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

[Tomasz Figa <tfiga@chromium.org>]

On Wed, Aug 22, 2018 at 6:16 PM Maxime Ripard <maxime.ripard@bootlin.com> wrote:
>
> Hi,
>
> On Tue, Aug 21, 2018 at 01:58:38PM -0300, Ezequiel Garcia wrote:
> > 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 */
>
> I'm not sure this is something we want to do at that point. libva
> doesn't give the start code, so this is only going to make the life of
> the sane controllers more difficult. And if you need to have the start
> code and parse it, then you're not so stateless anymore.

I might not remember correctly, but Rockchip decoder does some slice
parsing on its own (despite not doing any higher level parsing).
Probably that's why it needs those start codes.

I wonder if libva is the best reference here. It's been designed
almost entirely by Intel for Intel video hardware. We want something
that could work with a wide range of devices and avoid something like
a need to create a semi-stateless API few months later. In fact,
hardware from another vendor, we're working with, also does parsing of
slice headers internally. Moreover, we have some weird
kind-of-stateful decoders, which cannot fully deal with bitstream on
its own, e.g. cannot parse formats, cannot handle resolution changes,
need H264 bitstream NALUs split into separate buffers, etc.

As I suggested some time ago, having the full bitstream in the buffer,
with offsets of particular units included in respective controls,
would be the most scalable thing. If really needed, we could add flags
telling the driver that particular units are present, so one's
implementation of libva could put only raw slice data in the buffers.
But perhaps it's libva which needs some amendment?

Best regards,
Tomasz