LightDM Greeter: Laptop monitor unused when using an eGPU

[T-X]

Hardware description:

• Lenovo T14s AMD Gen3, iGPU: ATI/AMD Radeon 680M, Thunderbolt 3 via its 40gbit/s USB-C / USB4 port
• eGPU enclosure: Razer Core X Chroma -> USB-C / Thunderbolt 3
• GPU in eGPU enclosure: ATI/AMD Radeon RX 6650 XT

Software description:

• Linux Distribution: Debian Sid
• LightDM 1.26.0-8, lightdm-gtk-greeter: 2.0.8-2+b1
• egpu-switcher 0.19.0

When I add a Lenovo Thunderbolt 3 docking station with a DisplayPort monitor attached then the LightDM greeter appears on both the DP monitor and the laptop's eDP display, as expected.

However when adding an eGPU then I only see the LightDM greeter on any monitors connected to the eGPU, but not on the laptop's own eDP display anymore. The laptop's eDP display stays black during the LightDM greeter. Which is an issue in particular when I have nothing or just a VR headset (which has the X11 property "non-desktop 1" set by default and therefore is not shown in arandr/lxrandr etc.) connected to the eGPU.

On the other hand if I set "autologin-user=" in /etc/lightdm/lightdm.conf the login works fine. And on KDE both the eDP-1-1 display and if present any monitor connected to the eGPU is enabled by default. On the xmonad tiling window manager (my main, default window manager) by default only any eGPU connected display is enabled, but I can reenable the laptop's eDP display from a VT via xrandr as described here or via custom xmonad login autostart scripts. So this seems to confirm that the LightDM greeter works fine in general with en eGPU enabled, but just does not enable the laptop's eDP display, which is connected to the ATI/AMD Radeon 680M iGPU by default.

My typical procedure to enable the eGPU is to first boot the laptop normally and login to xmonad with no eGPU connected. Then connect the eGPU via the USB-C cable. And then run:

$ sudo egpu-switcher switch --override
$ sudo /etc/init.d/lightdm restart

The --override is necessary when I have no or only the VR headset connected to the eGPU. egpu-switcher then creates the following X11 config for me:

$ cat /etc/X11/xorg.conf.d/99-egpu-switcher.conf
# autogenerated by egpu-switcher

Section "Module"
    Load           "modesetting"
EndSection


Section "Device"
    Identifier     "Device0"
    Driver         "amdgpu"
    BusID          "PCI:7@0:0:0"
    Option         "AllowEmptyInitialConfiguration"
    Option         "AllowExternalGpus" "True"
EndSection

The issue was originally reported here first: hertg/egpu-switcher#106