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SubjectRe: [PATCH v2 1/2] media: docs-rst: Document memory-to-memory video decoder interface
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On 10/22/2018 04:48 PM, Tomasz Figa wrote:
> 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@chromium.org>
> ---
> Documentation/media/uapi/v4l/dev-decoder.rst | 1082 +++++++++++++++++
> Documentation/media/uapi/v4l/devices.rst | 1 +
> Documentation/media/uapi/v4l/pixfmt-v4l2.rst | 5 +
> Documentation/media/uapi/v4l/v4l2.rst | 10 +-
> .../media/uapi/v4l/vidioc-decoder-cmd.rst | 40 +-
> Documentation/media/uapi/v4l/vidioc-g-fmt.rst | 14 +
> 6 files changed, 1137 insertions(+), 15 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..09c7a6621b8e
> --- /dev/null
> +++ b/Documentation/media/uapi/v4l/dev-decoder.rst
> @@ -0,0 +1,1082 @@
> +.. -*- coding: utf-8; mode: rst -*-
> +
> +.. _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.
> +
> +3. All steps not marked “optional” are required.
> +
> +4. :c:func:`VIDIOC_G_EXT_CTRLS`, :c:func:`VIDIOC_S_EXT_CTRLS` may be used
> + interchangeably with :c:func:`VIDIOC_G_CTRL`, :c:func:`VIDIOC_S_CTRL`,
> + unless specified otherwise.
> +
> +5. Single-plane API (see spec) and applicable structures may be used
> + interchangeably with Multi-plane 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, A’ represents a buffer on the ``CAPTURE``
> + queue containing data (decoded frame/stream) that resulted from processing
> + buffer A.
> +
> +.. _decoder-glossary:
> +
> +Glossary
> +========
> +
> +CAPTURE
> + the destination buffer queue; for decoder, the queue of buffers containing
> + decoded frames; for encoder, the queue of buffers containing encoded
> + bitstream; ``V4L2_BUF_TYPE_VIDEO_CAPTURE```` or
> + ``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE``; data are captured from the hardware
> + into ``CAPTURE`` buffers
> +
> +client
> + application client 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; ``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 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 H.264-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
> + 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 H.264 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); 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; ``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 H.264 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
> + formats 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 given format, if applicable, may be
> + queried using their respective controls via :c:func:`VIDIOC_QUERYCTRL`.
> +
> +Initialization
> +==============
> +
> +1. **Optional.** Enumerate supported ``OUTPUT`` formats and resolutions. See
> + `Querying capabilities` above.
> +
> +2. 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``
> + required only if cannot be parsed from the stream for the given
> + coded format; optional otherwise - set to zero to ignore
> +
> + ``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 ``CAPTURE`` buffers
> +
> + * If width and height are set to non-zero values, the ``CAPTURE`` format
> + will be updated with an appropriate frame buffer resolution instantly.
> + 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.
> +
> + .. warning::
> +
> + Changing the ``OUTPUT`` format may change the currently set ``CAPTURE``
> + format. The decoder will derive a new ``CAPTURE`` format from the
> + ``OUTPUT`` format being set, including resolution, colorimetry
> + parameters, etc. If the client needs a specific ``CAPTURE`` format, it
> + must adjust it afterwards.
> +
> +3. **Optional.** Query the minimum number of buffers required for ``OUTPUT``
> + queue via :c:func:`VIDIOC_G_CTRL`. This is useful if the client intends to
> + use more buffers than the minimum required by hardware/format.
> +
> + * **Required fields:**
> +
> + ``id``
> + set to ``V4L2_CID_MIN_BUFFERS_FOR_OUTPUT``
> +
> + * **Return fields:**
> +
> + ``value``
> + the minimum number of ``OUTPUT`` buffers required for the currently
> + set format
> +
> +4. 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.
> +
> + .. note::
> +
> + To allocate more than the minimum number of buffers (for pipeline
> + depth), the client may query the ``V4L2_CID_MIN_BUFFERS_FOR_OUTPUT``
> + control to get the minimum number of buffers required by the
> + decoder/format, 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 ``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.
> +
> +5. Start streaming on the ``OUTPUT`` queue via :c:func:`VIDIOC_STREAMON`.
> +
> +6. **This step only applies to coded formats that contain resolution information
> + in the stream.**

As far as I know all codecs have resolution/metadata in the stream.

As discussed in the "[PATCH vicodec v4 0/3] Add support to more pixel formats in
vicodec" thread, it is easiest to assume that there is always metadata.

Perhaps there should be a single mention somewhere that such codecs are not
supported at the moment, but to be frank how can you decode a stream without
it containing such essential information? You are much more likely to implement
such a codec as a stateless codec.

So I would just drop this sentence here (and perhaps at other places in this
document or the encoder document as well).

Regards,

Hans

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