On 4/8/19 10:36 AM, Tomasz Figa wrote: > 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...) 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 >
