Hi Nicolas, On 6/6/20 12:24 AM, Nicolas Dufresne wrote: > Le vendredi 05 juin 2020 à 10:19 +0300, Stanimir Varbanov a écrit : >> >> On 5/26/20 1:09 PM, Hans Verkuil wrote: >>> From: Tomasz Figa <tfiga@xxxxxxxxxxxx> >>> >>> Due to complexity of the video encoding process, the V4L2 drivers of >>> stateful encoder hardware require specific sequences of V4L2 API calls >>> to be followed. These include capability enumeration, initialization, >>> encoding, encode parameters change, drain and reset. >>> >>> 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 encoder part of >>> the Codec API. >>> >>> Signed-off-by: Tomasz Figa <tfiga@xxxxxxxxxxxx> >>> Signed-off-by: Hans Verkuil <hverkuil-cisco@xxxxxxxxx> >>> --- >>> .../userspace-api/media/v4l/dev-encoder.rst | 728 ++++++++++++++++++ >>> .../userspace-api/media/v4l/dev-mem2mem.rst | 1 + >>> .../userspace-api/media/v4l/pixfmt-v4l2.rst | 5 + >>> .../userspace-api/media/v4l/v4l2.rst | 2 + >>> .../media/v4l/vidioc-encoder-cmd.rst | 51 +- >>> 5 files changed, 767 insertions(+), 20 deletions(-) >>> create mode 100644 Documentation/userspace-api/media/v4l/dev-encoder.rst >>> >>> diff --git a/Documentation/userspace-api/media/v4l/dev-encoder.rst b/Documentation/userspace-api/media/v4l/dev-encoder.rst >>> new file mode 100644 >>> index 000000000000..aace7b812a9c >>> --- /dev/null >>> +++ b/Documentation/userspace-api/media/v4l/dev-encoder.rst >>> @@ -0,0 +1,728 @@ >>> +.. SPDX-License-Identifier: GPL-2.0 >>> + >>> +.. _encoder: >>> + >>> +************************************************* >>> +Memory-to-Memory Stateful Video Encoder Interface >>> +************************************************* >>> + >>> +A stateful video encoder takes raw video frames in display order and encodes >>> +them into a bytestream. It generates complete chunks of the bytestream, including >>> +all metadata, headers, etc. The resulting bytestream does not require any >>> +further post-processing by the client. >>> + >>> +Performing software stream processing, header generation etc. in the driver >>> +in order to support this interface is strongly discouraged. In case such >>> +operations are needed, use of the Stateless Video Encoder Interface (in >>> +development) is strongly advised. >>> + >> >> <cut> >> >>> +Encoding Parameter Changes >>> +========================== >>> + >>> +The client is allowed to use :c:func:`VIDIOC_S_CTRL` to change encoder >>> +parameters at any time. The availability of parameters is encoder-specific >>> +and the client must query the encoder to find the set of available controls. >>> + >>> +The ability to change each parameter during encoding is encoder-specific, as >>> +per the standard semantics of the V4L2 control interface. The client may >>> +attempt to set a control during encoding and if the operation fails with the >>> +-EBUSY error code, the ``CAPTURE`` queue needs to be stopped for the >>> +configuration change to be allowed. To do this, it may follow the `Drain` >>> +sequence to avoid losing the already queued/encoded frames. >>> + >>> +The timing of parameter updates is encoder-specific, as per the standard >>> +semantics of the V4L2 control interface. If the client needs to apply the >>> +parameters exactly at specific frame, using the Request API >>> +(:ref:`media-request-api`) should be considered, if supported by the encoder. >> >> In request-api case does the control will return EBUSY immediately when >> S_CTRL is called from the client? I'm asking in the context of the >> controls which are not dynamic (cannot set during streaming and Reset >> sequence is needed). > > This is all hypothetical, as nothing of that has been implemented. But > I suppose there should be instant validation to allow that, even if > that means more plumbing inside the kernel. It would be better then > just running the request ignoring silently that control. Ideally > userspace should not have to go into trial and errors, so controls that > are means for the should be marked. Meanwhile I looked into the controls code and found that at least .try_ctrl will be called. Do you have something to add in below subject for SPS/PPS? > >> >>> + >>> +Drain >>> +===== >>> + >>> +To ensure that all the queued ``OUTPUT`` buffers have been processed and the >>> +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 encoded frames for all ``OUTPUT`` buffers queued before the >>> +sequence was started. >>> + >>> +1. Begin the drain sequence by issuing :c:func:`VIDIOC_ENCODER_CMD`. >>> + >>> + * **Required fields:** >>> + >>> + ``cmd`` >>> + set to ``V4L2_ENC_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_ENCODER_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_ENCODER_CMD` was issued will be processed and encoded as >>> + normal. The client must continue to handle both queues independently, >>> + similarly to normal encode operation. This includes: >>> + >>> + * 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 an encoded 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_ENC_CMD_STOP`` command are dequeued, >>> + >>> + * dequeuing the ``V4L2_EVENT_EOS`` event, if the client subscribes to it. >>> + >>> + .. note:: >>> + >>> + For backwards compatibility, the encoder will signal a ``V4L2_EVENT_EOS`` >>> + event when the last frame has been encoded and all frames are ready to be >>> + dequeued. It is deprecated behavior and the client must not rely on it. >>> + The ``V4L2_BUF_FLAG_LAST`` buffer flag should be used instead. >>> + >>> +3. Once all ``OUTPUT`` buffers queued before the ``V4L2_ENC_CMD_STOP`` call are >>> + dequeued and the last ``CAPTURE`` buffer is dequeued, the encoder is stopped >>> + and it will accept, but not process any newly queued ``OUTPUT`` buffers >>> + until the client issues any of the following operations: >>> + >>> + * ``V4L2_ENC_CMD_START`` - the encoder 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 encoder will be reset (see the `Reset` sequence) >>> + and then resume encoding, >>> + >>> + * a pair of :c:func:`VIDIOC_STREAMOFF` and :c:func:`VIDIOC_STREAMON` on the >>> + ``OUTPUT`` queue - the encoder will resume operation normally, however any >>> + source frames queued to the ``OUTPUT`` queue between ``V4L2_ENC_CMD_STOP`` >>> + and :c:func:`VIDIOC_STREAMOFF` will be discarded. >>> + >>> +.. 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_ENC_CMD_START`` or >>> + ``V4L2_ENC_CMD_STOP`` again while the drain sequence is in progress and they >>> + will fail with -EBUSY error code if attempted. >>> + >>> + For reference, handling of various corner cases is described below: >>> + >>> + * In case of no buffer in the ``OUTPUT`` queue at the time the >>> + ``V4L2_ENC_CMD_STOP`` command was issued, the drain sequence completes >>> + immediately and the encoder returns an empty ``CAPTURE`` buffer with the >>> + ``V4L2_BUF_FLAG_LAST`` flag set. >>> + >>> + * In case of no buffer in the ``CAPTURE`` queue at the time the drain >>> + sequence completes, the next time the client queues a ``CAPTURE`` buffer >>> + it is returned at once as an empty buffer with the ``V4L2_BUF_FLAG_LAST`` >>> + flag set. >>> + >>> + * If :c:func:`VIDIOC_STREAMOFF` is called on the ``CAPTURE`` queue in the >>> + middle of the drain sequence, the drain sequence is canceled and all >>> + ``CAPTURE`` buffers are implicitly returned to the client. >>> + >>> + * If :c:func:`VIDIOC_STREAMOFF` is called on the ``OUTPUT`` queue in the >>> + middle of the drain sequence, the drain sequence completes immediately and >>> + next ``CAPTURE`` buffer will be returned empty with the >>> + ``V4L2_BUF_FLAG_LAST`` flag set. >>> + >>> + Although not mandatory, the availability of encoder commands may be queried >>> + using :c:func:`VIDIOC_TRY_ENCODER_CMD`. >>> + >>> +Reset >>> +===== >>> + >>> +The client may want to request the encoder to reinitialize the encoding, so >>> +that the following stream data becomes independent from the stream data >>> +generated before. Depending on the coded format, that may imply that: >>> + >>> +* encoded frames produced after the restart must not reference any frames >>> + produced before the stop, e.g. no long term references for H.264/HEVC, >>> + >>> +* any headers that must be included in a standalone stream must be produced >>> + again, e.g. SPS and PPS for H.264/HEVC. >> >> It seems that clients needs to know SPS/PPS (for muxers?) and thus they >> will need to extract or parse the encoder output buffers to find them. >> Maybe the spec should consider adding some buffer flag to mark such >> buffers which contain SPS/PPS only. >> >> [1] - see at "type AvcCBox struct" >> >> Nicolas, how the gstreamer handle this? >> >>> + >>> +This can be achieved by performing the reset sequence. >>> + >>> +1. Perform the `Drain` sequence to ensure all the in-flight encoding finishes >>> + and respective buffers are dequeued. >>> + >>> +2. Stop streaming on the ``CAPTURE`` queue via :c:func:`VIDIOC_STREAMOFF`. This >>> + will return all currently queued ``CAPTURE`` buffers to the client, without >>> + valid frame data. >>> + >>> +3. Start streaming on the ``CAPTURE`` queue via :c:func:`VIDIOC_STREAMON` and >>> + continue with regular encoding sequence. The encoded frames produced into >>> + ``CAPTURE`` buffers from now on will contain a standalone stream that can be >>> + decoded without the need for frames encoded before the reset sequence, >>> + starting at the first ``OUTPUT`` buffer queued after issuing the >>> + `V4L2_ENC_CMD_STOP` of the `Drain` sequence. >>> + >>> +This sequence may be also used to change encoding parameters for encoders >>> +without the ability to change the parameters on the fly. >>> + >> >> <cut> >> > -- regards, Stan
