Le vendredi 19 juillet 2019 à 14:45 +0900, Tomasz Figa a écrit : > On Wed, Jul 17, 2019 at 9:18 PM Nicolas Dufresne <nicolas@xxxxxxxxxxxx> wrote: > > Hello, > > > > there is one little detail/proposal, see inline . > > > > Le lundi 03 juin 2019 à 13:28 +0200, Hans Verkuil a écrit : > > > From: Tomasz Figa <tfiga@xxxxxxxxxxxx> > > > > > > Due to complexity of the video decoding process, the V4L2 drivers of > > > stateful decoder hardware require specific sequences of V4L2 API calls > > > to be followed. These include capability enumeration, initialization, > > > decoding, seek, pause, dynamic resolution change, drain and end of > > > stream. > > > > > > Specifics of the above have been discussed during Media Workshops at > > > LinuxCon Europe 2012 in Barcelona and then later Embedded Linux > > > Conference Europe 2014 in Düsseldorf. The de facto Codec API that > > > originated at those events was later implemented by the drivers we already > > > have merged in mainline, such as s5p-mfc or coda. > > > > > > The only thing missing was the real specification included as a part of > > > Linux Media documentation. Fix it now and document the decoder part of > > > the Codec API. > > > > > > Signed-off-by: Tomasz Figa <tfiga@xxxxxxxxxxxx> > > > Signed-off-by: Hans Verkuil <hverkuil-cisco@xxxxxxxxx> > > > --- > > > Documentation/media/uapi/v4l/dev-decoder.rst | 1084 +++++++++++++++++ > > > Documentation/media/uapi/v4l/dev-mem2mem.rst | 8 +- > > > Documentation/media/uapi/v4l/pixfmt-v4l2.rst | 5 + > > > Documentation/media/uapi/v4l/v4l2.rst | 10 +- > > > .../media/uapi/v4l/vidioc-decoder-cmd.rst | 41 +- > > > 5 files changed, 1132 insertions(+), 16 deletions(-) > > > create mode 100644 Documentation/media/uapi/v4l/dev-decoder.rst > > > > > > diff --git a/Documentation/media/uapi/v4l/dev-decoder.rst b/Documentation/media/uapi/v4l/dev-decoder.rst > > > new file mode 100644 > > > index 000000000000..b106f2d97c48 > > > --- /dev/null > > > +++ b/Documentation/media/uapi/v4l/dev-decoder.rst > > > @@ -0,0 +1,1084 @@ > > > +.. SPDX-License-Identifier: GPL-2.0 > > > + > > > +.. _decoder: > > > + > > > +************************************************* > > > +Memory-to-memory Stateful Video Decoder Interface > > > +************************************************* > > > + > > > +A stateful video decoder takes complete chunks of the bitstream (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 notation 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 > > > + bitstream; ``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 bitstream 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 bitstream; 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 bitstream. > > > + > > > +raw format > > > + uncompressed format containing raw pixel data (e.g. YUV, RGB formats). > > > + > > > +resume point > > > + a point in the bitstream 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 bitstream. > > > + > > > +stream metadata > > > + additional (non-visual) information contained inside encoded bitstream; > > > + 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``. > > > + > > > +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 (bitstream) 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 bitstream 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 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 bitstream 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 bitstream. > > > + > > > + * **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. > > > + > > > + * 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 last will be copied. > > > > I'd like to propose to change this into " timestamp of the OUTPUT > > buffer queued first will be copied". The rationale is the following, if > > you have an OUTPUT buffer that is unaligned in the following form: > > > > [ remaining | complete frame ] > > > > The remaining data is the last bits of the current frame. So it should > > share a timestamp with the bits that came before. But then you have > > another frame which by accident could fit entirely in your buffer. With > > the last approach, it is not possible to give that frame a timestamp. > > > > The first approach fixes this problem. If there was a remaining on the > > very first buffer, it will be dropped anyway, so it also work for this > > case. This modification is aligned with MPEG TS container specification > > which says the the timestamp of the packet applies to the first ES > > packet that starts, the remaining bits will reuse the previous packet > > timestamp. The rest is common sense, e.g. the same ES packet should > > only have one timestamp etc. > > Wouldn't that replace one problem with another? How about the case below? > > [ complete frame | partial frame ] [ remaining ] Right, both methods are kind of bad (even though I still like sticking with something used elsewhere). Personally, I think it's probably not worth giving that case a standard and it could be left undefined. > > Best regards, > Tomasz
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