Bug 511281

Created attachment 186267 [details]
All output from "startplasma-wayland" launched from tty1 until eventual exit code 0

SUMMARY
With HDMI/DP external monitor(s) plugged in kwin_wayland 6.5.0 on my Snapdragon X Elite ARM64 Asus Vivobook S 15 laptop is unusable: everything looks fine on both monitors without cursor initially if I do nothing, but as soon as I move the mouse even a bit on the built-in display plasmashell disappears and doesn't come back until another SDDM relogin/startplasma-wayland invocation while kwin_wayland is seemingly restarted by kwin_wayland_wrapper.

Sometimes in this second state one frame with the cursor visible on a black background will be stuck on my built-in display and the I can blindly move it fine to my HDMI/DP display, until causing kwin_wayland to close/restart if I try the top-left hot corner. I can only get startplasma-wayland to give up exiting with code 0 if I make the shell quickly crash 5 times in a row by moving my mouse about while it's launching. There's no coredumps or dmesg logs after launching "startplasma-wayland" from a TTY or otherwise.

I already bisected this to https://invent.kde.org/plasma/kwin/-/commit/ecff9f9467 (backends/drm: remove the cursor plane from crtcs). There's nothing output to dmesg post-launch but I could share it too if potentially interesting. Downgrading back to 6.4.5 resolves the issue

With another display I had config as "disable built-in display but keep the external one" and that also is fine in the currently broken kwin 6.5.0, it's just the extended/whatever multi-display case which is completely broken.

STEPS TO REPRODUCE
1. Log into Plasma desktop 6.5.0 on an Asus Vivobook S 15 (Snapdragon X Elite) ARM64 laptop with HDMI (initially or later) plugged in

OBSERVED RESULT
plasmashell & kwin_wayland close on just about any interaction making the mouse visible/move (specifically on the built-in display?)

EXPECTED RESULT
Everything works just fine with only built-in display or external monitors plugged in

SOFTWARE/OS VERSIONS
(available in the Info Center app, or by running `kinfo` in a terminal window)
Linux/KDE Plasma: Chimera Linux aarch64
KDE Plasma Version: 6.5.0
KDE Frameworks Version: 6.19.0
Qt Version: 6.10.0

ADDITIONAL INFORMATION
My kernel tree is https://github.com/SpieringsAE/linux/tree/wip/x1e80100-6.17-rc6 cleanly rebased on https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tag/?h=v6.17.5 running https://paste.c-net.org/knyfhwbpghmy as .config if interesting