Re: [PATCH RESEND v7 1/2] media: uapi: Add H264 low-level decoder API compound controls.

[Maxime Ripard]

Hi,

On Fri, Apr 05, 2019 at 12:27:48PM -0400, Nicolas Dufresne wrote:
> Le vendredi 05 avril 2019 à 17:15 +0200, Maxime Ripard a écrit :
> > Hi Nicolas,
> >
> > On Thu, Apr 04, 2019 at 11:41:13AM -0400, Nicolas Dufresne wrote:
> > > > > > +    * - __u16
> > > > > > +      - ``pic_width_in_mbs_minus1``
> > > > > > +      -
> > > > > > +    * - __u16
> > > > > > +      - ``pic_height_in_map_units_minus1``
> > > > > > +      -
> > > > >
> > > > > We recently had some reflection with Alex that this is redundant with
> > > > > the width and height in the OUTPUT format. It may also apply to some
> > > > > other fields in these structs. I feel like they should be removed and
> > > > > passed via corresponding generic V4L2 properties - format, selection,
> > > > > etc.
> > > > >
> > > > > The same problem is also present in the MPEG2 controls. In fact, there
> > > > > was a patch already which used some fields from the controls to
> > > > > calculate the destination buffer strides, rather than bytesperline in
> > > > > the format.
> > > > >
> > > > > Since we're in staging, it could be done with a follow-up patch, though.
> > > >
> > > > Just my two cents. I played with some codecs a while back. IIRC some
> > > > specify a "codec" size in addition to the actual picture size, like
> > > > when the encoder does padding to fit the requirements of the codec
> > > > (spec). Is this needed anywhere?
> > >
> > > With state-less encoders, the headers, which contains the crop
> > > information is created by userspace and for state less decoder, the
> > > headers that contains this information is parsed by userspace. So I
> > > believe that in theory, the accelerator does not strictly need to be
> > > aware of the cropped dimensions.
> > >
> > > Another thing, is that there is not guarantied matches between e.g.
> > > depth of the chrome/luma and the final image buffers. Some hardware may
> > > have bandwidth limitation or internal converter and could possibly
> > > decode 10bit data into 8bit buffers.
> > >
> > > A third reason why I would not try and encode this header information
> > > is that there can be multiple PPS/SPS at the same time, and I think
> > > it's confusing if the relevant information to differentiate them is
> > > removed.
> >
> > Sorry if that sounds a bit dumb, but it's not really clear to me if
> > you're arguing for the removal of the data as Tomasz suggests, or if
> > you want to keep them.
> >
> > The first paragrah seems to advocate for the former, but the two
> > others for the latter.
>
> I think the data should stay. As I said, there can be multiple SPS/PPS,
> while there is only one format. That being said, how does SPS/PPS
> activation works ? How do you tell the driver about all the SPS/PPS and
> which one is being activated ?

The current way of dealing with this is that the _SLICE_PARAMS control
actually takes an array of SPS's. And same thing for PPS.

There's no difference between activated and deactivated ones
though. What is the use case for this?

Thanks!
Maxime

--
Maxime Ripard, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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