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SubjectRe: [PATCH 1/9] CHROMIUM: v4l: Add H264 low-level decoder API compound controls.
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Hi,

On Tue, 2018-08-21 at 13:07 -0400, Nicolas Dufresne wrote:
> Le mardi 21 août 2018 à 13:58 -0300, Ezequiel Garcia a écrit :
> > On Wed, 2018-06-13 at 16:07 +0200, Maxime Ripard wrote:
> > > From: Pawel Osciak <posciak@chromium.org>
> > >
> > > Signed-off-by: Pawel Osciak <posciak@chromium.org>
> > > Reviewed-by: Wu-cheng Li <wuchengli@chromium.org>
> > > Tested-by: Tomasz Figa <tfiga@chromium.org>
> > > [rebase44(groeck): include linux/types.h in v4l2-controls.h]
> > > Signed-off-by: Guenter Roeck <groeck@chromium.org>
> > > Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
> > > ---
> > >
> >
> > [..]
> > > diff --git a/include/uapi/linux/videodev2.h
> > > b/include/uapi/linux/videodev2.h
> > > index 242a6bfa1440..4b4a1b25a0db 100644
> > > --- a/include/uapi/linux/videodev2.h
> > > +++ b/include/uapi/linux/videodev2.h
> > > @@ -626,6 +626,7 @@ struct v4l2_pix_format {
> > > #define V4L2_PIX_FMT_H264 v4l2_fourcc('H', '2', '6', '4') /*
> > > H264 with start codes */
> > > #define V4L2_PIX_FMT_H264_NO_SC v4l2_fourcc('A', 'V', 'C', '1') /*
> > > H264 without start codes */
> > > #define V4L2_PIX_FMT_H264_MVC v4l2_fourcc('M', '2', '6', '4') /*
> > > H264 MVC */
> > > +#define V4L2_PIX_FMT_H264_SLICE v4l2_fourcc('S', '2', '6', '4') /*
> > > H264 parsed slices */
> >
> > As pointed out by Tomasz, the Rockchip VPU driver expects start codes
> > [1], so the userspace
> > should be aware of it. Perhaps we could document this pixel format
> > better as:
> >
> > #define V4L2_PIX_FMT_H264_SLICE v4l2_fourcc('S', '2', '6', '4') /*
> > H264 parsed slices with start codes */
> >
> > And introduce another pixel format:
> >
> > #define V4L2_PIX_FMT_H264_SLICE_NO_SC v4l2_fourcc(TODO) /* H264
> > parsed slices without start codes */
> >
> > For cedrus to use, as it seems it doesn't need start codes.
>
> I must admit that this RK requirement is a bit weird for slice data.
> Though, userspace wise, always adding start-code would be compatible,
> as the driver can just offset to remove it.

This would mean that the stateless API no longer takes parsed bitstream
data but effectively the full bitstream, which defeats the purpose of
the _SLICE pixel formats.

> Another option, because I'm not fan of adding dedicated formats for
> this, the RK driver could use data_offset (in mplane v4l2 buffers),
> just write a start code there. I like this solution because I would not
> be surprise if some drivers requires in fact an HW specific header,
> that the driver can generate as needed.

I like this idea, because it implies that the driver should deal with
the specificities of the hardware, instead of making the blurrying the
lines of stateless API for covering these cases.

> >
> > How does it sound?
> >
> > [1]
> > https://cs.chromium.org/chromium/src/media/gpu/v4l2/v4l2_slice_video_decode_accelerator.cc?rcl=63129434aeacf0f54bbae96814f10cf64e3e6c35&l=2438
--
Paul Kocialkowski, Bootlin (formerly Free Electrons)
Embedded Linux and kernel engineering
https://bootlin.com
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