Hi Hans, On 5/20/20 1:01 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 | 727 ++++++++++++++++++ > .../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, 766 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..b30ef9c5d970 > --- /dev/null > +++ b/Documentation/userspace-api/media/v4l/dev-encoder.rst > @@ -0,0 +1,727 @@ > +.. 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 > +======== > + > +This state is reached after the `Initialization` 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 encoded ``CAPTURE`` 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 standard behavior of V4L2 buffer > +queues and memory-to-memory devices. In addition, the order of encoded frames > +dequeued from the ``CAPTURE`` queue may differ from the order of queuing raw > +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 more than one buffer produced on > + ``CAPTURE`` (for example, if returning an encoded frame allowed the encoder > + to return a frame that preceded it in display, but succeeded it in the decode > + order; however, there may be other reasons for this as well), > + > +* a buffer queued to ``OUTPUT`` may result in a buffer being produced on > + ``CAPTURE`` later into encode 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 ^^^ encoding > + available at later time, due to specifics of the decoding process, ^^^ encoding > + > +* buffers queued to ``OUTPUT`` may not become available to dequeue instantly > + after being encoded into a corresponding ``CAPTURE`` buffer, e.g. if the > + encoder needs to use the frame as a reference for encoding further frames. > + <cut> > +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 mandatory, the availability of encoder commands may be queried maybe "Although not mandatory" ? > + using :c:func:`VIDIOC_TRY_ENCODER_CMD`. > + <cut> -- regards, Stan
