Re: [PATCH v3 2/2] media: docs-rst: Document memory-to-memory video encoder interface

[Michael Tretter <m.tretter@pengutronix.de>]

On Wed, 22 May 2019 17:43:49 +0900, Tomasz Figa wrote:
> On Tue, Apr 9, 2019 at 6:43 PM Tomasz Figa <tfiga@chromium.org> wrote:
> >
> > On Tue, Apr 9, 2019 at 6:37 PM Hans Verkuil <hverkuil@xs4all.nl> wrote:  
> > >
> > > On 4/9/19 9:11 AM, Tomasz Figa wrote:  
> > > > On Mon, Apr 8, 2019 at 5:43 PM Hans Verkuil <hverkuil@xs4all.nl> wrote:  
> > > >>
> > > >> On 4/8/19 10:36 AM, Tomasz Figa wrote:  
> > > >>> On Mon, Mar 25, 2019 at 10:12 PM Hans Verkuil <hverkuil@xs4all.nl> wrote:  
> > > >>>>
> > > >>>> Another comment found while creating compliance tests:
> > > >>>>
> > > >>>> On 1/24/19 11:04 AM, Tomasz Figa wrote:  
> > > >>>>> +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 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 decoded 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,  
> > > >>>>
> > > >>>> I assume that calling CMD_START when *not* draining will succeed but does nothing.
> > > >>>>
> > > >>>> In other words: while draining is in progress START will return EBUSY. When draining
> > > >>>> was finished, then START will resume the encoder. In all other cases it just returns
> > > >>>> 0 since the encoder is really already started.
> > > >>>>  
> > > >>>
> > > >>> Yes, that was the intention and seems to be the closest to the
> > > >>> behavior described in the existing documentation.
> > > >>>  
> > > >>>>> +
> > > >>>>> +   * 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.  
> > > >>>>
> > > >>>> I assume calling STOP again once the drain sequence completed just returns 0 and
> > > >>>> doesn't do anything else (since we're already stopped).
> > > >>>>  
> > > >>>
> > > >>> Right.
> > > >>>  
> > > >>>>> +
> > > >>>>> +   Although mandatory, the availability of encoder commands may be queried
> > > >>>>> +   using :c:func:`VIDIOC_TRY_ENCODER_CMD`.  
> > > >>>>
> > > >>>> Some corner cases:
> > > >>>>
> > > >>>> 1) No buffers are queued on either vb2_queue, but STREAMON is called for both queues.
> > > >>>>    Now ENC_CMD_STOP is issued. What should happen?
> > > >>>>
> > > >>>>    Proposal: the next time the applications queues a CAPTURE buffer it is returned
> > > >>>>    at once as an empty buffer with FLAG_LAST set.
> > > >>>>  
> > > >>>
> > > >>> SGTM. It's actually similar to a relatively common case where all
> > > >>> CAPTURE buffers have been dequeued and the application has to refill
> > > >>> the CAPTURE queue, but in the meantime a drain request needs to be
> > > >>> issued.
> > > >>>  
> > > >>>> 2) Both queues are streaming and buffers have been encoded, but currently no buffers
> > > >>>>    are queued on either vb2_queue. Now ENC_CMD_STOP is issued. What should happen?
> > > >>>>
> > > >>>>    Proposal: the next time the applications queues a CAPTURE buffer it is returned
> > > >>>>    at once as an empty buffer with FLAG_LAST set. This is consistent with the
> > > >>>>    previous corner case.  
> > > >>>
> > > >>> Agreed.
> > > >>>  
> > > >>>>
> > > >>>> 3) The CAPTURE queue contains buffers, the OUTPUT queue does not. Now ENC_CMD_STOP
> > > >>>>    is issued. What should happen?
> > > >>>>
> > > >>>>    Proposal: the oldest CAPTURE buffer in the ready queue is returned as an empty
> > > >>>>    buffer with FLAG_LAST set.  
> > > >>>
> > > >>> Generally agreed, but not sure if there is a reason to specifically
> > > >>> refer to the oldest buffer. (I'm personally for keeping the queues
> > > >>> ordered, though...)  
> > > >>
> > > >> Feel free to rephrase. Perhaps: "an empty CAPTURE buffer with FLAG_LAST set should be
> > > >> queued up for userspace to signal that the encoder has stopped." Or something along
> > > >> those lines.  
> > > >
> > > > I've added a note:
> > > >
> > > >    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 cancelled and all  
> > >
> > > cancelled -> canceled
> > >  
> > > >      ``CAPTURE`` buffers are implicitly returned to the userpace.  
> > >
> > > userpace -> userspace
> > >  
> >
> > Actually changed it to "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.
> > > >
> > > > Slightly changed the split into cases to cover behaviors rather than
> > > > conditions. WDYT?  
> > >
> > > Looks good (with those two typos fixed).
> > >  
> >
> > Thanks.  
> 
> Hmm, we actually looked into implementing this in mtk-vcodec and
> handling of this corner case gets quite complicated. When stopping the
> streaming on OUTPUT, you may not have any available CAPTURE buffer, so
> you need to keep some extra state in the driver and check it in vb2
> .buf_queue for CAPTURE to return the first buffer and complete the
> drain.
> 
> The general handling of drain would have to look like this:
>  - VIDIOC_DECODER_CMD must check if a drain isn't already in progress
> and also whether the queues are streaming,
>  - STREAMOFF(CAPTURE) needs to cancel any pending drain,
>  - STREAMOFF(OUTPUT) needs to return a CAPTURE buffer with LAST set if
> there is one or postpone it until a buffer is queued,
>  - QBUF(CAPTURE) must return the buffer instantly if such return was
> postponed by STREAMOFF(OUTPUT).
>  - DQBUF(CAPTURE) of a buffer with the LAST flag set would finish the sequence.
> 
> Sounds like we definitely need some generic code to handle this...

I implemented this as well for allegro. I ended up exactly with an
additional state (ALLEGRO_STATE_WAIT_FOR_BUFFER) to handle this corner
case. The implementation was tricky and I would highly appreciate
generic code that is used by other drivers as well.

Michael


> 
> >  
> > >         Hans
> > >  
> > > >
> > > > Best regards,
> > > > Tomasz
> > > >  .
> > > >>
> > > >> Regards,
> > > >>
> > > >>         Hans
> > > >>  
> > > >>>  
> > > >>>>
> > > >>>> 4) Both queues have queued buffers. ENC_CMD_STOP is issued to start the drain process.
> > > >>>>    Before the drain process completes STREAMOFF is called for either CAPTURE or
> > > >>>>    OUTPUT queue. What should happen?
> > > >>>>
> > > >>>>    Proposal for STREAMOFF(CAPTURE): aborts the drain process and all CAPTURE buffers are
> > > >>>>    returned to userspace. If encoding is restarted, then any remaining OUTPUT buffers
> > > >>>>    will be used as input to the encoder.
> > > >>>>  
> > > >>>
> > > >>> Agreed.
> > > >>>  
> > > >>>>    Proposal for STREAMOFF(OUTPUT): the next capture buffer will be empty and have
> > > >>>>    FLAG_LAST set.  
> > > >>>
> > > >>> Agreed.
> > > >>>  
> > > >>>>
> > > >>>> Some of this might have to be documented, but these corner cases should certainly be
> > > >>>> tested by v4l2-compliance. Before I write those tests I'd like to know if you agree
> > > >>>> with this.  
> > > >>>
> > > >>> Agreed with just one minor comment. Thanks for checking with me!
> > > >>>
> > > >>> Best regards,
> > > >>> Tomasz
> > > >>>  
> > > >>  
> > >  
>