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[209.85.167.176]) by smtp.gmail.com with ESMTPSA id 189sm12555926oid.35.2019.04.09.00.11.29 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 09 Apr 2019 00:11:29 -0700 (PDT) Received: by mail-oi1-f176.google.com with SMTP id l203so12694589oia.3 for ; Tue, 09 Apr 2019 00:11:29 -0700 (PDT) X-Received: by 2002:aca:edc7:: with SMTP id l190mr2709412oih.92.1554793888684; Tue, 09 Apr 2019 00:11:28 -0700 (PDT) MIME-Version: 1.0 References: <20190124100419.26492-1-tfiga@chromium.org> <20190124100419.26492-3-tfiga@chromium.org> In-Reply-To: From: Tomasz Figa Date: Tue, 9 Apr 2019 16:11:17 +0900 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v3 2/2] media: docs-rst: Document memory-to-memory video encoder interface To: Hans Verkuil Cc: Linux Media Mailing List , Linux Kernel Mailing List , Mauro Carvalho Chehab , Pawel Osciak , Alexandre Courbot , Kamil Debski , Andrzej Hajda , Kyungmin Park , Jeongtae Park , Philipp Zabel , =?UTF-8?B?VGlmZmFueSBMaW4gKOael+aFp+ePiik=?= , =?UTF-8?B?QW5kcmV3LUNUIENoZW4gKOmZs+aZuui/qik=?= , Stanimir Varbanov , Todor Tomov , Nicolas Dufresne , Paul Kocialkowski , Laurent Pinchart , dave.stevenson@raspberrypi.org, Ezequiel Garcia , Maxime Jourdan Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Apr 8, 2019 at 5:43 PM Hans Verkuil wrote: > > On 4/8/19 10:36 AM, Tomasz Figa wrote: > > On Mon, Mar 25, 2019 at 10:12 PM Hans Verkuil 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 > > >