Hi,
I have a system with a GeForce 1050 Ti on bullseye.
On this system, if I log in as a regular user and run startx, everything
works fine; KDE Plasma comes up and it's all good.
But sddm doesn't work. In fact, when it starts, it causes my monitor to
go "no signal". Oddly, though, if I can log in blindly, then once I hit
enter after putting in my password, KDE will come up and work like it
should.
I also tried lightdm and xdm. Both of them also had "no signal" when
starting.
It is using the nouveau driver. There are no errors in Xorg.0.log,
journalctl, dmesg, syslog, or the xsession log. lspci doesn't show any
other graphics adapter. xrandr on the sddm session shows it detected
the appropriate output at the appropriate resolution. Xorg.0.log looks
completely appropriate; detecting devices, setting them up, etc.
If I boot the same drive on a different box with Intel graphics, sddm
works fine.
This is a fresh bullseye install.
A am utterly baffled; I'd think at least xdm should work!
I had something similar happen with Debian 8 a long time ago. It turned out to be because the driver/firmware/grafx-chipset had the wrong idea about the monitor's available resolutions and modes. It was using too high a resolution.