Linux kernel mirror (for testing)
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linux
1.. SPDX-License-Identifier: GPL-2.0 OR GFDL-1.1-no-invariants-or-later
2.. c:namespace:: V4L
3
4.. _encoder:
5
6*************************************************
7Memory-to-Memory Stateful Video Encoder Interface
8*************************************************
9
10A stateful video encoder takes raw video frames in display order and encodes
11them into a bytestream. It generates complete chunks of the bytestream, including
12all metadata, headers, etc. The resulting bytestream does not require any
13further post-processing by the client.
14
15Performing software stream processing, header generation etc. in the driver
16in order to support this interface is strongly discouraged. In case such
17operations are needed, use of the Stateless Video Encoder Interface (in
18development) is strongly advised.
19
20Conventions and Notations Used in This Document
21===============================================
22
231. The general V4L2 API rules apply if not specified in this document
24 otherwise.
25
262. The meaning of words "must", "may", "should", etc. is as per `RFC
27 2119 <https://tools.ietf.org/html/rfc2119>`_.
28
293. All steps not marked "optional" are required.
30
314. :c:func:`VIDIOC_G_EXT_CTRLS` and :c:func:`VIDIOC_S_EXT_CTRLS` may be used
32 interchangeably with :c:func:`VIDIOC_G_CTRL` and :c:func:`VIDIOC_S_CTRL`,
33 unless specified otherwise.
34
355. Single-planar API (see :ref:`planar-apis`) and applicable structures may be
36 used interchangeably with multi-planar API, unless specified otherwise,
37 depending on encoder capabilities and following the general V4L2 guidelines.
38
396. i = [a..b]: sequence of integers from a to b, inclusive, i.e. i =
40 [0..2]: i = 0, 1, 2.
41
427. Given an ``OUTPUT`` buffer A, then A' represents a buffer on the ``CAPTURE``
43 queue containing data that resulted from processing buffer A.
44
45Glossary
46========
47
48Refer to :ref:`decoder-glossary`.
49
50State Machine
51=============
52
53.. kernel-render:: DOT
54 :alt: DOT digraph of encoder state machine
55 :caption: Encoder State Machine
56
57 digraph encoder_state_machine {
58 node [shape = doublecircle, label="Encoding"] Encoding;
59
60 node [shape = circle, label="Initialization"] Initialization;
61 node [shape = circle, label="Stopped"] Stopped;
62 node [shape = circle, label="Drain"] Drain;
63 node [shape = circle, label="Reset"] Reset;
64
65 node [shape = point]; qi
66 qi -> Initialization [ label = "open()" ];
67
68 Initialization -> Encoding [ label = "Both queues streaming" ];
69
70 Encoding -> Drain [ label = "V4L2_ENC_CMD_STOP" ];
71 Encoding -> Reset [ label = "VIDIOC_STREAMOFF(CAPTURE)" ];
72 Encoding -> Stopped [ label = "VIDIOC_STREAMOFF(OUTPUT)" ];
73 Encoding -> Encoding;
74
75 Drain -> Stopped [ label = "All CAPTURE\nbuffers dequeued\nor\nVIDIOC_STREAMOFF(OUTPUT)" ];
76 Drain -> Reset [ label = "VIDIOC_STREAMOFF(CAPTURE)" ];
77
78 Reset -> Encoding [ label = "VIDIOC_STREAMON(CAPTURE)" ];
79 Reset -> Initialization [ label = "VIDIOC_REQBUFS(OUTPUT, 0)" ];
80
81 Stopped -> Encoding [ label = "V4L2_ENC_CMD_START\nor\nVIDIOC_STREAMON(OUTPUT)" ];
82 Stopped -> Reset [ label = "VIDIOC_STREAMOFF(CAPTURE)" ];
83 }
84
85Querying Capabilities
86=====================
87
881. To enumerate the set of coded formats supported by the encoder, the
89 client may call :c:func:`VIDIOC_ENUM_FMT` on ``CAPTURE``.
90
91 * The full set of supported formats will be returned, regardless of the
92 format set on ``OUTPUT``.
93
942. To enumerate the set of supported raw formats, the client may call
95 :c:func:`VIDIOC_ENUM_FMT` on ``OUTPUT``.
96
97 * Only the formats supported for the format currently active on ``CAPTURE``
98 will be returned.
99
100 * In order to enumerate raw formats supported by a given coded format,
101 the client must first set that coded format on ``CAPTURE`` and then
102 enumerate the formats on ``OUTPUT``.
103
1043. The client may use :c:func:`VIDIOC_ENUM_FRAMESIZES` to detect supported
105 resolutions for a given format, passing the desired pixel format in
106 :c:type:`v4l2_frmsizeenum` ``pixel_format``.
107
108 * Values returned by :c:func:`VIDIOC_ENUM_FRAMESIZES` for a coded pixel
109 format will include all possible coded resolutions supported by the
110 encoder for the given coded pixel format.
111
112 * Values returned by :c:func:`VIDIOC_ENUM_FRAMESIZES` for a raw pixel format
113 will include all possible frame buffer resolutions supported by the
114 encoder for the given raw pixel format and coded format currently set on
115 ``CAPTURE``.
116
1174. The client may use :c:func:`VIDIOC_ENUM_FRAMEINTERVALS` to detect supported
118 frame intervals for a given format and resolution, passing the desired pixel
119 format in :c:type:`v4l2_frmivalenum` ``pixel_format`` and the resolution
120 in :c:type:`v4l2_frmivalenum` ``width`` and :c:type:`v4l2_frmivalenum`
121 ``height``.
122
123 * Values returned by :c:func:`VIDIOC_ENUM_FRAMEINTERVALS` for a coded pixel
124 format and coded resolution will include all possible frame intervals
125 supported by the encoder for the given coded pixel format and resolution.
126
127 * Values returned by :c:func:`VIDIOC_ENUM_FRAMEINTERVALS` for a raw pixel
128 format and resolution will include all possible frame intervals supported
129 by the encoder for the given raw pixel format and resolution and for the
130 coded format, coded resolution and coded frame interval currently set on
131 ``CAPTURE``.
132
133 * Support for :c:func:`VIDIOC_ENUM_FRAMEINTERVALS` is optional. If it is
134 not implemented, then there are no special restrictions other than the
135 limits of the codec itself.
136
1375. Supported profiles and levels for the coded format currently set on
138 ``CAPTURE``, if applicable, may be queried using their respective controls
139 via :c:func:`VIDIOC_QUERYCTRL`.
140
1416. Any additional encoder capabilities may be discovered by querying
142 their respective controls.
143
144Initialization
145==============
146
1471. Set the coded format on the ``CAPTURE`` queue via :c:func:`VIDIOC_S_FMT`.
148
149 * **Required fields:**
150
151 ``type``
152 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``CAPTURE``.
153
154 ``pixelformat``
155 the coded format to be produced.
156
157 ``sizeimage``
158 desired size of ``CAPTURE`` buffers; the encoder may adjust it to
159 match hardware requirements.
160
161 ``width``, ``height``
162 ignored (read-only).
163
164 other fields
165 follow standard semantics.
166
167 * **Returned fields:**
168
169 ``sizeimage``
170 adjusted size of ``CAPTURE`` buffers.
171
172 ``width``, ``height``
173 the coded size selected by the encoder based on current state, e.g.
174 ``OUTPUT`` format, selection rectangles, etc. (read-only).
175
176 .. important::
177
178 Changing the ``CAPTURE`` format may change the currently set ``OUTPUT``
179 format. How the new ``OUTPUT`` format is determined is up to the encoder
180 and the client must ensure it matches its needs afterwards.
181
1822. **Optional.** Enumerate supported ``OUTPUT`` formats (raw formats for
183 source) for the selected coded format via :c:func:`VIDIOC_ENUM_FMT`.
184
185 * **Required fields:**
186
187 ``type``
188 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``.
189
190 other fields
191 follow standard semantics.
192
193 * **Returned fields:**
194
195 ``pixelformat``
196 raw format supported for the coded format currently selected on
197 the ``CAPTURE`` queue.
198
199 other fields
200 follow standard semantics.
201
2023. Set the raw source format on the ``OUTPUT`` queue via
203 :c:func:`VIDIOC_S_FMT`.
204
205 * **Required fields:**
206
207 ``type``
208 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``.
209
210 ``pixelformat``
211 raw format of the source.
212
213 ``width``, ``height``
214 source resolution.
215
216 other fields
217 follow standard semantics.
218
219 * **Returned fields:**
220
221 ``width``, ``height``
222 may be adjusted to match encoder minimums, maximums and alignment
223 requirements, as required by the currently selected formats, as
224 reported by :c:func:`VIDIOC_ENUM_FRAMESIZES`.
225
226 other fields
227 follow standard semantics.
228
229 * Setting the ``OUTPUT`` format will reset the selection rectangles to their
230 default values, based on the new resolution, as described in the next
231 step.
232
2334. Set the raw frame interval on the ``OUTPUT`` queue via
234 :c:func:`VIDIOC_S_PARM`. This also sets the coded frame interval on the
235 ``CAPTURE`` queue to the same value.
236
237 * **Required fields:**
238
239 ``type``
240 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``.
241
242 ``parm.output``
243 set all fields except ``parm.output.timeperframe`` to 0.
244
245 ``parm.output.timeperframe``
246 the desired frame interval; the encoder may adjust it to
247 match hardware requirements.
248
249 * **Returned fields:**
250
251 ``parm.output.timeperframe``
252 the adjusted frame interval.
253
254 .. important::
255
256 Changing the ``OUTPUT`` frame interval *also* sets the framerate that
257 the encoder uses to encode the video. So setting the frame interval
258 to 1/24 (or 24 frames per second) will produce a coded video stream
259 that can be played back at that speed. The frame interval for the
260 ``OUTPUT`` queue is just a hint, the application may provide raw
261 frames at a different rate. It can be used by the driver to help
262 schedule multiple encoders running in parallel.
263
264 In the next step the ``CAPTURE`` frame interval can optionally be
265 changed to a different value. This is useful for off-line encoding
266 were the coded frame interval can be different from the rate at
267 which raw frames are supplied.
268
269 .. important::
270
271 ``timeperframe`` deals with *frames*, not fields. So for interlaced
272 formats this is the time per two fields, since a frame consists of
273 a top and a bottom field.
274
275 .. note::
276
277 It is due to historical reasons that changing the ``OUTPUT`` frame
278 interval also changes the coded frame interval on the ``CAPTURE``
279 queue. Ideally these would be independent settings, but that would
280 break the existing API.
281
2825. **Optional** Set the coded frame interval on the ``CAPTURE`` queue via
283 :c:func:`VIDIOC_S_PARM`. This is only necessary if the coded frame
284 interval is different from the raw frame interval, which is typically
285 the case for off-line encoding. Support for this feature is signalled
286 by the :ref:`V4L2_FMT_FLAG_ENC_CAP_FRAME_INTERVAL <fmtdesc-flags>` format flag.
287
288 * **Required fields:**
289
290 ``type``
291 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``CAPTURE``.
292
293 ``parm.capture``
294 set all fields except ``parm.capture.timeperframe`` to 0.
295
296 ``parm.capture.timeperframe``
297 the desired coded frame interval; the encoder may adjust it to
298 match hardware requirements.
299
300 * **Returned fields:**
301
302 ``parm.capture.timeperframe``
303 the adjusted frame interval.
304
305 .. important::
306
307 Changing the ``CAPTURE`` frame interval sets the framerate for the
308 coded video. It does *not* set the rate at which buffers arrive on the
309 ``CAPTURE`` queue, that depends on how fast the encoder is and how
310 fast raw frames are queued on the ``OUTPUT`` queue.
311
312 .. important::
313
314 ``timeperframe`` deals with *frames*, not fields. So for interlaced
315 formats this is the time per two fields, since a frame consists of
316 a top and a bottom field.
317
318 .. note::
319
320 Not all drivers support this functionality, in that case just set
321 the desired coded frame interval for the ``OUTPUT`` queue.
322
323 However, drivers that can schedule multiple encoders based on the
324 ``OUTPUT`` frame interval must support this optional feature.
325
3266. **Optional.** Set the visible resolution for the stream metadata via
327 :c:func:`VIDIOC_S_SELECTION` on the ``OUTPUT`` queue if it is desired
328 to be different than the full OUTPUT resolution.
329
330 * **Required fields:**
331
332 ``type``
333 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``.
334
335 ``target``
336 set to ``V4L2_SEL_TGT_CROP``.
337
338 ``r.left``, ``r.top``, ``r.width``, ``r.height``
339 visible rectangle; this must fit within the `V4L2_SEL_TGT_CROP_BOUNDS`
340 rectangle and may be subject to adjustment to match codec and
341 hardware constraints.
342
343 * **Returned fields:**
344
345 ``r.left``, ``r.top``, ``r.width``, ``r.height``
346 visible rectangle adjusted by the encoder.
347
348 * The following selection targets are supported on ``OUTPUT``:
349
350 ``V4L2_SEL_TGT_CROP_BOUNDS``
351 equal to the full source frame, matching the active ``OUTPUT``
352 format.
353
354 ``V4L2_SEL_TGT_CROP_DEFAULT``
355 equal to ``V4L2_SEL_TGT_CROP_BOUNDS``.
356
357 ``V4L2_SEL_TGT_CROP``
358 rectangle within the source buffer to be encoded into the
359 ``CAPTURE`` stream; defaults to ``V4L2_SEL_TGT_CROP_DEFAULT``.
360
361 .. note::
362
363 A common use case for this selection target is encoding a source
364 video with a resolution that is not a multiple of a macroblock,
365 e.g. the common 1920x1080 resolution may require the source
366 buffers to be aligned to 1920x1088 for codecs with 16x16 macroblock
367 size. To avoid encoding the padding, the client needs to explicitly
368 configure this selection target to 1920x1080.
369
370 .. warning::
371
372 The encoder may adjust the crop/compose rectangles to the nearest
373 supported ones to meet codec and hardware requirements. The client needs
374 to check the adjusted rectangle returned by :c:func:`VIDIOC_S_SELECTION`.
375
3767. Allocate buffers for both ``OUTPUT`` and ``CAPTURE`` via
377 :c:func:`VIDIOC_REQBUFS`. This may be performed in any order.
378
379 * **Required fields:**
380
381 ``count``
382 requested number of buffers to allocate; greater than zero.
383
384 ``type``
385 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT`` or
386 ``CAPTURE``.
387
388 other fields
389 follow standard semantics.
390
391 * **Returned fields:**
392
393 ``count``
394 actual number of buffers allocated.
395
396 .. warning::
397
398 The actual number of allocated buffers may differ from the ``count``
399 given. The client must check the updated value of ``count`` after the
400 call returns.
401
402 .. note::
403
404 To allocate more than the minimum number of OUTPUT buffers (for pipeline
405 depth), the client may query the ``V4L2_CID_MIN_BUFFERS_FOR_OUTPUT``
406 control to get the minimum number of buffers required, and pass the
407 obtained value plus the number of additional buffers needed in the
408 ``count`` field to :c:func:`VIDIOC_REQBUFS`.
409
410 Alternatively, :c:func:`VIDIOC_CREATE_BUFS` can be used to have more
411 control over buffer allocation.
412
413 * **Required fields:**
414
415 ``count``
416 requested number of buffers to allocate; greater than zero.
417
418 ``type``
419 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``.
420
421 other fields
422 follow standard semantics.
423
424 * **Returned fields:**
425
426 ``count``
427 adjusted to the number of allocated buffers.
428
4298. Begin streaming on both ``OUTPUT`` and ``CAPTURE`` queues via
430 :c:func:`VIDIOC_STREAMON`. This may be performed in any order. The actual
431 encoding process starts when both queues start streaming.
432
433.. note::
434
435 If the client stops the ``CAPTURE`` queue during the encode process and then
436 restarts it again, the encoder will begin generating a stream independent
437 from the stream generated before the stop. The exact constraints depend
438 on the coded format, but may include the following implications:
439
440 * encoded frames produced after the restart must not reference any
441 frames produced before the stop, e.g. no long term references for
442 H.264/HEVC,
443
444 * any headers that must be included in a standalone stream must be
445 produced again, e.g. SPS and PPS for H.264/HEVC.
446
447Encoding
448========
449
450This state is reached after the `Initialization` sequence finishes
451successfully. In this state, the client queues and dequeues buffers to both
452queues via :c:func:`VIDIOC_QBUF` and :c:func:`VIDIOC_DQBUF`, following the
453standard semantics.
454
455The content of encoded ``CAPTURE`` buffers depends on the active coded pixel
456format and may be affected by codec-specific extended controls, as stated
457in the documentation of each format.
458
459Both queues operate independently, following standard behavior of V4L2 buffer
460queues and memory-to-memory devices. In addition, the order of encoded frames
461dequeued from the ``CAPTURE`` queue may differ from the order of queuing raw
462frames to the ``OUTPUT`` queue, due to properties of the selected coded format,
463e.g. frame reordering.
464
465The client must not assume any direct relationship between ``CAPTURE`` and
466``OUTPUT`` buffers and any specific timing of buffers becoming
467available to dequeue. Specifically:
468
469* a buffer queued to ``OUTPUT`` may result in more than one buffer produced on
470 ``CAPTURE`` (for example, if returning an encoded frame allowed the encoder
471 to return a frame that preceded it in display, but succeeded it in the decode
472 order; however, there may be other reasons for this as well),
473
474* a buffer queued to ``OUTPUT`` may result in a buffer being produced on
475 ``CAPTURE`` later into encode process, and/or after processing further
476 ``OUTPUT`` buffers, or be returned out of order, e.g. if display
477 reordering is used,
478
479* buffers may become available on the ``CAPTURE`` queue without additional
480 buffers queued to ``OUTPUT`` (e.g. during drain or ``EOS``), because of the
481 ``OUTPUT`` buffers queued in the past whose encoding results are only
482 available at later time, due to specifics of the encoding process,
483
484* buffers queued to ``OUTPUT`` may not become available to dequeue instantly
485 after being encoded into a corresponding ``CAPTURE`` buffer, e.g. if the
486 encoder needs to use the frame as a reference for encoding further frames.
487
488.. note::
489
490 To allow matching encoded ``CAPTURE`` buffers with ``OUTPUT`` buffers they
491 originated from, the client can set the ``timestamp`` field of the
492 :c:type:`v4l2_buffer` struct when queuing an ``OUTPUT`` buffer. The
493 ``CAPTURE`` buffer(s), which resulted from encoding that ``OUTPUT`` buffer
494 will have their ``timestamp`` field set to the same value when dequeued.
495
496 In addition to the straightforward case of one ``OUTPUT`` buffer producing
497 one ``CAPTURE`` buffer, the following cases are defined:
498
499 * one ``OUTPUT`` buffer generates multiple ``CAPTURE`` buffers: the same
500 ``OUTPUT`` timestamp will be copied to multiple ``CAPTURE`` buffers,
501
502 * the encoding order differs from the presentation order (i.e. the
503 ``CAPTURE`` buffers are out-of-order compared to the ``OUTPUT`` buffers):
504 ``CAPTURE`` timestamps will not retain the order of ``OUTPUT`` timestamps.
505
506.. note::
507
508 To let the client distinguish between frame types (keyframes, intermediate
509 frames; the exact list of types depends on the coded format), the
510 ``CAPTURE`` buffers will have corresponding flag bits set in their
511 :c:type:`v4l2_buffer` struct when dequeued. See the documentation of
512 :c:type:`v4l2_buffer` and each coded pixel format for exact list of flags
513 and their meanings.
514
515Should an encoding error occur, it will be reported to the client with the level
516of details depending on the encoder capabilities. Specifically:
517
518* the ``CAPTURE`` buffer (if any) that contains the results of the failed encode
519 operation will be returned with the ``V4L2_BUF_FLAG_ERROR`` flag set,
520
521* if the encoder is able to precisely report the ``OUTPUT`` buffer(s) that triggered
522 the error, such buffer(s) will be returned with the ``V4L2_BUF_FLAG_ERROR`` flag
523 set.
524
525.. note::
526
527 If a ``CAPTURE`` buffer is too small then it is just returned with the
528 ``V4L2_BUF_FLAG_ERROR`` flag set. More work is needed to detect that this
529 error occurred because the buffer was too small, and to provide support to
530 free existing buffers that were too small.
531
532In case of a fatal failure that does not allow the encoding to continue, any
533further operations on corresponding encoder file handle will return the -EIO
534error code. The client may close the file handle and open a new one, or
535alternatively reinitialize the instance by stopping streaming on both queues,
536releasing all buffers and performing the Initialization sequence again.
537
538Encoding Parameter Changes
539==========================
540
541The client is allowed to use :c:func:`VIDIOC_S_CTRL` to change encoder
542parameters at any time. The availability of parameters is encoder-specific
543and the client must query the encoder to find the set of available controls.
544
545The ability to change each parameter during encoding is encoder-specific, as
546per the standard semantics of the V4L2 control interface. The client may
547attempt to set a control during encoding and if the operation fails with the
548-EBUSY error code, the ``CAPTURE`` queue needs to be stopped for the
549configuration change to be allowed. To do this, it may follow the `Drain`
550sequence to avoid losing the already queued/encoded frames.
551
552The timing of parameter updates is encoder-specific, as per the standard
553semantics of the V4L2 control interface. If the client needs to apply the
554parameters exactly at specific frame, using the Request API
555(:ref:`media-request-api`) should be considered, if supported by the encoder.
556
557Drain
558=====
559
560To ensure that all the queued ``OUTPUT`` buffers have been processed and the
561related ``CAPTURE`` buffers are given to the client, the client must follow the
562drain sequence described below. After the drain sequence ends, the client has
563received all encoded frames for all ``OUTPUT`` buffers queued before the
564sequence was started.
565
5661. Begin the drain sequence by issuing :c:func:`VIDIOC_ENCODER_CMD`.
567
568 * **Required fields:**
569
570 ``cmd``
571 set to ``V4L2_ENC_CMD_STOP``.
572
573 ``flags``
574 set to 0.
575
576 ``pts``
577 set to 0.
578
579 .. warning::
580
581 The sequence can be only initiated if both ``OUTPUT`` and ``CAPTURE``
582 queues are streaming. For compatibility reasons, the call to
583 :c:func:`VIDIOC_ENCODER_CMD` will not fail even if any of the queues is
584 not streaming, but at the same time it will not initiate the `Drain`
585 sequence and so the steps described below would not be applicable.
586
5872. Any ``OUTPUT`` buffers queued by the client before the
588 :c:func:`VIDIOC_ENCODER_CMD` was issued will be processed and encoded as
589 normal. The client must continue to handle both queues independently,
590 similarly to normal encode operation. This includes:
591
592 * queuing and dequeuing ``CAPTURE`` buffers, until a buffer marked with the
593 ``V4L2_BUF_FLAG_LAST`` flag is dequeued,
594
595 .. warning::
596
597 The last buffer may be empty (with :c:type:`v4l2_buffer`
598 ``bytesused`` = 0) and in that case it must be ignored by the client,
599 as it does not contain an encoded frame.
600
601 .. note::
602
603 Any attempt to dequeue more ``CAPTURE`` buffers beyond the buffer
604 marked with ``V4L2_BUF_FLAG_LAST`` will result in a -EPIPE error from
605 :c:func:`VIDIOC_DQBUF`.
606
607 * dequeuing processed ``OUTPUT`` buffers, until all the buffers queued
608 before the ``V4L2_ENC_CMD_STOP`` command are dequeued,
609
610 * dequeuing the ``V4L2_EVENT_EOS`` event, if the client subscribes to it.
611
612 .. note::
613
614 For backwards compatibility, the encoder will signal a ``V4L2_EVENT_EOS``
615 event when the last frame has been encoded and all frames are ready to be
616 dequeued. It is deprecated behavior and the client must not rely on it.
617 The ``V4L2_BUF_FLAG_LAST`` buffer flag should be used instead.
618
6193. Once all ``OUTPUT`` buffers queued before the ``V4L2_ENC_CMD_STOP`` call are
620 dequeued and the last ``CAPTURE`` buffer is dequeued, the encoder is stopped
621 and it will accept, but not process any newly queued ``OUTPUT`` buffers
622 until the client issues any of the following operations:
623
624 * ``V4L2_ENC_CMD_START`` - the encoder will not be reset and will resume
625 operation normally, with all the state from before the drain,
626
627 * a pair of :c:func:`VIDIOC_STREAMOFF` and :c:func:`VIDIOC_STREAMON` on the
628 ``CAPTURE`` queue - the encoder will be reset (see the `Reset` sequence)
629 and then resume encoding,
630
631 * a pair of :c:func:`VIDIOC_STREAMOFF` and :c:func:`VIDIOC_STREAMON` on the
632 ``OUTPUT`` queue - the encoder will resume operation normally, however any
633 source frames queued to the ``OUTPUT`` queue between ``V4L2_ENC_CMD_STOP``
634 and :c:func:`VIDIOC_STREAMOFF` will be discarded.
635
636.. note::
637
638 Once the drain sequence is initiated, the client needs to drive it to
639 completion, as described by the steps above, unless it aborts the process by
640 issuing :c:func:`VIDIOC_STREAMOFF` on any of the ``OUTPUT`` or ``CAPTURE``
641 queues. The client is not allowed to issue ``V4L2_ENC_CMD_START`` or
642 ``V4L2_ENC_CMD_STOP`` again while the drain sequence is in progress and they
643 will fail with -EBUSY error code if attempted.
644
645 For reference, handling of various corner cases is described below:
646
647 * In case of no buffer in the ``OUTPUT`` queue at the time the
648 ``V4L2_ENC_CMD_STOP`` command was issued, the drain sequence completes
649 immediately and the encoder returns an empty ``CAPTURE`` buffer with the
650 ``V4L2_BUF_FLAG_LAST`` flag set.
651
652 * In case of no buffer in the ``CAPTURE`` queue at the time the drain
653 sequence completes, the next time the client queues a ``CAPTURE`` buffer
654 it is returned at once as an empty buffer with the ``V4L2_BUF_FLAG_LAST``
655 flag set.
656
657 * If :c:func:`VIDIOC_STREAMOFF` is called on the ``CAPTURE`` queue in the
658 middle of the drain sequence, the drain sequence is canceled and all
659 ``CAPTURE`` buffers are implicitly returned to the client.
660
661 * If :c:func:`VIDIOC_STREAMOFF` is called on the ``OUTPUT`` queue in the
662 middle of the drain sequence, the drain sequence completes immediately and
663 next ``CAPTURE`` buffer will be returned empty with the
664 ``V4L2_BUF_FLAG_LAST`` flag set.
665
666 Although not mandatory, the availability of encoder commands may be queried
667 using :c:func:`VIDIOC_TRY_ENCODER_CMD`.
668
669Reset
670=====
671
672The client may want to request the encoder to reinitialize the encoding, so
673that the following stream data becomes independent from the stream data
674generated before. Depending on the coded format, that may imply that:
675
676* encoded frames produced after the restart must not reference any frames
677 produced before the stop, e.g. no long term references for H.264/HEVC,
678
679* any headers that must be included in a standalone stream must be produced
680 again, e.g. SPS and PPS for H.264/HEVC.
681
682This can be achieved by performing the reset sequence.
683
6841. Perform the `Drain` sequence to ensure all the in-flight encoding finishes
685 and respective buffers are dequeued.
686
6872. Stop streaming on the ``CAPTURE`` queue via :c:func:`VIDIOC_STREAMOFF`. This
688 will return all currently queued ``CAPTURE`` buffers to the client, without
689 valid frame data.
690
6913. Start streaming on the ``CAPTURE`` queue via :c:func:`VIDIOC_STREAMON` and
692 continue with regular encoding sequence. The encoded frames produced into
693 ``CAPTURE`` buffers from now on will contain a standalone stream that can be
694 decoded without the need for frames encoded before the reset sequence,
695 starting at the first ``OUTPUT`` buffer queued after issuing the
696 `V4L2_ENC_CMD_STOP` of the `Drain` sequence.
697
698This sequence may be also used to change encoding parameters for encoders
699without the ability to change the parameters on the fly.
700
701Commit Points
702=============
703
704Setting formats and allocating buffers triggers changes in the behavior of the
705encoder.
706
7071. Setting the format on the ``CAPTURE`` queue may change the set of formats
708 supported/advertised on the ``OUTPUT`` queue. In particular, it also means
709 that the ``OUTPUT`` format may be reset and the client must not rely on the
710 previously set format being preserved.
711
7122. Enumerating formats on the ``OUTPUT`` queue always returns only formats
713 supported for the current ``CAPTURE`` format.
714
7153. Setting the format on the ``OUTPUT`` queue does not change the list of
716 formats available on the ``CAPTURE`` queue. An attempt to set the ``OUTPUT``
717 format that is not supported for the currently selected ``CAPTURE`` format
718 will result in the encoder adjusting the requested ``OUTPUT`` format to a
719 supported one.
720
7214. Enumerating formats on the ``CAPTURE`` queue always returns the full set of
722 supported coded formats, irrespective of the current ``OUTPUT`` format.
723
7245. While buffers are allocated on any of the ``OUTPUT`` or ``CAPTURE`` queues,
725 the client must not change the format on the ``CAPTURE`` queue. Drivers will
726 return the -EBUSY error code for any such format change attempt.
727
728To summarize, setting formats and allocation must always start with the
729``CAPTURE`` queue and the ``CAPTURE`` queue is the master that governs the
730set of supported formats for the ``OUTPUT`` queue.