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

[Tomasz Figa <tfiga@chromium.org>]

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
     ``CAPTURE`` buffers are implicitly returned to the userpace.

   * 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?

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
> >
>