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SubjectRe: [PATCH] media: videobuf2: add V4L2_BUF_FLAG_CODECCONFIG flag
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Le mardi 05 juillet 2022 à 13:34 +0100, Dave Stevenson a écrit :
> Hi Ming and Nicolas
>
> On Mon, 4 Jul 2022 at 16:53, Nicolas Dufresne <nicolas@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@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.

I see, the spec needs to be more clear then.

>
> 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.

I did miss the addition of this API, thanks for the reminder. The problem right
now is that things are being added with the needed cross-reference.

>
> 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?

What about:
- Document better that in absence of V4L2_CID_MPEG_VIDEO_HEADER_MODE in a
driver, V4L2_MPEG_VIDEO_HEADER_MODE_JOINED_WITH_1ST_FRAME is to be assumed
- Document that in V4L2_MPEG_VIDEO_HEADER_MODE_SEPARATE, driver should signal
the header buffer with the newly added buffer flag (I would suggest
V4L2_BUF_FLAG_HEADERS_ONLY)
- Document for each support CODECs if V4L2_CID_MPEG_VIDEO_HEADER_MODE can be
supported, and the expected headers and the packing of these
- Document that decoders that didn't implement V4L2_CID_MPEG_VIDEO_HEADER_MODE
should be assumed to only support
V4L2_MPEG_VIDEO_HEADER_MODE_JOINED_WITH_1ST_FRAME decoding (even though it
allowed to support any alignment, even random).
- Cross-reference V4L2_CID_MPEG_VIDEO_HEADER_MODE into the new flag
documentation (whatever this new flag will be called).

>
> 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
> > >
> >

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