SUMMARY Hardware: Steam Deck/ROG Ally Dock : Dell WD-19S Monitors: 3 Lenovo Q27q-10 1440p 75 Hz With all three monitors connected (MST and Display Stream Compression) Plasma X11 can output to all three at their native 1440p 75 Hz. When running Plasma Wayland, it's either * 2 1440 75 Hz and the third monitor never initializes video (Does however show up to KScreen) * 1 1440 75 Hz and 2 1080 60 Hz (As if Display Stream Compression wasn't even working to begin with) I have confirmed on Gnome Wayland Mutter that all three monitors do work at their native resolution for Wayland. STEPS TO REPRODUCE 1. Plug in monitors 2. Go to Display Settings 3. Try to configure all three at native reosolution OBSERVED RESULT Acts like DSC isn't available and either 2 work or 3 work at reduced resolution. EXPECTED RESULT All three to work at native resolution (As they do in Gnome) SOFTWARE/OS VERSIONS Linux/KDE Plasma: Arch Linux KDE Plasma Version: 6.0.2 KDE Frameworks Version: 24.0.2 Qt Version: 6.6.2 ADDITIONAL INFORMATION Dmesg from ROG Ally https://gist.github.com/parkerlreed/10da3f1ff55340c5b1f317948ee8f7ac Journalctl (kernel + plasma) from Steam Deck https://gist.github.com/parkerlreed/0b153848d2513d0ad7d8d8696aeffe2f amdgpu thread https://gitlab.freedesktop.org/drm/amd/-/issues/3278
Additional note: Inoperability with Wayland was present on 5.27 as well going back quite a ways. So this isn't new to Plasma 6.
Just tried enabling 2 1440 75 and 1 1080 60 and got this on kernel messages [ 717.144654] amdgpu 0000:09:00.0: [drm] REG_WAIT timeout 1us * 1000 tries - dcn314_dsc_pg_control line:264 [ 717.351967] [drm] Send DSC enable to synaptics Waiting for the monitor and it's just slow enough to be past the timeout?
This is a driver bug; the reason you're not seeing this with Xorg or Gnome Wayland is that they only do 8 bits per color, while KWin does 10. As a workaround you can use the KWIN_DRM_PREFER_COLOR_DEPTH=24 environment variable, which makes KWin drop down to 8bpc as well.
Oh my. Thank you! Is there an existing bug triage for this I can follow?
There is https://gitlab.freedesktop.org/drm/amd/-/issues/2598 already, though I'm not sure if it's the exact same issue