How should "max bpc" KMS property work?

[Ville Syrjälä]

On Tue, Apr 26, 2022 at 03:38:25PM -0400, Alex Deucher wrote:
> On Tue, Apr 26, 2022 at 4:35 AM Pekka Paalanen <ppaalanen at gmail.com> wrote:
> >
> > Hi all,
> >
> > I'm working on setting HDR & WCG video modes in Weston, and I thought
> > setting "max bpc" KMS property on the connector would be a good idea.
> > I'm confused about how it works though.
> >
> > I did some digging in https://gitlab.freedesktop.org/wayland/weston/-/issues/612
> >
> > Summary:
> >
> > - Apparently the property was originally added as a manual workaround
> >   for sink hardware behaving badly with high depth. A simple end user
> >   setting for "max bpc" would suffice for this use.
> >
> > - Drivers will sometimes automatically choose a lower bpc than the "max
> >   bpc" value, but never bigger.
> >
> > - amdgpu seems to (did?) default "max bpc" to 8, meaning that I
> >   definitely want to raise it.
> >
> > If I always slam "max bpc" to the highest supported value for that
> > property, do I lose more than workarounds for bad sink hardware?
> >
> > Do I lose the ability to set video modes that take too much bandwidth
> > at uncapped driver-selected bpc while capping the bpc lower would allow
> > me to use those video modes?
> 
> You wouldn't lose workarounds for amdgpu, you'd just lose potential
> modes.  The reason we added this feature in the first place was
> because users bought new 4K monitors and the driver capped them at
> 30Hz because we always defaulted to the highest supported bpc.  We got
> tons of bug reports about 4k at 60 not being available and that was due
> to the fact that the bpc was set to something greater than 8.  I'm not
> sure what the right answer is.  It really depends on whether the user
> wants higher bpc or faster refresh rates and possibly additional
> higher res modes.

IMO the right answer is to do mode filtering based on the min bpc.

-- 
Ville Syrjälä
Intel