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

[Tomasz Figa <tfiga@chromium.org>]

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.

>         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
> >>>
> >>
>