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

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

On Thu, 2019-01-31 at 13:30 +0100, Hans Verkuil wrote:
> On 1/31/19 11:45 AM, Hans Verkuil wrote:
> > On 1/24/19 11:04 AM, 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
> > > 
> > 
> > <snip>
> > 
> > > +4.  **This step only applies to coded formats that contain resolution information
> > > +    in the stream.** Continue queuing/dequeuing bitstream buffers to/from the
> > > +    ``OUTPUT`` queue via :c:func:`VIDIOC_QBUF` and :c:func:`VIDIOC_DQBUF`. The
> > > +    buffers will be processed and returned to the client in order, until
> > > +    required metadata to configure the ``CAPTURE`` queue are found. This is
> > > +    indicated by the decoder sending a ``V4L2_EVENT_SOURCE_CHANGE`` event with
> > > +    ``V4L2_EVENT_SRC_CH_RESOLUTION`` source change type.
> > > +
> > > +    * It is not an error if the first buffer does not contain enough data for
> > > +      this to occur. Processing of the buffers will continue as long as more
> > > +      data is needed.
> > > +
> > > +    * If data in a buffer that triggers the event is required to decode the
> > > +      first frame, it will not be returned to the client, until the
> > > +      initialization sequence completes and the frame is decoded.
> > > +
> > > +    * If the client sets width and height of the ``OUTPUT`` format to 0,
> > > +      calling :c:func:`VIDIOC_G_FMT`, :c:func:`VIDIOC_S_FMT`,
> > > +      :c:func:`VIDIOC_TRY_FMT` or :c:func:`VIDIOC_REQBUFS` on the ``CAPTURE``
> > > +      queue will return the ``-EACCES`` error code, until the decoder
> > > +      configures ``CAPTURE`` format according to stream metadata.
> > 
> > I think this should also include the G/S_SELECTION ioctls, right?
> 
> I've started work on adding compliance tests for codecs to v4l2-compliance and
> I quickly discovered that this 'EACCES' error code is not nice at all.
> 
> The problem is that it is really inconsistent with V4L2 behavior: the basic
> rule is that there always is a format defined, i.e. G_FMT will always return
> a format.
> 
> Suddenly returning an error is actually quite painful to handle because it is
> a weird exception just for the capture queue of a stateful decoder if no
> output resolution is known.
> 
> Just writing that sentence is painful.
> 
> Why not just return some default driver defined format? It will automatically
> be updated once the decoder parsed the bitstream and knows the new resolution.
> 
> It really is just the same behavior as with a resolution change.
> 
> It is also perfectly fine to request buffers for the capture queue for that
> default format. It's pointless, but not a bug.
> 
> Unless I am missing something I strongly recommend changing this behavior.

I just wrote the same in my reply to Nicolas, the CODA driver currently
sets the capture queue width/height to the output queue's crop rectangle
(rounded to macroblock size) without ever having seen the SPS.

regards
Philipp