Variable Refresh Rate & flickering screens

[Michel Dänzer]

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.


-- 
Earthling Michel Dänzer               |               https://redhat.com
Libre software enthusiast             |             Mesa and X developer