Re: [PATCH v3 1/2] media: docs-rst: Document memory-to-memory video decoder interface

[Philipp Zabel <p.zabel@pengutronix.de>]

On Fri, 2019-04-05 at 13:21 +0200, Hans Verkuil wrote:
> On 4/5/19 12:59 PM, Philipp Zabel wrote:
> > On Thu, 2019-01-24 at 19:04 +0900, Tomasz Figa wrote:
> > > Due to complexity of the video decoding process, the V4L2 drivers of
> > > stateful decoder hardware require specific sequences of V4L2 API calls
> > > to be followed. These include capability enumeration, initialization,
> > > decoding, seek, pause, dynamic resolution change, drain and end of
> > > stream.
> > > 
> > > Specifics of the above have been discussed during Media Workshops at
> > > LinuxCon Europe 2012 in Barcelona and then later Embedded Linux
> > > Conference Europe 2014 in Düsseldorf. The de facto Codec API that
> > > originated at those events was later implemented by the drivers we already
> > > have merged in mainline, such as s5p-mfc or coda.
> > > 
> > > The only thing missing was the real specification included as a part of
> > > Linux Media documentation. Fix it now and document the decoder part of
> > > the Codec API.
> > > 
> > > Signed-off-by: Tomasz Figa <tfiga@chromium.org>
> > > ---
> > >  Documentation/media/uapi/v4l/dev-decoder.rst  | 1076 +++++++++++++++++
> > >  Documentation/media/uapi/v4l/dev-mem2mem.rst  |    5 +
> > >  Documentation/media/uapi/v4l/pixfmt-v4l2.rst  |    5 +
> > >  Documentation/media/uapi/v4l/v4l2.rst         |   10 +-
> > >  .../media/uapi/v4l/vidioc-decoder-cmd.rst     |   40 +-
> > >  Documentation/media/uapi/v4l/vidioc-g-fmt.rst |   14 +
> > >  6 files changed, 1135 insertions(+), 15 deletions(-)
> > >  create mode 100644 Documentation/media/uapi/v4l/dev-decoder.rst
> > > 
> > > diff --git a/Documentation/media/uapi/v4l/dev-decoder.rst b/Documentation/media/uapi/v4l/dev-decoder.rst
> > > new file mode 100644
> > > index 000000000000..b7db2352ad41
> > > --- /dev/null
> > > +++ b/Documentation/media/uapi/v4l/dev-decoder.rst
> > > @@ -0,0 +1,1076 @@
> > > +.. -*- coding: utf-8; mode: rst -*-
> > > +
> > > +.. _decoder:
> > > +
> > > +*************************************************
> > > +Memory-to-memory Stateful Video Decoder Interface
> > > +*************************************************
> > > +
> > 
> > [...]
> > > +
> > > +Drain
> > > +=====
> > > +
> > 
> > [...]
> > > +
> > > +3. Once all the ``OUTPUT`` buffers queued before the ``V4L2_DEC_CMD_STOP`` call
> > > +   are dequeued and the last ``CAPTURE`` buffer is dequeued, the decoder is
> > > +   stopped and it will accept, but not process, any newly queued ``OUTPUT``
> > > +   buffers until the client issues any of the following operations:
> > > +
> > 
> > [...]
> > > +
> > > +.. 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_DEC_CMD_START`` or
> > > +   ``V4L2_DEC_CMD_STOP`` again while the drain sequence is in progress and they
> > > +   will fail with -EBUSY error code if attempted.
> > 
> > Should the V4L2_DEC_CMD_STOP documentation in
> > Documentation/media/uapi/v4l/vidioc-decoder-cmd.rst:
> > 
> >   * - ``V4L2_DEC_CMD_STOP``
> >     - 1
> >     - Stop the decoder. When the decoder is already stopped, this
> >       command does nothing.
> > 
> > be updated to mention that for mem2mem decoders this not only does
> > nothing, but returns an -EBUSY error return value?
> 
> Yes. Basically in this case the STOP is still in progress, so returning
> EBUSY makes sense and should be documented.
> 
> > 
> > Also, if V4L2_DEC_CMD_STOP_IMMEDIATELY is set, should the decoder drop
> > all pending buffers and signal EOS immediately? Should this be mentioned
> > here?
> 
> Hmm, the decoder spec says that flags should be set to 0, and current codec
> drivers explicitly disallow a non-0 flags value.
> 
> I'm not sure if there is a practical use-case for V4L2_DEC_CMD_STOP_IMMEDIATELY
> in the context of a decoder. If you want to stop immediately, then just call
> streamoff on the OUTPUT queue.

v4l2-compliance currently complains if V4L2_DEC_CMD_STOP_IMMEDIATELY is
not supported:

                fail: v4l2-test-codecs.cpp(104): ret != 0
        test VIDIOC_(TRY_)DECODER_CMD: FAIL

Should testDecoder be changed to set the V4L2_DEC_CMD_STOP_IMMEDIATELY
flag only if node->codec_mask does not have the STATEFUL_DECODER bit
set?

regards
Philipp