with gcc11 forced, kwin works, but same issues with displays; though unsure if I just missed the error in the firehose of errors, but there's those:

kwin_wayland_drm: Atomic commit failed! Permission denied
kwin_wayland_drm: Presentation failed! Permission denied

love to see errno creeping in

from there, found a single useful result, which hinted at:

bool noAMS = qEnvironmentVariableIntValue("KWIN_DRM_NO_AMS", &isEnvVarSet)

which changes the errors to basically be those in a tight loop

kwin_core: Applying KScreen config failed!
kwin_wayland_drm: Setting vrr failed! Permission denied
kwin_wayland_drm: Setting vrr failed! Permission denied
kwin_wayland_drm: Setting vrr failed! Permission denied
kwin_wayland_drm: Setting vrr failed! Permission denied

since it's errno on DRM operations, I suspect the issue might be one of those two:

• kwin assuming too much about what it can do
• the DRM driver(?) exposes features it can't actually do

and since VRR is new in 5.27 I believe, on 5.26 when disabling atomic mode setting

kwin_wayland_drm: Page flip failed: Permission denied
kwin_wayland_drm: Presentation failed! Permission denied

but not in a loop, just once

and since VRR is new in 5.27 I believe, on 5.26 when disabling atomic mode setting

kwin_wayland_drm: Page flip failed: Permission denied
kwin_wayland_drm: Presentation failed! Permission denied

but not in a loop, just ~~once~~ twice (when disconnecting)

and since VRR is new in 5.27 I believe, on 5.26 when disabling atomic mode setting

kwin_wayland_drm: Page flip failed: Permission denied
kwin_wayland_drm: Presentation failed! Permission denied

but not in a loop, just once twice (when disconnecting)