On Tue, Apr 26, 2022 at 4:35 AM Pekka Paalanen ppaalanen@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@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