--- /dev/null
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _decoder:
+
+*************************************************
+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.
+
+Conventions and Notations Used in This Document
+===============================================
+
+1. The general V4L2 API rules apply if not specified in this document
+ otherwise.
+
+2. The meaning of words "must", "may", "should", etc. is as per `RFC
+ 2119 <https://tools.ietf.org/html/rfc2119>`_.
+
+3. All steps not marked "optional" are required.
+
+4. :c:func:`VIDIOC_G_EXT_CTRLS` and :c:func:`VIDIOC_S_EXT_CTRLS` may be used
+ interchangeably with :c:func:`VIDIOC_G_CTRL` and :c:func:`VIDIOC_S_CTRL`,
+ unless specified otherwise.
+
+5. Single-planar API (see :ref:`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.
+
+6. i = [a..b]: sequence of integers from a to b, inclusive, i.e. i =
+ [0..2]: i = 0, 1, 2.
+
+7. Given an ``OUTPUT`` buffer A, then A’ represents a buffer on the ``CAPTURE``
+ queue containing data that resulted from processing buffer A.
+
+.. _decoder-glossary:
+
+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.
+
+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).
+
+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.
+
+State Machine
+=============
+
+.. kernel-render:: DOT
+ :alt: DOT digraph of decoder state machine
+ :caption: Decoder State Machine
+
+ digraph decoder_state_machine {
+ node [shape = doublecircle, label="Decoding"] Decoding;
+
+ node [shape = circle, label="Initialization"] Initialization;
+ node [shape = circle, label="Capture\nsetup"] CaptureSetup;
+ node [shape = circle, label="Dynamic\nResolution\nChange"] ResChange;
+ node [shape = circle, label="Stopped"] Stopped;
+ node [shape = circle, label="Drain"] Drain;
+ node [shape = circle, label="Seek"] Seek;
+ node [shape = circle, label="End of Stream"] EoS;
+
+ node [shape = point]; qi
+ qi -> Initialization [ label = "open()" ];
+
+ Initialization -> CaptureSetup [ label = "CAPTURE\nformat\nestablished" ];
+
+ CaptureSetup -> Stopped [ label = "CAPTURE\nbuffers\nready" ];
+
+ Decoding -> ResChange [ label = "Stream\nresolution\nchange" ];
+ Decoding -> Drain [ label = "V4L2_DEC_CMD_STOP" ];
+ Decoding -> EoS [ label = "EoS mark\nin the stream" ];
+ Decoding -> Seek [ label = "VIDIOC_STREAMOFF(OUTPUT)" ];
+ Decoding -> Stopped [ label = "VIDIOC_STREAMOFF(CAPTURE)" ];
+ Decoding -> Decoding;
+
+ ResChange -> CaptureSetup [ label = "CAPTURE\nformat\nestablished" ];
+ ResChange -> Seek [ label = "VIDIOC_STREAMOFF(OUTPUT)" ];
+
+ EoS -> Drain [ label = "Implicit\ndrain" ];
+
+ Drain -> Stopped [ label = "All CAPTURE\nbuffers dequeued\nor\nVIDIOC_STREAMOFF(CAPTURE)" ];
+ Drain -> Seek [ label = "VIDIOC_STREAMOFF(OUTPUT)" ];
+
+ Seek -> Decoding [ label = "VIDIOC_STREAMON(OUTPUT)" ];
+ Seek -> Initialization [ label = "VIDIOC_REQBUFS(OUTPUT, 0)" ];
+
+ Stopped -> Decoding [ label = "V4L2_DEC_CMD_START\nor\nVIDIOC_STREAMON(CAPTURE)" ];
+ Stopped -> Seek [ label = "VIDIOC_STREAMOFF(OUTPUT)" ];
+ }
+
+Querying Capabilities
+=====================
+
+1. To enumerate the set of coded formats supported by the decoder, the
+ client may call :c:func:`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 :c:type:`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.
+
+2. To enumerate the set of supported raw formats, the client may call
+ :c:func:`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``.
+
+3. The client may use :c:func:`VIDIOC_ENUM_FRAMESIZES` to detect supported
+ resolutions for a given format, passing desired pixel format in
+ :c:type:`v4l2_frmsizeenum` ``pixel_format``.
+
+ * Values returned by :c:func:`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 :c:func:`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``.
+
+4. Supported profiles and levels for the coded format currently set on
+ ``OUTPUT``, if applicable, may be queried using their respective controls
+ via :c:func:`VIDIOC_QUERYCTRL`.
+
+Initialization
+==============
+
+1. Set the coded format on ``OUTPUT`` via :c:func:`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
+ :c:func:`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.
+
+2. Allocate source (bytestream) buffers via :c:func:`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, :c:func:`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.
+
+3. Start streaming on the ``OUTPUT`` queue via :c:func:`VIDIOC_STREAMON`.
+
+4. **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 :c:func:`VIDIOC_QBUF` and :c:func:`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 :c:func:`VIDIOC_G_FMT`, :c:func:`VIDIOC_S_FMT`,
+ :c:func:`VIDIOC_TRY_FMT` or :c:func:`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.
+
+5. Continue with the `Capture Setup` sequence.
+
+Capture Setup
+=============
+
+1. Call :c:func:`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.
+
+2. **Optional.** Acquire the visible resolution via
+ :c:func:`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 :c:func:`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.
+
+3. **Optional.** Enumerate ``CAPTURE`` formats via :c:func:`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 :c:func:`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,
+ :c:func:`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 :c:func:`VIDIOC_ENUM_FMT`
+ would return RGB formats again in that case.
+
+4. **Optional.** Set the ``CAPTURE`` format via :c:func:`VIDIOC_S_FMT` on the
+ ``CAPTURE`` queue. The client may choose a different format than
+ selected/suggested by the decoder in :c:func:`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 :c:func:`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.
+
+5. **Optional.** Set the compose rectangle via :c:func:`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 :c:func:`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 :c:func:`VIDIOC_S_SELECTION`.
+
+6. 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 :c:func:`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 :c:func:`VIDIOC_DECODER_CMD`
+ with the ``V4L2_DEC_CMD_START`` command,
+
+ * if the ``CAPTURE`` queue is not streaming, call :c:func:`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.
+
+7. **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.
+
+8. **If the** ``CAPTURE`` **queue is streaming,** call :c:func:`VIDIOC_STREAMOFF`
+ on the ``CAPTURE`` queue to stop streaming.
+
+ .. warning::
+
+ The ``OUTPUT`` queue must remain streaming. Calling
+ :c:func:`VIDIOC_STREAMOFF` on it would abort the sequence and trigger a
+ seek.
+
+9. **If the** ``CAPTURE`` **queue has buffers allocated,** free the ``CAPTURE``
+ buffers using :c:func:`VIDIOC_REQBUFS`.
+
+ * **Required fields:**
+
+ ``count``
+ set to 0.
+
+ ``type``
+ a ``V4L2_BUF_TYPE_*`` enum appropriate for ``CAPTURE``.
+
+ ``memory``
+ follows standard semantics.
+
+10. Allocate ``CAPTURE`` buffers via :c:func:`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 :c:func:`VIDIOC_REQBUFS`.
+
+ Alternatively, :c:func:`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 :c:func:`VIDIOC_G_FMT` and save it
+ until this step.
+
+ The format obtained in the query may be then used with
+ :c:func:`VIDIOC_CREATE_BUFS` in this step to allocate the buffers.
+
+11. Call :c:func:`VIDIOC_STREAMON` on the ``CAPTURE`` queue to start decoding
+ frames.
+
+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
+:c:func:`VIDIOC_QBUF` and :c:func:`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
+ :c:type:`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.
+
+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.
+
+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.
+
+1. Stop the ``OUTPUT`` queue to begin the seek sequence via
+ :c:func:`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).
+
+2. Restart the ``OUTPUT`` queue via :c:func:`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.
+
+3. 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.
+
+4. 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.
+
+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
+ :c:func:`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.
+
+Whenever that happens, the decoder must proceed as follows:
+
+1. 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.
+
+2. 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 :c:type:`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
+ :c:func:`VIDIOC_DQBUF`.
+
+The client must continue the sequence as described below to continue the
+decoding process.
+
+1. 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
+ :c:func:`VIDIOC_STREAMOFF` and :c:func:`VIDIOC_STREAMON` on the
+ ``CAPTURE`` queue, or a call to :c:func:`VIDIOC_DECODER_CMD` with the
+ ``V4L2_DEC_CMD_START`` command.
+
+2. Continue with the `Capture Setup` sequence.
+
+.. note::
+
+ During the resolution change sequence, the ``OUTPUT`` queue must remain
+ streaming. Calling :c:func:`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.
+
+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.
+
+1. Begin drain by issuing :c:func:`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
+ :c:func:`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.
+
+2. Any ``OUTPUT`` buffers queued by the client before the
+ :c:func:`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 :c:type:`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
+ :c:func:`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.
+
+3. 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 :c:func:`VIDIOC_STREAMOFF` and :c:func:`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 :c:func:`VIDIOC_STREAMOFF` and :c:func:`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 :c:func:`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 mandatory, the availability of decoder commands may be queried
+ using :c:func:`VIDIOC_TRY_DECODER_CMD`.
+
+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 :c:func:`VIDIOC_DECODER_CMD`.
+
+Commit Points
+=============
+
+Setting formats and allocating buffers trigger changes in the behavior of the
+decoder.
+
+1. 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.
+
+2. Enumerating formats on the ``CAPTURE`` queue always returns only formats
+ supported for the current ``OUTPUT`` format.
+
+3. 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.
+
+4. Enumerating formats on the ``OUTPUT`` queue always returns the full set of
+ supported coded formats, irrespectively of the current ``CAPTURE`` format.
+
+5. 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.
projects / users / hch / configfs.git / commitdiff