I have setup a second Debian Stable partition. Booting this, kernel 4.19.0-6, graphics work fine in 1920x1080. No drm "Link Training Unsuccessful" errors.
But booting old partition, graphics still fail and set 1024x768. I have then compared packages on these 2 partition and cleaned up, I could see xorg packages was missing in old partition. By this I could see a change. Now lightdm would startup in high resolution. But XFCE would not start, giving black display, set monitor to sleep. Clearing files in .cache/sessions and .config/xfce4 of user have some effect on this, now XFCE start fine in 1920x1080. I will now monitor and see if this is stable. Looks like first boot, when monitor is cold, still go for 1024x768. /Torben -- Torben Schou Jensen Swamp Thing Homepage: http://swampthing.dk/~tsj/ Skype: swampthing38