Variable Refresh Rate & flickering screens

[Harry Wentland]

On 2020-03-12 7:25 a.m., Simon Ser wrote:
> 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.

Not the main VRR expert and we're still discussing this internally but I
think it'll very much depend on the display whether you'll see flicker
in this case.

The other complication is that for gaming we don't want to use the
cursor as a VRR trigger and only look at page flips in order to allow
for smooth gameplay. For a desktop use-case that's probably not the
right policy.

Harry

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