Re: [PATCH] media: uapi: h264: clarify num_ref_idx_l[01]_(default_)active fields

[Paul Kocialkowski <paul.kocialkowski@bootlin.com>]

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On Sat 05 Oct 19, 22:54, Tomasz Figa wrote:
> On Sat, Oct 5, 2019 at 10:39 PM Paul Kocialkowski
> <paul.kocialkowski@bootlin.com> wrote:
> >
> > Hi,
> >
> > On Sat 05 Oct 19, 17:22, Tomasz Figa wrote:
> > > On Fri, Oct 4, 2019 at 6:12 AM Paul Kocialkowski
> > > <paul.kocialkowski@bootlin.com> wrote:
> > > >
> > > > Hi,
> > > >
> > > > On Thu 05 Sep 19, 13:42, Philipp Zabel wrote:
> > > > > To explain why num_ref_idx_active_override_flag is not part of the API,
> > > > > describe how the num_ref_idx_l[01]_active_minus1 fields and the
> > > > > num_ref_idx_l[01]_default_active_minus1 fields are used, depending on
> > > > > whether the decoder parses slice headers itself or not.
> > > >
> > > > Is there any particular reason why this is preferable to exposing the flag?
> > > > It feels like having the flag around sticks closer to the bitstream,
> > > > so it's more straightforward for everyone.
> > > >
> > > > In case there's only one set of fields exposed by the hardware (and it doesn't
> > > > do slice parsing itself), we could always check the flag in the driver and use
> > > > either the default PPS values or the slice-specific values there.
> > > >
> > > > What do you think?
> > >
> > > IMHO it would only add further logic to the drivers, because they
> > > would need to have a conditional block that selects default or
> > > per-slice value based on the flag. The goal was to be able for the
> > > driver to just passively write num_ref_idx_l[01]_active_minus1 (or the
> > > default one for slice-parsing hardware) to corresponding hardware
> > > registers.
> >
> > Well, the Allwinner block has both set of fields and a field for the flag,
> > so in this case I find that it is cleaner to just copy the values and flag
> > rather than always setting the flag even when it's the default value used.
> >
> > More generally, the two sets + flag are closer to bitstream which feels less
> > confusing than re-purposing these fields from the slice header to fit the
> > result of flag ? per-slice : per-picture.
> >
> > > We're talking about kernel drivers here and for security reasons any
> > > logic should be reduced to the strictly necessary minimum.
> >
> > I definitely agree that bitstream parsing in the kernel is to be avoided for
> > security reasons (among other things), but don't see the harm in considering
> > the flags in-driver if needed. We do it for a number of other flags already
> > (and strongly need to).
> 
> If the fields are well documented, does it really matter? I'd suggest
> just keeping it as is, rather than overpolishing things and preventing
> the interface from stabilizing.

I just don't see the benefit of changing the purpose of a bitstream element.
Even with documentation, it adds some unnecessary confusion and I find this to
be a complicated enough topic without it ;)

Especially for the case of hardware that does slice header parsing itself, it
feels particularly unsettling to have to take the default PPS values for the
fields from the slice header control rather than PPS.

The bottomline is that we have use cases for each of the two set of fields
independently, so I feel like this is reason enough to avoid mixing them
together.

We are still in the process of polishing the API anyway, so now feels like a
good time to change these things.

Cheers,

Paul

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