On Mon, Mar 25, 2019 at 10:12 PM Hans Verkuil <hverkuil@xxxxxxxxx> 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...) > > 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
