Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
tjh.dev
kernel
os
linux
1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2.. c:namespace:: V4L
3
4.. _buffer:
5
6*******
7Buffers
8*******
9
10A buffer contains data exchanged by application and driver using one of
11the Streaming I/O methods. In the multi-planar API, the data is held in
12planes, while the buffer structure acts as a container for the planes.
13Only pointers to buffers (planes) are exchanged, the data itself is not
14copied. These pointers, together with meta-information like timestamps
15or field parity, are stored in a struct :c:type:`v4l2_buffer`,
16argument to the :ref:`VIDIOC_QUERYBUF`,
17:ref:`VIDIOC_QBUF <VIDIOC_QBUF>` and
18:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl. In the multi-planar API,
19some plane-specific members of struct :c:type:`v4l2_buffer`,
20such as pointers and sizes for each plane, are stored in
21struct :c:type:`v4l2_plane` instead. In that case,
22struct :c:type:`v4l2_buffer` contains an array of plane structures.
23
24Dequeued video buffers come with timestamps. The driver decides at which
25part of the frame and with which clock the timestamp is taken. Please
26see flags in the masks ``V4L2_BUF_FLAG_TIMESTAMP_MASK`` and
27``V4L2_BUF_FLAG_TSTAMP_SRC_MASK`` in :ref:`buffer-flags`. These flags
28are always valid and constant across all buffers during the whole video
29stream. Changes in these flags may take place as a side effect of
30:ref:`VIDIOC_S_INPUT <VIDIOC_G_INPUT>` or
31:ref:`VIDIOC_S_OUTPUT <VIDIOC_G_OUTPUT>` however. The
32``V4L2_BUF_FLAG_TIMESTAMP_COPY`` timestamp type which is used by e.g. on
33mem-to-mem devices is an exception to the rule: the timestamp source
34flags are copied from the OUTPUT video buffer to the CAPTURE video
35buffer.
36
37Interactions between formats, controls and buffers
38==================================================
39
40V4L2 exposes parameters that influence the buffer size, or the way data is
41laid out in the buffer. Those parameters are exposed through both formats and
42controls. One example of such a control is the ``V4L2_CID_ROTATE`` control
43that modifies the direction in which pixels are stored in the buffer, as well
44as the buffer size when the selected format includes padding at the end of
45lines.
46
47The set of information needed to interpret the content of a buffer (e.g. the
48pixel format, the line stride, the tiling orientation or the rotation) is
49collectively referred to in the rest of this section as the buffer layout.
50
51Controls that can modify the buffer layout shall set the
52``V4L2_CTRL_FLAG_MODIFY_LAYOUT`` flag.
53
54Modifying formats or controls that influence the buffer size or layout require
55the stream to be stopped. Any attempt at such a modification while the stream
56is active shall cause the ioctl setting the format or the control to return
57the ``EBUSY`` error code. In that case drivers shall also set the
58``V4L2_CTRL_FLAG_GRABBED`` flag when calling
59:c:func:`VIDIOC_QUERYCTRL` or :c:func:`VIDIOC_QUERY_EXT_CTRL` for such a
60control while the stream is active.
61
62.. note::
63
64 The :c:func:`VIDIOC_S_SELECTION` ioctl can, depending on the hardware (for
65 instance if the device doesn't include a scaler), modify the format in
66 addition to the selection rectangle. Similarly, the
67 :c:func:`VIDIOC_S_INPUT`, :c:func:`VIDIOC_S_OUTPUT`, :c:func:`VIDIOC_S_STD`
68 and :c:func:`VIDIOC_S_DV_TIMINGS` ioctls can also modify the format and
69 selection rectangles. When those ioctls result in a buffer size or layout
70 change, drivers shall handle that condition as they would handle it in the
71 :c:func:`VIDIOC_S_FMT` ioctl in all cases described in this section.
72
73Controls that only influence the buffer layout can be modified at any time
74when the stream is stopped. As they don't influence the buffer size, no
75special handling is needed to synchronize those controls with buffer
76allocation and the ``V4L2_CTRL_FLAG_GRABBED`` flag is cleared once the
77stream is stopped.
78
79Formats and controls that influence the buffer size interact with buffer
80allocation. The simplest way to handle this is for drivers to always require
81buffers to be reallocated in order to change those formats or controls. In
82that case, to perform such changes, userspace applications shall first stop
83the video stream with the :c:func:`VIDIOC_STREAMOFF` ioctl if it is running
84and free all buffers with the :c:func:`VIDIOC_REQBUFS` ioctl if they are
85allocated. After freeing all buffers the ``V4L2_CTRL_FLAG_GRABBED`` flag
86for controls is cleared. The format or controls can then be modified, and
87buffers shall then be reallocated and the stream restarted. A typical ioctl
88sequence is
89
90 #. VIDIOC_STREAMOFF
91 #. VIDIOC_REQBUFS(0)
92 #. VIDIOC_S_EXT_CTRLS
93 #. VIDIOC_S_FMT
94 #. VIDIOC_REQBUFS(n)
95 #. VIDIOC_QBUF
96 #. VIDIOC_STREAMON
97
98The second :c:func:`VIDIOC_REQBUFS` call will take the new format and control
99value into account to compute the buffer size to allocate. Applications can
100also retrieve the size by calling the :c:func:`VIDIOC_G_FMT` ioctl if needed.
101
102.. note::
103
104 The API doesn't mandate the above order for control (3.) and format (4.)
105 changes. Format and controls can be set in a different order, or even
106 interleaved, depending on the device and use case. For instance some
107 controls might behave differently for different pixel formats, in which
108 case the format might need to be set first.
109
110When reallocation is required, any attempt to modify format or controls that
111influences the buffer size while buffers are allocated shall cause the format
112or control set ioctl to return the ``EBUSY`` error. Any attempt to queue a
113buffer too small for the current format or controls shall cause the
114:c:func:`VIDIOC_QBUF` ioctl to return a ``EINVAL`` error.
115
116Buffer reallocation is an expensive operation. To avoid that cost, drivers can
117(and are encouraged to) allow format or controls that influence the buffer
118size to be changed with buffers allocated. In that case, a typical ioctl
119sequence to modify format and controls is
120
121 #. VIDIOC_STREAMOFF
122 #. VIDIOC_S_EXT_CTRLS
123 #. VIDIOC_S_FMT
124 #. VIDIOC_QBUF
125 #. VIDIOC_STREAMON
126
127For this sequence to operate correctly, queued buffers need to be large enough
128for the new format or controls. Drivers shall return a ``ENOSPC`` error in
129response to format change (:c:func:`VIDIOC_S_FMT`) or control changes
130(:c:func:`VIDIOC_S_CTRL` or :c:func:`VIDIOC_S_EXT_CTRLS`) if buffers too small
131for the new format are currently queued. As a simplification, drivers are
132allowed to return a ``EBUSY`` error from these ioctls if any buffer is
133currently queued, without checking the queued buffers sizes.
134
135Additionally, drivers shall return a ``EINVAL`` error from the
136:c:func:`VIDIOC_QBUF` ioctl if the buffer being queued is too small for the
137current format or controls. Together, these requirements ensure that queued
138buffers will always be large enough for the configured format and controls.
139
140Userspace applications can query the buffer size required for a given format
141and controls by first setting the desired control values and then trying the
142desired format. The :c:func:`VIDIOC_TRY_FMT` ioctl will return the required
143buffer size.
144
145 #. VIDIOC_S_EXT_CTRLS(x)
146 #. VIDIOC_TRY_FMT()
147 #. VIDIOC_S_EXT_CTRLS(y)
148 #. VIDIOC_TRY_FMT()
149
150The :c:func:`VIDIOC_CREATE_BUFS` ioctl can then be used to allocate buffers
151based on the queried sizes (for instance by allocating a set of buffers large
152enough for all the desired formats and controls, or by allocating separate set
153of appropriately sized buffers for each use case).
154
155.. c:type:: v4l2_buffer
156
157struct v4l2_buffer
158==================
159
160.. tabularcolumns:: |p{2.9cm}|p{2.4cm}|p{12.0cm}|
161
162.. cssclass:: longtable
163
164.. flat-table:: struct v4l2_buffer
165 :header-rows: 0
166 :stub-columns: 0
167 :widths: 1 2 10
168
169 * - __u32
170 - ``index``
171 - Number of the buffer, set by the application except when calling
172 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>`, then it is set by the
173 driver. This field can range from zero to the number of buffers
174 allocated with the :ref:`VIDIOC_REQBUFS` ioctl
175 (struct :c:type:`v4l2_requestbuffers`
176 ``count``), plus any buffers allocated with
177 :ref:`VIDIOC_CREATE_BUFS` minus one.
178 * - __u32
179 - ``type``
180 - Type of the buffer, same as struct
181 :c:type:`v4l2_format` ``type`` or struct
182 :c:type:`v4l2_requestbuffers` ``type``, set
183 by the application. See :c:type:`v4l2_buf_type`
184 * - __u32
185 - ``bytesused``
186 - The number of bytes occupied by the data in the buffer. It depends
187 on the negotiated data format and may change with each buffer for
188 compressed variable size data like JPEG images. Drivers must set
189 this field when ``type`` refers to a capture stream, applications
190 when it refers to an output stream. For multiplanar formats this field
191 is ignored and the
192 ``planes`` pointer is used instead.
193 * - __u32
194 - ``flags``
195 - Flags set by the application or driver, see :ref:`buffer-flags`.
196 * - __u32
197 - ``field``
198 - Indicates the field order of the image in the buffer, see
199 :c:type:`v4l2_field`. This field is not used when the buffer
200 contains VBI data. Drivers must set it when ``type`` refers to a
201 capture stream, applications when it refers to an output stream.
202 * - struct timeval
203 - ``timestamp``
204 - For capture streams this is time when the first data byte was
205 captured, as returned by the :c:func:`clock_gettime()` function
206 for the relevant clock id; see ``V4L2_BUF_FLAG_TIMESTAMP_*`` in
207 :ref:`buffer-flags`. For output streams the driver stores the
208 time at which the last data byte was actually sent out in the
209 ``timestamp`` field. This permits applications to monitor the
210 drift between the video and system clock. For output streams that
211 use ``V4L2_BUF_FLAG_TIMESTAMP_COPY`` the application has to fill
212 in the timestamp which will be copied by the driver to the capture
213 stream.
214 * - struct :c:type:`v4l2_timecode`
215 - ``timecode``
216 - When the ``V4L2_BUF_FLAG_TIMECODE`` flag is set in ``flags``, this
217 structure contains a frame timecode. In
218 :c:type:`V4L2_FIELD_ALTERNATE <v4l2_field>` mode the top and
219 bottom field contain the same timecode. Timecodes are intended to
220 help video editing and are typically recorded on video tapes, but
221 also embedded in compressed formats like MPEG. This field is
222 independent of the ``timestamp`` and ``sequence`` fields.
223 * - __u32
224 - ``sequence``
225 - Set by the driver, counting the frames (not fields!) in sequence.
226 This field is set for both input and output devices.
227 * - :cspan:`2`
228
229 In :c:type:`V4L2_FIELD_ALTERNATE <v4l2_field>` mode the top and
230 bottom field have the same sequence number. The count starts at
231 zero and includes dropped or repeated frames. A dropped frame was
232 received by an input device but could not be stored due to lack of
233 free buffer space. A repeated frame was displayed again by an
234 output device because the application did not pass new data in
235 time.
236
237 .. note::
238
239 This may count the frames received e.g. over USB, without
240 taking into account the frames dropped by the remote hardware due
241 to limited compression throughput or bus bandwidth. These devices
242 identify by not enumerating any video standards, see
243 :ref:`standard`.
244
245 * - __u32
246 - ``memory``
247 - This field must be set by applications and/or drivers in
248 accordance with the selected I/O method. See :c:type:`v4l2_memory`
249 * - union {
250 - ``m``
251 * - __u32
252 - ``offset``
253 - For the single-planar API and when ``memory`` is
254 ``V4L2_MEMORY_MMAP`` this is the offset of the buffer from the
255 start of the device memory. The value is returned by the driver
256 and apart of serving as parameter to the
257 :c:func:`mmap()` function not useful for applications.
258 See :ref:`mmap` for details
259 * - unsigned long
260 - ``userptr``
261 - For the single-planar API and when ``memory`` is
262 ``V4L2_MEMORY_USERPTR`` this is a pointer to the buffer (casted to
263 unsigned long type) in virtual memory, set by the application. See
264 :ref:`userp` for details.
265 * - struct v4l2_plane
266 - ``*planes``
267 - When using the multi-planar API, contains a userspace pointer to
268 an array of struct :c:type:`v4l2_plane`. The size of
269 the array should be put in the ``length`` field of this
270 struct :c:type:`v4l2_buffer` structure.
271 * - int
272 - ``fd``
273 - For the single-plane API and when ``memory`` is
274 ``V4L2_MEMORY_DMABUF`` this is the file descriptor associated with
275 a DMABUF buffer.
276 * - }
277 -
278 * - __u32
279 - ``length``
280 - Size of the buffer (not the payload) in bytes for the
281 single-planar API. This is set by the driver based on the calls to
282 :ref:`VIDIOC_REQBUFS` and/or
283 :ref:`VIDIOC_CREATE_BUFS`. For the
284 multi-planar API the application sets this to the number of
285 elements in the ``planes`` array. The driver will fill in the
286 actual number of valid elements in that array.
287 * - __u32
288 - ``reserved2``
289 - A place holder for future extensions. Drivers and applications
290 must set this to 0.
291 * - __u32
292 - ``request_fd``
293 - The file descriptor of the request to queue the buffer to. If the flag
294 ``V4L2_BUF_FLAG_REQUEST_FD`` is set, then the buffer will be
295 queued to this request. If the flag is not set, then this field will
296 be ignored.
297
298 The ``V4L2_BUF_FLAG_REQUEST_FD`` flag and this field are only used by
299 :ref:`ioctl VIDIOC_QBUF <VIDIOC_QBUF>` and ignored by other ioctls that
300 take a :c:type:`v4l2_buffer` as argument.
301
302 Applications should not set ``V4L2_BUF_FLAG_REQUEST_FD`` for any ioctls
303 other than :ref:`VIDIOC_QBUF <VIDIOC_QBUF>`.
304
305 If the device does not support requests, then ``EBADR`` will be returned.
306 If requests are supported but an invalid request file descriptor is
307 given, then ``EINVAL`` will be returned.
308
309
310.. c:type:: v4l2_plane
311
312struct v4l2_plane
313=================
314
315.. tabularcolumns:: |p{3.5cm}|p{3.5cm}|p{10.3cm}|
316
317.. cssclass:: longtable
318
319.. flat-table::
320 :header-rows: 0
321 :stub-columns: 0
322 :widths: 1 1 2
323
324 * - __u32
325 - ``bytesused``
326 - The number of bytes occupied by data in the plane (its payload).
327 Drivers must set this field when ``type`` refers to a capture
328 stream, applications when it refers to an output stream.
329
330 .. note::
331
332 Note that the actual image data starts at ``data_offset``
333 which may not be 0.
334 * - __u32
335 - ``length``
336 - Size in bytes of the plane (not its payload). This is set by the
337 driver based on the calls to
338 :ref:`VIDIOC_REQBUFS` and/or
339 :ref:`VIDIOC_CREATE_BUFS`.
340 * - union {
341 - ``m``
342 * - __u32
343 - ``mem_offset``
344 - When the memory type in the containing struct
345 :c:type:`v4l2_buffer` is ``V4L2_MEMORY_MMAP``, this
346 is the value that should be passed to :c:func:`mmap()`,
347 similar to the ``offset`` field in struct
348 :c:type:`v4l2_buffer`.
349 * - unsigned long
350 - ``userptr``
351 - When the memory type in the containing struct
352 :c:type:`v4l2_buffer` is ``V4L2_MEMORY_USERPTR``,
353 this is a userspace pointer to the memory allocated for this plane
354 by an application.
355 * - int
356 - ``fd``
357 - When the memory type in the containing struct
358 :c:type:`v4l2_buffer` is ``V4L2_MEMORY_DMABUF``,
359 this is a file descriptor associated with a DMABUF buffer, similar
360 to the ``fd`` field in struct :c:type:`v4l2_buffer`.
361 * - }
362 -
363 * - __u32
364 - ``data_offset``
365 - Offset in bytes to video data in the plane. Drivers must set this
366 field when ``type`` refers to a capture stream, applications when
367 it refers to an output stream.
368
369 .. note::
370
371 That data_offset is included in ``bytesused``. So the
372 size of the image in the plane is ``bytesused``-``data_offset``
373 at offset ``data_offset`` from the start of the plane.
374 * - __u32
375 - ``reserved[11]``
376 - Reserved for future use. Should be zeroed by drivers and
377 applications.
378
379
380.. c:type:: v4l2_buf_type
381
382enum v4l2_buf_type
383==================
384
385.. cssclass:: longtable
386
387.. tabularcolumns:: |p{7.8cm}|p{0.6cm}|p{8.9cm}|
388
389.. flat-table::
390 :header-rows: 0
391 :stub-columns: 0
392 :widths: 4 1 9
393
394 * - ``V4L2_BUF_TYPE_VIDEO_CAPTURE``
395 - 1
396 - Buffer of a single-planar video capture stream, see
397 :ref:`capture`.
398 * - ``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE``
399 - 9
400 - Buffer of a multi-planar video capture stream, see
401 :ref:`capture`.
402 * - ``V4L2_BUF_TYPE_VIDEO_OUTPUT``
403 - 2
404 - Buffer of a single-planar video output stream, see
405 :ref:`output`.
406 * - ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``
407 - 10
408 - Buffer of a multi-planar video output stream, see :ref:`output`.
409 * - ``V4L2_BUF_TYPE_VIDEO_OVERLAY``
410 - 3
411 - Buffer for video overlay, see :ref:`overlay`.
412 * - ``V4L2_BUF_TYPE_VBI_CAPTURE``
413 - 4
414 - Buffer of a raw VBI capture stream, see :ref:`raw-vbi`.
415 * - ``V4L2_BUF_TYPE_VBI_OUTPUT``
416 - 5
417 - Buffer of a raw VBI output stream, see :ref:`raw-vbi`.
418 * - ``V4L2_BUF_TYPE_SLICED_VBI_CAPTURE``
419 - 6
420 - Buffer of a sliced VBI capture stream, see :ref:`sliced`.
421 * - ``V4L2_BUF_TYPE_SLICED_VBI_OUTPUT``
422 - 7
423 - Buffer of a sliced VBI output stream, see :ref:`sliced`.
424 * - ``V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY``
425 - 8
426 - Buffer for video output overlay (OSD), see :ref:`osd`.
427 * - ``V4L2_BUF_TYPE_SDR_CAPTURE``
428 - 11
429 - Buffer for Software Defined Radio (SDR) capture stream, see
430 :ref:`sdr`.
431 * - ``V4L2_BUF_TYPE_SDR_OUTPUT``
432 - 12
433 - Buffer for Software Defined Radio (SDR) output stream, see
434 :ref:`sdr`.
435 * - ``V4L2_BUF_TYPE_META_CAPTURE``
436 - 13
437 - Buffer for metadata capture, see :ref:`metadata`.
438 * - ``V4L2_BUF_TYPE_META_OUTPUT``
439 - 14
440 - Buffer for metadata output, see :ref:`metadata`.
441
442
443.. _buffer-flags:
444
445Buffer Flags
446============
447
448.. raw:: latex
449
450 \footnotesize
451
452.. tabularcolumns:: |p{6.5cm}|p{1.8cm}|p{9.0cm}|
453
454.. cssclass:: longtable
455
456.. flat-table::
457 :header-rows: 0
458 :stub-columns: 0
459 :widths: 65 18 70
460
461 * .. _`V4L2-BUF-FLAG-MAPPED`:
462
463 - ``V4L2_BUF_FLAG_MAPPED``
464 - 0x00000001
465 - The buffer resides in device memory and has been mapped into the
466 application's address space, see :ref:`mmap` for details.
467 Drivers set or clear this flag when the
468 :ref:`VIDIOC_QUERYBUF`,
469 :ref:`VIDIOC_QBUF` or
470 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl is called. Set by the
471 driver.
472 * .. _`V4L2-BUF-FLAG-QUEUED`:
473
474 - ``V4L2_BUF_FLAG_QUEUED``
475 - 0x00000002
476 - Internally drivers maintain two buffer queues, an incoming and
477 outgoing queue. When this flag is set, the buffer is currently on
478 the incoming queue. It automatically moves to the outgoing queue
479 after the buffer has been filled (capture devices) or displayed
480 (output devices). Drivers set or clear this flag when the
481 ``VIDIOC_QUERYBUF`` ioctl is called. After (successful) calling
482 the ``VIDIOC_QBUF``\ ioctl it is always set and after
483 ``VIDIOC_DQBUF`` always cleared.
484 * .. _`V4L2-BUF-FLAG-DONE`:
485
486 - ``V4L2_BUF_FLAG_DONE``
487 - 0x00000004
488 - When this flag is set, the buffer is currently on the outgoing
489 queue, ready to be dequeued from the driver. Drivers set or clear
490 this flag when the ``VIDIOC_QUERYBUF`` ioctl is called. After
491 calling the ``VIDIOC_QBUF`` or ``VIDIOC_DQBUF`` it is always
492 cleared. Of course a buffer cannot be on both queues at the same
493 time, the ``V4L2_BUF_FLAG_QUEUED`` and ``V4L2_BUF_FLAG_DONE`` flag
494 are mutually exclusive. They can be both cleared however, then the
495 buffer is in "dequeued" state, in the application domain so to
496 say.
497 * .. _`V4L2-BUF-FLAG-ERROR`:
498
499 - ``V4L2_BUF_FLAG_ERROR``
500 - 0x00000040
501 - When this flag is set, the buffer has been dequeued successfully,
502 although the data might have been corrupted. This is recoverable,
503 streaming may continue as normal and the buffer may be reused
504 normally. Drivers set this flag when the ``VIDIOC_DQBUF`` ioctl is
505 called.
506 * .. _`V4L2-BUF-FLAG-IN-REQUEST`:
507
508 - ``V4L2_BUF_FLAG_IN_REQUEST``
509 - 0x00000080
510 - This buffer is part of a request that hasn't been queued yet.
511 * .. _`V4L2-BUF-FLAG-KEYFRAME`:
512
513 - ``V4L2_BUF_FLAG_KEYFRAME``
514 - 0x00000008
515 - Drivers set or clear this flag when calling the ``VIDIOC_DQBUF``
516 ioctl. It may be set by video capture devices when the buffer
517 contains a compressed image which is a key frame (or field), i. e.
518 can be decompressed on its own. Also known as an I-frame.
519 Applications can set this bit when ``type`` refers to an output
520 stream.
521 * .. _`V4L2-BUF-FLAG-PFRAME`:
522
523 - ``V4L2_BUF_FLAG_PFRAME``
524 - 0x00000010
525 - Similar to ``V4L2_BUF_FLAG_KEYFRAME`` this flags predicted frames
526 or fields which contain only differences to a previous key frame.
527 Applications can set this bit when ``type`` refers to an output
528 stream.
529 * .. _`V4L2-BUF-FLAG-BFRAME`:
530
531 - ``V4L2_BUF_FLAG_BFRAME``
532 - 0x00000020
533 - Similar to ``V4L2_BUF_FLAG_KEYFRAME`` this flags a bi-directional
534 predicted frame or field which contains only the differences
535 between the current frame and both the preceding and following key
536 frames to specify its content. Applications can set this bit when
537 ``type`` refers to an output stream.
538 * .. _`V4L2-BUF-FLAG-TIMECODE`:
539
540 - ``V4L2_BUF_FLAG_TIMECODE``
541 - 0x00000100
542 - The ``timecode`` field is valid. Drivers set or clear this flag
543 when the ``VIDIOC_DQBUF`` ioctl is called. Applications can set
544 this bit and the corresponding ``timecode`` structure when
545 ``type`` refers to an output stream.
546 * .. _`V4L2-BUF-FLAG-PREPARED`:
547
548 - ``V4L2_BUF_FLAG_PREPARED``
549 - 0x00000400
550 - The buffer has been prepared for I/O and can be queued by the
551 application. Drivers set or clear this flag when the
552 :ref:`VIDIOC_QUERYBUF <VIDIOC_QUERYBUF>`,
553 :ref:`VIDIOC_PREPARE_BUF <VIDIOC_QBUF>`,
554 :ref:`VIDIOC_QBUF <VIDIOC_QBUF>` or
555 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl is called.
556 * .. _`V4L2-BUF-FLAG-NO-CACHE-INVALIDATE`:
557
558 - ``V4L2_BUF_FLAG_NO_CACHE_INVALIDATE``
559 - 0x00000800
560 - Caches do not have to be invalidated for this buffer. Typically
561 applications shall use this flag if the data captured in the
562 buffer is not going to be touched by the CPU, instead the buffer
563 will, probably, be passed on to a DMA-capable hardware unit for
564 further processing or output. This flag is ignored unless the
565 queue is used for :ref:`memory mapping <mmap>` streaming I/O and
566 reports :ref:`V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS
567 <V4L2-BUF-CAP-SUPPORTS-MMAP-CACHE-HINTS>` capability.
568 * .. _`V4L2-BUF-FLAG-NO-CACHE-CLEAN`:
569
570 - ``V4L2_BUF_FLAG_NO_CACHE_CLEAN``
571 - 0x00001000
572 - Caches do not have to be cleaned for this buffer. Typically
573 applications shall use this flag for output buffers if the data in
574 this buffer has not been created by the CPU but by some
575 DMA-capable unit, in which case caches have not been used. This flag
576 is ignored unless the queue is used for :ref:`memory mapping <mmap>`
577 streaming I/O and reports :ref:`V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS
578 <V4L2-BUF-CAP-SUPPORTS-MMAP-CACHE-HINTS>` capability.
579 * .. _`V4L2-BUF-FLAG-M2M-HOLD-CAPTURE-BUF`:
580
581 - ``V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF``
582 - 0x00000200
583 - Only valid if struct :c:type:`v4l2_requestbuffers` flag ``V4L2_BUF_CAP_SUPPORTS_M2M_HOLD_CAPTURE_BUF`` is
584 set. It is typically used with stateless decoders where multiple
585 output buffers each decode to a slice of the decoded frame.
586 Applications can set this flag when queueing the output buffer
587 to prevent the driver from dequeueing the capture buffer after
588 the output buffer has been decoded (i.e. the capture buffer is
589 'held'). If the timestamp of this output buffer differs from that
590 of the previous output buffer, then that indicates the start of a
591 new frame and the previously held capture buffer is dequeued.
592 * .. _`V4L2-BUF-FLAG-LAST`:
593
594 - ``V4L2_BUF_FLAG_LAST``
595 - 0x00100000
596 - Last buffer produced by the hardware. mem2mem codec drivers set
597 this flag on the capture queue for the last buffer when the
598 :ref:`VIDIOC_QUERYBUF` or
599 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl is called. Due to
600 hardware limitations, the last buffer may be empty. In this case
601 the driver will set the ``bytesused`` field to 0, regardless of
602 the format. Any subsequent call to the
603 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl will not block anymore,
604 but return an ``EPIPE`` error code.
605 * .. _`V4L2-BUF-FLAG-REQUEST-FD`:
606
607 - ``V4L2_BUF_FLAG_REQUEST_FD``
608 - 0x00800000
609 - The ``request_fd`` field contains a valid file descriptor.
610 * .. _`V4L2-BUF-FLAG-TIMESTAMP-MASK`:
611
612 - ``V4L2_BUF_FLAG_TIMESTAMP_MASK``
613 - 0x0000e000
614 - Mask for timestamp types below. To test the timestamp type, mask
615 out bits not belonging to timestamp type by performing a logical
616 and operation with buffer flags and timestamp mask.
617 * .. _`V4L2-BUF-FLAG-TIMESTAMP-UNKNOWN`:
618
619 - ``V4L2_BUF_FLAG_TIMESTAMP_UNKNOWN``
620 - 0x00000000
621 - Unknown timestamp type. This type is used by drivers before Linux
622 3.9 and may be either monotonic (see below) or realtime (wall
623 clock). Monotonic clock has been favoured in embedded systems
624 whereas most of the drivers use the realtime clock. Either kinds
625 of timestamps are available in user space via
626 :c:func:`clock_gettime` using clock IDs ``CLOCK_MONOTONIC``
627 and ``CLOCK_REALTIME``, respectively.
628 * .. _`V4L2-BUF-FLAG-TIMESTAMP-MONOTONIC`:
629
630 - ``V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC``
631 - 0x00002000
632 - The buffer timestamp has been taken from the ``CLOCK_MONOTONIC``
633 clock. To access the same clock outside V4L2, use
634 :c:func:`clock_gettime`.
635 * .. _`V4L2-BUF-FLAG-TIMESTAMP-COPY`:
636
637 - ``V4L2_BUF_FLAG_TIMESTAMP_COPY``
638 - 0x00004000
639 - The CAPTURE buffer timestamp has been taken from the corresponding
640 OUTPUT buffer. This flag applies only to mem2mem devices.
641 * .. _`V4L2-BUF-FLAG-TSTAMP-SRC-MASK`:
642
643 - ``V4L2_BUF_FLAG_TSTAMP_SRC_MASK``
644 - 0x00070000
645 - Mask for timestamp sources below. The timestamp source defines the
646 point of time the timestamp is taken in relation to the frame.
647 Logical 'and' operation between the ``flags`` field and
648 ``V4L2_BUF_FLAG_TSTAMP_SRC_MASK`` produces the value of the
649 timestamp source. Applications must set the timestamp source when
650 ``type`` refers to an output stream and
651 ``V4L2_BUF_FLAG_TIMESTAMP_COPY`` is set.
652 * .. _`V4L2-BUF-FLAG-TSTAMP-SRC-EOF`:
653
654 - ``V4L2_BUF_FLAG_TSTAMP_SRC_EOF``
655 - 0x00000000
656 - End Of Frame. The buffer timestamp has been taken when the last
657 pixel of the frame has been received or the last pixel of the
658 frame has been transmitted. In practice, software generated
659 timestamps will typically be read from the clock a small amount of
660 time after the last pixel has been received or transmitten,
661 depending on the system and other activity in it.
662 * .. _`V4L2-BUF-FLAG-TSTAMP-SRC-SOE`:
663
664 - ``V4L2_BUF_FLAG_TSTAMP_SRC_SOE``
665 - 0x00010000
666 - Start Of Exposure. The buffer timestamp has been taken when the
667 exposure of the frame has begun. This is only valid for the
668 ``V4L2_BUF_TYPE_VIDEO_CAPTURE`` buffer type.
669
670.. c:enum:: v4l2_memory
671
672.. raw:: latex
673
674 \normalsize
675
676enum v4l2_memory
677================
678
679.. tabularcolumns:: |p{5.0cm}|p{0.8cm}|p{11.5cm}|
680
681.. flat-table::
682 :header-rows: 0
683 :stub-columns: 0
684 :widths: 3 1 4
685
686 * - ``V4L2_MEMORY_MMAP``
687 - 1
688 - The buffer is used for :ref:`memory mapping <mmap>` I/O.
689 * - ``V4L2_MEMORY_USERPTR``
690 - 2
691 - The buffer is used for :ref:`user pointer <userp>` I/O.
692 * - ``V4L2_MEMORY_OVERLAY``
693 - 3
694 - [to do]
695 * - ``V4L2_MEMORY_DMABUF``
696 - 4
697 - The buffer is used for :ref:`DMA shared buffer <dmabuf>` I/O.
698
699.. raw:: latex
700
701 \normalsize
702
703Timecodes
704=========
705
706The :c:type:`v4l2_buffer_timecode` structure is designed to hold a
707:ref:`smpte12m` or similar timecode.
708(struct :c:type:`timeval` timestamps are stored in the struct
709:c:type:`v4l2_buffer` ``timestamp`` field.)
710
711.. c:type:: v4l2_timecode
712
713struct v4l2_timecode
714--------------------
715
716.. tabularcolumns:: |p{1.4cm}|p{2.8cm}|p{13.1cm}|
717
718.. flat-table::
719 :header-rows: 0
720 :stub-columns: 0
721 :widths: 1 1 2
722
723 * - __u32
724 - ``type``
725 - Frame rate the timecodes are based on, see :ref:`timecode-type`.
726 * - __u32
727 - ``flags``
728 - Timecode flags, see :ref:`timecode-flags`.
729 * - __u8
730 - ``frames``
731 - Frame count, 0 ... 23/24/29/49/59, depending on the type of
732 timecode.
733 * - __u8
734 - ``seconds``
735 - Seconds count, 0 ... 59. This is a binary, not BCD number.
736 * - __u8
737 - ``minutes``
738 - Minutes count, 0 ... 59. This is a binary, not BCD number.
739 * - __u8
740 - ``hours``
741 - Hours count, 0 ... 29. This is a binary, not BCD number.
742 * - __u8
743 - ``userbits``\ [4]
744 - The "user group" bits from the timecode.
745
746
747.. _timecode-type:
748
749Timecode Types
750--------------
751
752.. flat-table::
753 :header-rows: 0
754 :stub-columns: 0
755 :widths: 3 1 4
756
757 * - ``V4L2_TC_TYPE_24FPS``
758 - 1
759 - 24 frames per second, i. e. film.
760 * - ``V4L2_TC_TYPE_25FPS``
761 - 2
762 - 25 frames per second, i. e. PAL or SECAM video.
763 * - ``V4L2_TC_TYPE_30FPS``
764 - 3
765 - 30 frames per second, i. e. NTSC video.
766 * - ``V4L2_TC_TYPE_50FPS``
767 - 4
768 -
769 * - ``V4L2_TC_TYPE_60FPS``
770 - 5
771 -
772
773
774.. _timecode-flags:
775
776Timecode Flags
777--------------
778
779.. tabularcolumns:: |p{6.6cm}|p{1.4cm}|p{9.3cm}|
780
781.. flat-table::
782 :header-rows: 0
783 :stub-columns: 0
784 :widths: 3 1 4
785
786 * - ``V4L2_TC_FLAG_DROPFRAME``
787 - 0x0001
788 - Indicates "drop frame" semantics for counting frames in 29.97 fps
789 material. When set, frame numbers 0 and 1 at the start of each
790 minute, except minutes 0, 10, 20, 30, 40, 50 are omitted from the
791 count.
792 * - ``V4L2_TC_FLAG_COLORFRAME``
793 - 0x0002
794 - The "color frame" flag.
795 * - ``V4L2_TC_USERBITS_field``
796 - 0x000C
797 - Field mask for the "binary group flags".
798 * - ``V4L2_TC_USERBITS_USERDEFINED``
799 - 0x0000
800 - Unspecified format.
801 * - ``V4L2_TC_USERBITS_8BITCHARS``
802 - 0x0008
803 - 8-bit ISO characters.