I've installed plasma-workspace-wayland which install kwin-wayland which
installed kwin-wayland-backend-wayland.
When I tried to start up kde plasma in a wayland session, the screen got black
for a few seconds, and then I was back at the login screen (sddm I guess).
No error messages or easily findable logs or anything.
After quite some research I've found ~/.local/share/sddm/wayland-session.log
which explained me that it couldn't find any viable backend. Seems like the
installed kwin-wayland-backend-wayland doesn't work. So I've just tried to
install kwin-wayland-backend-drm and then the sessions was able to properly
start up.
Don't ask my why kwin-wayland-backend-wayland was installed instead of kwin-
wayland-backend-drm. But I remember that I've tried wayland out, maybe 1-1.5
years ago, maybe there were some rest overs?
But I guess for most installations the drm should be preferred. Or at least
mentioned in the description of plasma-workspace-wayland?
Then sddm should show a proper message, which would help a lot, and instead of
only logging to ~/.local/share/sddm/wayland-session.log which is basically
impossible to find, it should also log to journalctl.
Here ~/.local/share/sddm/wayland-session.log:
No backend specified through command line argument, trying auto resolution
FATAL ERROR: could not find a backend
No backend specified through command line argument, trying auto resolution
FATAL ERROR: could not find a backend
No backend specified through command line argument, trying auto resolution
FATAL ERROR: could not find a backend
No backend specified through command line argument, trying auto resolution
FATAL ERROR: could not find a backend
No backend specified through command line argument, trying auto resolution
FATAL ERROR: could not find a backend
No backend specified through command line argument, trying auto resolution
FATAL ERROR: could not find a backend
No backend specified through command line argument, trying auto resolution
FATAL ERROR: could not find a backend
No backend specified through command line argument, trying auto resolution
FATAL ERROR: could not find a backend
No backend specified through command line argument, trying auto resolution
FATAL ERROR: could not find a backend
No backend specified through command line argument, trying auto resolution
FATAL ERROR: could not find a backend
No backend specified through command line argument, trying auto resolution
FATAL ERROR: could not find a backend
startplasmacompositor: Shutting down...
startplasmacompositor: Done.