https://bugs.kde.org/show_bug.cgi?id=383023
Bug ID: 383023
Summary: Firefox and mpv stutter with 75Hz on Intel Skylake IGP
Product: kwin
Version: 5.10.3
Platform: Archlinux Packages
OS: Linux
Status: UNCONFIRMED
Severity: normal
Priority: NOR
Component: compositing
Assignee: kwin-bugs-n...@kde.org
Reporter: tempel.jul...@gmail.com
Target Milestone: ---
Kernel 4.12, Mesa 17.1, Plasma 5.10.3 (5.10.4 wasn't available at the time of
testing, but can retry with this version), Firefox 54, mpv 0.25.0 (was the
current version at that time)
I got an old LCD 1280x1024 display connected via DVI which supports both 60Hz
and 75Hz.
When I set it to 75Hz, automatic scrolling in Firefox (can be enabled in the
options for 3rd mouse button, full OGL acceleration via about:config) is
stuttery. It looks very much like it was scrolling with 60fps, which naturally
looks stuttery at 75Hz display refreshrate. This stuttering disappears when I
set the display to 60Hz.
This is not an issue of Firefox but Plasma's compositor since the stuttering
with 75Hz disappears when I disable Plasma's compositor. It runs with totally
fluid 75fps @ 75Hz then.
mpv also suffers some stuttering, even with 60Hz and 60fps video. This issue
disappears as well with disabled Plasma compositing.
I also tried a custom edid file for the display which always forces it to 75Hz
(60Hz aren't reported as available then), that unfortunately didn't help.
Plasma's compositing of moving windows looks totally smooth to me on the
contrary (also without the custom edid and normally selected 75Hz in Plasma
System Settings), so it "only" seems to be a problem with the content of
windows, not the windows themselves (but I'm just guessing).
Now the thing is: When I enforce KWIN_USE_INTEL_SWAP_EVENT, every mentioned
problem disappears. Both Firefox and mpv are absolutely smooth then at any
refreshrate, and so is KWin compositing still in general.
Unfotunately, the lock-up bug described on the environment variables help site
still exists. This seems to usually hapen when the compositing gets turned off,
so probably simply not allowing applications to turn off compositing could work
well enough.
However, it of course would be better if everything worked right ootb and
without drawbacks.
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