Re: [PATCHv4 1/2] media: docs-rst: Document memory-to-memory video decoder interface

[Tomasz Figa <tfiga@chromium.org>]

On Wed, Jul 10, 2019 at 5:09 PM Hans Verkuil <hverkuil-cisco@xs4all.nl> wrote:
>
> On 7/3/19 6:58 AM, Tomasz Figa wrote:
> > Hi Hans,
> >
> > On Mon, Jun 3, 2019 at 8:28 PM Hans Verkuil <hverkuil-cisco@xs4all.nl> wrote:
> >>
> >> From: Tomasz Figa <tfiga@chromium.org>
> >>
> >> 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>
> >> Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
> >> ---
> >>  Documentation/media/uapi/v4l/dev-decoder.rst  | 1084 +++++++++++++++++
> >>  Documentation/media/uapi/v4l/dev-mem2mem.rst  |    8 +-
> >>  Documentation/media/uapi/v4l/pixfmt-v4l2.rst  |    5 +
> >>  Documentation/media/uapi/v4l/v4l2.rst         |   10 +-
> >>  .../media/uapi/v4l/vidioc-decoder-cmd.rst     |   41 +-
> >>  5 files changed, 1132 insertions(+), 16 deletions(-)
> >>  create mode 100644 Documentation/media/uapi/v4l/dev-decoder.rst
> >>
> >
> > Thanks a lot for helping with remaining changes.
> >
> > Just one thing inline our team member found recently.
> >
> > [snip]
> >> +Capture setup
> >> +=============
> >> +
> > [snip]
> >> +4.  **Optional.** Set the ``CAPTURE`` format via :c:func:`VIDIOC_S_FMT` on the
> >> +    ``CAPTURE`` queue. The client may choose a different format than
> >> +    selected/suggested by the decoder in :c:func:`VIDIOC_G_FMT`.
> >> +
> >> +    * **Required fields:**
> >> +
> >> +      ``type``
> >> +          a ``V4L2_BUF_TYPE_*`` enum appropriate for ``CAPTURE``.
> >> +
> >> +      ``pixelformat``
> >> +          a raw pixel format.
> >
> > The client should be able to set the width and height as well. It's a
> > quite frequent case, especially in DMA-buf import mode, that the
> > buffers are actually bigger (e.g. more alignment) than what we could
> > get from the decoder by default. For sane hardware platforms it's
> > reasonable to expect that such bigger buffers could be handled as
> > well, as long as we update the width and height here.
>
> I've added this:
>
>      ``width``, ``height``
>          frame buffer resolution of the decoded stream; typically unchanged from
>          what was returned with :c:func:`VIDIOC_G_FMT`, but it may be different
>          if the hardware supports composition and/or scaling.
>
> Is that what you were looking for?
>

Not sure if composition is a requirement here, but I guess it depends
on how we define composition. Most of the hardware today at least
support arbitrary strides (+/- some alignment), but still write the
pixels at (0,0)x(w,h).

In fact, there would be already some composition happening, even
without arbitrary strides, because G_FMT would return values aligned
in some way, but only the visible rectangle would contain meaningful
pixel data.

Best regards,
Tomasz