1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 .. _decoder: 3 .. _decoder:
4 4
5 ********************************************** 5 *************************************************
6 Memory-to-Memory Stateful Video Decoder Interf 6 Memory-to-Memory Stateful Video Decoder Interface
7 ********************************************** 7 *************************************************
8 8
9 A stateful video decoder takes complete chunks 9 A stateful video decoder takes complete chunks of the bytestream (e.g. Annex-B
10 H.264/HEVC stream, raw VP8/9 stream) and decod 10 H.264/HEVC stream, raw VP8/9 stream) and decodes them into raw video frames in
11 display order. The decoder is expected not to 11 display order. The decoder is expected not to require any additional information
12 from the client to process these buffers. 12 from the client to process these buffers.
13 13
14 Performing software parsing, processing etc. o 14 Performing software parsing, processing etc. of the stream in the driver in
15 order to support this interface is strongly di 15 order to support this interface is strongly discouraged. In case such
16 operations are needed, use of the Stateless Vi 16 operations are needed, use of the Stateless Video Decoder Interface (in
17 development) is strongly advised. 17 development) is strongly advised.
18 18
19 Conventions and Notations Used in This Documen 19 Conventions and Notations Used in This Document
20 ============================================== 20 ===============================================
21 21
22 1. The general V4L2 API rules apply if not spe 22 1. The general V4L2 API rules apply if not specified in this document
23 otherwise. 23 otherwise.
24 24
25 2. The meaning of words "must", "may", "should 25 2. The meaning of words "must", "may", "should", etc. is as per `RFC
26 2119 <https://tools.ietf.org/html/rfc2119>` 26 2119 <https://tools.ietf.org/html/rfc2119>`_.
27 27
28 3. All steps not marked "optional" are require 28 3. All steps not marked "optional" are required.
29 29
30 4. :c:func:`VIDIOC_G_EXT_CTRLS` and :c:func:`V 30 4. :c:func:`VIDIOC_G_EXT_CTRLS` and :c:func:`VIDIOC_S_EXT_CTRLS` may be used
31 interchangeably with :c:func:`VIDIOC_G_CTRL 31 interchangeably with :c:func:`VIDIOC_G_CTRL` and :c:func:`VIDIOC_S_CTRL`,
32 unless specified otherwise. 32 unless specified otherwise.
33 33
34 5. Single-planar API (see :ref:`planar-apis`) 34 5. Single-planar API (see :ref:`planar-apis`) and applicable structures may be
35 used interchangeably with multi-planar API, 35 used interchangeably with multi-planar API, unless specified otherwise,
36 depending on decoder capabilities and follo 36 depending on decoder capabilities and following the general V4L2 guidelines.
37 37
38 6. i = [a..b]: sequence of integers from a to 38 6. i = [a..b]: sequence of integers from a to b, inclusive, i.e. i =
39 [0..2]: i = 0, 1, 2. 39 [0..2]: i = 0, 1, 2.
40 40
41 7. Given an ``OUTPUT`` buffer A, then A' repre 41 7. Given an ``OUTPUT`` buffer A, then A' represents a buffer on the ``CAPTURE``
42 queue containing data that resulted from pr 42 queue containing data that resulted from processing buffer A.
43 43
44 .. _decoder-glossary: 44 .. _decoder-glossary:
45 45
46 Glossary 46 Glossary
47 ======== 47 ========
48 48
49 CAPTURE 49 CAPTURE
50 the destination buffer queue; for decoders, 50 the destination buffer queue; for decoders, the queue of buffers containing
51 decoded frames; for encoders, the queue of 51 decoded frames; for encoders, the queue of buffers containing an encoded
52 bytestream; ``V4L2_BUF_TYPE_VIDEO_CAPTURE`` 52 bytestream; ``V4L2_BUF_TYPE_VIDEO_CAPTURE`` or
53 ``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE``; dat 53 ``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE``; data is captured from the hardware
54 into ``CAPTURE`` buffers. 54 into ``CAPTURE`` buffers.
55 55
56 client 56 client
57 the application communicating with the deco 57 the application communicating with the decoder or encoder implementing
58 this interface. 58 this interface.
59 59
60 coded format 60 coded format
61 encoded/compressed video bytestream format 61 encoded/compressed video bytestream format (e.g. H.264, VP8, etc.); see
62 also: raw format. 62 also: raw format.
63 63
64 coded height 64 coded height
65 height for given coded resolution. 65 height for given coded resolution.
66 66
67 coded resolution 67 coded resolution
68 stream resolution in pixels aligned to code 68 stream resolution in pixels aligned to codec and hardware requirements;
69 typically visible resolution rounded up to 69 typically visible resolution rounded up to full macroblocks;
70 see also: visible resolution. 70 see also: visible resolution.
71 71
72 coded width 72 coded width
73 width for given coded resolution. 73 width for given coded resolution.
74 74
75 coding tree unit 75 coding tree unit
76 processing unit of the HEVC codec (correspo 76 processing unit of the HEVC codec (corresponds to macroblock units in
77 H.264, VP8, VP9), 77 H.264, VP8, VP9),
78 can use block structures of up to 64×64 pi 78 can use block structures of up to 64×64 pixels.
79 Good at sub-partitioning the picture into v 79 Good at sub-partitioning the picture into variable sized structures.
80 80
81 decode order 81 decode order
82 the order in which frames are decoded; may 82 the order in which frames are decoded; may differ from display order if the
83 coded format includes a feature of frame re 83 coded format includes a feature of frame reordering; for decoders,
84 ``OUTPUT`` buffers must be queued by the cl 84 ``OUTPUT`` buffers must be queued by the client in decode order; for
85 encoders ``CAPTURE`` buffers must be return 85 encoders ``CAPTURE`` buffers must be returned by the encoder in decode order.
86 86
87 destination 87 destination
88 data resulting from the decode process; see 88 data resulting from the decode process; see ``CAPTURE``.
89 89
90 display order 90 display order
91 the order in which frames must be displayed 91 the order in which frames must be displayed; for encoders, ``OUTPUT``
92 buffers must be queued by the client in dis 92 buffers must be queued by the client in display order; for decoders,
93 ``CAPTURE`` buffers must be returned by the 93 ``CAPTURE`` buffers must be returned by the decoder in display order.
94 94
95 DPB 95 DPB
96 Decoded Picture Buffer; an H.264/HEVC term 96 Decoded Picture Buffer; an H.264/HEVC term for a buffer that stores a decoded
97 raw frame available for reference in furthe 97 raw frame available for reference in further decoding steps.
98 98
99 EOS 99 EOS
100 end of stream. 100 end of stream.
101 101
102 IDR 102 IDR
103 Instantaneous Decoder Refresh; a type of a 103 Instantaneous Decoder Refresh; a type of a keyframe in an H.264/HEVC-encoded
104 stream, which clears the list of earlier re 104 stream, which clears the list of earlier reference frames (DPBs).
105 105
106 keyframe 106 keyframe
107 an encoded frame that does not reference fr 107 an encoded frame that does not reference frames decoded earlier, i.e.
108 can be decoded fully on its own. 108 can be decoded fully on its own.
109 109
110 macroblock 110 macroblock
111 a processing unit in image and video compre 111 a processing unit in image and video compression formats based on linear
112 block transforms (e.g. H.264, VP8, VP9); co 112 block transforms (e.g. H.264, VP8, VP9); codec-specific, but for most of
113 popular codecs the size is 16x16 samples (p 113 popular codecs the size is 16x16 samples (pixels). The HEVC codec uses a
114 slightly more flexible processing unit call 114 slightly more flexible processing unit called coding tree unit (CTU).
115 115
116 OUTPUT 116 OUTPUT
117 the source buffer queue; for decoders, the 117 the source buffer queue; for decoders, the queue of buffers containing
118 an encoded bytestream; for encoders, the qu 118 an encoded bytestream; for encoders, the queue of buffers containing raw
119 frames; ``V4L2_BUF_TYPE_VIDEO_OUTPUT`` or 119 frames; ``V4L2_BUF_TYPE_VIDEO_OUTPUT`` or
120 ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``; the 120 ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``; the hardware is fed with data
121 from ``OUTPUT`` buffers. 121 from ``OUTPUT`` buffers.
122 122
123 PPS 123 PPS
124 Picture Parameter Set; a type of metadata e 124 Picture Parameter Set; a type of metadata entity in an H.264/HEVC bytestream.
125 125
126 raw format 126 raw format
127 uncompressed format containing raw pixel da 127 uncompressed format containing raw pixel data (e.g. YUV, RGB formats).
128 128
129 resume point 129 resume point
130 a point in the bytestream from which decodi 130 a point in the bytestream from which decoding may start/continue, without
131 any previous state/data present, e.g.: a ke 131 any previous state/data present, e.g.: a keyframe (VP8/VP9) or
132 SPS/PPS/IDR sequence (H.264/HEVC); a resume 132 SPS/PPS/IDR sequence (H.264/HEVC); a resume point is required to start decode
133 of a new stream, or to resume decoding afte 133 of a new stream, or to resume decoding after a seek.
134 134
135 source 135 source
136 data fed to the decoder or encoder; see ``O 136 data fed to the decoder or encoder; see ``OUTPUT``.
137 137
138 source height 138 source height
139 height in pixels for given source resolutio 139 height in pixels for given source resolution; relevant to encoders only.
140 140
141 source resolution 141 source resolution
142 resolution in pixels of source frames being 142 resolution in pixels of source frames being source to the encoder and
143 subject to further cropping to the bounds o 143 subject to further cropping to the bounds of visible resolution; relevant to
144 encoders only. 144 encoders only.
145 145
146 source width 146 source width
147 width in pixels for given source resolution 147 width in pixels for given source resolution; relevant to encoders only.
148 148
149 SPS 149 SPS
150 Sequence Parameter Set; a type of metadata 150 Sequence Parameter Set; a type of metadata entity in an H.264/HEVC bytestream.
151 151
152 stream metadata 152 stream metadata
153 additional (non-visual) information contain 153 additional (non-visual) information contained inside encoded bytestream;
154 for example: coded resolution, visible reso 154 for example: coded resolution, visible resolution, codec profile.
155 155
156 visible height 156 visible height
157 height for given visible resolution; displa 157 height for given visible resolution; display height.
158 158
159 visible resolution 159 visible resolution
160 stream resolution of the visible picture, i 160 stream resolution of the visible picture, in pixels, to be used for
161 display purposes; must be smaller or equal 161 display purposes; must be smaller or equal to coded resolution;
162 display resolution. 162 display resolution.
163 163
164 visible width 164 visible width
165 width for given visible resolution; display 165 width for given visible resolution; display width.
166 166
167 State Machine 167 State Machine
168 ============= 168 =============
169 169
170 .. kernel-render:: DOT 170 .. kernel-render:: DOT
171 :alt: DOT digraph of decoder state machine 171 :alt: DOT digraph of decoder state machine
172 :caption: Decoder State Machine 172 :caption: Decoder State Machine
173 173
174 digraph decoder_state_machine { 174 digraph decoder_state_machine {
175 node [shape = doublecircle, label="Deco 175 node [shape = doublecircle, label="Decoding"] Decoding;
176 176
177 node [shape = circle, label="Initializa 177 node [shape = circle, label="Initialization"] Initialization;
178 node [shape = circle, label="Capture\ns 178 node [shape = circle, label="Capture\nsetup"] CaptureSetup;
179 node [shape = circle, label="Dynamic\nR 179 node [shape = circle, label="Dynamic\nResolution\nChange"] ResChange;
180 node [shape = circle, label="Stopped"] 180 node [shape = circle, label="Stopped"] Stopped;
181 node [shape = circle, label="Drain"] Dr 181 node [shape = circle, label="Drain"] Drain;
182 node [shape = circle, label="Seek"] See 182 node [shape = circle, label="Seek"] Seek;
183 node [shape = circle, label="End of Str 183 node [shape = circle, label="End of Stream"] EoS;
184 184
185 node [shape = point]; qi 185 node [shape = point]; qi
186 qi -> Initialization [ label = "open()" 186 qi -> Initialization [ label = "open()" ];
187 187
188 Initialization -> CaptureSetup [ label 188 Initialization -> CaptureSetup [ label = "CAPTURE\nformat\nestablished" ];
189 189
190 CaptureSetup -> Stopped [ label = "CAPT 190 CaptureSetup -> Stopped [ label = "CAPTURE\nbuffers\nready" ];
191 191
192 Decoding -> ResChange [ label = "Stream 192 Decoding -> ResChange [ label = "Stream\nresolution\nchange" ];
193 Decoding -> Drain [ label = "V4L2_DEC_C 193 Decoding -> Drain [ label = "V4L2_DEC_CMD_STOP" ];
194 Decoding -> EoS [ label = "EoS mark\nin 194 Decoding -> EoS [ label = "EoS mark\nin the stream" ];
195 Decoding -> Seek [ label = "VIDIOC_STRE 195 Decoding -> Seek [ label = "VIDIOC_STREAMOFF(OUTPUT)" ];
196 Decoding -> Stopped [ label = "VIDIOC_S 196 Decoding -> Stopped [ label = "VIDIOC_STREAMOFF(CAPTURE)" ];
197 Decoding -> Decoding; 197 Decoding -> Decoding;
198 198
199 ResChange -> CaptureSetup [ label = "CA 199 ResChange -> CaptureSetup [ label = "CAPTURE\nformat\nestablished" ];
200 ResChange -> Seek [ label = "VIDIOC_STR 200 ResChange -> Seek [ label = "VIDIOC_STREAMOFF(OUTPUT)" ];
201 201
202 EoS -> Drain [ label = "Implicit\ndrain 202 EoS -> Drain [ label = "Implicit\ndrain" ];
203 203
204 Drain -> Stopped [ label = "All CAPTURE 204 Drain -> Stopped [ label = "All CAPTURE\nbuffers dequeued\nor\nVIDIOC_STREAMOFF(CAPTURE)" ];
205 Drain -> Seek [ label = "VIDIOC_STREAMO 205 Drain -> Seek [ label = "VIDIOC_STREAMOFF(OUTPUT)" ];
206 206
207 Seek -> Decoding [ label = "VIDIOC_STRE 207 Seek -> Decoding [ label = "VIDIOC_STREAMON(OUTPUT)" ];
208 Seek -> Initialization [ label = "VIDIO 208 Seek -> Initialization [ label = "VIDIOC_REQBUFS(OUTPUT, 0)" ];
209 209
210 Stopped -> Decoding [ label = "V4L2_DEC 210 Stopped -> Decoding [ label = "V4L2_DEC_CMD_START\nor\nVIDIOC_STREAMON(CAPTURE)" ];
211 Stopped -> Seek [ label = "VIDIOC_STREA 211 Stopped -> Seek [ label = "VIDIOC_STREAMOFF(OUTPUT)" ];
212 } 212 }
213 213
214 Querying Capabilities 214 Querying Capabilities
215 ===================== 215 =====================
216 216
217 1. To enumerate the set of coded formats suppo 217 1. To enumerate the set of coded formats supported by the decoder, the
218 client may call :c:func:`VIDIOC_ENUM_FMT` o 218 client may call :c:func:`VIDIOC_ENUM_FMT` on ``OUTPUT``.
219 219
220 * The full set of supported formats will be 220 * The full set of supported formats will be returned, regardless of the
221 format set on ``CAPTURE``. 221 format set on ``CAPTURE``.
222 * Check the flags field of :c:type:`v4l2_fm 222 * Check the flags field of :c:type:`v4l2_fmtdesc` for more information
223 about the decoder's capabilities with res 223 about the decoder's capabilities with respect to each coded format.
224 In particular whether or not the decoder 224 In particular whether or not the decoder has a full-fledged bytestream
225 parser and if the decoder supports dynami 225 parser and if the decoder supports dynamic resolution changes.
226 226
227 2. To enumerate the set of supported raw forma 227 2. To enumerate the set of supported raw formats, the client may call
228 :c:func:`VIDIOC_ENUM_FMT` on ``CAPTURE``. 228 :c:func:`VIDIOC_ENUM_FMT` on ``CAPTURE``.
229 229
230 * Only the formats supported for the format 230 * Only the formats supported for the format currently active on ``OUTPUT``
231 will be returned. 231 will be returned.
232 232
233 * In order to enumerate raw formats support 233 * In order to enumerate raw formats supported by a given coded format,
234 the client must first set that coded form 234 the client must first set that coded format on ``OUTPUT`` and then
235 enumerate formats on ``CAPTURE``. 235 enumerate formats on ``CAPTURE``.
236 236
237 3. The client may use :c:func:`VIDIOC_ENUM_FRA 237 3. The client may use :c:func:`VIDIOC_ENUM_FRAMESIZES` to detect supported
238 resolutions for a given format, passing des 238 resolutions for a given format, passing desired pixel format in
239 :c:type:`v4l2_frmsizeenum` ``pixel_format`` 239 :c:type:`v4l2_frmsizeenum` ``pixel_format``.
240 240
241 * Values returned by :c:func:`VIDIOC_ENUM_F 241 * Values returned by :c:func:`VIDIOC_ENUM_FRAMESIZES` for a coded pixel
242 format will include all possible coded re 242 format will include all possible coded resolutions supported by the
243 decoder for given coded pixel format. 243 decoder for given coded pixel format.
244 244
245 * Values returned by :c:func:`VIDIOC_ENUM_F 245 * Values returned by :c:func:`VIDIOC_ENUM_FRAMESIZES` for a raw pixel format
246 will include all possible frame buffer re 246 will include all possible frame buffer resolutions supported by the
247 decoder for given raw pixel format and th 247 decoder for given raw pixel format and the coded format currently set on
248 ``OUTPUT``. 248 ``OUTPUT``.
249 249
250 4. Supported profiles and levels for the coded 250 4. Supported profiles and levels for the coded format currently set on
251 ``OUTPUT``, if applicable, may be queried u 251 ``OUTPUT``, if applicable, may be queried using their respective controls
252 via :c:func:`VIDIOC_QUERYCTRL`. 252 via :c:func:`VIDIOC_QUERYCTRL`.
253 253
254 Initialization 254 Initialization
255 ============== 255 ==============
256 256
257 1. Set the coded format on ``OUTPUT`` via :c:f 257 1. Set the coded format on ``OUTPUT`` via :c:func:`VIDIOC_S_FMT`.
258 258
259 * **Required fields:** 259 * **Required fields:**
260 260
261 ``type`` 261 ``type``
262 a ``V4L2_BUF_TYPE_*`` enum appropriat 262 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``.
263 263
264 ``pixelformat`` 264 ``pixelformat``
265 a coded pixel format. 265 a coded pixel format.
266 266
267 ``width``, ``height`` 267 ``width``, ``height``
268 coded resolution of the stream; requi 268 coded resolution of the stream; required only if it cannot be parsed
269 from the stream for the given coded f 269 from the stream for the given coded format; otherwise the decoder will
270 use this resolution as a placeholder 270 use this resolution as a placeholder resolution that will likely change
271 as soon as it can parse the actual co 271 as soon as it can parse the actual coded resolution from the stream.
272 272
273 ``sizeimage`` 273 ``sizeimage``
274 desired size of ``OUTPUT`` buffers; t 274 desired size of ``OUTPUT`` buffers; the decoder may adjust it to
275 match hardware requirements. 275 match hardware requirements.
276 276
277 other fields 277 other fields
278 follow standard semantics. 278 follow standard semantics.
279 279
280 * **Returned fields:** !! 280 * **Return fields:**
281 281
282 ``sizeimage`` 282 ``sizeimage``
283 adjusted size of ``OUTPUT`` buffers. 283 adjusted size of ``OUTPUT`` buffers.
284 284
285 * The ``CAPTURE`` format will be updated wi 285 * The ``CAPTURE`` format will be updated with an appropriate frame buffer
286 resolution instantly based on the width a 286 resolution instantly based on the width and height returned by
287 :c:func:`VIDIOC_S_FMT`. 287 :c:func:`VIDIOC_S_FMT`.
288 However, for coded formats that include s 288 However, for coded formats that include stream resolution information,
289 after the decoder is done parsing the inf 289 after the decoder is done parsing the information from the stream, it will
290 update the ``CAPTURE`` format with new va 290 update the ``CAPTURE`` format with new values and signal a source change
291 event, regardless of whether they match t 291 event, regardless of whether they match the values set by the client or
292 not. 292 not.
293 293
294 .. important:: 294 .. important::
295 295
296 Changing the ``OUTPUT`` format may chang 296 Changing the ``OUTPUT`` format may change the currently set ``CAPTURE``
297 format. How the new ``CAPTURE`` format i 297 format. How the new ``CAPTURE`` format is determined is up to the decoder
298 and the client must ensure it matches it 298 and the client must ensure it matches its needs afterwards.
299 299
300 2. Allocate source (bytestream) buffers via : 300 2. Allocate source (bytestream) buffers via :c:func:`VIDIOC_REQBUFS` on
301 ``OUTPUT``. 301 ``OUTPUT``.
302 302
303 * **Required fields:** 303 * **Required fields:**
304 304
305 ``count`` 305 ``count``
306 requested number of buffers to alloc 306 requested number of buffers to allocate; greater than zero.
307 307
308 ``type`` 308 ``type``
309 a ``V4L2_BUF_TYPE_*`` enum appropria 309 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``.
310 310
311 ``memory`` 311 ``memory``
312 follows standard semantics. 312 follows standard semantics.
313 313
314 * **Returned fields:** !! 314 * **Return fields:**
315 315
316 ``count`` 316 ``count``
317 the actual number of buffers allocat 317 the actual number of buffers allocated.
318 318
319 .. warning:: 319 .. warning::
320 320
321 The actual number of allocated buffers 321 The actual number of allocated buffers may differ from the ``count``
322 given. The client must check the update 322 given. The client must check the updated value of ``count`` after the
323 call returns. 323 call returns.
324 324
325 Alternatively, :c:func:`VIDIOC_CREATE_BUFS 325 Alternatively, :c:func:`VIDIOC_CREATE_BUFS` on the ``OUTPUT`` queue can be
326 used to have more control over buffer allo 326 used to have more control over buffer allocation.
327 327
328 * **Required fields:** 328 * **Required fields:**
329 329
330 ``count`` 330 ``count``
331 requested number of buffers to alloc 331 requested number of buffers to allocate; greater than zero.
332 332
333 ``type`` 333 ``type``
334 a ``V4L2_BUF_TYPE_*`` enum appropria 334 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``.
335 335
336 ``memory`` 336 ``memory``
337 follows standard semantics. 337 follows standard semantics.
338 338
339 ``format`` 339 ``format``
340 follows standard semantics. 340 follows standard semantics.
341 341
342 * **Returned fields:** !! 342 * **Return fields:**
343 343
344 ``count`` 344 ``count``
345 adjusted to the number of allocated 345 adjusted to the number of allocated buffers.
346 346
347 .. warning:: 347 .. warning::
348 348
349 The actual number of allocated buffers 349 The actual number of allocated buffers may differ from the ``count``
350 given. The client must check the update 350 given. The client must check the updated value of ``count`` after the
351 call returns. 351 call returns.
352 352
353 3. Start streaming on the ``OUTPUT`` queue vi 353 3. Start streaming on the ``OUTPUT`` queue via :c:func:`VIDIOC_STREAMON`.
354 354
355 4. **This step only applies to coded formats 355 4. **This step only applies to coded formats that contain resolution information
356 in the stream.** Continue queuing/dequeuin 356 in the stream.** Continue queuing/dequeuing bytestream buffers to/from the
357 ``OUTPUT`` queue via :c:func:`VIDIOC_QBUF` 357 ``OUTPUT`` queue via :c:func:`VIDIOC_QBUF` and :c:func:`VIDIOC_DQBUF`. The
358 buffers will be processed and returned to 358 buffers will be processed and returned to the client in order, until
359 required metadata to configure the ``CAPTU 359 required metadata to configure the ``CAPTURE`` queue are found. This is
360 indicated by the decoder sending a ``V4L2_ 360 indicated by the decoder sending a ``V4L2_EVENT_SOURCE_CHANGE`` event with
361 ``changes`` set to ``V4L2_EVENT_SRC_CH_RES 361 ``changes`` set to ``V4L2_EVENT_SRC_CH_RESOLUTION``.
362 362
363 * It is not an error if the first buffer d 363 * It is not an error if the first buffer does not contain enough data for
364 this to occur. Processing of the buffers 364 this to occur. Processing of the buffers will continue as long as more
365 data is needed. 365 data is needed.
366 366
367 * If data in a buffer that triggers the ev 367 * If data in a buffer that triggers the event is required to decode the
368 first frame, it will not be returned to 368 first frame, it will not be returned to the client, until the
369 initialization sequence completes and th 369 initialization sequence completes and the frame is decoded.
370 370
371 * If the client has not set the coded reso 371 * If the client has not set the coded resolution of the stream on its own,
372 calling :c:func:`VIDIOC_G_FMT`, :c:func: 372 calling :c:func:`VIDIOC_G_FMT`, :c:func:`VIDIOC_S_FMT`,
373 :c:func:`VIDIOC_TRY_FMT` or :c:func:`VID 373 :c:func:`VIDIOC_TRY_FMT` or :c:func:`VIDIOC_REQBUFS` on the ``CAPTURE``
374 queue will not return the real values fo 374 queue will not return the real values for the stream until a
375 ``V4L2_EVENT_SOURCE_CHANGE`` event with 375 ``V4L2_EVENT_SOURCE_CHANGE`` event with ``changes`` set to
376 ``V4L2_EVENT_SRC_CH_RESOLUTION`` is sign 376 ``V4L2_EVENT_SRC_CH_RESOLUTION`` is signaled.
377 377
378 .. important:: 378 .. important::
379 379
380 Any client query issued after the decod 380 Any client query issued after the decoder queues the event will return
381 values applying to the just parsed stre 381 values applying to the just parsed stream, including queue formats,
382 selection rectangles and controls. 382 selection rectangles and controls.
383 383
384 .. note:: 384 .. note::
385 385
386 A client capable of acquiring stream pa 386 A client capable of acquiring stream parameters from the bytestream on
387 its own may attempt to set the width an 387 its own may attempt to set the width and height of the ``OUTPUT`` format
388 to non-zero values matching the coded s 388 to non-zero values matching the coded size of the stream, skip this step
389 and continue with the `Capture Setup` s 389 and continue with the `Capture Setup` sequence. However, it must not
390 rely on any driver queries regarding st 390 rely on any driver queries regarding stream parameters, such as
391 selection rectangles and controls, sinc 391 selection rectangles and controls, since the decoder has not parsed them
392 from the stream yet. If the values conf 392 from the stream yet. If the values configured by the client do not match
393 those parsed by the decoder, a `Dynamic 393 those parsed by the decoder, a `Dynamic Resolution Change` will be
394 triggered to reconfigure them. 394 triggered to reconfigure them.
395 395
396 .. note:: 396 .. note::
397 397
398 No decoded frames are produced during t 398 No decoded frames are produced during this phase.
399 399
400 5. Continue with the `Capture Setup` sequence 400 5. Continue with the `Capture Setup` sequence.
401 401
402 Capture Setup 402 Capture Setup
403 ============= 403 =============
404 404
405 1. Call :c:func:`VIDIOC_G_FMT` on the ``CAPTU 405 1. Call :c:func:`VIDIOC_G_FMT` on the ``CAPTURE`` queue to get format for the
406 destination buffers parsed/decoded from th 406 destination buffers parsed/decoded from the bytestream.
407 407
408 * **Required fields:** 408 * **Required fields:**
409 409
410 ``type`` 410 ``type``
411 a ``V4L2_BUF_TYPE_*`` enum appropria 411 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``CAPTURE``.
412 412
413 * **Returned fields:** !! 413 * **Return fields:**
414 414
415 ``width``, ``height`` 415 ``width``, ``height``
416 frame buffer resolution for the deco 416 frame buffer resolution for the decoded frames.
417 417
418 ``pixelformat`` 418 ``pixelformat``
419 pixel format for decoded frames. 419 pixel format for decoded frames.
420 420
421 ``num_planes`` (for _MPLANE ``type`` onl 421 ``num_planes`` (for _MPLANE ``type`` only)
422 number of planes for pixelformat. 422 number of planes for pixelformat.
423 423
424 ``sizeimage``, ``bytesperline`` 424 ``sizeimage``, ``bytesperline``
425 as per standard semantics; matching 425 as per standard semantics; matching frame buffer format.
426 426
427 .. note:: 427 .. note::
428 428
429 The value of ``pixelformat`` may be any 429 The value of ``pixelformat`` may be any pixel format supported by the
430 decoder for the current stream. The dec 430 decoder for the current stream. The decoder should choose a
431 preferred/optimal format for the defaul 431 preferred/optimal format for the default configuration. For example, a
432 YUV format may be preferred over an RGB 432 YUV format may be preferred over an RGB format if an additional
433 conversion step would be required for t 433 conversion step would be required for the latter.
434 434
435 2. **Optional.** Acquire the visible resoluti 435 2. **Optional.** Acquire the visible resolution via
436 :c:func:`VIDIOC_G_SELECTION`. 436 :c:func:`VIDIOC_G_SELECTION`.
437 437
438 * **Required fields:** 438 * **Required fields:**
439 439
440 ``type`` 440 ``type``
441 a ``V4L2_BUF_TYPE_*`` enum appropria 441 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``CAPTURE``.
442 442
443 ``target`` 443 ``target``
444 set to ``V4L2_SEL_TGT_COMPOSE``. 444 set to ``V4L2_SEL_TGT_COMPOSE``.
445 445
446 * **Returned fields:** !! 446 * **Return fields:**
447 447
448 ``r.left``, ``r.top``, ``r.width``, ``r. 448 ``r.left``, ``r.top``, ``r.width``, ``r.height``
449 the visible rectangle; it must fit w 449 the visible rectangle; it must fit within the frame buffer resolution
450 returned by :c:func:`VIDIOC_G_FMT` o 450 returned by :c:func:`VIDIOC_G_FMT` on ``CAPTURE``.
451 451
452 * The following selection targets are supp 452 * The following selection targets are supported on ``CAPTURE``:
453 453
454 ``V4L2_SEL_TGT_CROP_BOUNDS`` 454 ``V4L2_SEL_TGT_CROP_BOUNDS``
455 corresponds to the coded resolution 455 corresponds to the coded resolution of the stream.
456 456
457 ``V4L2_SEL_TGT_CROP_DEFAULT`` 457 ``V4L2_SEL_TGT_CROP_DEFAULT``
458 the rectangle covering the part of t 458 the rectangle covering the part of the ``CAPTURE`` buffer that
459 contains meaningful picture data (vi 459 contains meaningful picture data (visible area); width and height
460 will be equal to the visible resolut 460 will be equal to the visible resolution of the stream.
461 461
462 ``V4L2_SEL_TGT_CROP`` 462 ``V4L2_SEL_TGT_CROP``
463 the rectangle within the coded resol 463 the rectangle within the coded resolution to be output to
464 ``CAPTURE``; defaults to ``V4L2_SEL_ 464 ``CAPTURE``; defaults to ``V4L2_SEL_TGT_CROP_DEFAULT``; read-only on
465 hardware without additional compose/ 465 hardware without additional compose/scaling capabilities.
466 466
467 ``V4L2_SEL_TGT_COMPOSE_BOUNDS`` 467 ``V4L2_SEL_TGT_COMPOSE_BOUNDS``
468 the maximum rectangle within a ``CAP 468 the maximum rectangle within a ``CAPTURE`` buffer, which the cropped
469 frame can be composed into; equal to 469 frame can be composed into; equal to ``V4L2_SEL_TGT_CROP`` if the
470 hardware does not support compose/sc 470 hardware does not support compose/scaling.
471 471
472 ``V4L2_SEL_TGT_COMPOSE_DEFAULT`` 472 ``V4L2_SEL_TGT_COMPOSE_DEFAULT``
473 equal to ``V4L2_SEL_TGT_CROP``. 473 equal to ``V4L2_SEL_TGT_CROP``.
474 474
475 ``V4L2_SEL_TGT_COMPOSE`` 475 ``V4L2_SEL_TGT_COMPOSE``
476 the rectangle inside a ``CAPTURE`` b 476 the rectangle inside a ``CAPTURE`` buffer into which the cropped
477 frame is written; defaults to ``V4L2 477 frame is written; defaults to ``V4L2_SEL_TGT_COMPOSE_DEFAULT``;
478 read-only on hardware without additi 478 read-only on hardware without additional compose/scaling capabilities.
479 479
480 ``V4L2_SEL_TGT_COMPOSE_PADDED`` 480 ``V4L2_SEL_TGT_COMPOSE_PADDED``
481 the rectangle inside a ``CAPTURE`` b 481 the rectangle inside a ``CAPTURE`` buffer which is overwritten by the
482 hardware; equal to ``V4L2_SEL_TGT_CO 482 hardware; equal to ``V4L2_SEL_TGT_COMPOSE`` if the hardware does not
483 write padding pixels. 483 write padding pixels.
484 484
485 .. warning:: 485 .. warning::
486 486
487 The values are guaranteed to be meaning 487 The values are guaranteed to be meaningful only after the decoder
488 successfully parses the stream metadata 488 successfully parses the stream metadata. The client must not rely on the
489 query before that happens. 489 query before that happens.
490 490
491 3. **Optional.** Enumerate ``CAPTURE`` format 491 3. **Optional.** Enumerate ``CAPTURE`` formats via :c:func:`VIDIOC_ENUM_FMT` on
492 the ``CAPTURE`` queue. Once the stream inf 492 the ``CAPTURE`` queue. Once the stream information is parsed and known, the
493 client may use this ioctl to discover whic 493 client may use this ioctl to discover which raw formats are supported for
494 given stream and select one of them via :c 494 given stream and select one of them via :c:func:`VIDIOC_S_FMT`.
495 495
496 .. important:: 496 .. important::
497 497
498 The decoder will return only formats su 498 The decoder will return only formats supported for the currently
499 established coded format, as per the `` 499 established coded format, as per the ``OUTPUT`` format and/or stream
500 metadata parsed in this initialization 500 metadata parsed in this initialization sequence, even if more formats
501 may be supported by the decoder in gene 501 may be supported by the decoder in general. In other words, the set
502 returned will be a subset of the initia 502 returned will be a subset of the initial query mentioned in the
503 `Querying Capabilities` section. 503 `Querying Capabilities` section.
504 504
505 For example, a decoder may support YUV 505 For example, a decoder may support YUV and RGB formats for resolutions
506 1920x1088 and lower, but only YUV for h 506 1920x1088 and lower, but only YUV for higher resolutions (due to
507 hardware limitations). After parsing a 507 hardware limitations). After parsing a resolution of 1920x1088 or lower,
508 :c:func:`VIDIOC_ENUM_FMT` may return a 508 :c:func:`VIDIOC_ENUM_FMT` may return a set of YUV and RGB pixel formats,
509 but after parsing resolution higher tha 509 but after parsing resolution higher than 1920x1088, the decoder will not
510 return RGB, unsupported for this resolu 510 return RGB, unsupported for this resolution.
511 511
512 However, subsequent resolution change e 512 However, subsequent resolution change event triggered after
513 discovering a resolution change within 513 discovering a resolution change within the same stream may switch
514 the stream into a lower resolution and 514 the stream into a lower resolution and :c:func:`VIDIOC_ENUM_FMT`
515 would return RGB formats again in that 515 would return RGB formats again in that case.
516 516
517 4. **Optional.** Set the ``CAPTURE`` format v 517 4. **Optional.** Set the ``CAPTURE`` format via :c:func:`VIDIOC_S_FMT` on the
518 ``CAPTURE`` queue. The client may choose a 518 ``CAPTURE`` queue. The client may choose a different format than
519 selected/suggested by the decoder in :c:fu 519 selected/suggested by the decoder in :c:func:`VIDIOC_G_FMT`.
520 520
521 * **Required fields:** 521 * **Required fields:**
522 522
523 ``type`` 523 ``type``
524 a ``V4L2_BUF_TYPE_*`` enum appropria 524 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``CAPTURE``.
525 525
526 ``pixelformat`` 526 ``pixelformat``
527 a raw pixel format. 527 a raw pixel format.
528 528
529 ``width``, ``height`` 529 ``width``, ``height``
530 frame buffer resolution of the decode 530 frame buffer resolution of the decoded stream; typically unchanged from
531 what was returned with :c:func:`VIDIO 531 what was returned with :c:func:`VIDIOC_G_FMT`, but it may be different
532 if the hardware supports composition 532 if the hardware supports composition and/or scaling.
533 533
534 * Setting the ``CAPTURE`` format will reset 534 * Setting the ``CAPTURE`` format will reset the compose selection rectangles
535 to their default values, based on the new 535 to their default values, based on the new resolution, as described in the
536 previous step. 536 previous step.
537 537
538 5. **Optional.** Set the compose rectangle via 538 5. **Optional.** Set the compose rectangle via :c:func:`VIDIOC_S_SELECTION` on
539 the ``CAPTURE`` queue if it is desired and 539 the ``CAPTURE`` queue if it is desired and if the decoder has compose and/or
540 scaling capabilities. 540 scaling capabilities.
541 541
542 * **Required fields:** 542 * **Required fields:**
543 543
544 ``type`` 544 ``type``
545 a ``V4L2_BUF_TYPE_*`` enum appropriat 545 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``CAPTURE``.
546 546
547 ``target`` 547 ``target``
548 set to ``V4L2_SEL_TGT_COMPOSE``. 548 set to ``V4L2_SEL_TGT_COMPOSE``.
549 549
550 ``r.left``, ``r.top``, ``r.width``, ``r.h 550 ``r.left``, ``r.top``, ``r.width``, ``r.height``
551 the rectangle inside a ``CAPTURE`` bu 551 the rectangle inside a ``CAPTURE`` buffer into which the cropped
552 frame is written; defaults to ``V4L2_ 552 frame is written; defaults to ``V4L2_SEL_TGT_COMPOSE_DEFAULT``;
553 read-only on hardware without additio 553 read-only on hardware without additional compose/scaling capabilities.
554 554
555 * **Returned fields:** !! 555 * **Return fields:**
556 556
557 ``r.left``, ``r.top``, ``r.width``, ``r.h 557 ``r.left``, ``r.top``, ``r.width``, ``r.height``
558 the visible rectangle; it must fit wi 558 the visible rectangle; it must fit within the frame buffer resolution
559 returned by :c:func:`VIDIOC_G_FMT` on 559 returned by :c:func:`VIDIOC_G_FMT` on ``CAPTURE``.
560 560
561 .. warning:: 561 .. warning::
562 562
563 The decoder may adjust the compose recta 563 The decoder may adjust the compose rectangle to the nearest
564 supported one to meet codec and hardware 564 supported one to meet codec and hardware requirements. The client needs
565 to check the adjusted rectangle returned 565 to check the adjusted rectangle returned by :c:func:`VIDIOC_S_SELECTION`.
566 566
567 6. If all the following conditions are met, t 567 6. If all the following conditions are met, the client may resume the decoding
568 instantly: 568 instantly:
569 569
570 * ``sizeimage`` of the new format (determi 570 * ``sizeimage`` of the new format (determined in previous steps) is less
571 than or equal to the size of currently a 571 than or equal to the size of currently allocated buffers,
572 572
573 * the number of buffers currently allocate 573 * the number of buffers currently allocated is greater than or equal to the
574 minimum number of buffers acquired in pr 574 minimum number of buffers acquired in previous steps. To fulfill this
575 requirement, the client may use :c:func: 575 requirement, the client may use :c:func:`VIDIOC_CREATE_BUFS` to add new
576 buffers. 576 buffers.
577 577
578 In that case, the remaining steps do not a 578 In that case, the remaining steps do not apply and the client may resume
579 the decoding by one of the following actio 579 the decoding by one of the following actions:
580 580
581 * if the ``CAPTURE`` queue is streaming, c 581 * if the ``CAPTURE`` queue is streaming, call :c:func:`VIDIOC_DECODER_CMD`
582 with the ``V4L2_DEC_CMD_START`` command, 582 with the ``V4L2_DEC_CMD_START`` command,
583 583
584 * if the ``CAPTURE`` queue is not streamin 584 * if the ``CAPTURE`` queue is not streaming, call :c:func:`VIDIOC_STREAMON`
585 on the ``CAPTURE`` queue. 585 on the ``CAPTURE`` queue.
586 586
587 However, if the client intends to change t 587 However, if the client intends to change the buffer set, to lower
588 memory usage or for any other reasons, it 588 memory usage or for any other reasons, it may be achieved by following
589 the steps below. 589 the steps below.
590 590
591 7. **If the** ``CAPTURE`` **queue is streamin 591 7. **If the** ``CAPTURE`` **queue is streaming,** keep queuing and dequeuing
592 buffers on the ``CAPTURE`` queue until a b 592 buffers on the ``CAPTURE`` queue until a buffer marked with the
593 ``V4L2_BUF_FLAG_LAST`` flag is dequeued. 593 ``V4L2_BUF_FLAG_LAST`` flag is dequeued.
594 594
595 8. **If the** ``CAPTURE`` **queue is streamin 595 8. **If the** ``CAPTURE`` **queue is streaming,** call :c:func:`VIDIOC_STREAMOFF`
596 on the ``CAPTURE`` queue to stop streaming 596 on the ``CAPTURE`` queue to stop streaming.
597 597
598 .. warning:: 598 .. warning::
599 599
600 The ``OUTPUT`` queue must remain stream 600 The ``OUTPUT`` queue must remain streaming. Calling
601 :c:func:`VIDIOC_STREAMOFF` on it would 601 :c:func:`VIDIOC_STREAMOFF` on it would abort the sequence and trigger a
602 seek. 602 seek.
603 603
604 9. **If the** ``CAPTURE`` **queue has buffers 604 9. **If the** ``CAPTURE`` **queue has buffers allocated,** free the ``CAPTURE``
605 buffers using :c:func:`VIDIOC_REQBUFS`. 605 buffers using :c:func:`VIDIOC_REQBUFS`.
606 606
607 * **Required fields:** 607 * **Required fields:**
608 608
609 ``count`` 609 ``count``
610 set to 0. 610 set to 0.
611 611
612 ``type`` 612 ``type``
613 a ``V4L2_BUF_TYPE_*`` enum appropria 613 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``CAPTURE``.
614 614
615 ``memory`` 615 ``memory``
616 follows standard semantics. 616 follows standard semantics.
617 617
618 10. Allocate ``CAPTURE`` buffers via :c:func:` 618 10. Allocate ``CAPTURE`` buffers via :c:func:`VIDIOC_REQBUFS` on the
619 ``CAPTURE`` queue. 619 ``CAPTURE`` queue.
620 620
621 * **Required fields:** 621 * **Required fields:**
622 622
623 ``count`` 623 ``count``
624 requested number of buffers to alloc 624 requested number of buffers to allocate; greater than zero.
625 625
626 ``type`` 626 ``type``
627 a ``V4L2_BUF_TYPE_*`` enum appropria 627 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``CAPTURE``.
628 628
629 ``memory`` 629 ``memory``
630 follows standard semantics. 630 follows standard semantics.
631 631
632 * **Returned fields:** !! 632 * **Return fields:**
633 633
634 ``count`` 634 ``count``
635 actual number of buffers allocated. 635 actual number of buffers allocated.
636 636
637 .. warning:: 637 .. warning::
638 638
639 The actual number of allocated buffers 639 The actual number of allocated buffers may differ from the ``count``
640 given. The client must check the update 640 given. The client must check the updated value of ``count`` after the
641 call returns. 641 call returns.
642 642
643 .. note:: 643 .. note::
644 644
645 To allocate more than the minimum numbe 645 To allocate more than the minimum number of buffers (for pipeline
646 depth), the client may query the ``V4L2 646 depth), the client may query the ``V4L2_CID_MIN_BUFFERS_FOR_CAPTURE``
647 control to get the minimum number of bu 647 control to get the minimum number of buffers required, and pass the
648 obtained value plus the number of addit 648 obtained value plus the number of additional buffers needed in the
649 ``count`` field to :c:func:`VIDIOC_REQB 649 ``count`` field to :c:func:`VIDIOC_REQBUFS`.
650 650
651 Alternatively, :c:func:`VIDIOC_CREATE_BUFS 651 Alternatively, :c:func:`VIDIOC_CREATE_BUFS` on the ``CAPTURE`` queue can be
652 used to have more control over buffer allo 652 used to have more control over buffer allocation. For example, by
653 allocating buffers larger than the current 653 allocating buffers larger than the current ``CAPTURE`` format, future
654 resolution changes can be accommodated. 654 resolution changes can be accommodated.
655 655
656 * **Required fields:** 656 * **Required fields:**
657 657
658 ``count`` 658 ``count``
659 requested number of buffers to alloc 659 requested number of buffers to allocate; greater than zero.
660 660
661 ``type`` 661 ``type``
662 a ``V4L2_BUF_TYPE_*`` enum appropria 662 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``CAPTURE``.
663 663
664 ``memory`` 664 ``memory``
665 follows standard semantics. 665 follows standard semantics.
666 666
667 ``format`` 667 ``format``
668 a format representing the maximum fr 668 a format representing the maximum framebuffer resolution to be
669 accommodated by newly allocated buff 669 accommodated by newly allocated buffers.
670 670
671 * **Returned fields:** !! 671 * **Return fields:**
672 672
673 ``count`` 673 ``count``
674 adjusted to the number of allocated 674 adjusted to the number of allocated buffers.
675 675
676 .. warning:: 676 .. warning::
677 677
678 The actual number of allocated buffers 678 The actual number of allocated buffers may differ from the ``count``
679 given. The client must check the updat 679 given. The client must check the updated value of ``count`` after the
680 call returns. 680 call returns.
681 681
682 .. note:: 682 .. note::
683 683
684 To allocate buffers for a format differ 684 To allocate buffers for a format different than parsed from the stream
685 metadata, the client must proceed as fo 685 metadata, the client must proceed as follows, before the metadata
686 parsing is initiated: 686 parsing is initiated:
687 687
688 * set width and height of the ``OUTPUT` 688 * set width and height of the ``OUTPUT`` format to desired coded resolution to
689 let the decoder configure the ``CAPTU 689 let the decoder configure the ``CAPTURE`` format appropriately,
690 690
691 * query the ``CAPTURE`` format using :c 691 * query the ``CAPTURE`` format using :c:func:`VIDIOC_G_FMT` and save it
692 until this step. 692 until this step.
693 693
694 The format obtained in the query may be 694 The format obtained in the query may be then used with
695 :c:func:`VIDIOC_CREATE_BUFS` in this st 695 :c:func:`VIDIOC_CREATE_BUFS` in this step to allocate the buffers.
696 696
697 11. Call :c:func:`VIDIOC_STREAMON` on the ``CA 697 11. Call :c:func:`VIDIOC_STREAMON` on the ``CAPTURE`` queue to start decoding
698 frames. 698 frames.
699 699
700 Decoding 700 Decoding
701 ======== 701 ========
702 702
703 This state is reached after the `Capture Setup 703 This state is reached after the `Capture Setup` sequence finishes successfully.
704 In this state, the client queues and dequeues 704 In this state, the client queues and dequeues buffers to both queues via
705 :c:func:`VIDIOC_QBUF` and :c:func:`VIDIOC_DQBU 705 :c:func:`VIDIOC_QBUF` and :c:func:`VIDIOC_DQBUF`, following the standard
706 semantics. 706 semantics.
707 707
708 The content of the source ``OUTPUT`` buffers d 708 The content of the source ``OUTPUT`` buffers depends on the active coded pixel
709 format and may be affected by codec-specific e 709 format and may be affected by codec-specific extended controls, as stated in
710 the documentation of each format. 710 the documentation of each format.
711 711
712 Both queues operate independently, following t 712 Both queues operate independently, following the standard behavior of V4L2
713 buffer queues and memory-to-memory devices. In 713 buffer queues and memory-to-memory devices. In addition, the order of decoded
714 frames dequeued from the ``CAPTURE`` queue may 714 frames dequeued from the ``CAPTURE`` queue may differ from the order of queuing
715 coded frames to the ``OUTPUT`` queue, due to p 715 coded frames to the ``OUTPUT`` queue, due to properties of the selected coded
716 format, e.g. frame reordering. 716 format, e.g. frame reordering.
717 717
718 The client must not assume any direct relation 718 The client must not assume any direct relationship between ``CAPTURE``
719 and ``OUTPUT`` buffers and any specific timing 719 and ``OUTPUT`` buffers and any specific timing of buffers becoming
720 available to dequeue. Specifically: 720 available to dequeue. Specifically:
721 721
722 * a buffer queued to ``OUTPUT`` may result in 722 * a buffer queued to ``OUTPUT`` may result in no buffers being produced
723 on ``CAPTURE`` (e.g. if it does not contain 723 on ``CAPTURE`` (e.g. if it does not contain encoded data, or if only
724 metadata syntax structures are present in it 724 metadata syntax structures are present in it),
725 725
726 * a buffer queued to ``OUTPUT`` may result in 726 * a buffer queued to ``OUTPUT`` may result in more than one buffer produced
727 on ``CAPTURE`` (if the encoded data containe 727 on ``CAPTURE`` (if the encoded data contained more than one frame, or if
728 returning a decoded frame allowed the decode 728 returning a decoded frame allowed the decoder to return a frame that
729 preceded it in decode, but succeeded it in t 729 preceded it in decode, but succeeded it in the display order),
730 730
731 * a buffer queued to ``OUTPUT`` may result in 731 * a buffer queued to ``OUTPUT`` may result in a buffer being produced on
732 ``CAPTURE`` later into decode process, and/o 732 ``CAPTURE`` later into decode process, and/or after processing further
733 ``OUTPUT`` buffers, or be returned out of or 733 ``OUTPUT`` buffers, or be returned out of order, e.g. if display
734 reordering is used, 734 reordering is used,
735 735
736 * buffers may become available on the ``CAPTUR 736 * buffers may become available on the ``CAPTURE`` queue without additional
737 buffers queued to ``OUTPUT`` (e.g. during dr 737 buffers queued to ``OUTPUT`` (e.g. during drain or ``EOS``), because of the
738 ``OUTPUT`` buffers queued in the past whose 738 ``OUTPUT`` buffers queued in the past whose decoding results are only
739 available at later time, due to specifics of 739 available at later time, due to specifics of the decoding process.
740 740
741 .. note:: 741 .. note::
742 742
743 To allow matching decoded ``CAPTURE`` buffe 743 To allow matching decoded ``CAPTURE`` buffers with ``OUTPUT`` buffers they
744 originated from, the client can set the ``t 744 originated from, the client can set the ``timestamp`` field of the
745 :c:type:`v4l2_buffer` struct when queuing a 745 :c:type:`v4l2_buffer` struct when queuing an ``OUTPUT`` buffer. The
746 ``CAPTURE`` buffer(s), which resulted from 746 ``CAPTURE`` buffer(s), which resulted from decoding that ``OUTPUT`` buffer
747 will have their ``timestamp`` field set to 747 will have their ``timestamp`` field set to the same value when dequeued.
748 748
749 In addition to the straightforward case of 749 In addition to the straightforward case of one ``OUTPUT`` buffer producing
750 one ``CAPTURE`` buffer, the following cases 750 one ``CAPTURE`` buffer, the following cases are defined:
751 751
752 * one ``OUTPUT`` buffer generates multiple 752 * one ``OUTPUT`` buffer generates multiple ``CAPTURE`` buffers: the same
753 ``OUTPUT`` timestamp will be copied to mu 753 ``OUTPUT`` timestamp will be copied to multiple ``CAPTURE`` buffers.
754 754
755 * multiple ``OUTPUT`` buffers generate one 755 * multiple ``OUTPUT`` buffers generate one ``CAPTURE`` buffer: timestamp of
756 the ``OUTPUT`` buffer queued first will b 756 the ``OUTPUT`` buffer queued first will be copied.
757 757
758 * the decoding order differs from the displ 758 * the decoding order differs from the display order (i.e. the ``CAPTURE``
759 buffers are out-of-order compared to the 759 buffers are out-of-order compared to the ``OUTPUT`` buffers): ``CAPTURE``
760 timestamps will not retain the order of ` 760 timestamps will not retain the order of ``OUTPUT`` timestamps.
761 761
762 .. note:: 762 .. note::
763 763
764 The backing memory of ``CAPTURE`` buffers t 764 The backing memory of ``CAPTURE`` buffers that are used as reference frames
765 by the stream may be read by the hardware e 765 by the stream may be read by the hardware even after they are dequeued.
766 Consequently, the client should avoid writi 766 Consequently, the client should avoid writing into this memory while the
767 ``CAPTURE`` queue is streaming. Failure to 767 ``CAPTURE`` queue is streaming. Failure to observe this may result in
768 corruption of decoded frames. 768 corruption of decoded frames.
769 769
770 Similarly, when using a memory type other t 770 Similarly, when using a memory type other than ``V4L2_MEMORY_MMAP``, the
771 client should make sure that each ``CAPTURE 771 client should make sure that each ``CAPTURE`` buffer is always queued with
772 the same backing memory for as long as the 772 the same backing memory for as long as the ``CAPTURE`` queue is streaming.
773 The reason for this is that V4L2 buffer ind 773 The reason for this is that V4L2 buffer indices can be used by drivers to
774 identify frames. Thus, if the backing memor 774 identify frames. Thus, if the backing memory of a reference frame is
775 submitted under a different buffer ID, the 775 submitted under a different buffer ID, the driver may misidentify it and
776 decode a new frame into it while it is stil 776 decode a new frame into it while it is still in use, resulting in corruption
777 of the following frames. 777 of the following frames.
778 778
779 During the decoding, the decoder may initiate 779 During the decoding, the decoder may initiate one of the special sequences, as
780 listed below. The sequences will result in the 780 listed below. The sequences will result in the decoder returning all the
781 ``CAPTURE`` buffers that originated from all t 781 ``CAPTURE`` buffers that originated from all the ``OUTPUT`` buffers processed
782 before the sequence started. Last of the buffe 782 before the sequence started. Last of the buffers will have the
783 ``V4L2_BUF_FLAG_LAST`` flag set. To determine 783 ``V4L2_BUF_FLAG_LAST`` flag set. To determine the sequence to follow, the client
784 must check if there is any pending event and: 784 must check if there is any pending event and:
785 785
786 * if a ``V4L2_EVENT_SOURCE_CHANGE`` event with 786 * if a ``V4L2_EVENT_SOURCE_CHANGE`` event with ``changes`` set to
787 ``V4L2_EVENT_SRC_CH_RESOLUTION`` is pending, 787 ``V4L2_EVENT_SRC_CH_RESOLUTION`` is pending, the `Dynamic Resolution
788 Change` sequence needs to be followed, 788 Change` sequence needs to be followed,
789 789
790 * if a ``V4L2_EVENT_EOS`` event is pending, th 790 * if a ``V4L2_EVENT_EOS`` event is pending, the `End of Stream` sequence needs
791 to be followed. 791 to be followed.
792 792
793 Some of the sequences can be intermixed with e 793 Some of the sequences can be intermixed with each other and need to be handled
794 as they happen. The exact operation is documen 794 as they happen. The exact operation is documented for each sequence.
795 795
796 Should a decoding error occur, it will be repo 796 Should a decoding error occur, it will be reported to the client with the level
797 of details depending on the decoder capabiliti 797 of details depending on the decoder capabilities. Specifically:
798 798
799 * the CAPTURE buffer that contains the results 799 * the CAPTURE buffer that contains the results of the failed decode operation
800 will be returned with the V4L2_BUF_FLAG_ERRO 800 will be returned with the V4L2_BUF_FLAG_ERROR flag set,
801 801
802 * if the decoder is able to precisely report t 802 * if the decoder is able to precisely report the OUTPUT buffer that triggered
803 the error, such buffer will be returned with 803 the error, such buffer will be returned with the V4L2_BUF_FLAG_ERROR flag
804 set. 804 set.
805 805
806 In case of a fatal failure that does not allow 806 In case of a fatal failure that does not allow the decoding to continue, any
807 further operations on corresponding decoder fi 807 further operations on corresponding decoder file handle will return the -EIO
808 error code. The client may close the file hand 808 error code. The client may close the file handle and open a new one, or
809 alternatively reinitialize the instance by sto 809 alternatively reinitialize the instance by stopping streaming on both queues,
810 releasing all buffers and performing the Initi 810 releasing all buffers and performing the Initialization sequence again.
811 811
812 Seek 812 Seek
813 ==== 813 ====
814 814
815 Seek is controlled by the ``OUTPUT`` queue, as 815 Seek is controlled by the ``OUTPUT`` queue, as it is the source of coded data.
816 The seek does not require any specific operati 816 The seek does not require any specific operation on the ``CAPTURE`` queue, but
817 it may be affected as per normal decoder opera 817 it may be affected as per normal decoder operation.
818 818
819 1. Stop the ``OUTPUT`` queue to begin the seek 819 1. Stop the ``OUTPUT`` queue to begin the seek sequence via
820 :c:func:`VIDIOC_STREAMOFF`. 820 :c:func:`VIDIOC_STREAMOFF`.
821 821
822 * **Required fields:** 822 * **Required fields:**
823 823
824 ``type`` 824 ``type``
825 a ``V4L2_BUF_TYPE_*`` enum appropriat 825 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``.
826 826
827 * The decoder will drop all the pending ``O 827 * The decoder will drop all the pending ``OUTPUT`` buffers and they must be
828 treated as returned to the client (follow 828 treated as returned to the client (following standard semantics).
829 829
830 2. Restart the ``OUTPUT`` queue via :c:func:`V 830 2. Restart the ``OUTPUT`` queue via :c:func:`VIDIOC_STREAMON`.
831 831
832 * **Required fields:** 832 * **Required fields:**
833 833
834 ``type`` 834 ``type``
835 a ``V4L2_BUF_TYPE_*`` enum appropriat 835 a ``V4L2_BUF_TYPE_*`` enum appropriate for ``OUTPUT``.
836 836
837 * The decoder will start accepting new sour 837 * The decoder will start accepting new source bytestream buffers after the
838 call returns. 838 call returns.
839 839
840 3. Start queuing buffers containing coded data 840 3. Start queuing buffers containing coded data after the seek to the ``OUTPUT``
841 queue until a suitable resume point is foun 841 queue until a suitable resume point is found.
842 842
843 .. note:: 843 .. note::
844 844
845 There is no requirement to begin queuing 845 There is no requirement to begin queuing coded data starting exactly
846 from a resume point (e.g. SPS or a keyfr 846 from a resume point (e.g. SPS or a keyframe). Any queued ``OUTPUT``
847 buffers will be processed and returned t 847 buffers will be processed and returned to the client until a suitable
848 resume point is found. While looking fo 848 resume point is found. While looking for a resume point, the decoder
849 should not produce any decoded frames in 849 should not produce any decoded frames into ``CAPTURE`` buffers.
850 850
851 Some hardware is known to mishandle seek 851 Some hardware is known to mishandle seeks to a non-resume point. Such an
852 operation may result in an unspecified n 852 operation may result in an unspecified number of corrupted decoded frames
853 being made available on the ``CAPTURE`` 853 being made available on the ``CAPTURE`` queue. Drivers must ensure that
854 no fatal decoding errors or crashes occu 854 no fatal decoding errors or crashes occur, and implement any necessary
855 handling and workarounds for hardware is 855 handling and workarounds for hardware issues related to seek operations.
856 856
857 .. warning:: 857 .. warning::
858 858
859 In case of the H.264/HEVC codec, the cli 859 In case of the H.264/HEVC codec, the client must take care not to seek
860 over a change of SPS/PPS. Even though th 860 over a change of SPS/PPS. Even though the target frame could be a
861 keyframe, the stale SPS/PPS inside decod 861 keyframe, the stale SPS/PPS inside decoder state would lead to undefined
862 results when decoding. Although the deco 862 results when decoding. Although the decoder must handle that case without
863 a crash or a fatal decode error, the cli 863 a crash or a fatal decode error, the client must not expect a sensible
864 decode output. 864 decode output.
865 865
866 If the hardware can detect such corrupte 866 If the hardware can detect such corrupted decoded frames, then
867 corresponding buffers will be returned t 867 corresponding buffers will be returned to the client with the
868 V4L2_BUF_FLAG_ERROR set. See the `Decodi 868 V4L2_BUF_FLAG_ERROR set. See the `Decoding` section for further
869 description of decode error reporting. 869 description of decode error reporting.
870 870
871 4. After a resume point is found, the decoder 871 4. After a resume point is found, the decoder will start returning ``CAPTURE``
872 buffers containing decoded frames. 872 buffers containing decoded frames.
873 873
874 .. important:: 874 .. important::
875 875
876 A seek may result in the `Dynamic Resolutio 876 A seek may result in the `Dynamic Resolution Change` sequence being
877 initiated, due to the seek target having de 877 initiated, due to the seek target having decoding parameters different from
878 the part of the stream decoded before the s 878 the part of the stream decoded before the seek. The sequence must be handled
879 as per normal decoder operation. 879 as per normal decoder operation.
880 880
881 .. warning:: 881 .. warning::
882 882
883 It is not specified when the ``CAPTURE`` qu 883 It is not specified when the ``CAPTURE`` queue starts producing buffers
884 containing decoded data from the ``OUTPUT`` 884 containing decoded data from the ``OUTPUT`` buffers queued after the seek,
885 as it operates independently from the ``OUT 885 as it operates independently from the ``OUTPUT`` queue.
886 886
887 The decoder may return a number of remainin 887 The decoder may return a number of remaining ``CAPTURE`` buffers containing
888 decoded frames originating from the ``OUTPU 888 decoded frames originating from the ``OUTPUT`` buffers queued before the
889 seek sequence is performed. 889 seek sequence is performed.
890 890
891 The ``VIDIOC_STREAMOFF`` operation discards 891 The ``VIDIOC_STREAMOFF`` operation discards any remaining queued
892 ``OUTPUT`` buffers, which means that not al 892 ``OUTPUT`` buffers, which means that not all of the ``OUTPUT`` buffers
893 queued before the seek sequence may have ma 893 queued before the seek sequence may have matching ``CAPTURE`` buffers
894 produced. For example, given the sequence 894 produced. For example, given the sequence of operations on the
895 ``OUTPUT`` queue: 895 ``OUTPUT`` queue:
896 896
897 QBUF(A), QBUF(B), STREAMOFF(), STREAMON() 897 QBUF(A), QBUF(B), STREAMOFF(), STREAMON(), QBUF(G), QBUF(H),
898 898
899 any of the following results on the ``CAPTU 899 any of the following results on the ``CAPTURE`` queue is allowed:
900 900
901 {A', B', G', H'}, {A', G', H'}, {G', H'}. 901 {A', B', G', H'}, {A', G', H'}, {G', H'}.
902 902
903 To determine the CAPTURE buffer containing 903 To determine the CAPTURE buffer containing the first decoded frame after the
904 seek, the client may observe the timestamps 904 seek, the client may observe the timestamps to match the CAPTURE and OUTPUT
905 buffers or use V4L2_DEC_CMD_STOP and V4L2_D 905 buffers or use V4L2_DEC_CMD_STOP and V4L2_DEC_CMD_START to drain the
906 decoder. 906 decoder.
907 907
908 .. note:: 908 .. note::
909 909
910 To achieve instantaneous seek, the client m 910 To achieve instantaneous seek, the client may restart streaming on the
911 ``CAPTURE`` queue too to discard decoded, b 911 ``CAPTURE`` queue too to discard decoded, but not yet dequeued buffers.
912 912
913 Dynamic Resolution Change 913 Dynamic Resolution Change
914 ========================= 914 =========================
915 915
916 Streams that include resolution metadata in th 916 Streams that include resolution metadata in the bytestream may require switching
917 to a different resolution during the decoding. 917 to a different resolution during the decoding.
918 918
919 .. note:: 919 .. note::
920 920
921 Not all decoders can detect resolution chan 921 Not all decoders can detect resolution changes. Those that do set the
922 ``V4L2_FMT_FLAG_DYN_RESOLUTION`` flag for t 922 ``V4L2_FMT_FLAG_DYN_RESOLUTION`` flag for the coded format when
923 :c:func:`VIDIOC_ENUM_FMT` is called. 923 :c:func:`VIDIOC_ENUM_FMT` is called.
924 924
925 The sequence starts when the decoder detects a 925 The sequence starts when the decoder detects a coded frame with one or more of
926 the following parameters different from those 926 the following parameters different from those previously established (and
927 reflected by corresponding queries): 927 reflected by corresponding queries):
928 928
929 * coded resolution (``OUTPUT`` width and heigh 929 * coded resolution (``OUTPUT`` width and height),
930 930
931 * visible resolution (selection rectangles), 931 * visible resolution (selection rectangles),
932 932
933 * the minimum number of buffers needed for dec 933 * the minimum number of buffers needed for decoding,
934 934
935 * bit-depth of the bitstream has been changed. 935 * bit-depth of the bitstream has been changed.
936 936
937 Whenever that happens, the decoder must procee 937 Whenever that happens, the decoder must proceed as follows:
938 938
939 1. After encountering a resolution change in 939 1. After encountering a resolution change in the stream, the decoder sends a
940 ``V4L2_EVENT_SOURCE_CHANGE`` event with `` 940 ``V4L2_EVENT_SOURCE_CHANGE`` event with ``changes`` set to
941 ``V4L2_EVENT_SRC_CH_RESOLUTION``. 941 ``V4L2_EVENT_SRC_CH_RESOLUTION``.
942 942
943 .. important:: 943 .. important::
944 944
945 Any client query issued after the decod 945 Any client query issued after the decoder queues the event will return
946 values applying to the stream after the 946 values applying to the stream after the resolution change, including
947 queue formats, selection rectangles and 947 queue formats, selection rectangles and controls.
948 948
949 2. The decoder will then process and decode a 949 2. The decoder will then process and decode all remaining buffers from before
950 the resolution change point. 950 the resolution change point.
951 951
952 * The last buffer from before the change m 952 * The last buffer from before the change must be marked with the
953 ``V4L2_BUF_FLAG_LAST`` flag, similarly t 953 ``V4L2_BUF_FLAG_LAST`` flag, similarly to the `Drain` sequence above.
954 954
955 .. warning:: 955 .. warning::
956 956
957 The last buffer may be empty (with :c:t 957 The last buffer may be empty (with :c:type:`v4l2_buffer` ``bytesused``
958 = 0) and in that case it must be ignore 958 = 0) and in that case it must be ignored by the client, as it does not
959 contain a decoded frame. 959 contain a decoded frame.
960 960
961 .. note:: 961 .. note::
962 962
963 Any attempt to dequeue more ``CAPTURE`` 963 Any attempt to dequeue more ``CAPTURE`` buffers beyond the buffer marked
964 with ``V4L2_BUF_FLAG_LAST`` will result 964 with ``V4L2_BUF_FLAG_LAST`` will result in a -EPIPE error from
965 :c:func:`VIDIOC_DQBUF`. 965 :c:func:`VIDIOC_DQBUF`.
966 966
967 The client must continue the sequence as descr 967 The client must continue the sequence as described below to continue the
968 decoding process. 968 decoding process.
969 969
970 1. Dequeue the source change event. 970 1. Dequeue the source change event.
971 971
972 .. important:: 972 .. important::
973 973
974 A source change triggers an implicit de 974 A source change triggers an implicit decoder drain, similar to the
975 explicit `Drain` sequence. The decoder 975 explicit `Drain` sequence. The decoder is stopped after it completes.
976 The decoding process must be resumed wi 976 The decoding process must be resumed with either a pair of calls to
977 :c:func:`VIDIOC_STREAMOFF` and :c:func: 977 :c:func:`VIDIOC_STREAMOFF` and :c:func:`VIDIOC_STREAMON` on the
978 ``CAPTURE`` queue, or a call to :c:func 978 ``CAPTURE`` queue, or a call to :c:func:`VIDIOC_DECODER_CMD` with the
979 ``V4L2_DEC_CMD_START`` command. 979 ``V4L2_DEC_CMD_START`` command.
980 980
981 2. Continue with the `Capture Setup` sequence 981 2. Continue with the `Capture Setup` sequence.
982 982
983 .. note:: 983 .. note::
984 984
985 During the resolution change sequence, the 985 During the resolution change sequence, the ``OUTPUT`` queue must remain
986 streaming. Calling :c:func:`VIDIOC_STREAMOF 986 streaming. Calling :c:func:`VIDIOC_STREAMOFF` on the ``OUTPUT`` queue would
987 abort the sequence and initiate a seek. 987 abort the sequence and initiate a seek.
988 988
989 In principle, the ``OUTPUT`` queue operates 989 In principle, the ``OUTPUT`` queue operates separately from the ``CAPTURE``
990 queue and this remains true for the duratio 990 queue and this remains true for the duration of the entire resolution change
991 sequence as well. 991 sequence as well.
992 992
993 The client should, for best performance and 993 The client should, for best performance and simplicity, keep queuing/dequeuing
994 buffers to/from the ``OUTPUT`` queue even w 994 buffers to/from the ``OUTPUT`` queue even while processing this sequence.
995 995
996 Drain 996 Drain
997 ===== 997 =====
998 998
999 To ensure that all queued ``OUTPUT`` buffers h 999 To ensure that all queued ``OUTPUT`` buffers have been processed and related
1000 ``CAPTURE`` buffers are given to the client, 1000 ``CAPTURE`` buffers are given to the client, the client must follow the drain
1001 sequence described below. After the drain seq 1001 sequence described below. After the drain sequence ends, the client has
1002 received all decoded frames for all ``OUTPUT` 1002 received all decoded frames for all ``OUTPUT`` buffers queued before the
1003 sequence was started. 1003 sequence was started.
1004 1004
1005 1. Begin drain by issuing :c:func:`VIDIOC_DEC 1005 1. Begin drain by issuing :c:func:`VIDIOC_DECODER_CMD`.
1006 1006
1007 * **Required fields:** 1007 * **Required fields:**
1008 1008
1009 ``cmd`` 1009 ``cmd``
1010 set to ``V4L2_DEC_CMD_STOP``. 1010 set to ``V4L2_DEC_CMD_STOP``.
1011 1011
1012 ``flags`` 1012 ``flags``
1013 set to 0. 1013 set to 0.
1014 1014
1015 ``pts`` 1015 ``pts``
1016 set to 0. 1016 set to 0.
1017 1017
1018 .. warning:: 1018 .. warning::
1019 1019
1020 The sequence can be only initiated if b 1020 The sequence can be only initiated if both ``OUTPUT`` and ``CAPTURE``
1021 queues are streaming. For compatibility 1021 queues are streaming. For compatibility reasons, the call to
1022 :c:func:`VIDIOC_DECODER_CMD` will not f 1022 :c:func:`VIDIOC_DECODER_CMD` will not fail even if any of the queues is
1023 not streaming, but at the same time it 1023 not streaming, but at the same time it will not initiate the `Drain`
1024 sequence and so the steps described bel 1024 sequence and so the steps described below would not be applicable.
1025 1025
1026 2. Any ``OUTPUT`` buffers queued by the clien 1026 2. Any ``OUTPUT`` buffers queued by the client before the
1027 :c:func:`VIDIOC_DECODER_CMD` was issued wi 1027 :c:func:`VIDIOC_DECODER_CMD` was issued will be processed and decoded as
1028 normal. The client must continue to handle 1028 normal. The client must continue to handle both queues independently,
1029 similarly to normal decode operation. This 1029 similarly to normal decode operation. This includes:
1030 1030
1031 * handling any operations triggered as a r 1031 * handling any operations triggered as a result of processing those buffers,
1032 such as the `Dynamic Resolution Change` 1032 such as the `Dynamic Resolution Change` sequence, before continuing with
1033 the drain sequence, 1033 the drain sequence,
1034 1034
1035 * queuing and dequeuing ``CAPTURE`` buffer 1035 * queuing and dequeuing ``CAPTURE`` buffers, until a buffer marked with the
1036 ``V4L2_BUF_FLAG_LAST`` flag is dequeued, 1036 ``V4L2_BUF_FLAG_LAST`` flag is dequeued,
1037 1037
1038 .. warning:: 1038 .. warning::
1039 1039
1040 The last buffer may be empty (with :c 1040 The last buffer may be empty (with :c:type:`v4l2_buffer`
1041 ``bytesused`` = 0) and in that case i 1041 ``bytesused`` = 0) and in that case it must be ignored by the client,
1042 as it does not contain a decoded fram 1042 as it does not contain a decoded frame.
1043 1043
1044 .. note:: 1044 .. note::
1045 1045
1046 Any attempt to dequeue more ``CAPTURE 1046 Any attempt to dequeue more ``CAPTURE`` buffers beyond the buffer
1047 marked with ``V4L2_BUF_FLAG_LAST`` wi 1047 marked with ``V4L2_BUF_FLAG_LAST`` will result in a -EPIPE error from
1048 :c:func:`VIDIOC_DQBUF`. 1048 :c:func:`VIDIOC_DQBUF`.
1049 1049
1050 * dequeuing processed ``OUTPUT`` buffers, 1050 * dequeuing processed ``OUTPUT`` buffers, until all the buffers queued
1051 before the ``V4L2_DEC_CMD_STOP`` command 1051 before the ``V4L2_DEC_CMD_STOP`` command are dequeued,
1052 1052
1053 * dequeuing the ``V4L2_EVENT_EOS`` event, 1053 * dequeuing the ``V4L2_EVENT_EOS`` event, if the client subscribed to it.
1054 1054
1055 .. note:: 1055 .. note::
1056 1056
1057 For backwards compatibility, the decode 1057 For backwards compatibility, the decoder will signal a ``V4L2_EVENT_EOS``
1058 event when the last frame has been deco 1058 event when the last frame has been decoded and all frames are ready to be
1059 dequeued. It is a deprecated behavior a 1059 dequeued. It is a deprecated behavior and the client must not rely on it.
1060 The ``V4L2_BUF_FLAG_LAST`` buffer flag 1060 The ``V4L2_BUF_FLAG_LAST`` buffer flag should be used instead.
1061 1061
1062 3. Once all the ``OUTPUT`` buffers queued bef 1062 3. Once all the ``OUTPUT`` buffers queued before the ``V4L2_DEC_CMD_STOP`` call
1063 are dequeued and the last ``CAPTURE`` buff 1063 are dequeued and the last ``CAPTURE`` buffer is dequeued, the decoder is
1064 stopped and it will accept, but not proces 1064 stopped and it will accept, but not process, any newly queued ``OUTPUT``
1065 buffers until the client issues any of the 1065 buffers until the client issues any of the following operations:
1066 1066
1067 * ``V4L2_DEC_CMD_START`` - the decoder wil 1067 * ``V4L2_DEC_CMD_START`` - the decoder will not be reset and will resume
1068 operation normally, with all the state f 1068 operation normally, with all the state from before the drain,
1069 1069
1070 * a pair of :c:func:`VIDIOC_STREAMOFF` and 1070 * a pair of :c:func:`VIDIOC_STREAMOFF` and :c:func:`VIDIOC_STREAMON` on the
1071 ``CAPTURE`` queue - the decoder will res 1071 ``CAPTURE`` queue - the decoder will resume the operation normally,
1072 however any ``CAPTURE`` buffers still in 1072 however any ``CAPTURE`` buffers still in the queue will be returned to the
1073 client, 1073 client,
1074 1074
1075 * a pair of :c:func:`VIDIOC_STREAMOFF` and 1075 * a pair of :c:func:`VIDIOC_STREAMOFF` and :c:func:`VIDIOC_STREAMON` on the
1076 ``OUTPUT`` queue - any pending source bu 1076 ``OUTPUT`` queue - any pending source buffers will be returned to the
1077 client and the `Seek` sequence will be t 1077 client and the `Seek` sequence will be triggered.
1078 1078
1079 .. note:: 1079 .. note::
1080 1080
1081 Once the drain sequence is initiated, the 1081 Once the drain sequence is initiated, the client needs to drive it to
1082 completion, as described by the steps abov 1082 completion, as described by the steps above, unless it aborts the process by
1083 issuing :c:func:`VIDIOC_STREAMOFF` on any 1083 issuing :c:func:`VIDIOC_STREAMOFF` on any of the ``OUTPUT`` or ``CAPTURE``
1084 queues. The client is not allowed to issu 1084 queues. The client is not allowed to issue ``V4L2_DEC_CMD_START`` or
1085 ``V4L2_DEC_CMD_STOP`` again while the drai 1085 ``V4L2_DEC_CMD_STOP`` again while the drain sequence is in progress and they
1086 will fail with -EBUSY error code if attemp 1086 will fail with -EBUSY error code if attempted.
1087 1087
1088 Although not mandatory, the availability o 1088 Although not mandatory, the availability of decoder commands may be queried
1089 using :c:func:`VIDIOC_TRY_DECODER_CMD`. 1089 using :c:func:`VIDIOC_TRY_DECODER_CMD`.
1090 1090
1091 End of Stream 1091 End of Stream
1092 ============= 1092 =============
1093 1093
1094 If the decoder encounters an end of stream ma 1094 If the decoder encounters an end of stream marking in the stream, the decoder
1095 will initiate the `Drain` sequence, which the 1095 will initiate the `Drain` sequence, which the client must handle as described
1096 above, skipping the initial :c:func:`VIDIOC_D 1096 above, skipping the initial :c:func:`VIDIOC_DECODER_CMD`.
1097 1097
1098 Commit Points 1098 Commit Points
1099 ============= 1099 =============
1100 1100
1101 Setting formats and allocating buffers trigge 1101 Setting formats and allocating buffers trigger changes in the behavior of the
1102 decoder. 1102 decoder.
1103 1103
1104 1. Setting the format on the ``OUTPUT`` queue 1104 1. Setting the format on the ``OUTPUT`` queue may change the set of formats
1105 supported/advertised on the ``CAPTURE`` qu 1105 supported/advertised on the ``CAPTURE`` queue. In particular, it also means
1106 that the ``CAPTURE`` format may be reset a 1106 that the ``CAPTURE`` format may be reset and the client must not rely on the
1107 previously set format being preserved. 1107 previously set format being preserved.
1108 1108
1109 2. Enumerating formats on the ``CAPTURE`` que 1109 2. Enumerating formats on the ``CAPTURE`` queue always returns only formats
1110 supported for the current ``OUTPUT`` forma 1110 supported for the current ``OUTPUT`` format.
1111 1111
1112 3. Setting the format on the ``CAPTURE`` queu 1112 3. Setting the format on the ``CAPTURE`` queue does not change the list of
1113 formats available on the ``OUTPUT`` queue. 1113 formats available on the ``OUTPUT`` queue. An attempt to set a ``CAPTURE``
1114 format that is not supported for the curre 1114 format that is not supported for the currently selected ``OUTPUT`` format
1115 will result in the decoder adjusting the r 1115 will result in the decoder adjusting the requested ``CAPTURE`` format to a
1116 supported one. 1116 supported one.
1117 1117
1118 4. Enumerating formats on the ``OUTPUT`` queu 1118 4. Enumerating formats on the ``OUTPUT`` queue always returns the full set of
1119 supported coded formats, irrespectively of 1119 supported coded formats, irrespectively of the current ``CAPTURE`` format.
1120 1120
1121 5. While buffers are allocated on any of the 1121 5. While buffers are allocated on any of the ``OUTPUT`` or ``CAPTURE`` queues,
1122 the client must not change the format on t 1122 the client must not change the format on the ``OUTPUT`` queue. Drivers will
1123 return the -EBUSY error code for any such 1123 return the -EBUSY error code for any such format change attempt.
1124 1124
1125 To summarize, setting formats and allocation 1125 To summarize, setting formats and allocation must always start with the
1126 ``OUTPUT`` queue and the ``OUTPUT`` queue is 1126 ``OUTPUT`` queue and the ``OUTPUT`` queue is the master that governs the
1127 set of supported formats for the ``CAPTURE`` 1127 set of supported formats for the ``CAPTURE`` queue.
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