Description of problem: It seems that the window managers (sddm/gdm) are now activly checking the CanGraphical property from logind. This means they fail to start X on seats that don't have this properly. The efifb device exports a perfectly functional graphical framebuffer which tends to get taken over by native linux drivers (amdgpu/etc). If that doesn't happen then X can go ahead and start on the efifb device anway but with fixed resolution/color depth. AKA switching the DM to xdm allows normal login.
Version-Release number of selected component (if applicable): F29, sddm 0.18.0-1, systemd-udev 239-11. kernel-4.20.10-200
How reproducible: 100% with UEFI-Rpi firmware and latest fedora. I suspect its probably reproducable on any UEFI system where a dedicated kernel module doesn't take over the efifb device. AKA modprobe.blacklist=amdgpu or noveau leaves the efifb in place, and it never gets cangraphcial set.
Steps to Reproduce:
1. Grab RPI+upstream EDK2 firmware for rpi, build/install machine with f29
2. check `loginctl show-seat seat0` CanGraphical=XXX
3. Note `loginctrl seat-status seat0`
graphics:fb0 "EFI VGA"
# loginctl show-seat seat0
Cangraphical should be set on working fbX devices.
BTW: SDDM & the rpi in this config works in a basic F29 install, its only after doing an upgrade does it start to fail. Not sure if it was SDDM that caused the problem or systemd-udev or something else irritating the problem (its not the kernel, booting the older kernel displays the same problem).
udev is now part of systemd
As this problem was reproduced on x86, was causing a lot of problem, I'm going to dupe this against the gdm changes to remove the master-of-seat check.
*** This bug has been marked as a duplicate of bug 1683197 ***