with a Lenovo dock (ref 40AF0135EU) and two monitors :
Samsung LU28R55 ;
and old Terra monitor, FHD, which I use in portrait mode.
Problem
I can’t use the two monitors at the same time anymore. I have to unplug the Terra from the dock to be able to output on the Samsung only (with the built-in screen of course).
Investigation so far…
Everything was working perfectly before Christmas : I can’t remember which kernel I was using and in snapper, I only see the last few kernels so… I don’t know which was the last working kernel.
My laptop can output to the two monitors separately (to the Samsung one using an HDMI port, to the Terra one using an adapter USB-C to the DVI port of the screen).
I booted from the latest Linux Mint and everything works fine with the dock : I get the output on the Samsung, the Terra monitors and also on the built-in screen.
So, I guess the problem I encounter has something to do with an update I did while travelling on Christmas holidays (that’s why I only saw there was a problem with my home setup a few days ago).
Would you have any suggestions ? What can I try to pinpoint and hopefully solve the problem ?
P.S. : although I mentioned Wayland, the same problem happens with X11.
Plasma has historically been giving multiple display users lots of problems, and been working lately on solving them. That progress is unofficially making 5.26.x beta software for these users, giving many users more trouble than they’re used to having. In about a month, 5.27 will begin its official beta, and when 5.27 is released these problems will suppose to have been solved.
If you can’t avoid using the dock for a couple of months, you can try disabling KScreen2 in background startup settings, then do any screen manipulation you need to do without involving Plasma, using xrandr (which is what KScreen uses) directly. You can install and use arandr to develop an appropriate xrandr script(s) that can be placed in /etc/X11/xinit/xinitrc.d/ and/or linked to a hotkey(s) for convenient application.
There are many bugs about this in KDE’s bug tracker, where you may find more suggestions.
If you need more help, please provide input/output from running inxi -Gaz in Konsole, with and without using the dock, with as many displays working as you can manage.
Yesterday I stumbled onto something by accident that may affect your dock. Docks can (maybe must?) employ MST to do their job. The 6.1 kernel just received a patch upstream to fix broken MST, so with the next or second next kernel this might just go away.
Do you still have kernel 6.0.12 installed? If yes, does booting it fix this? Do you have a snapshot that contains it to revert to?
Thanks for your very detailed answer. I managed to boot into a snapshot that contained 6.0.10-1-default kernel and, as you suspected, my dock works just fine ! So it was probably the kernel I used before going on holidays.
For now, I’ll just revert to this one snapshot I think, and wait for the next or second next kernel
