buffer` contains an array of plane structures.
  29
  30 Dequeued video buffers come with timestamps. The driver decides at which
  31 part of the frame and with which clock the timestamp is taken. Please
  32 see flags in the masks ``V4L2_BUF_FLAG_TIMESTAMP_MASK`` and
  33 ``V4L2_BUF_FLAG_TSTAMP_SRC_MASK`` in :ref:`buffer-flags`. These flags
  34 are always valid and constant across all buffers during the whole video
  35 stream. Changes in these flags may take place as a side effect of
  36 :ref:`VIDIOC_S_INPUT <VIDIOC_G_INPUT>` or
  37 :ref:`VIDIOC_S_OUTPUT <VIDIOC_G_OUTPUT>` however. The
  38 ``V4L2_BUF_FLAG_TIMESTAMP_COPY`` timestamp type which is used by e.g. on
  39 mem-to-mem devices is an exception to the rule: the timestamp source
  40 flags are copied from the OUTPUT video buffer to the CAPTURE video
  41 buffer.
  42
  43
  44 Interactions between formats, controls and buffers
  45 ==================================================
  46
  47 V4L2 exposes parameters that influence the buffer size, or the way data is
  48 laid out in the buffer. Those parameters are exposed through both formats and
  49 controls. One example of such a control is the ``V4L2_CID_ROTATE`` control
  50 that modifies the direction in which pixels are stored in the buffer, as well
  51 as the buffer size when the selected format includes padding at the end of
  52 lines.
  53
  54 The set of information needed to interpret the content of a buffer (e.g. the
  55 pixel format, the line stride, the tiling orientation or the rotation) is
  56 collectively referred to in the rest of this section as the buffer layout.
  57
  58 Controls that can modify the buffer layout shall set the
  59 ``V4L2_CTRL_FLAG_MODIFY_LAYOUT`` flag.
  60
  61 Modifying formats or controls that influence the buffer size or layout require
  62 the stream to be stopped. Any attempt at such a modification while the stream
  63 is active shall cause the ioctl setting the format or the control to return
  64 the ``EBUSY`` error code. In that case drivers shall also set the
  65 ``V4L2_CTRL_FLAG_GRABBED`` flag when calling
  66 :c:func:`VIDIOC_QUERYCTRL` or :c:func:`VIDIOC_QUERY_EXT_CTRL` for such a
  67 control while the stream is active.
  68
  69 .. note::
  70
  71    The :c:func:`VIDIOC_S_SELECTION` ioctl can, depending on the hardware (for
  72    instance if the device doesn't include a scaler), modify the format in
  73    addition to the selection rectangle. Similarly, the
  74    :c:func:`VIDIOC_S_INPUT`, :c:func:`VIDIOC_S_OUTPUT`, :c:func:`VIDIOC_S_STD`
  75    and :c:func:`VIDIOC_S_DV_TIMINGS` ioctls can also modify the format and
  76    selection rectangles. When those ioctls result in a buffer size or layout
  77    change, drivers shall handle that condition as they would handle it in the
  78    :c:func:`VIDIOC_S_FMT` ioctl in all cases described in this section.
  79
  80 Controls that only influence the buffer layout can be modified at any time
  81 when the stream is stopped. As they don't influence the buffer size, no
  82 special handling is needed to synchronize those controls with buffer
  83 allocation and the ``V4L2_CTRL_FLAG_GRABBED`` flag is cleared once the
  84 stream is stopped.
  85
  86 Formats and controls that influence the buffer size interact with buffer
  87 allocation. The simplest way to handle this is for drivers to always require
  88 buffers to be reallocated in order to change those formats or controls. In
  89 that case, to perform such changes, userspace applications shall first stop
  90 the video stream with the :c:func:`VIDIOC_STREAMOFF` ioctl if it is running
  91 and free all buffers with the :c:func:`VIDIOC_REQBUFS` ioctl if they are
  92 allocated. After freeing all buffers the ``V4L2_CTRL_FLAG_GRABBED`` flag
  93 for controls is cleared. The format or controls can then be modified, and
  94 buffers shall then be reallocated and the stream restarted. A typical ioctl
  95 sequence is
  96
  97  #. VIDIOC_STREAMOFF
  98  #. VIDIOC_REQBUFS(0)
  99  #. VIDIOC_S_EXT_CTRLS
 100  #. VIDIOC_S_FMT
 101  #. VIDIOC_REQBUFS(n)
 102  #. VIDIOC_QBUF
 103  #. VIDIOC_STREAMON
 104
 105 The second :c:func:`VIDIOC_REQBUFS` call will take the new format and control
 106 value into account to compute the buffer size to allocate. Applications can
 107 also retrieve the size by calling the :c:func:`VIDIOC_G_FMT` ioctl if needed.
 108
 109 .. note::
 110
 111    The API doesn't mandate the above order for control (3.) and format (4.)
 112    changes. Format and controls can be set in a different order, or even
 113    interleaved, depending on the device and use case. For instance some
 114    controls might behave differently for different pixel formats, in which
 115    case the format might need to be set first.
 116
 117 When reallocation is required, any attempt to modify format or controls that
 118 influences the buffer size while buffers are allocated shall cause the format
 119 or control set ioctl to return the ``EBUSY`` error. Any attempt to queue a
 120 buffer too small for the current format or controls shall cause the
 121 :c:func:`VIDIOC_QBUF` ioctl to return a ``EINVAL`` error.
 122
 123 Buffer reallocation is an expensive operation. To avoid that cost, drivers can
 124 (and are encouraged to) allow format or controls that influence the buffer
 125 size to be changed with buffers allocated. In that case, a typical ioctl
 126 sequence to modify format and controls is
 127
 128  #. VIDIOC_STREAMOFF
 129  #. VIDIOC_S_EXT_CTRLS
 130  #. VIDIOC_S_FMT
 131  #. VIDIOC_QBUF
 132  #. VIDIOC_STREAMON
 133
 134 For this sequence to operate correctly, queued buffers need to be large enough
 135 for the new format or controls. Drivers shall return a ``ENOSPC`` error in
 136 response to format change (:c:func:`VIDIOC_S_FMT`) or control changes
 137 (:c:func:`VIDIOC_S_CTRL` or :c:func:`VIDIOC_S_EXT_CTRLS`) if buffers too small
 138 for the new format are currently queued. As a simplification, drivers are
 139 allowed to return a ``EBUSY`` error from these ioctls if any buffer is
 140 currently queued, without checking the queued buffers sizes.
 141
 142 Additionally, drivers shall return a ``EINVAL`` error from the
 143 :c:func:`VIDIOC_QBUF` ioctl if the buffer being queued is too small for the
 144 current format or controls. Together, these requirements ensure that queued
 145 buffers will always be large enough for the configured format and controls.
 146
 147 Userspace applications can query the buffer size required for a given format
 148 and controls by first setting the desired control values and then trying the
 149 desired format. The :c:func:`VIDIOC_TRY_FMT` ioctl will return the required
 150 buffer size.
 151
 152  #. VIDIOC_S_EXT_CTRLS(x)
 153  #. VIDIOC_TRY_FMT()
 154  #. VIDIOC_S_EXT_CTRLS(y)
 155  #. VIDIOC_TRY_FMT()
 156
 157 The :c:func:`VIDIOC_CREATE_BUFS` ioctl can then be used to allocate buffers
 158 based on the queried sizes (for instance by allocating a set of buffers large
 159 enough for all the desired formats and controls, or by allocating separate set
 160 of appropriately sized buffers for each use case).
 161
 162
 163 .. c:type:: v4l2_buffer
 164
 165 struct v4l2_buffer
 166 ==================
 167
 168 .. tabularcolumns:: |p{2.8cm}|p{2.5cm}|p{1.3cm}|p{10.5cm}|
 169
 170 .. cssclass:: longtable
 171
 172 .. flat-table:: struct v4l2_buffer
 173     :header-rows:  0
 174     :stub-columns: 0
 175     :widths:       1 2 1 10
 176
 177     * - __u32
 178       - ``index``
 179       -
 180       - Number of the buffer, set by the application except when calling
 181         :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>`, then it is set by the
 182         driver. This field can range from zero to the number of buffers
 183         allocated with the :ref:`VIDIOC_REQBUFS` ioctl
 184         (struct :c:type:`v4l2_requestbuffers`
 185         ``count``), plus any buffers allocated with
 186         :ref:`VIDIOC_CREATE_BUFS` minus one.
 187     * - __u32
 188       - ``type``
 189       -
 190       - Type of the buffer, same as struct
 191         :c:type:`v4l2_format` ``type`` or struct
 192         :c:type:`v4l2_requestbuffers` ``type``, set
 193         by the application. See :c:type:`v4l2_buf_type`
 194     * - __u32
 195       - ``bytesused``
 196       -
 197       - The number of bytes occupied by the data in the buffer. It depends
 198         on the negotiated data format and may change with each buffer for
 199         compressed variable size data like JPEG images. Drivers must set
 200         this field when ``type`` refers to a capture stream, applications
 201         when it refers to an output stream. If the application sets this
 202         to 0 for an output stream, then ``bytesused`` will be set to the
 203         size of the buffer (see the ``length`` field of this struct) by
 204         the driver. For multiplanar formats this field is ignored and the
 205         ``planes`` pointer is used instead.
 206     * - __u32
 207       - ``flags``
 208       -
 209       - Flags set by the application or driver, see :ref:`buffer-flags`.
 210     * - __u32
 211       - ``field``
 212       -
 213       - Indicates the field order of the image in the buffer, see
 214         :c:type:`v4l2_field`. This field is not used when the buffer
 215         contains VBI data. Drivers must set it when ``type`` refers to a
 216         capture stream, applications when it refers to an output stream.
 217     * - struct timeval
 218       - ``timestamp``
 219       -
 220       - For capture streams this is time when the first data byte was
 221         captured, as returned by the :c:func:`clock_gettime()` function
 222         for the relevant clock id; see ``V4L2_BUF_FLAG_TIMESTAMP_*`` in
 223         :ref:`buffer-flags`. For output streams the driver stores the
 224         time at which the last data byte was actually sent out in the
 225         ``timestamp`` field. This permits applications to monitor the
 226         drift between the video and system clock. For output streams that
 227         use ``V4L2_BUF_FLAG_TIMESTAMP_COPY`` the application has to fill
 228         in the timestamp which will be copied by the driver to the capture
 229         stream.
 230     * - struct :c:type:`v4l2_timecode`
 231       - ``timecode``
 232       -
 233       - When the ``V4L2_BUF_FLAG_TIMECODE`` flag is set in ``flags``, this
 234         structure contains a frame timecode. In
 235         :c:type:`V4L2_FIELD_ALTERNATE <v4l2_field>` mode the top and
 236         bottom field contain the same timecode. Timecodes are intended to
 237         help video editing and are typically recorded on video tapes, but
 238         also embedded in compressed formats like MPEG. This field is
 239         independent of the ``timestamp`` and ``sequence`` fields.
 240     * - __u32
 241       - ``sequence``
 242       -
 243       - Set by the driver, counting the frames (not fields!) in sequence.
 244         This field is set for both input and output devices.
 245     * - :cspan:`3`
 246
 247         In :c:type:`V4L2_FIELD_ALTERNATE <v4l2_field>` mode the top and
 248         bottom field have the same sequence number. The count starts at
 249         zero and includes dropped or repeated frames. A dropped frame was
 250         received by an input device but could not be stored due to lack of
 251         free buffer space. A repeated frame was displayed again by an
 252         output device because the application did not pass new data in
 253         time.
 254
 255         .. note::
 256
 257            This may count the frames received e.g. over USB, without
 258            taking into account the frames dropped by the remote hardware due
 259            to limited compression throughput or bus bandwidth. These devices
 260            identify by not enumerating any video standards, see
 261            :ref:`standard`.
 262
 263     * - __u32
 264       - ``memory``
 265       -
 266       - This field must be set by applications and/or drivers in
 267         accordance with the selected I/O method. See :c:type:`v4l2_memory`
 268     * - union
 269       - ``m``
 270     * -
 271       - __u32
 272       - ``offset``
 273       - For the single-planar API and when ``memory`` is
 274         ``V4L2_MEMORY_MMAP`` this is the offset of the buffer from the
 275         start of the device memory. The value is returned by the driver
 276         and apart of serving as parameter to the
 277         :ref:`mmap() <func-mmap>` function not useful for applications.
 278         See :ref:`mmap` for details
 279     * -
 280       - unsigned long
 281       - ``userptr``
 282       - For the single-planar API and when ``memory`` is
 283         ``V4L2_MEMORY_USERPTR`` this is a pointer to the buffer (casted to
 284         unsigned long type) in virtual memory, set by the application. See
 285         :ref:`userp` for details.
 286     * -
 287       - struct v4l2_plane
 288       - ``*planes``
 289       - When using the multi-planar API, contains a userspace pointer to
 290         an array of struct :c:type:`v4l2_plane`. The size of
 291         the array should be put in the ``length`` field of this
 292         struct :c:type:`v4l2_buffer` structure.
 293     * -
 294       - int
 295       - ``fd``
 296       - For the single-plane API and when ``memory`` is
 297         ``V4L2_MEMORY_DMABUF`` this is the file descriptor associated with
 298         a DMABUF buffer.
 299     * - __u32
 300       - ``length``
 301       -
 302       - Size of the buffer (not the payload) in bytes for the
 303         single-planar API. This is set by the driver based on the calls to
 304         :ref:`VIDIOC_REQBUFS` and/or
 305         :ref:`VIDIOC_CREATE_BUFS`. For the
 306         multi-planar API the application sets this to the number of
 307         elements in the ``planes`` array. The driver will fill in the
 308         actual number of valid elements in that array.
 309     * - __u32
 310       - ``reserved2``
 311       -
 312       - A place holder for future extensions. Drivers and applications
 313         must set this to 0.
 314     * - __u32
 315       - ``request_fd``
 316       -
 317       - The file descriptor of the request to queue the buffer to. If the flag
 318         ``V4L2_BUF_FLAG_REQUEST_FD`` is set, then the buffer will be
 319         queued to this request. If the flag is not set, then this field will
 320         be ignored.
 321
 322         The ``V4L2_BUF_FLAG_REQUEST_FD`` flag and this field are only used by
 323         :ref:`ioctl VIDIOC_QBUF <VIDIOC_QBUF>` and ignored by other ioctls that
 324         take a :c:type:`v4l2_buffer` as argument.
 325
 326         Applications should not set ``V4L2_BUF_FLAG_REQUEST_FD`` for any ioctls
 327         other than :ref:`VIDIOC_QBUF <VIDIOC_QBUF>`.
 328
 329         If the device does not support requests, then ``EACCES`` will be returned.
 330         If requests are supported but an invalid request file descriptor is
 331         given, then ``EINVAL`` will be returned.
 332
 333
 334
 335 .. c:type:: v4l2_plane
 336
 337 struct v4l2_plane
 338 =================
 339
 340 .. tabularcolumns:: |p{3.5cm}|p{3.5cm}|p{3.5cm}|p{7.0cm}|
 341
 342 .. cssclass:: longtable
 343
 344 .. flat-table::
 345     :header-rows:  0
 346     :stub-columns: 0
 347     :widths:       1 1 1 2
 348
 349     * - __u32
 350       - ``bytesused``
 351       -
 352       - The number of bytes occupied by data in the plane (its payload).
 353         Drivers must set this field when ``type`` refers to a capture
 354         stream, applications when it refers to an output stream. If the
 355         application sets this to 0 for an output stream, then
 356         ``bytesused`` will be set to the size of the plane (see the
 357         ``length`` field of this struct) by the driver.
 358
 359         .. note::
 360
 361            Note that the actual image data starts at ``data_offset``
 362            which may not be 0.
 363     * - __u32
 364       - ``length``
 365       -
 366       - Size in bytes of the plane (not its payload). This is set by the
 367         driver based on the calls to
 368         :ref:`VIDIOC_REQBUFS` and/or
 369         :ref:`VIDIOC_CREATE_BUFS`.
 370     * - union
 371       - ``m``
 372       -
 373       -
 374     * -
 375       - __u32
 376       - ``mem_offset``
 377       - When the memory type in the containing struct
 378         :c:type:`v4l2_buffer` is ``V4L2_MEMORY_MMAP``, this
 379         is the value that should be passed to :ref:`mmap() <func-mmap>`,
 380         similar to the ``offset`` field in struct
 381         :c:type:`v4l2_buffer`.
 382     * -
 383       - unsigned long
 384       - ``userptr``
 385       - When the memory type in the containing struct
 386         :c:type:`v4l2_buffer` is ``V4L2_MEMORY_USERPTR``,
 387         this is a userspace pointer to the memory allocated for this plane
 388         by an application.
 389     * -
 390       - int
 391       - ``fd``
 392       - When the memory type in the containing struct
 393         :c:type:`v4l2_buffer` is ``V4L2_MEMORY_DMABUF``,
 394         this is a file descriptor associated with a DMABUF buffer, similar
 395         to the ``fd`` field in struct :c:type:`v4l2_buffer`.
 396     * - __u32
 397       - ``data_offset``
 398       -
 399       - Offset in bytes to video data in the plane. Drivers must set this
 400         field when ``type`` refers to a capture stream, applications when
 401         it refers to an output stream.
 402
 403         .. note::
 404
 405            That data_offset is included  in ``bytesused``. So the
 406            size of the image in the plane is ``bytesused``-``data_offset``
 407            at offset ``data_offset`` from the start of the plane.
 408     * - __u32
 409       - ``reserved[11]``
 410       -
 411       - Reserved for future use. Should be zeroed by drivers and
 412         applications.
 413
 414
 415
 416 .. c:type:: v4l2_buf_type
 417
 418 enum v4l2_buf_type
 419 ==================
 420
 421 .. cssclass:: longtable
 422
 423 .. tabularcolumns:: |p{7.2cm}|p{0.6cm}|p{9.7cm}|
 424
 425 .. flat-table::
 426     :header-rows:  0
 427     :stub-columns: 0
 428     :widths:       4 1 9
 429
 430     * - ``V4L2_BUF_TYPE_VIDEO_CAPTURE``
 431       - 1
 432       - Buffer of a single-planar video capture stream, see
 433         :ref:`capture`.
 434     * - ``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE``
 435       - 9
 436       - Buffer of a multi-planar video capture stream, see
 437         :ref:`capture`.
 438     * - ``V4L2_BUF_TYPE_VIDEO_OUTPUT``
 439       - 2
 440       - Buffer of a single-planar video output stream, see
 441         :ref:`output`.
 442     * - ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``
 443       - 10
 444       - Buffer of a multi-planar video output stream, see :ref:`output`.
 445     * - ``V4L2_BUF_TYPE_VIDEO_OVERLAY``
 446       - 3
 447       - Buffer for video overlay, see :ref:`overlay`.
 448     * - ``V4L2_BUF_TYPE_VBI_CAPTURE``
 449       - 4
 450       - Buffer of a raw VBI capture stream, see :ref:`raw-vbi`.
 451     * - ``V4L2_BUF_TYPE_VBI_OUTPUT``
 452       - 5
 453       - Buffer of a raw VBI output stream, see :ref:`raw-vbi`.
 454     * - ``V4L2_BUF_TYPE_SLICED_VBI_CAPTURE``
 455       - 6
 456       - Buffer of a sliced VBI capture stream, see :ref:`sliced`.
 457     * - ``V4L2_BUF_TYPE_SLICED_VBI_OUTPUT``
 458       - 7
 459       - Buffer of a sliced VBI output stream, see :ref:`sliced`.
 460     * - ``V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY``
 461       - 8
 462       - Buffer for video output overlay (OSD), see :ref:`osd`.
 463     * - ``V4L2_BUF_TYPE_SDR_CAPTURE``
 464       - 11
 465       - Buffer for Software Defined Radio (SDR) capture stream, see
 466         :ref:`sdr`.
 467     * - ``V4L2_BUF_TYPE_SDR_OUTPUT``
 468       - 12
 469       - Buffer for Software Defined Radio (SDR) output stream, see
 470         :ref:`sdr`.
 471     * - ``V4L2_BUF_TYPE_META_CAPTURE``
 472       - 13
 473       - Buffer for metadata capture, see :ref:`metadata`.
 474     * - ``V4L2_BUF_TYPE_META_OUTPUT``
 475       - 14
 476       - Buffer for metadata output, see :ref:`metadata`.
 477
 478
 479
 480 .. _buffer-flags:
 481
 482 Buffer Flags
 483 ============
 484
 485 .. tabularcolumns:: |p{7.0cm}|p{2.2cm}|p{8.3cm}|
 486
 487 .. cssclass:: longtable
 488
 489 .. flat-table::
 490     :header-rows:  0
 491     :stub-columns: 0
 492     :widths:       3 1 4
 493
 494     * .. _`V4L2-BUF-FLAG-MAPPED`:
 495
 496       - ``V4L2_BUF_FLAG_MAPPED``
 497       - 0x00000001
 498       - The buffer resides in device memory and has been mapped into the
 499         application's address space, see :ref:`mmap` for details.
 500         Drivers set or clear this flag when the
 501         :ref:`VIDIOC_QUERYBUF`,
 502         :ref:`VIDIOC_QBUF` or
 503         :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl is called. Set by the
 504         driver.
 505     * .. _`V4L2-BUF-FLAG-QUEUED`:
 506
 507       - ``V4L2_BUF_FLAG_QUEUED``
 508       - 0x00000002
 509       - Internally drivers maintain two buffer queues, an incoming and
 510         outgoing queue. When this flag is set, the buffer is currently on
 511         the incoming queue. It automatically moves to the outgoing queue
 512         after the buffer has been filled (capture devices) or displayed
 513         (output devices). Drivers set or clear this flag when the
 514         ``VIDIOC_QUERYBUF`` ioctl is called. After (successful) calling
 515         the ``VIDIOC_QBUF``\ ioctl it is always set and after
 516         ``VIDIOC_DQBUF`` always cleared.
 517     * .. _`V4L2-BUF-FLAG-DONE`:
 518
 519       - ``V4L2_BUF_FLAG_DONE``
 520       - 0x00000004
 521       - When this flag is set, the buffer is currently on the outgoing
 522         queue, ready to be dequeued from the driver. Drivers set or clear
 523         this flag when the ``VIDIOC_QUERYBUF`` ioctl is called. After
 524         calling the ``VIDIOC_QBUF`` or ``VIDIOC_DQBUF`` it is always
 525         cleared. Of course a buffer cannot be on both queues at the same
 526         time, the ``V4L2_BUF_FLAG_QUEUED`` and ``V4L2_BUF_FLAG_DONE`` flag
 527         are mutually exclusive. They can be both cleared however, then the
 528         buffer is in "dequeued" state, in the application domain so to
 529         say.
 530     * .. _`V4L2-BUF-FLAG-ERROR`:
 531
 532       - ``V4L2_BUF_FLAG_ERROR``
 533       - 0x00000040
 534       - When this flag is set, the buffer has been dequeued successfully,
 535         although the data might have been corrupted. This is recoverable,
 536         streaming may continue as normal and the buffer may be reused
 537         normally. Drivers set this flag when the ``VIDIOC_DQBUF`` ioctl is
 538         called.
 539     * .. _`V4L2-BUF-FLAG-IN-REQUEST`:
 540
 541       - ``V4L2_BUF_FLAG_IN_REQUEST``
 542       - 0x00000080
 543       - This buffer is part of a request that hasn't been queued yet.
 544     * .. _`V4L2-BUF-FLAG-KEYFRAME`:
 545
 546       - ``V4L2_BUF_FLAG_KEYFRAME``
 547       - 0x00000008
 548       - Drivers set or clear this flag when calling the ``VIDIOC_DQBUF``
 549         ioctl. It may be set by video capture devices when the buffer
 550         contains a compressed image which is a key frame (or field), i. e.
 551         can be decompressed on its own. Also known as an I-frame.
 552         Applications can set this bit when ``type`` refers to an output
 553         stream.
 554     * .. _`V4L2-BUF-FLAG-PFRAME`:
 555
 556       - ``V4L2_BUF_FLAG_PFRAME``
 557       - 0x00000010
 558       - Similar to ``V4L2_BUF_FLAG_KEYFRAME`` this flags predicted frames
 559         or fields which contain only differences to a previous key frame.
 560         Applications can set this bit when ``type`` refers to an output
 561         stream.
 562     * .. _`V4L2-BUF-FLAG-BFRAME`:
 563
 564       - ``V4L2_BUF_FLAG_BFRAME``
 565       - 0x00000020
 566       - Similar to ``V4L2_BUF_FLAG_KEYFRAME`` this flags a bi-directional
 567         predicted frame or field which contains only the differences
 568         between the current frame and both the preceding and following key
 569         frames to specify its content. Applications can set this bit when
 570         ``type`` refers to an output stream.
 571     * .. _`V4L2-BUF-FLAG-TIMECODE`:
 572
 573       - ``V4L2_BUF_FLAG_TIMECODE``
 574       - 0x00000100
 575       - The ``timecode`` field is valid. Drivers set or clear this flag
 576         when the ``VIDIOC_DQBUF`` ioctl is called. Applications can set
 577         this bit and the corresponding ``timecode`` structure when
 578         ``type`` refers to an output stream.
 579     * .. _`V4L2-BUF-FLAG-PREPARED`:
 580
 581       - ``V4L2_BUF_FLAG_PREPARED``
 582       - 0x00000400
 583       - The buffer has been prepared for I/O and can be queued by the
 584         application. Drivers set or clear this flag when the
 585         :ref:`VIDIOC_QUERYBUF`,
 586         :ref:`VIDIOC_PREPARE_BUF <VIDIOC_QBUF>`,
 587         :ref:`VIDIOC_QBUF` or
 588         :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl is called.
 589     * .. _`V4L2-BUF-FLAG-NO-CACHE-INVALIDATE`:
 590
 591       - ``V4L2_BUF_FLAG_NO_CACHE_INVALIDATE``
 592       - 0x00000800
 593       - Caches do not have to be invalidated for this buffer. Typically
 594         applications shall use this flag if the data captured in the
 595         buffer is not going to be touched by the CPU, instead the buffer
 596         will, probably, be passed on to a DMA-capable hardware unit for
 597         further processing or output.
 598     * .. _`V4L2-BUF-FLAG-NO-CACHE-CLEAN`:
 599
 600       - ``V4L2_BUF_FLAG_NO_CACHE_CLEAN``
 601       - 0x00001000
 602       - Caches do not have to be cleaned for this buffer. Typically
 603         applications shall use this flag for output buffers if the data in
 604         this buffer has not been created by the CPU but by some
 605         DMA-capable unit, in which case caches have not been used.
 606     * .. _`V4L2-BUF-FLAG-LAST`:
 607
 608       - ``V4L2_BUF_FLAG_LAST``
 609       - 0x00100000
 610       - Last buffer produced by the hardware. mem2mem codec drivers set
 611         this flag on the capture queue for the last buffer when the
 612         :ref:`VIDIOC_QUERYBUF` or
 613         :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl is called. Due to
 614         hardware limitations, the last buffer may be empty. In this case
 615         the driver will set the ``bytesused`` field to 0, regardless of
 616         the format. Any Any subsequent call to the
 617         :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl will not block anymore,
 618         but return an ``EPIPE`` error code.
 619     * .. _`V4L2-BUF-FLAG-REQUEST-FD`:
 620
 621       - ``V4L2_BUF_FLAG_REQUEST_FD``
 622       - 0x00800000
 623       - The ``request_fd`` field contains a valid file descriptor.
 624     * .. _`V4L2-BUF-FLAG-TIMESTAMP-MASK`:
 625
 626       - ``V4L2_BUF_FLAG_TIMESTAMP_MASK``
 627       - 0x0000e000
 628       - Mask for timestamp types below. To test the timestamp type, mask
 629         out bits not belonging to timestamp type by performing a logical
 630         and operation with buffer flags and timestamp mask.
 631     * .. _`V4L2-BUF-FLAG-TIMESTAMP-UNKNOWN`:
 632
 633       - ``V4L2_BUF_FLAG_TIMESTAMP_UNKNOWN``
 634       - 0x00000000
 635       - Unknown timestamp type. This type is used by drivers before Linux
 636         3.9 and may be either monotonic (see below) or realtime (wall
 637         clock). Monotonic clock has been favoured in embedded systems
 638         whereas most of the drivers use the realtime clock. Either kinds
 639         of timestamps are available in user space via
 640         :c:func:`clock_gettime` using clock IDs ``CLOCK_MONOTONIC``
 641         and ``CLOCK_REALTIME``, respectively.
 642     * .. _`V4L2-BUF-FLAG-TIMESTAMP-MONOTONIC`:
 643
 644       - ``V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC``
 645       - 0x00002000
 646       - The buffer timestamp has been taken from the ``CLOCK_MONOTONIC``
 647         clock. To access the same clock outside V4L2, use
 648         :c:func:`clock_gettime`.
 649     * .. _`V4L2-BUF-FLAG-TIMESTAMP-COPY`:
 650
 651       - ``V4L2_BUF_FLAG_TIMESTAMP_COPY``
 652       - 0x00004000
 653       - The CAPTURE buffer timestamp has been taken from the corresponding
 654         OUTPUT buffer. This flag applies only to mem2mem devices.
 655     * .. _`V4L2-BUF-FLAG-TSTAMP-SRC-MASK`:
 656
 657       - ``V4L2_BUF_FLAG_TSTAMP_SRC_MASK``
 658       - 0x00070000
 659       - Mask for timestamp sources below. The timestamp source defines the
 660         point of time the timestamp is taken in relation to the frame.
 661         Logical 'and' operation between the ``flags`` field and
 662         ``V4L2_BUF_FLAG_TSTAMP_SRC_MASK`` produces the value of the
 663         timestamp source. Applications must set the timestamp source when
 664         ``type`` refers to an output stream and
 665         ``V4L2_BUF_FLAG_TIMESTAMP_COPY`` is set.
 666     * .. _`V4L2-BUF-FLAG-TSTAMP-SRC-EOF`:
 667
 668       - ``V4L2_BUF_FLAG_TSTAMP_SRC_EOF``
 669       - 0x00000000
 670       - End Of Frame. The buffer timestamp has been taken when the last
 671         pixel of the frame has been received or the last pixel of the
 672         frame has been transmitted. In practice, software generated
 673         timestamps will typically be read from the clock a small amount of
 674         time after the last pixel has been received or transmitten,
 675         depending on the system and other activity in it.
 676     * .. _`V4L2-BUF-FLAG-TSTAMP-SRC-SOE`:
 677
 678       - ``V4L2_BUF_FLAG_TSTAMP_SRC_SOE``
 679       - 0x00010000
 680       - Start Of Exposure. The buffer timestamp has been taken when the
 681         exposure of the frame has begun. This is only valid for the
 682         ``V4L2_BUF_TYPE_VIDEO_CAPTURE`` buffer type.
 683
 684
 685
 686 .. c:type:: v4l2_memory
 687
 688 enum v4l2_memory
 689 ================
 690
 691 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
 692
 693 .. flat-table::
 694     :header-rows:  0
 695     :stub-columns: 0
 696     :widths:       3 1 4
 697
 698     * - ``V4L2_MEMORY_MMAP``
 699       - 1
 700       - The buffer is used for :ref:`memory mapping <mmap>` I/O.
 701     * - ``V4L2_MEMORY_USERPTR``
 702       - 2
 703       - The buffer is used for :ref:`user pointer <userp>` I/O.
 704     * - ``V4L2_MEMORY_OVERLAY``
 705       - 3
 706       - [to do]
 707     * - ``V4L2_MEMORY_DMABUF``
 708       - 4
 709       - The buffer is used for :ref:`DMA shared buffer <dmabuf>` I/O.
 710
 711
 712
 713 Timecodes
 714 =========
 715
 716 The :c:type:`v4l2_buffer_timecode` structure is designed to hold a
 717 :ref:`smpte12m` or similar timecode.
 718 (struct :c:type:`timeval` timestamps are stored in the struct
 719 :c:type:`v4l2_buffer` ``timestamp`` field.)
 720
 721
 722 .. c:type:: v4l2_timecode
 723
 724 struct v4l2_timecode
 725 --------------------
 726
 727 .. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
 728
 729 .. flat-table::
 730     :header-rows:  0
 731     :stub-columns: 0
 732     :widths:       1 1 2
 733
 734     * - __u32
 735       - ``type``
 736       - Frame rate the timecodes are based on, see :ref:`timecode-type`.
 737     * - __u32
 738       - ``flags``
 739       - Timecode flags, see :ref:`timecode-flags`.
 740     * - __u8
 741       - ``frames``
 742       - Frame count, 0 ... 23/24/29/49/59, depending on the type of
 743         timecode.
 744     * - __u8
 745       - ``seconds``
 746       - Seconds count, 0 ... 59. This is a binary, not BCD number.
 747     * - __u8
 748       - ``minutes``
 749       - Minutes count, 0 ... 59. This is a binary, not BCD number.
 750     * - __u8
 751       - ``hours``
 752       - Hours count, 0 ... 29. This is a binary, not BCD number.
 753     * - __u8
 754       - ``userbits``\ [4]
 755       - The "user group" bits from the timecode.
 756
 757
 758
 759 .. _timecode-type:
 760
 761 Timecode Types
 762 --------------
 763
 764 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
 765
 766 .. flat-table::
 767     :header-rows:  0
 768     :stub-columns: 0
 769     :widths:       3 1 4
 770
 771     * - ``V4L2_TC_TYPE_24FPS``
 772       - 1
 773       - 24 frames per second, i. e. film.
 774     * - ``V4L2_TC_TYPE_25FPS``
 775       - 2
 776       - 25 frames per second, i. e. PAL or SECAM video.
 777     * - ``V4L2_TC_TYPE_30FPS``
 778       - 3
 779       - 30 frames per second, i. e. NTSC video.
 780     * - ``V4L2_TC_TYPE_50FPS``
 781       - 4
 782       -
 783     * - ``V4L2_TC_TYPE_60FPS``
 784       - 5
 785       -
 786
 787
 788
 789 .. _timecode-flags:
 790
 791 Timecode Flags
 792 --------------
 793
 794 .. tabularcolumns:: |p{6.6cm}|p{1.4cm}|p{9.5cm}|
 795
 796 .. flat-table::
 797     :header-rows:  0
 798     :stub-columns: 0
 799     :widths:       3 1 4
 800
 801     * - ``V4L2_TC_FLAG_DROPFRAME``
 802       - 0x0001
 803       - Indicates "drop frame" semantics for counting frames in 29.97 fps
 804         material. When set, frame numbers 0 and 1 at the start of each
 805         minute, except minutes 0, 10, 20, 30, 40, 50 are omitted from the
 806         count.
 807     * - ``V4L2_TC_FLAG_COLORFRAME``
 808       - 0x0002
 809       - The "color frame" flag.
 810     * - ``V4L2_TC_USERBITS_field``
 811       - 0x000C
 812       - Field mask for the "binary group flags".
 813     * - ``V4L2_TC_USERBITS_USERDEFINED``
 814       - 0x0000
 815       - Unspecified format.
 816     * - ``V4L2_TC_USERBITS_8BITCHARS``
 817       - 0x0008
 818       - 8-bit ISO characters.