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

[Tomasz Figa]

On Wed, Aug 22, 2018 at 11:45 PM Paul Kocialkowski
<paul.kocialkowski@xxxxxxxxxxx> wrote:
>
> Hi,
>
> On Wed, 2018-08-22 at 22:38 +0900, Tomasz Figa wrote:
> > On Wed, Aug 22, 2018 at 10:07 PM Paul Kocialkowski
> > <paul.kocialkowski@xxxxxxxxxxx> wrote:
> > >
> > > Hi,
> > >
> > > On Tue, 2018-08-21 at 13:07 -0400, Nicolas Dufresne wrote:
> > > > Le mardi 21 aoÃt 2018 Ã 13:58 -0300, Ezequiel Garcia a Ãcrit :
> > > > > On Wed, 2018-06-13 at 16:07 +0200, Maxime Ripard wrote:
> > > > > > From: Pawel Osciak <posciak@xxxxxxxxxxxx>
> > > > > >
> > > > > > Signed-off-by: Pawel Osciak <posciak@xxxxxxxxxxxx>
> > > > > > Reviewed-by: Wu-cheng Li <wuchengli@xxxxxxxxxxxx>
> > > > > > Tested-by: Tomasz Figa <tfiga@xxxxxxxxxxxx>
> > > > > > [rebase44(groeck): include linux/types.h in v4l2-controls.h]
> > > > > > Signed-off-by: Guenter Roeck <groeck@xxxxxxxxxxxx>
> > > > > > Signed-off-by: Maxime Ripard <maxime.ripard@xxxxxxxxxxx>
> > > > > > ---
> > > > > >
> > > > >
> > > > > [..]
> > > > > > 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.
> > >
> >
> > Not entirely. One of the purposes of the _SLICE pixel format was to
> > specify it in a way that adds a requirement of providing the required
> > controls by the client.
>
> I think we need to define what we want the stateless APIs (and these
> formats) to precisely reflect conceptually. I've started discussing this
> in the Request API and V4L2 capabilities thread.
>
> > > > 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.
> >
> > The spec says
> >
> > "Offset in bytes to video data in the plane. Drivers must set this
> > field when type refers to a capture stream, applications when it
> > refers to an output stream."
> >
> > which would mean that user space would have to know to reserve some
> > bytes at the beginning for the driver to add the start code there. (Or
> > the driver memmove()ing the data forward when the buffer is queued,
> > assuming that there is enough space in the buffer, but it should
> > normally be the case.)
> >
> > Sounds like a pixel format with full bitstream data and some offsets
> > to particular parts inside given inside a control might be the most
> > flexible and cleanest solution.
>
> I can't help but think that bringing the whole bitstream over to the
> kernel with a dedicated pix fmt just for the sake of having 3 start code
> bytes is rather overkill anyway.
>
> I believe moving the data around to be the best call for this situation.
> Or maybe there's a way to alloc more data *before* the bufer that is
> exposed to userspace, so userspace can fill it normally and the driver
> can bring-in the necessary heading start code bytes before the buffer?

After thinking this over for some time, I believe it boils down to
whether we can have an in-kernel library for turning H264 (and other
codec) header structs back into a bitstream, if we end up with more
than one driver need to do it. If that's fine, I think we're okay with
having just the parsed pixel format around.

Note that I didn't think about this with the Rockchip driver in mind,
since it indeed only needs few bytes.

Best regards,
Tomasz