How should "max bpc" KMS property work?

[Alex Deucher]

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.

Alex

>
> Or, are drivers required to choose a lower-than-usual but highest
> usable bpc to make the requested video mode squeeze through the
> connector and link?
>
> Do I need to implement a fallback strategy in a display server,
> starting from the highest possible "max bpc" value, and if my modeset
> is rejected, repeatedly try with lower "max bpc" setting until it works
> or I'm out of bpc options?
>
>
> Thanks,
> pq