On Fri, Jun 5, 2020 at 11:24 PM Nicolas Dufresne <nicolas@xxxxxxxxxxxx> 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. > There isn't much to be implemented here, as it's the same standard V4L2 behavior as existed for a while. If a device supports changing the control on the fly, a call to S_CTRL succeeds, if not, it fails with -EBUSY. This is regardless of whether requests are used or not. Requests in this case simply synchronize applying the new control values with the right frame. > > > > > + > > > +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> > > >
