[PATCH] media: videobuf2: add V4L2_BUF_FLAG_CODECCONFIG flag

[Dave Stevenson]

Hi Ming and Nicolas

On Mon, 4 Jul 2022 at 16:53, Nicolas Dufresne <nicolas at ndufresne.ca> wrote:
>
> Le mardi 28 juin 2022 à 10:19 +0800, Ming Qian a écrit :
> > By setting the V4L2_BUF_FLAG_CODECCONFIG flag,
> > user-space should be able to hint decoder
> > the vb2 only contains codec config header,
> > but does not contain any frame data.
> > It's only used for parsing header, and can't be decoded.
>
> This is copied from OMX specification. I think we we import this, we should at
> least refer to the original.
>
> >
> > Current, it's usually used by android.
> >
> > Signed-off-by: Ming Qian <ming.qian at nxp.com>
> > ---
> >  Documentation/userspace-api/media/v4l/buffer.rst | 9 +++++++++
> >  include/uapi/linux/videodev2.h                   | 2 ++
> >  2 files changed, 11 insertions(+)
> >
> > diff --git a/Documentation/userspace-api/media/v4l/buffer.rst b/Documentation/userspace-api/media/v4l/buffer.rst
> > index 4638ec64db00..acdc4556f4f4 100644
> > --- a/Documentation/userspace-api/media/v4l/buffer.rst
> > +++ b/Documentation/userspace-api/media/v4l/buffer.rst
> > @@ -607,6 +607,15 @@ Buffer Flags
> >       the format. Any subsequent call to the
> >       :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl will not block anymore,
> >       but return an ``EPIPE`` error code.
> > +    * .. _`V4L2-BUF-FLAG-CODECCONFIG`:
> > +
> > +      - ``V4L2_BUF_FLAG_CODECCONFIG``
> > +      - 0x00200000
> > +      - This flag may be set when the buffer only contains codec config
> > +    header, but does not contain any frame data. Usually the codec config
> > +    header is merged to the next idr frame, with the flag
> > +    ``V4L2_BUF_FLAG_KEYFRAME``, but there is still some scenes that will
> > +    split the header and queue it separately.
>
> I think the documentation is clear. Now, if a driver uses this, will existing
> userland (perhaps good to check GStreamer, FFMPEG and Chromium ?) will break ?
> So we need existing driver to do this when flagged to, and just copy/append when
> the userland didn't opt-in that feature ?

The commit text says it is for userspace feeding data into a video
decoder, so it's a userspace choice instead of driver.

For encoders there is already V4L2_CID_MPEG_VIDEO_HEADER_MODE [1]
which allows for V4L2_MPEG_VIDEO_HEADER_MODE_SEPARATE or
V4L2_MPEG_VIDEO_HEADER_MODE_JOINED_WITH_1ST_FRAME. FFmpeg selects
_SEPARATE by default [2], whilst the default is normally
_JOINED_WITH_1ST_FRAME.

It does raise the question as to whether all decoders will support
header byte only buffers, and does there need to be a capabilities
flag to denote that it is supported.
And should encoders in V4L2_MPEG_VIDEO_HEADER_MODE_SEPARATE mode set
it on the headers only buffers?

A number of undefined elements of how this should be implemented/used :-(

  Dave

[1] https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/ext-ctrls-codec.html
[2] https://github.com/FFmpeg/FFmpeg/blob/master/libavcodec/v4l2_m2m_enc.c#L196

> >      * .. _`V4L2-BUF-FLAG-REQUEST-FD`:
> >
> >        - ``V4L2_BUF_FLAG_REQUEST_FD``
> > diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> > index 5311ac4fde35..8708ef257710 100644
> > --- a/include/uapi/linux/videodev2.h
> > +++ b/include/uapi/linux/videodev2.h
> > @@ -1131,6 +1131,8 @@ static inline __u64 v4l2_timeval_to_ns(const struct timeval *tv)
> >  #define V4L2_BUF_FLAG_TSTAMP_SRC_SOE         0x00010000
> >  /* mem2mem encoder/decoder */
> >  #define V4L2_BUF_FLAG_LAST                   0x00100000
> > +/* Buffer only contains codec header */
> > +#define V4L2_BUF_FLAG_CODECCONFIG            0x00200000
> >  /* request_fd is valid */
> >  #define V4L2_BUF_FLAG_REQUEST_FD             0x00800000
> >
>