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

[Hans Verkuil <hverkuil@xs4all.nl>]

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.

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
>