Variable Refresh Rate & flickering screens

[Simon Ser]

On Thursday, March 12, 2020 11:45 AM, Michel Dänzer <michel at daenzer.net> wrote:

> On 2020-03-11 9:09 p.m., Simon Ser wrote:
>
> > 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.
>
> One potential issue I see with this is the cursor, which can feel
> awkward if it only moves at 30 Hz. I wonder if a slew rate which can
> reliably prevent flickering allows the framerate to ramp up quickly
> enough for this not to be annoying.

Thanks for pointing this out, that's a valid concern.

My VRR monitor has a 40-60Hz VRR range. With the current Sway
implementation and drm_monitor [1], I can check that moving my mouse on
the monitor makes the refresh interval go from ~25ms to ~16.6ms (ie.
40Hz to 60Hz). Right now there's no slew rate, so it goes from min to
max instantly. I don't notice any flickering and there's no lag.

Looking at the list of monitors [2] on Wikipedia, it doesn't seem like
there's any monitor supporting less than 35Hz.

We'd need to perform experiments on more hardware, but flickering has
been noticed by our users on higher-end monitors which support 144Hz.
I think the slew rate would prevent going from 35Hz to 144Hz directly,
but probably not from 40Hz to 60Hz. I think that would be acceptable.

If it's not, we can always tweak the minimum refresh rate.

tl;dr need to test on more hardware, but should probably be fine.

[1]: https://github.com/emersion/drm_monitor
[2]: https://en.wikipedia.org/wiki/FreeSync#List_of_supported_monitors