4.5.1. Memory-to-Memory Stateful Video Decoder Interface¶

A stateful video decoder takes complete chunks of the bytestream (e.g. Annex-B H.264/HEVC stream, raw VP8/9 stream) and decodes them into raw video frames in display order. The decoder is expected not to require any additional information from the client to process these buffers.

Performing software parsing, processing etc. of the stream in the driver in order to support this interface is strongly discouraged. In case such operations are needed, use of the Stateless Video Decoder Interface (in development) is strongly advised.

4.5.1.1. Conventions and Notations Used in This Document¶

The general V4L2 API rules apply if not specified in this document otherwise.
The meaning of words “must”, “may”, “should”, etc. is as per RFC 2119.
All steps not marked “optional” are required.
VIDIOC_G_EXT_CTRLS() and VIDIOC_S_EXT_CTRLS() may be used interchangeably with VIDIOC_G_CTRL() and VIDIOC_S_CTRL(), unless specified otherwise.
Single-planar API (see Single- and multi-planar APIs) and applicable structures may be used interchangeably with multi-planar API, unless specified otherwise, depending on decoder capabilities and following the general V4L2 guidelines.
i = [a..b]: sequence of integers from a to b, inclusive, i.e. i = [0..2]: i = 0, 1, 2.
Given an OUTPUT buffer A, then A’ represents a buffer on the CAPTURE queue containing data that resulted from processing buffer A.

4.5.1.2. Glossary¶

CAPTURE: the destination buffer queue; for decoders, the queue of buffers containing decoded frames; for encoders, the queue of buffers containing an encoded bytestream; V4L2_BUF_TYPE_VIDEO_CAPTURE or V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE; data is captured from the hardware into CAPTURE buffers.
client: the application communicating with the decoder or encoder implementing this interface.
coded format: encoded/compressed video bytestream format (e.g. H.264, VP8, etc.); see also: raw format.
coded height: height for given coded resolution.
coded resolution: stream resolution in pixels aligned to codec and hardware requirements; typically visible resolution rounded up to full macroblocks; see also: visible resolution.
coded width: width for given coded resolution.
coding tree unit: processing unit of the HEVC codec (corresponds to macroblock units in H.264, VP8, VP9), can use block structures of up to 64×64 pixels. Good at sub-partitioning the picture into variable sized structures.
decode order: the order in which frames are decoded; may differ from display order if the coded format includes a feature of frame reordering; for decoders, OUTPUT buffers must be queued by the client in decode order; for encoders CAPTURE buffers must be returned by the encoder in decode order.
destination: data resulting from the decode process; see CAPTURE.
display order: the order in which frames must be displayed; for encoders, OUTPUT buffers must be queued by the client in display order; for decoders, CAPTURE buffers must be returned by the decoder in display order.
DPB: Decoded Picture Buffer; an H.264/HEVC term for a buffer that stores a decoded raw frame available for reference in further decoding steps.
EOS: end of stream.
IDR: Instantaneous Decoder Refresh; a type of a keyframe in an H.264/HEVC-encoded stream, which clears the list of earlier reference frames (DPBs).
keyframe: an encoded frame that does not reference frames decoded earlier, i.e. can be decoded fully on its own.
macroblock: a processing unit in image and video compression formats based on linear block transforms (e.g. H.264, VP8, VP9); codec-specific, but for most of popular codecs the size is 16x16 samples (pixels). The HEVC codec uses a slightly more flexible processing unit called coding tree unit (CTU).
OUTPUT: the source buffer queue; for decoders, the queue of buffers containing an encoded bytestream; for encoders, the queue of buffers containing raw frames; V4L2_BUF_TYPE_VIDEO_OUTPUT or V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE; the hardware is fed with data from OUTPUT buffers.
PPS: Picture Parameter Set; a type of metadata entity in an H.264/HEVC bytestream.
raw format: uncompressed format containing raw pixel data (e.g. YUV, RGB formats).
resume point: a point in the bytestream from which decoding may start/continue, without any previous state/data present, e.g.: a keyframe (VP8/VP9) or SPS/PPS/IDR sequence (H.264/HEVC); a resume point is required to start decode of a new stream, or to resume decoding after a seek.
source: data fed to the decoder or encoder; see OUTPUT.
source height: height in pixels for given source resolution; relevant to encoders only.
source resolution: resolution in pixels of source frames being source to the encoder and subject to further cropping to the bounds of visible resolution; relevant to encoders only.
source width: width in pixels for given source resolution; relevant to encoders only.
SPS: Sequence Parameter Set; a type of metadata entity in an H.264/HEVC bytestream.
stream metadata: additional (non-visual) information contained inside encoded bytestream; for example: coded resolution, visible resolution, codec profile.
visible height: height for given visible resolution; display height.
visible resolution: stream resolution of the visible picture, in pixels, to be used for display purposes; must be smaller or equal to coded resolution; display resolution.
visible width: width for given visible resolution; display width.

4.5.1.3. State Machine¶

DOT digraph of decoder state machine — Decoder State Machine¶

4.5.1.4. Querying Capabilities¶

To enumerate the set of coded formats supported by the decoder, the client may call VIDIOC_ENUM_FMT() on OUTPUT.
- The full set of supported formats will be returned, regardless of the format set on CAPTURE.
- Check the flags field of v4l2_fmtdesc for more information about the decoder’s capabilities with respect to each coded format. In particular whether or not the decoder has a full-fledged bytestream parser and if the decoder supports dynamic resolution changes.
To enumerate the set of supported raw formats, the client may call VIDIOC_ENUM_FMT() on CAPTURE.
- Only the formats supported for the format currently active on OUTPUT will be returned.
- In order to enumerate raw formats supported by a given coded format, the client must first set that coded format on OUTPUT and then enumerate formats on CAPTURE.
The client may use VIDIOC_ENUM_FRAMESIZES() to detect supported resolutions for a given format, passing desired pixel format in v4l2_frmsizeenum pixel_format.
- Values returned by VIDIOC_ENUM_FRAMESIZES() for a coded pixel format will include all possible coded resolutions supported by the decoder for given coded pixel format.
- Values returned by VIDIOC_ENUM_FRAMESIZES() for a raw pixel format will include all possible frame buffer resolutions supported by the decoder for given raw pixel format and the coded format currently set on OUTPUT.
Supported profiles and levels for the coded format currently set on OUTPUT, if applicable, may be queried using their respective controls via VIDIOC_QUERYCTRL().

4.5.1.5. Initialization¶

Set the coded format on OUTPUT via VIDIOC_S_FMT().
- Required fields:
  
  type
  a V4L2_BUF_TYPE_* enum appropriate for OUTPUT.
  
  pixelformat
  a coded pixel format.
  
  width, height
  coded resolution of the stream; required only if it cannot be parsed from the stream for the given coded format; otherwise the decoder will use this resolution as a placeholder resolution that will likely change as soon as it can parse the actual coded resolution from the stream.
  
  sizeimage
  desired size of OUTPUT buffers; the decoder may adjust it to match hardware requirements.
  
  other fields
  follow standard semantics.
- Return fields:
  
  sizeimage
  adjusted size of OUTPUT buffers.
- The CAPTURE format will be updated with an appropriate frame buffer resolution instantly based on the width and height returned by VIDIOC_S_FMT(). However, for coded formats that include stream resolution information, after the decoder is done parsing the information from the stream, it will update the CAPTURE format with new values and signal a source change event, regardless of whether they match the values set by the client or not.
Important

Changing the OUTPUT format may change the currently set CAPTURE format. How the new CAPTURE format is determined is up to the decoder and the client must ensure it matches its needs afterwards.
Allocate source (bytestream) buffers via VIDIOC_REQBUFS() on OUTPUT.
- Required fields:
  
  count
  requested number of buffers to allocate; greater than zero.
  
  type
  a V4L2_BUF_TYPE_* enum appropriate for OUTPUT.
  
  memory
  follows standard semantics.
- Return fields:
  
  count
  the actual number of buffers allocated.
Warning

The actual number of allocated buffers may differ from the count given. The client must check the updated value of count after the call returns.

Alternatively, VIDIOC_CREATE_BUFS() on the OUTPUT queue can be used to have more control over buffer allocation.
- Required fields:
  
  count
  requested number of buffers to allocate; greater than zero.
  
  type
  a V4L2_BUF_TYPE_* enum appropriate for OUTPUT.
  
  memory
  follows standard semantics.
  
  format
  follows standard semantics.
- Return fields:
  
  count
  adjusted to the number of allocated buffers.
Warning

The actual number of allocated buffers may differ from the count given. The client must check the updated value of count after the call returns.
Start streaming on the OUTPUT queue via VIDIOC_STREAMON().
This step only applies to coded formats that contain resolution information in the stream. Continue queuing/dequeuing bytestream buffers to/from the OUTPUT queue via VIDIOC_QBUF() and VIDIOC_DQBUF(). The buffers will be processed and returned to the client in order, until required metadata to configure the CAPTURE queue are found. This is indicated by the decoder sending a V4L2_EVENT_SOURCE_CHANGE event with changes set to V4L2_EVENT_SRC_CH_RESOLUTION.
- It is not an error if the first buffer does not contain enough data for this to occur. Processing of the buffers will continue as long as more data is needed.
- If data in a buffer that triggers the event is required to decode the first frame, it will not be returned to the client, until the initialization sequence completes and the frame is decoded.
- If the client has not set the coded resolution of the stream on its own, calling VIDIOC_G_FMT(), VIDIOC_S_FMT(), VIDIOC_TRY_FMT() or VIDIOC_REQBUFS() on the CAPTURE queue will not return the real values for the stream until a V4L2_EVENT_SOURCE_CHANGE event with changes set to V4L2_EVENT_SRC_CH_RESOLUTION is signaled.
Important

Any client query issued after the decoder queues the event will return values applying to the just parsed stream, including queue formats, selection rectangles and controls.

Note

A client capable of acquiring stream parameters from the bytestream on its own may attempt to set the width and height of the OUTPUT format to non-zero values matching the coded size of the stream, skip this step and continue with the Capture Setup sequence. However, it must not rely on any driver queries regarding stream parameters, such as selection rectangles and controls, since the decoder has not parsed them from the stream yet. If the values configured by the client do not match those parsed by the decoder, a Dynamic Resolution Change will be triggered to reconfigure them.

Note

No decoded frames are produced during this phase.
Continue with the Capture Setup sequence.

4.5.1.6. Capture Setup¶

Call VIDIOC_G_FMT() on the CAPTURE queue to get format for the destination buffers parsed/decoded from the bytestream.
- Required fields:
  
  type
  a V4L2_BUF_TYPE_* enum appropriate for CAPTURE.
- Return fields:
  
  width, height
  frame buffer resolution for the decoded frames.
  
  pixelformat
  pixel format for decoded frames.
  
  num_planes (for _MPLANE type only)
  number of planes for pixelformat.
  
  sizeimage, bytesperline
  as per standard semantics; matching frame buffer format.
Note

The value of pixelformat may be any pixel format supported by the decoder for the current stream. The decoder should choose a preferred/optimal format for the default configuration. For example, a YUV format may be preferred over an RGB format if an additional conversion step would be required for the latter.
Optional. Acquire the visible resolution via VIDIOC_G_SELECTION().
- Required fields:
  
  type
  a V4L2_BUF_TYPE_* enum appropriate for CAPTURE.
  
  target
  set to V4L2_SEL_TGT_COMPOSE.
- Return fields:
  
  r.left, r.top, r.width, r.height
  the visible rectangle; it must fit within the frame buffer resolution returned by VIDIOC_G_FMT() on CAPTURE.
- The following selection targets are supported on CAPTURE:
  
  V4L2_SEL_TGT_CROP_BOUNDS
  corresponds to the coded resolution of the stream.
  
  V4L2_SEL_TGT_CROP_DEFAULT
  the rectangle covering the part of the CAPTURE buffer that contains meaningful picture data (visible area); width and height will be equal to the visible resolution of the stream.
  
  V4L2_SEL_TGT_CROP
  the rectangle within the coded resolution to be output to CAPTURE; defaults to V4L2_SEL_TGT_CROP_DEFAULT; read-only on hardware without additional compose/scaling capabilities.
  
  V4L2_SEL_TGT_COMPOSE_BOUNDS
  the maximum rectangle within a CAPTURE buffer, which the cropped frame can be composed into; equal to V4L2_SEL_TGT_CROP if the hardware does not support compose/scaling.
  
  V4L2_SEL_TGT_COMPOSE_DEFAULT
  equal to V4L2_SEL_TGT_CROP.
  
  V4L2_SEL_TGT_COMPOSE
  the rectangle inside a CAPTURE buffer into which the cropped frame is written; defaults to V4L2_SEL_TGT_COMPOSE_DEFAULT; read-only on hardware without additional compose/scaling capabilities.
  
  V4L2_SEL_TGT_COMPOSE_PADDED
  the rectangle inside a CAPTURE buffer which is overwritten by the hardware; equal to V4L2_SEL_TGT_COMPOSE if the hardware does not write padding pixels.
Warning

The values are guaranteed to be meaningful only after the decoder successfully parses the stream metadata. The client must not rely on the query before that happens.
Optional. Enumerate CAPTURE formats via VIDIOC_ENUM_FMT() on the CAPTURE queue. Once the stream information is parsed and known, the client may use this ioctl to discover which raw formats are supported for given stream and select one of them via VIDIOC_S_FMT().

Important

The decoder will return only formats supported for the currently established coded format, as per the OUTPUT format and/or stream metadata parsed in this initialization sequence, even if more formats may be supported by the decoder in general. In other words, the set returned will be a subset of the initial query mentioned in the Querying Capabilities section.

For example, a decoder may support YUV and RGB formats for resolutions 1920x1088 and lower, but only YUV for higher resolutions (due to hardware limitations). After parsing a resolution of 1920x1088 or lower, VIDIOC_ENUM_FMT() may return a set of YUV and RGB pixel formats, but after parsing resolution higher than 1920x1088, the decoder will not return RGB, unsupported for this resolution.

However, subsequent resolution change event triggered after discovering a resolution change within the same stream may switch the stream into a lower resolution and VIDIOC_ENUM_FMT() would return RGB formats again in that case.
Optional. Set the CAPTURE format via VIDIOC_S_FMT() on the CAPTURE queue. The client may choose a different format than selected/suggested by the decoder in VIDIOC_G_FMT().
- Required fields:
  
  type
  a V4L2_BUF_TYPE_* enum appropriate for CAPTURE.
  
  pixelformat
  a raw pixel format.
  
  width, height
  frame buffer resolution of the decoded stream; typically unchanged from what was returned with VIDIOC_G_FMT(), but it may be different if the hardware supports composition and/or scaling.

Setting the CAPTURE format will reset the compose selection rectangles to their default values, based on the new resolution, as described in the previous step.

Optional. Set the compose rectangle via VIDIOC_S_SELECTION() on the CAPTURE queue if it is desired and if the decoder has compose and/or scaling capabilities.
- Required fields:
  
  type
  a V4L2_BUF_TYPE_* enum appropriate for CAPTURE.
  
  target
  set to V4L2_SEL_TGT_COMPOSE.
  
  r.left, r.top, r.width, r.height
  the rectangle inside a CAPTURE buffer into which the cropped frame is written; defaults to V4L2_SEL_TGT_COMPOSE_DEFAULT; read-only on hardware without additional compose/scaling capabilities.
- Return fields:
  
  r.left, r.top, r.width, r.height
  the visible rectangle; it must fit within the frame buffer resolution returned by VIDIOC_G_FMT() on CAPTURE.
Warning

The decoder may adjust the compose rectangle to the nearest supported one to meet codec and hardware requirements. The client needs to check the adjusted rectangle returned by VIDIOC_S_SELECTION().
If all the following conditions are met, the client may resume the decoding instantly:
- sizeimage of the new format (determined in previous steps) is less than or equal to the size of currently allocated buffers,
- the number of buffers currently allocated is greater than or equal to the minimum number of buffers acquired in previous steps. To fulfill this requirement, the client may use VIDIOC_CREATE_BUFS() to add new buffers.
In that case, the remaining steps do not apply and the client may resume the decoding by one of the following actions:
- if the CAPTURE queue is streaming, call VIDIOC_DECODER_CMD() with the V4L2_DEC_CMD_START command,
- if the CAPTURE queue is not streaming, call VIDIOC_STREAMON() on the CAPTURE queue.
However, if the client intends to change the buffer set, to lower memory usage or for any other reasons, it may be achieved by following the steps below.
If the CAPTURE queue is streaming, keep queuing and dequeuing buffers on the CAPTURE queue until a buffer marked with the V4L2_BUF_FLAG_LAST flag is dequeued.
If the CAPTURE queue is streaming, call VIDIOC_STREAMOFF() on the CAPTURE queue to stop streaming.

Warning

The OUTPUT queue must remain streaming. Calling VIDIOC_STREAMOFF() on it would abort the sequence and trigger a seek.
If the CAPTURE queue has buffers allocated, free the CAPTURE buffers using VIDIOC_REQBUFS().
- Required fields:
  
  count
  set to 0.
  
  type
  a V4L2_BUF_TYPE_* enum appropriate for CAPTURE.
  
  memory
  follows standard semantics.
Allocate CAPTURE buffers via VIDIOC_REQBUFS() on the CAPTURE queue.
- Required fields:
  
  count
  requested number of buffers to allocate; greater than zero.
  
  type
  a V4L2_BUF_TYPE_* enum appropriate for CAPTURE.
  
  memory
  follows standard semantics.
- Return fields:
  
  count
  actual number of buffers allocated.
Warning

The actual number of allocated buffers may differ from the count given. The client must check the updated value of count after the call returns.

Note

To allocate more than the minimum number of buffers (for pipeline depth), the client may query the V4L2_CID_MIN_BUFFERS_FOR_CAPTURE control to get the minimum number of buffers required, and pass the obtained value plus the number of additional buffers needed in the count field to VIDIOC_REQBUFS().

Alternatively, VIDIOC_CREATE_BUFS() on the CAPTURE queue can be used to have more control over buffer allocation. For example, by allocating buffers larger than the current CAPTURE format, future resolution changes can be accommodated.
- Required fields:
  
  count
  requested number of buffers to allocate; greater than zero.
  
  type
  a V4L2_BUF_TYPE_* enum appropriate for CAPTURE.
  
  memory
  follows standard semantics.
  
  format
  a format representing the maximum framebuffer resolution to be accommodated by newly allocated buffers.
- Return fields:
  
  count
  adjusted to the number of allocated buffers.
Warning

The actual number of allocated buffers may differ from the count given. The client must check the updated value of count after the call returns.
Note

To allocate buffers for a format different than parsed from the stream metadata, the client must proceed as follows, before the metadata parsing is initiated:
- set width and height of the OUTPUT format to desired coded resolution to let the decoder configure the CAPTURE format appropriately,
- query the CAPTURE format using VIDIOC_G_FMT() and save it until this step.
The format obtained in the query may be then used with VIDIOC_CREATE_BUFS() in this step to allocate the buffers.
Call VIDIOC_STREAMON() on the CAPTURE queue to start decoding frames.

4.5.1.7. Decoding¶

This state is reached after the Capture Setup sequence finishes successfully. In this state, the client queues and dequeues buffers to both queues via VIDIOC_QBUF() and VIDIOC_DQBUF(), following the standard semantics.

The content of the source OUTPUT buffers depends on the active coded pixel format and may be affected by codec-specific extended controls, as stated in the documentation of each format.

Both queues operate independently, following the standard behavior of V4L2 buffer queues and memory-to-memory devices. In addition, the order of decoded frames dequeued from the CAPTURE queue may differ from the order of queuing coded frames to the OUTPUT queue, due to properties of the selected coded format, e.g. frame reordering.

The client must not assume any direct relationship between CAPTURE and OUTPUT buffers and any specific timing of buffers becoming available to dequeue. Specifically:

a buffer queued to OUTPUT may result in no buffers being produced on CAPTURE (e.g. if it does not contain encoded data, or if only metadata syntax structures are present in it),
a buffer queued to OUTPUT may result in more than one buffer produced on CAPTURE (if the encoded data contained more than one frame, or if returning a decoded frame allowed the decoder to return a frame that preceded it in decode, but succeeded it in the display order),
a buffer queued to OUTPUT may result in a buffer being produced on CAPTURE later into decode process, and/or after processing further OUTPUT buffers, or be returned out of order, e.g. if display reordering is used,
buffers may become available on the CAPTURE queue without additional buffers queued to OUTPUT (e.g. during drain or EOS), because of the OUTPUT buffers queued in the past whose decoding results are only available at later time, due to specifics of the decoding process.

Note

To allow matching decoded CAPTURE buffers with OUTPUT buffers they originated from, the client can set the timestamp field of the v4l2_buffer struct when queuing an OUTPUT buffer. The CAPTURE buffer(s), which resulted from decoding that OUTPUT buffer will have their timestamp field set to the same value when dequeued.

In addition to the straightforward case of one OUTPUT buffer producing one CAPTURE buffer, the following cases are defined:

one OUTPUT buffer generates multiple CAPTURE buffers: the same OUTPUT timestamp will be copied to multiple CAPTURE buffers.
multiple OUTPUT buffers generate one CAPTURE buffer: timestamp of the OUTPUT buffer queued first will be copied.
the decoding order differs from the display order (i.e. the CAPTURE buffers are out-of-order compared to the OUTPUT buffers): CAPTURE timestamps will not retain the order of OUTPUT timestamps.

Note

The backing memory of CAPTURE buffers that are used as reference frames by the stream may be read by the hardware even after they are dequeued. Consequently, the client should avoid writing into this memory while the CAPTURE queue is streaming. Failure to observe this may result in corruption of decoded frames.

Similarly, when using a memory type other than V4L2_MEMORY_MMAP, the client should make sure that each CAPTURE buffer is always queued with the same backing memory for as long as the CAPTURE queue is streaming. The reason for this is that V4L2 buffer indices can be used by drivers to identify frames. Thus, if the backing memory of a reference frame is submitted under a different buffer ID, the driver may misidentify it and decode a new frame into it while it is still in use, resulting in corruption of the following frames.

During the decoding, the decoder may initiate one of the special sequences, as listed below. The sequences will result in the decoder returning all the CAPTURE buffers that originated from all the OUTPUT buffers processed before the sequence started. Last of the buffers will have the V4L2_BUF_FLAG_LAST flag set. To determine the sequence to follow, the client must check if there is any pending event and:

if a V4L2_EVENT_SOURCE_CHANGE event with changes set to V4L2_EVENT_SRC_CH_RESOLUTION is pending, the Dynamic Resolution Change sequence needs to be followed,
if a V4L2_EVENT_EOS event is pending, the End of Stream sequence needs to be followed.

Some of the sequences can be intermixed with each other and need to be handled as they happen. The exact operation is documented for each sequence.

Should a decoding error occur, it will be reported to the client with the level of details depending on the decoder capabilities. Specifically:

the CAPTURE buffer that contains the results of the failed decode operation will be returned with the V4L2_BUF_FLAG_ERROR flag set,
if the decoder is able to precisely report the OUTPUT buffer that triggered the error, such buffer will be returned with the V4L2_BUF_FLAG_ERROR flag set.

In case of a fatal failure that does not allow the decoding to continue, any further operations on corresponding decoder file handle will return the -EIO error code. The client may close the file handle and open a new one, or alternatively reinitialize the instance by stopping streaming on both queues, releasing all buffers and performing the Initialization sequence again.

4.5.1.8. Seek¶

Seek is controlled by the OUTPUT queue, as it is the source of coded data. The seek does not require any specific operation on the CAPTURE queue, but it may be affected as per normal decoder operation.

Stop the OUTPUT queue to begin the seek sequence via VIDIOC_STREAMOFF().
- Required fields:
  
  type
  a V4L2_BUF_TYPE_* enum appropriate for OUTPUT.
- The decoder will drop all the pending OUTPUT buffers and they must be treated as returned to the client (following standard semantics).
Restart the OUTPUT queue via VIDIOC_STREAMON().
- Required fields:
  
  type
  a V4L2_BUF_TYPE_* enum appropriate for OUTPUT.
- The decoder will start accepting new source bytestream buffers after the call returns.
Start queuing buffers containing coded data after the seek to the OUTPUT queue until a suitable resume point is found.

Note

There is no requirement to begin queuing coded data starting exactly from a resume point (e.g. SPS or a keyframe). Any queued OUTPUT buffers will be processed and returned to the client until a suitable resume point is found. While looking for a resume point, the decoder should not produce any decoded frames into CAPTURE buffers.

Some hardware is known to mishandle seeks to a non-resume point. Such an operation may result in an unspecified number of corrupted decoded frames being made available on the CAPTURE queue. Drivers must ensure that no fatal decoding errors or crashes occur, and implement any necessary handling and workarounds for hardware issues related to seek operations.

Warning

In case of the H.264/HEVC codec, the client must take care not to seek over a change of SPS/PPS. Even though the target frame could be a keyframe, the stale SPS/PPS inside decoder state would lead to undefined results when decoding. Although the decoder must handle that case without a crash or a fatal decode error, the client must not expect a sensible decode output.

If the hardware can detect such corrupted decoded frames, then corresponding buffers will be returned to the client with the V4L2_BUF_FLAG_ERROR set. See the Decoding section for further description of decode error reporting.
After a resume point is found, the decoder will start returning CAPTURE buffers containing decoded frames.

Important

A seek may result in the Dynamic Resolution Change sequence being initiated, due to the seek target having decoding parameters different from the part of the stream decoded before the seek. The sequence must be handled as per normal decoder operation.

Warning

It is not specified when the CAPTURE queue starts producing buffers containing decoded data from the OUTPUT buffers queued after the seek, as it operates independently from the OUTPUT queue.

The decoder may return a number of remaining CAPTURE buffers containing decoded frames originating from the OUTPUT buffers queued before the seek sequence is performed.

The VIDIOC_STREAMOFF operation discards any remaining queued OUTPUT buffers, which means that not all of the OUTPUT buffers queued before the seek sequence may have matching CAPTURE buffers produced. For example, given the sequence of operations on the OUTPUT queue:

QBUF(A), QBUF(B), STREAMOFF(), STREAMON(), QBUF(G), QBUF(H),

any of the following results on the CAPTURE queue is allowed:

{A’, B’, G’, H’}, {A’, G’, H’}, {G’, H’}.

To determine the CAPTURE buffer containing the first decoded frame after the seek, the client may observe the timestamps to match the CAPTURE and OUTPUT buffers or use V4L2_DEC_CMD_STOP and V4L2_DEC_CMD_START to drain the decoder.

Note

To achieve instantaneous seek, the client may restart streaming on the CAPTURE queue too to discard decoded, but not yet dequeued buffers.

4.5.1.9. Dynamic Resolution Change¶

Streams that include resolution metadata in the bytestream may require switching to a different resolution during the decoding.

Note

Not all decoders can detect resolution changes. Those that do set the V4L2_FMT_FLAG_DYN_RESOLUTION flag for the coded format when VIDIOC_ENUM_FMT() is called.

The sequence starts when the decoder detects a coded frame with one or more of the following parameters different from those previously established (and reflected by corresponding queries):

coded resolution (OUTPUT width and height),
visible resolution (selection rectangles),
the minimum number of buffers needed for decoding,
bit-depth of the bitstream has been changed.

Whenever that happens, the decoder must proceed as follows:

After encountering a resolution change in the stream, the decoder sends a V4L2_EVENT_SOURCE_CHANGE event with changes set to V4L2_EVENT_SRC_CH_RESOLUTION.

Important

Any client query issued after the decoder queues the event will return values applying to the stream after the resolution change, including queue formats, selection rectangles and controls.
The decoder will then process and decode all remaining buffers from before the resolution change point.
- The last buffer from before the change must be marked with the V4L2_BUF_FLAG_LAST flag, similarly to the Drain sequence above.
Warning

The last buffer may be empty (with v4l2_buffer bytesused = 0) and in that case it must be ignored by the client, as it does not contain a decoded frame.

Note

Any attempt to dequeue more CAPTURE buffers beyond the buffer marked with V4L2_BUF_FLAG_LAST will result in a -EPIPE error from VIDIOC_DQBUF().

The client must continue the sequence as described below to continue the decoding process.

Dequeue the source change event.

Important

A source change triggers an implicit decoder drain, similar to the explicit Drain sequence. The decoder is stopped after it completes. The decoding process must be resumed with either a pair of calls to VIDIOC_STREAMOFF() and VIDIOC_STREAMON() on the CAPTURE queue, or a call to VIDIOC_DECODER_CMD() with the V4L2_DEC_CMD_START command.
Continue with the Capture Setup sequence.

Note

During the resolution change sequence, the OUTPUT queue must remain streaming. Calling VIDIOC_STREAMOFF() on the OUTPUT queue would abort the sequence and initiate a seek.

In principle, the OUTPUT queue operates separately from the CAPTURE queue and this remains true for the duration of the entire resolution change sequence as well.

The client should, for best performance and simplicity, keep queuing/dequeuing buffers to/from the OUTPUT queue even while processing this sequence.

4.5.1.10. Drain¶

To ensure that all queued OUTPUT buffers have been processed and related CAPTURE buffers are given to the client, the client must follow the drain sequence described below. After the drain sequence ends, the client has received all decoded frames for all OUTPUT buffers queued before the sequence was started.

Begin drain by issuing VIDIOC_DECODER_CMD().
- Required fields:
  
  cmd
  set to V4L2_DEC_CMD_STOP.
  
  flags
  set to 0.
  
  pts
  set to 0.
Warning

The sequence can be only initiated if both OUTPUT and CAPTURE queues are streaming. For compatibility reasons, the call to VIDIOC_DECODER_CMD() will not fail even if any of the queues is not streaming, but at the same time it will not initiate the Drain sequence and so the steps described below would not be applicable.
Any OUTPUT buffers queued by the client before the VIDIOC_DECODER_CMD() was issued will be processed and decoded as normal. The client must continue to handle both queues independently, similarly to normal decode operation. This includes:
- handling any operations triggered as a result of processing those buffers, such as the Dynamic Resolution Change sequence, before continuing with the drain sequence,
- queuing and dequeuing CAPTURE buffers, until a buffer marked with the V4L2_BUF_FLAG_LAST flag is dequeued,
  
  Warning
  
  The last buffer may be empty (with v4l2_buffer bytesused = 0) and in that case it must be ignored by the client, as it does not contain a decoded frame.
  
  Note
  
  Any attempt to dequeue more CAPTURE buffers beyond the buffer marked with V4L2_BUF_FLAG_LAST will result in a -EPIPE error from VIDIOC_DQBUF().
- dequeuing processed OUTPUT buffers, until all the buffers queued before the V4L2_DEC_CMD_STOP command are dequeued,
- dequeuing the V4L2_EVENT_EOS event, if the client subscribed to it.
Note

For backwards compatibility, the decoder will signal a V4L2_EVENT_EOS event when the last frame has been decoded and all frames are ready to be dequeued. It is a deprecated behavior and the client must not rely on it. The V4L2_BUF_FLAG_LAST buffer flag should be used instead.
Once all the OUTPUT buffers queued before the V4L2_DEC_CMD_STOP call are dequeued and the last CAPTURE buffer is dequeued, the decoder is stopped and it will accept, but not process, any newly queued OUTPUT buffers until the client issues any of the following operations:
- V4L2_DEC_CMD_START - the decoder will not be reset and will resume operation normally, with all the state from before the drain,
- a pair of VIDIOC_STREAMOFF() and VIDIOC_STREAMON() on the CAPTURE queue - the decoder will resume the operation normally, however any CAPTURE buffers still in the queue will be returned to the client,
- a pair of VIDIOC_STREAMOFF() and VIDIOC_STREAMON() on the OUTPUT queue - any pending source buffers will be returned to the client and the Seek sequence will be triggered.

Note

Once the drain sequence is initiated, the client needs to drive it to completion, as described by the steps above, unless it aborts the process by issuing VIDIOC_STREAMOFF() on any of the OUTPUT or CAPTURE queues. The client is not allowed to issue V4L2_DEC_CMD_START or V4L2_DEC_CMD_STOP again while the drain sequence is in progress and they will fail with -EBUSY error code if attempted.

Although not mandatory, the availability of decoder commands may be queried using VIDIOC_TRY_DECODER_CMD().

4.5.1.11. End of Stream¶

If the decoder encounters an end of stream marking in the stream, the decoder will initiate the Drain sequence, which the client must handle as described above, skipping the initial VIDIOC_DECODER_CMD().

4.5.1.12. Commit Points¶

Setting formats and allocating buffers trigger changes in the behavior of the decoder.

Setting the format on the OUTPUT queue may change the set of formats supported/advertised on the CAPTURE queue. In particular, it also means that the CAPTURE format may be reset and the client must not rely on the previously set format being preserved.
Enumerating formats on the CAPTURE queue always returns only formats supported for the current OUTPUT format.
Setting the format on the CAPTURE queue does not change the list of formats available on the OUTPUT queue. An attempt to set a CAPTURE format that is not supported for the currently selected OUTPUT format will result in the decoder adjusting the requested CAPTURE format to a supported one.
Enumerating formats on the OUTPUT queue always returns the full set of supported coded formats, irrespectively of the current CAPTURE format.
While buffers are allocated on any of the OUTPUT or CAPTURE queues, the client must not change the format on the OUTPUT queue. Drivers will return the -EBUSY error code for any such format change attempt.

To summarize, setting formats and allocation must always start with the OUTPUT queue and the OUTPUT queue is the master that governs the set of supported formats for the CAPTURE queue.