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. If the application sets this
191	to 0 for an output stream, then ``bytesused`` will be set to the
192	size of the buffer (see the ``length`` field of this struct) by
193	the driver. For multiplanar formats this field is ignored and the
194	``planes`` pointer is used instead.
195    * - __u32
196      - ``flags``
197      - Flags set by the application or driver, see :ref:`buffer-flags`.
198    * - __u32
199      - ``field``
200      - Indicates the field order of the image in the buffer, see
201	:c:type:`v4l2_field`. This field is not used when the buffer
202	contains VBI data. Drivers must set it when ``type`` refers to a
203	capture stream, applications when it refers to an output stream.
204    * - struct timeval
205      - ``timestamp``
206      - For capture streams this is time when the first data byte was
207	captured, as returned by the :c:func:`clock_gettime()` function
208	for the relevant clock id; see ``V4L2_BUF_FLAG_TIMESTAMP_*`` in
209	:ref:`buffer-flags`. For output streams the driver stores the
210	time at which the last data byte was actually sent out in the
211	``timestamp`` field. This permits applications to monitor the
212	drift between the video and system clock. For output streams that
213	use ``V4L2_BUF_FLAG_TIMESTAMP_COPY`` the application has to fill
214	in the timestamp which will be copied by the driver to the capture
215	stream.
216    * - struct :c:type:`v4l2_timecode`
217      - ``timecode``
218      - When the ``V4L2_BUF_FLAG_TIMECODE`` flag is set in ``flags``, this
219	structure contains a frame timecode. In
220	:c:type:`V4L2_FIELD_ALTERNATE <v4l2_field>` mode the top and
221	bottom field contain the same timecode. Timecodes are intended to
222	help video editing and are typically recorded on video tapes, but
223	also embedded in compressed formats like MPEG. This field is
224	independent of the ``timestamp`` and ``sequence`` fields.
225    * - __u32
226      - ``sequence``
227      - Set by the driver, counting the frames (not fields!) in sequence.
228	This field is set for both input and output devices.
229    * - :cspan:`2`
230
231	In :c:type:`V4L2_FIELD_ALTERNATE <v4l2_field>` mode the top and
232	bottom field have the same sequence number. The count starts at
233	zero and includes dropped or repeated frames. A dropped frame was
234	received by an input device but could not be stored due to lack of
235	free buffer space. A repeated frame was displayed again by an
236	output device because the application did not pass new data in
237	time.
238
239	.. note::
240
241	   This may count the frames received e.g. over USB, without
242	   taking into account the frames dropped by the remote hardware due
243	   to limited compression throughput or bus bandwidth. These devices
244	   identify by not enumerating any video standards, see
245	   :ref:`standard`.
246
247    * - __u32
248      - ``memory``
249      - This field must be set by applications and/or drivers in
250	accordance with the selected I/O method. See :c:type:`v4l2_memory`
251    * - union {
252      - ``m``
253    * - __u32
254      - ``offset``
255      - For the single-planar API and when ``memory`` is
256	``V4L2_MEMORY_MMAP`` this is the offset of the buffer from the
257	start of the device memory. The value is returned by the driver
258	and apart of serving as parameter to the
259	:c:func:`mmap()` function not useful for applications.
260	See :ref:`mmap` for details
261    * - unsigned long
262      - ``userptr``
263      - For the single-planar API and when ``memory`` is
264	``V4L2_MEMORY_USERPTR`` this is a pointer to the buffer (casted to
265	unsigned long type) in virtual memory, set by the application. See
266	:ref:`userp` for details.
267    * - struct v4l2_plane
268      - ``*planes``
269      - When using the multi-planar API, contains a userspace pointer to
270	an array of struct :c:type:`v4l2_plane`. The size of
271	the array should be put in the ``length`` field of this
272	struct :c:type:`v4l2_buffer` structure.
273    * - int
274      - ``fd``
275      - For the single-plane API and when ``memory`` is
276	``V4L2_MEMORY_DMABUF`` this is the file descriptor associated with
277	a DMABUF buffer.
278    * - }
279      -
280    * - __u32
281      - ``length``
282      - Size of the buffer (not the payload) in bytes for the
283	single-planar API. This is set by the driver based on the calls to
284	:ref:`VIDIOC_REQBUFS` and/or
285	:ref:`VIDIOC_CREATE_BUFS`. For the
286	multi-planar API the application sets this to the number of
287	elements in the ``planes`` array. The driver will fill in the
288	actual number of valid elements in that array.
289    * - __u32
290      - ``reserved2``
291      - A place holder for future extensions. Drivers and applications
292	must set this to 0.
293    * - __u32
294      - ``request_fd``
295      - The file descriptor of the request to queue the buffer to. If the flag
296        ``V4L2_BUF_FLAG_REQUEST_FD`` is set, then the buffer will be
297	queued to this request. If the flag is not set, then this field will
298	be ignored.
299
300	The ``V4L2_BUF_FLAG_REQUEST_FD`` flag and this field are only used by
301	:ref:`ioctl VIDIOC_QBUF <VIDIOC_QBUF>` and ignored by other ioctls that
302	take a :c:type:`v4l2_buffer` as argument.
303
304	Applications should not set ``V4L2_BUF_FLAG_REQUEST_FD`` for any ioctls
305	other than :ref:`VIDIOC_QBUF <VIDIOC_QBUF>`.
306
307	If the device does not support requests, then ``EBADR`` will be returned.
308	If requests are supported but an invalid request file descriptor is
309	given, then ``EINVAL`` will be returned.
310
311
312.. c:type:: v4l2_plane
313
314struct v4l2_plane
315=================
316
317.. tabularcolumns:: |p{3.5cm}|p{3.5cm}|p{10.3cm}|
318
319.. cssclass:: longtable
320
321.. flat-table::
322    :header-rows:  0
323    :stub-columns: 0
324    :widths:       1 1 2
325
326    * - __u32
327      - ``bytesused``
328      - The number of bytes occupied by data in the plane (its payload).
329	Drivers must set this field when ``type`` refers to a capture
330	stream, applications when it refers to an output stream. If the
331	application sets this to 0 for an output stream, then
332	``bytesused`` will be set to the size of the plane (see the
333	``length`` field of this struct) by the driver.
334
335	.. note::
336
337	   Note that the actual image data starts at ``data_offset``
338	   which may not be 0.
339    * - __u32
340      - ``length``
341      - Size in bytes of the plane (not its payload). This is set by the
342	driver based on the calls to
343	:ref:`VIDIOC_REQBUFS` and/or
344	:ref:`VIDIOC_CREATE_BUFS`.
345    * - union {
346      - ``m``
347    * - __u32
348      - ``mem_offset``
349      - When the memory type in the containing struct
350	:c:type:`v4l2_buffer` is ``V4L2_MEMORY_MMAP``, this
351	is the value that should be passed to :c:func:`mmap()`,
352	similar to the ``offset`` field in struct
353	:c:type:`v4l2_buffer`.
354    * - unsigned long
355      - ``userptr``
356      - When the memory type in the containing struct
357	:c:type:`v4l2_buffer` is ``V4L2_MEMORY_USERPTR``,
358	this is a userspace pointer to the memory allocated for this plane
359	by an application.
360    * - int
361      - ``fd``
362      - When the memory type in the containing struct
363	:c:type:`v4l2_buffer` is ``V4L2_MEMORY_DMABUF``,
364	this is a file descriptor associated with a DMABUF buffer, similar
365	to the ``fd`` field in struct :c:type:`v4l2_buffer`.
366    * - }
367      -
368    * - __u32
369      - ``data_offset``
370      - Offset in bytes to video data in the plane. Drivers must set this
371	field when ``type`` refers to a capture stream, applications when
372	it refers to an output stream.
373
374	.. note::
375
376	   That data_offset is included  in ``bytesused``. So the
377	   size of the image in the plane is ``bytesused``-``data_offset``
378	   at offset ``data_offset`` from the start of the plane.
379    * - __u32
380      - ``reserved[11]``
381      - Reserved for future use. Should be zeroed by drivers and
382	applications.
383
384
385.. c:type:: v4l2_buf_type
386
387enum v4l2_buf_type
388==================
389
390.. cssclass:: longtable
391
392.. tabularcolumns:: |p{7.8cm}|p{0.6cm}|p{8.9cm}|
393
394.. flat-table::
395    :header-rows:  0
396    :stub-columns: 0
397    :widths:       4 1 9
398
399    * - ``V4L2_BUF_TYPE_VIDEO_CAPTURE``
400      - 1
401      - Buffer of a single-planar video capture stream, see
402	:ref:`capture`.
403    * - ``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE``
404      - 9
405      - Buffer of a multi-planar video capture stream, see
406	:ref:`capture`.
407    * - ``V4L2_BUF_TYPE_VIDEO_OUTPUT``
408      - 2
409      - Buffer of a single-planar video output stream, see
410	:ref:`output`.
411    * - ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``
412      - 10
413      - Buffer of a multi-planar video output stream, see :ref:`output`.
414    * - ``V4L2_BUF_TYPE_VIDEO_OVERLAY``
415      - 3
416      - Buffer for video overlay, see :ref:`overlay`.
417    * - ``V4L2_BUF_TYPE_VBI_CAPTURE``
418      - 4
419      - Buffer of a raw VBI capture stream, see :ref:`raw-vbi`.
420    * - ``V4L2_BUF_TYPE_VBI_OUTPUT``
421      - 5
422      - Buffer of a raw VBI output stream, see :ref:`raw-vbi`.
423    * - ``V4L2_BUF_TYPE_SLICED_VBI_CAPTURE``
424      - 6
425      - Buffer of a sliced VBI capture stream, see :ref:`sliced`.
426    * - ``V4L2_BUF_TYPE_SLICED_VBI_OUTPUT``
427      - 7
428      - Buffer of a sliced VBI output stream, see :ref:`sliced`.
429    * - ``V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY``
430      - 8
431      - Buffer for video output overlay (OSD), see :ref:`osd`.
432    * - ``V4L2_BUF_TYPE_SDR_CAPTURE``
433      - 11
434      - Buffer for Software Defined Radio (SDR) capture stream, see
435	:ref:`sdr`.
436    * - ``V4L2_BUF_TYPE_SDR_OUTPUT``
437      - 12
438      - Buffer for Software Defined Radio (SDR) output stream, see
439	:ref:`sdr`.
440    * - ``V4L2_BUF_TYPE_META_CAPTURE``
441      - 13
442      - Buffer for metadata capture, see :ref:`metadata`.
443    * - ``V4L2_BUF_TYPE_META_OUTPUT``
444      - 14
445      - Buffer for metadata output, see :ref:`metadata`.
446
447
448.. _buffer-flags:
449
450Buffer Flags
451============
452
453.. raw:: latex
454
455    \footnotesize
456
457.. tabularcolumns:: |p{6.5cm}|p{1.8cm}|p{9.0cm}|
458
459.. cssclass:: longtable
460
461.. flat-table::
462    :header-rows:  0
463    :stub-columns: 0
464    :widths:       65 18 70
465
466    * .. _`V4L2-BUF-FLAG-MAPPED`:
467
468      - ``V4L2_BUF_FLAG_MAPPED``
469      - 0x00000001
470      - The buffer resides in device memory and has been mapped into the
471	application's address space, see :ref:`mmap` for details.
472	Drivers set or clear this flag when the
473	:ref:`VIDIOC_QUERYBUF`,
474	:ref:`VIDIOC_QBUF` or
475	:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl is called. Set by the
476	driver.
477    * .. _`V4L2-BUF-FLAG-QUEUED`:
478
479      - ``V4L2_BUF_FLAG_QUEUED``
480      - 0x00000002
481      - Internally drivers maintain two buffer queues, an incoming and
482	outgoing queue. When this flag is set, the buffer is currently on
483	the incoming queue. It automatically moves to the outgoing queue
484	after the buffer has been filled (capture devices) or displayed
485	(output devices). Drivers set or clear this flag when the
486	``VIDIOC_QUERYBUF`` ioctl is called. After (successful) calling
487	the ``VIDIOC_QBUF``\ ioctl it is always set and after
488	``VIDIOC_DQBUF`` always cleared.
489    * .. _`V4L2-BUF-FLAG-DONE`:
490
491      - ``V4L2_BUF_FLAG_DONE``
492      - 0x00000004
493      - When this flag is set, the buffer is currently on the outgoing
494	queue, ready to be dequeued from the driver. Drivers set or clear
495	this flag when the ``VIDIOC_QUERYBUF`` ioctl is called. After
496	calling the ``VIDIOC_QBUF`` or ``VIDIOC_DQBUF`` it is always
497	cleared. Of course a buffer cannot be on both queues at the same
498	time, the ``V4L2_BUF_FLAG_QUEUED`` and ``V4L2_BUF_FLAG_DONE`` flag
499	are mutually exclusive. They can be both cleared however, then the
500	buffer is in "dequeued" state, in the application domain so to
501	say.
502    * .. _`V4L2-BUF-FLAG-ERROR`:
503
504      - ``V4L2_BUF_FLAG_ERROR``
505      - 0x00000040
506      - When this flag is set, the buffer has been dequeued successfully,
507	although the data might have been corrupted. This is recoverable,
508	streaming may continue as normal and the buffer may be reused
509	normally. Drivers set this flag when the ``VIDIOC_DQBUF`` ioctl is
510	called.
511    * .. _`V4L2-BUF-FLAG-IN-REQUEST`:
512
513      - ``V4L2_BUF_FLAG_IN_REQUEST``
514      - 0x00000080
515      - This buffer is part of a request that hasn't been queued yet.
516    * .. _`V4L2-BUF-FLAG-KEYFRAME`:
517
518      - ``V4L2_BUF_FLAG_KEYFRAME``
519      - 0x00000008
520      - Drivers set or clear this flag when calling the ``VIDIOC_DQBUF``
521	ioctl. It may be set by video capture devices when the buffer
522	contains a compressed image which is a key frame (or field), i. e.
523	can be decompressed on its own. Also known as an I-frame.
524	Applications can set this bit when ``type`` refers to an output
525	stream.
526    * .. _`V4L2-BUF-FLAG-PFRAME`:
527
528      - ``V4L2_BUF_FLAG_PFRAME``
529      - 0x00000010
530      - Similar to ``V4L2_BUF_FLAG_KEYFRAME`` this flags predicted frames
531	or fields which contain only differences to a previous key frame.
532	Applications can set this bit when ``type`` refers to an output
533	stream.
534    * .. _`V4L2-BUF-FLAG-BFRAME`:
535
536      - ``V4L2_BUF_FLAG_BFRAME``
537      - 0x00000020
538      - Similar to ``V4L2_BUF_FLAG_KEYFRAME`` this flags a bi-directional
539	predicted frame or field which contains only the differences
540	between the current frame and both the preceding and following key
541	frames to specify its content. Applications can set this bit when
542	``type`` refers to an output stream.
543    * .. _`V4L2-BUF-FLAG-TIMECODE`:
544
545      - ``V4L2_BUF_FLAG_TIMECODE``
546      - 0x00000100
547      - The ``timecode`` field is valid. Drivers set or clear this flag
548	when the ``VIDIOC_DQBUF`` ioctl is called. Applications can set
549	this bit and the corresponding ``timecode`` structure when
550	``type`` refers to an output stream.
551    * .. _`V4L2-BUF-FLAG-PREPARED`:
552
553      - ``V4L2_BUF_FLAG_PREPARED``
554      - 0x00000400
555      - The buffer has been prepared for I/O and can be queued by the
556	application. Drivers set or clear this flag when the
557	:ref:`VIDIOC_QUERYBUF`,
558	:ref:`VIDIOC_PREPARE_BUF <VIDIOC_QBUF>`,
559	:ref:`VIDIOC_QBUF` or
560	:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl is called.
561    * .. _`V4L2-BUF-FLAG-NO-CACHE-INVALIDATE`:
562
563      - ``V4L2_BUF_FLAG_NO_CACHE_INVALIDATE``
564      - 0x00000800
565      - Caches do not have to be invalidated for this buffer. Typically
566	applications shall use this flag if the data captured in the
567	buffer is not going to be touched by the CPU, instead the buffer
568	will, probably, be passed on to a DMA-capable hardware unit for
569	further processing or output. This flag is ignored unless the
570	queue is used for :ref:`memory mapping <mmap>` streaming I/O and
571	reports :ref:`V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS
572	<V4L2-BUF-CAP-SUPPORTS-MMAP-CACHE-HINTS>` capability.
573    * .. _`V4L2-BUF-FLAG-NO-CACHE-CLEAN`:
574
575      - ``V4L2_BUF_FLAG_NO_CACHE_CLEAN``
576      - 0x00001000
577      - Caches do not have to be cleaned for this buffer. Typically
578	applications shall use this flag for output buffers if the data in
579	this buffer has not been created by the CPU but by some
580	DMA-capable unit, in which case caches have not been used. This flag
581	is ignored unless the queue is used for :ref:`memory mapping <mmap>`
582	streaming I/O and reports :ref:`V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS
583	<V4L2-BUF-CAP-SUPPORTS-MMAP-CACHE-HINTS>` capability.
584    * .. _`V4L2-BUF-FLAG-M2M-HOLD-CAPTURE-BUF`:
585
586      - ``V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF``
587      - 0x00000200
588      - Only valid if struct :c:type:`v4l2_requestbuffers` flag ``V4L2_BUF_CAP_SUPPORTS_M2M_HOLD_CAPTURE_BUF`` is
589	set. It is typically used with stateless decoders where multiple
590	output buffers each decode to a slice of the decoded frame.
591	Applications can set this flag when queueing the output buffer
592	to prevent the driver from dequeueing the capture buffer after
593	the output buffer has been decoded (i.e. the capture buffer is
594	'held'). If the timestamp of this output buffer differs from that
595	of the previous output buffer, then that indicates the start of a
596	new frame and the previously held capture buffer is dequeued.
597    * .. _`V4L2-BUF-FLAG-LAST`:
598
599      - ``V4L2_BUF_FLAG_LAST``
600      - 0x00100000
601      - Last buffer produced by the hardware. mem2mem codec drivers set
602	this flag on the capture queue for the last buffer when the
603	:ref:`VIDIOC_QUERYBUF` or
604	:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl is called. Due to
605	hardware limitations, the last buffer may be empty. In this case
606	the driver will set the ``bytesused`` field to 0, regardless of
607	the format. Any subsequent call to the
608	:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl will not block anymore,
609	but return an ``EPIPE`` error code.
610    * .. _`V4L2-BUF-FLAG-REQUEST-FD`:
611
612      - ``V4L2_BUF_FLAG_REQUEST_FD``
613      - 0x00800000
614      - The ``request_fd`` field contains a valid file descriptor.
615    * .. _`V4L2-BUF-FLAG-TIMESTAMP-MASK`:
616
617      - ``V4L2_BUF_FLAG_TIMESTAMP_MASK``
618      - 0x0000e000
619      - Mask for timestamp types below. To test the timestamp type, mask
620	out bits not belonging to timestamp type by performing a logical
621	and operation with buffer flags and timestamp mask.
622    * .. _`V4L2-BUF-FLAG-TIMESTAMP-UNKNOWN`:
623
624      - ``V4L2_BUF_FLAG_TIMESTAMP_UNKNOWN``
625      - 0x00000000
626      - Unknown timestamp type. This type is used by drivers before Linux
627	3.9 and may be either monotonic (see below) or realtime (wall
628	clock). Monotonic clock has been favoured in embedded systems
629	whereas most of the drivers use the realtime clock. Either kinds
630	of timestamps are available in user space via
631	:c:func:`clock_gettime` using clock IDs ``CLOCK_MONOTONIC``
632	and ``CLOCK_REALTIME``, respectively.
633    * .. _`V4L2-BUF-FLAG-TIMESTAMP-MONOTONIC`:
634
635      - ``V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC``
636      - 0x00002000
637      - The buffer timestamp has been taken from the ``CLOCK_MONOTONIC``
638	clock. To access the same clock outside V4L2, use
639	:c:func:`clock_gettime`.
640    * .. _`V4L2-BUF-FLAG-TIMESTAMP-COPY`:
641
642      - ``V4L2_BUF_FLAG_TIMESTAMP_COPY``
643      - 0x00004000
644      - The CAPTURE buffer timestamp has been taken from the corresponding
645	OUTPUT buffer. This flag applies only to mem2mem devices.
646    * .. _`V4L2-BUF-FLAG-TSTAMP-SRC-MASK`:
647
648      - ``V4L2_BUF_FLAG_TSTAMP_SRC_MASK``
649      - 0x00070000
650      - Mask for timestamp sources below. The timestamp source defines the
651	point of time the timestamp is taken in relation to the frame.
652	Logical 'and' operation between the ``flags`` field and
653	``V4L2_BUF_FLAG_TSTAMP_SRC_MASK`` produces the value of the
654	timestamp source. Applications must set the timestamp source when
655	``type`` refers to an output stream and
656	``V4L2_BUF_FLAG_TIMESTAMP_COPY`` is set.
657    * .. _`V4L2-BUF-FLAG-TSTAMP-SRC-EOF`:
658
659      - ``V4L2_BUF_FLAG_TSTAMP_SRC_EOF``
660      - 0x00000000
661      - End Of Frame. The buffer timestamp has been taken when the last
662	pixel of the frame has been received or the last pixel of the
663	frame has been transmitted. In practice, software generated
664	timestamps will typically be read from the clock a small amount of
665	time after the last pixel has been received or transmitten,
666	depending on the system and other activity in it.
667    * .. _`V4L2-BUF-FLAG-TSTAMP-SRC-SOE`:
668
669      - ``V4L2_BUF_FLAG_TSTAMP_SRC_SOE``
670      - 0x00010000
671      - Start Of Exposure. The buffer timestamp has been taken when the
672	exposure of the frame has begun. This is only valid for the
673	``V4L2_BUF_TYPE_VIDEO_CAPTURE`` buffer type.
674
675.. raw:: latex
676
677    \normalsize
678
679enum v4l2_memory
680================
681
682.. tabularcolumns:: |p{5.0cm}|p{0.8cm}|p{11.5cm}|
683
684.. flat-table::
685    :header-rows:  0
686    :stub-columns: 0
687    :widths:       3 1 4
688
689    * - ``V4L2_MEMORY_MMAP``
690      - 1
691      - The buffer is used for :ref:`memory mapping <mmap>` I/O.
692    * - ``V4L2_MEMORY_USERPTR``
693      - 2
694      - The buffer is used for :ref:`user pointer <userp>` I/O.
695    * - ``V4L2_MEMORY_OVERLAY``
696      - 3
697      - [to do]
698    * - ``V4L2_MEMORY_DMABUF``
699      - 4
700      - The buffer is used for :ref:`DMA shared buffer <dmabuf>` I/O.
701
702.. _memory-flags:
703
704Memory Consistency Flags
705------------------------
706
707.. raw:: latex
708
709    \small
710
711.. tabularcolumns:: |p{7.0cm}|p{2.1cm}|p{8.4cm}|
712
713.. cssclass:: longtable
714
715.. flat-table::
716    :header-rows:  0
717    :stub-columns: 0
718    :widths:       3 1 4
719
720    * .. _`V4L2-MEMORY-FLAG-NON-COHERENT`:
721
722      - ``V4L2_MEMORY_FLAG_NON_COHERENT``
723      - 0x00000001
724      - A buffer is allocated either in coherent (it will be automatically
725	coherent between the CPU and the bus) or non-coherent memory. The
726	latter can provide performance gains, for instance the CPU cache
727	sync/flush operations can be avoided if the buffer is accessed by the
728	corresponding device only and the CPU does not read/write to/from that
729	buffer. However, this requires extra care from the driver -- it must
730	guarantee memory consistency by issuing a cache flush/sync when
731	consistency is needed. If this flag is set V4L2 will attempt to
732	allocate the buffer in non-coherent memory. The flag takes effect
733	only if the buffer is used for :ref:`memory mapping <mmap>` I/O and the
734	queue reports the :ref:`V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS
735	<V4L2-BUF-CAP-SUPPORTS-MMAP-CACHE-HINTS>` capability.
736
737.. raw:: latex
738
739    \normalsize
740
741Timecodes
742=========
743
744The :c:type:`v4l2_buffer_timecode` structure is designed to hold a
745:ref:`smpte12m` or similar timecode.
746(struct :c:type:`timeval` timestamps are stored in the struct
747:c:type:`v4l2_buffer` ``timestamp`` field.)
748
749.. c:type:: v4l2_timecode
750
751struct v4l2_timecode
752--------------------
753
754.. tabularcolumns:: |p{1.4cm}|p{2.8cm}|p{13.1cm}|
755
756.. flat-table::
757    :header-rows:  0
758    :stub-columns: 0
759    :widths:       1 1 2
760
761    * - __u32
762      - ``type``
763      - Frame rate the timecodes are based on, see :ref:`timecode-type`.
764    * - __u32
765      - ``flags``
766      - Timecode flags, see :ref:`timecode-flags`.
767    * - __u8
768      - ``frames``
769      - Frame count, 0 ... 23/24/29/49/59, depending on the type of
770	timecode.
771    * - __u8
772      - ``seconds``
773      - Seconds count, 0 ... 59. This is a binary, not BCD number.
774    * - __u8
775      - ``minutes``
776      - Minutes count, 0 ... 59. This is a binary, not BCD number.
777    * - __u8
778      - ``hours``
779      - Hours count, 0 ... 29. This is a binary, not BCD number.
780    * - __u8
781      - ``userbits``\ [4]
782      - The "user group" bits from the timecode.
783
784
785.. _timecode-type:
786
787Timecode Types
788--------------
789
790.. flat-table::
791    :header-rows:  0
792    :stub-columns: 0
793    :widths:       3 1 4
794
795    * - ``V4L2_TC_TYPE_24FPS``
796      - 1
797      - 24 frames per second, i. e. film.
798    * - ``V4L2_TC_TYPE_25FPS``
799      - 2
800      - 25 frames per second, i. e. PAL or SECAM video.
801    * - ``V4L2_TC_TYPE_30FPS``
802      - 3
803      - 30 frames per second, i. e. NTSC video.
804    * - ``V4L2_TC_TYPE_50FPS``
805      - 4
806      -
807    * - ``V4L2_TC_TYPE_60FPS``
808      - 5
809      -
810
811
812.. _timecode-flags:
813
814Timecode Flags
815--------------
816
817.. tabularcolumns:: |p{6.6cm}|p{1.4cm}|p{9.3cm}|
818
819.. flat-table::
820    :header-rows:  0
821    :stub-columns: 0
822    :widths:       3 1 4
823
824    * - ``V4L2_TC_FLAG_DROPFRAME``
825      - 0x0001
826      - Indicates "drop frame" semantics for counting frames in 29.97 fps
827	material. When set, frame numbers 0 and 1 at the start of each
828	minute, except minutes 0, 10, 20, 30, 40, 50 are omitted from the
829	count.
830    * - ``V4L2_TC_FLAG_COLORFRAME``
831      - 0x0002
832      - The "color frame" flag.
833    * - ``V4L2_TC_USERBITS_field``
834      - 0x000C
835      - Field mask for the "binary group flags".
836    * - ``V4L2_TC_USERBITS_USERDEFINED``
837      - 0x0000
838      - Unspecified format.
839    * - ``V4L2_TC_USERBITS_8BITCHARS``
840      - 0x0008
841      - 8-bit ISO characters.
842