Variable Refresh Rate & flickering screens

[Simon Ser]

Hi all,

I've been working on adding VRR support to Sway [1] (a Wayland
compositor). The compositor just sets the VRR_ENABLED property.

This works fine for some screens, but causes flcikering for other
screens as expected [2]. Fixing the flickering is something we've
talked about last XDC [3]. The flickering is caused by physical
limitations of the screen: changing the refresh rate too quickly
results in brightness issues.

The approach taken by xf86-video-amdgpu is to only enable VRR if an app
is fullscreen and not present in a special Mesa blacklist (e.g. Firefox
is in the blacklist because it doesn't render at a fixed interval).

For Wayland, I'd prefer to avoid having a blacklist. I'd like to be
able to use VRR in the general case (not just for fullscreen apps).
A way to fix the flickering would be to implement a slew rate and make
it so refresh rate variations are capped by the slew rate.

The main question is: should this be implemented in the kernel or in
user-space?

I personally think it'd make more sense to implement this in the
kernel. This would de-duplicate the slew rate logic between compositors
and would avoid unnecessarily waking up user-space.

Moreover, it seems Intel hardware supports programming a slew rate
directly. To take advantage of this feature the slew rate needs to be
implemented in the kernel.

Harry, what do you think?

Thanks,

Simon

[1]: https://github.com/swaywm/sway/pull/5063
[2]: https://github.com/swaywm/sway/issues/5076
[3]: https://xdc2019.x.org/event/5/contributions/331/