Black Screen on SDDM start since Plasma 5.12

[mspaulding06]

Issue description

When booting NixOS after updating to KDE 5.12 with the latest nixpkgs commit SDDM no longer starts.

Steps to reproduce

1. Fresh install of NixOS (with KDE and SDDM enabled)
2. Clone nixpkgs repository from GitHub
3. Run nixos-rebuild boot --upgrade -I nixpkgs=/my/path/to/nixpkgs
4. Reboot

Technical details

Hardware Configuration:
Encountered same problem on both VMWare Guest and Thinkpad X220 core-i7.

I had noticed this problem when building the KDE 5.12 packages before they were in the Nix cache and assumed that there was something wrong with my environment at the time. I've now updated to KDE 5.12 using the latest nixpkgs repository commit which pulled in KDE packages from the cache server and the problem still occurs.

Relevent errors from the log:

Feb 15 09:23:11 nixos sddm-greeter[862]: High-DPI autoscaling not Enabled
Feb 15 09:23:11 nixos sddm-greeter[862]: Reading from "/nix/store/ldyf3y8v0wd7h694lpva9h6vd5dksh2w-desktops/plasma5.desktop"
Feb 15 09:23:11 nixos sddm-greeter[862]: Reading from "/nix/store/ldyf3y8v0wd7h694lpva9h6vd5dksh2w-desktops/xterm.desktop"
Feb 15 09:23:11 nixos sddm-greeter[862]: inotify_add_watch("/usr/share/wayland-sessions") failed: "No such file or directory"
Feb 15 09:23:11 nixos sddm-greeter[862]: Connected to the daemon.
Feb 15 09:23:11 nixos sddm-greeter[862]: Loading file:///run/current-system/sw/share/sddm/themes/breeze/Main.qml...
Feb 15 09:23:12 nixos sddm-greeter[862]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:23:12 nixos sddm-greeter[862]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:23:12 nixos sddm-greeter[862]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:23:12 nixos sddm-greeter[862]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:23:12 nixos sddm-greeter[862]: QObject::installEventFilter(): Cannot filter events for objects in a different thread.
Feb 15 09:23:12 nixos sddm-greeter[862]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:23:12 nixos sddm-greeter[862]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:23:12 nixos sddm-greeter[862]: QObject::installEventFilter(): Cannot filter events for objects in a different thread.
Feb 15 09:23:12 nixos sddm-greeter[862]: file:///run/current-system/sw/share/sddm/themes/breeze/components/VirtualKeyboard.qml:20:1: module "QtQuick.VirtualKeyboard" is not installed
Feb 15 09:23:12 nixos sddm-greeter[862]: Adding view for "Virtual1" QRect(0,0 1024x768)
Feb 15 09:23:12 nixos sddm-greeter[862]: QDBusConnection: name 'org.freedesktop.UDisks2' had owner '' but we thought it was ':1.6'
Feb 15 09:23:12 nixos sddm-greeter[862]: Message received from daemon: Capabilities
Feb 15 09:23:12 nixos sddm-greeter[862]: Message received from daemon: HostName
Feb 15 09:23:14 nixos sddm-greeter[862]: Reading from "/nix/store/ldyf3y8v0wd7h694lpva9h6vd5dksh2w-desktops/plasma5.desktop"
Feb 15 09:23:16 nixos sddm-greeter[862]: Message received from daemon: LoginFailed
Feb 15 09:23:16 nixos sddm-greeter[862]: Message received from daemon: LoginFailed
Feb 15 09:23:44 nixos sddm-greeter[862]: The X11 connection broke: I/O error (code 1)
Feb 15 09:23:51 nixos sddm-greeter[874]: High-DPI autoscaling not Enabled
Feb 15 09:23:51 nixos sddm-greeter[874]: Reading from "/nix/store/ldyf3y8v0wd7h694lpva9h6vd5dksh2w-desktops/plasma5.desktop"
Feb 15 09:23:51 nixos sddm-greeter[874]: Reading from "/nix/store/ldyf3y8v0wd7h694lpva9h6vd5dksh2w-desktops/xterm.desktop"
Feb 15 09:23:51 nixos sddm-greeter[874]: inotify_add_watch("/usr/share/wayland-sessions") failed: "No such file or directory"
Feb 15 09:23:51 nixos sddm-greeter[874]: Connected to the daemon.
Feb 15 09:23:51 nixos sddm-greeter[874]: Loading file:///run/current-system/sw/share/sddm/themes/breeze/Main.qml...
Feb 15 09:23:51 nixos sddm-greeter[874]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:23:51 nixos sddm-greeter[874]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:23:51 nixos sddm-greeter[874]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:23:51 nixos sddm-greeter[874]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:23:51 nixos sddm-greeter[874]: QObject::installEventFilter(): Cannot filter events for objects in a different thread.
Feb 15 09:23:51 nixos sddm-greeter[874]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:23:52 nixos sddm-greeter[874]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:23:52 nixos sddm-greeter[874]: QObject::installEventFilter(): Cannot filter events for objects in a different thread.
Feb 15 09:23:52 nixos sddm-greeter[874]: file:///run/current-system/sw/share/sddm/themes/breeze/components/VirtualKeyboard.qml:20:1: module "QtQuick.VirtualKeyboard" is not installed
Feb 15 09:23:52 nixos sddm-greeter[874]: Adding view for "Virtual1" QRect(0,0 1024x768)
Feb 15 09:23:52 nixos sddm-greeter[874]: QDBusConnection: name 'org.freedesktop.UDisks2' had owner '' but we thought it was ':1.6'
Feb 15 09:23:52 nixos sddm-greeter[874]: Message received from daemon: Capabilities
Feb 15 09:23:52 nixos sddm-greeter[874]: Message received from daemon: HostName
Feb 15 09:23:53 nixos sddm-greeter[874]: Reading from "/nix/store/ldyf3y8v0wd7h694lpva9h6vd5dksh2w-desktops/plasma5.desktop"
Feb 15 09:23:53 nixos sddm-greeter[874]: Message received from daemon: LoginSucceeded
Feb 15 09:25:08 nixos sddm-greeter[860]: High-DPI autoscaling not Enabled
Feb 15 09:25:08 nixos sddm-greeter[860]: Reading from "/nix/store/ldyf3y8v0wd7h694lpva9h6vd5dksh2w-desktops/plasma5.desktop"
Feb 15 09:25:08 nixos sddm-greeter[860]: Reading from "/nix/store/ldyf3y8v0wd7h694lpva9h6vd5dksh2w-desktops/xterm.desktop"
Feb 15 09:25:08 nixos sddm-greeter[860]: inotify_add_watch("/usr/share/wayland-sessions") failed: "No such file or directory"
Feb 15 09:25:08 nixos sddm-greeter[860]: Connected to the daemon.
Feb 15 09:25:08 nixos sddm-greeter[860]: Loading file:///run/current-system/sw/share/sddm/themes/breeze/Main.qml...
Feb 15 09:25:09 nixos sddm-greeter[860]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:25:09 nixos sddm-greeter[860]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:25:09 nixos sddm-greeter[860]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:25:09 nixos sddm-greeter[860]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:25:09 nixos sddm-greeter[860]: QObject::installEventFilter(): Cannot filter events for objects in a different thread.
Feb 15 09:25:09 nixos sddm-greeter[860]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:25:09 nixos sddm-greeter[860]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:25:09 nixos sddm-greeter[860]: QObject::installEventFilter(): Cannot filter events for objects in a different thread.
Feb 15 09:25:09 nixos sddm-greeter[860]: file:///run/current-system/sw/share/sddm/themes/breeze/components/VirtualKeyboard.qml:20:1: module "QtQuick.VirtualKeyboard" is not installed
Feb 15 09:25:09 nixos sddm-greeter[860]: Adding view for "Virtual1" QRect(0,0 1024x768)
Feb 15 09:25:09 nixos sddm-greeter[860]: QDBusConnection: name 'org.freedesktop.UDisks2' had owner '' but we thought it was ':1.6'
Feb 15 09:25:09 nixos sddm-greeter[860]: Message received from daemon: Capabilities
Feb 15 09:25:09 nixos sddm-greeter[860]: Message received from daemon: HostName
Feb 15 09:25:45 nixos sddm-greeter[860]: Reading from "/nix/store/ldyf3y8v0wd7h694lpva9h6vd5dksh2w-desktops/plasma5.desktop"
Feb 15 09:25:45 nixos sddm-greeter[860]: Message received from daemon: LoginSucceeded
Feb 15 09:39:16 nixos sddm-greeter[886]: High-DPI autoscaling not Enabled
Feb 15 09:39:16 nixos sddm-greeter[886]: Loading theme configuration from "/run/current-system/sw/share/sddm/themes/breeze/theme.conf"
Feb 15 09:39:16 nixos sddm-greeter[886]: Reading from "/nix/store/71w3sf41c36ci6kdpxsdbp9ch2bnsj2r-desktops/plasma5.desktop"
Feb 15 09:39:16 nixos sddm-greeter[886]: Reading from "/nix/store/71w3sf41c36ci6kdpxsdbp9ch2bnsj2r-desktops/xterm.desktop"
Feb 15 09:39:16 nixos sddm-greeter[886]: inotify_add_watch("/usr/share/wayland-sessions") failed: "No such file or directory"
Feb 15 09:39:16 nixos sddm-greeter[886]: Connected to the daemon.
Feb 15 09:39:17 nixos sddm-greeter[886]: Loading file:///run/current-system/sw/share/sddm/themes/breeze/Main.qml...
Feb 15 09:39:17 nixos sddm-greeter[886]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:39:17 nixos sddm-greeter[886]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:39:17 nixos sddm-greeter[886]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:39:17 nixos sddm-greeter[886]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:39:17 nixos sddm-greeter[886]: QObject::installEventFilter(): Cannot filter events for objects in a different thread.
Feb 15 09:39:17 nixos sddm-greeter[886]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:39:17 nixos sddm-greeter[886]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:39:17 nixos sddm-greeter[886]: QObject::installEventFilter(): Cannot filter events for objects in a different thread.
Feb 15 09:39:17 nixos sddm-greeter[886]: Empty filename passed to function
Feb 15 09:39:17 nixos sddm-greeter[886]: Empty filename passed to function
Feb 15 09:39:17 nixos sddm-greeter[886]: Empty filename passed to function
Feb 15 09:39:17 nixos sddm-greeter[886]: Adding view for "Virtual1" QRect(0,0 1024x768)
Feb 15 09:39:17 nixos sddm-greeter[886]: QDBusConnection: name 'org.freedesktop.UDisks2' had owner '' but we thought it was ':1.6'
Feb 15 09:39:17 nixos sddm-greeter[886]: Message received from daemon: Capabilities
Feb 15 09:39:17 nixos sddm-greeter[886]: Message received from daemon: HostName
Feb 15 09:39:38 nixos sddm-greeter[886]: Reading from "/nix/store/71w3sf41c36ci6kdpxsdbp9ch2bnsj2r-desktops/plasma5.desktop"
Feb 15 09:39:38 nixos sddm-greeter[886]: Message received from daemon: LoginSucceeded
Feb 15 09:47:36 nixos sddm-greeter[860]: High-DPI autoscaling not Enabled
Feb 15 09:47:36 nixos sddm-greeter[860]: Loading theme configuration from "/run/current-system/sw/share/sddm/themes/breeze/theme.conf"
Feb 15 09:47:36 nixos sddm-greeter[860]: Reading from "/nix/store/209rnrb281yaypi033djq5gc646dyv0z-desktops/plasma5.desktop"
Feb 15 09:47:36 nixos sddm-greeter[860]: Reading from "/nix/store/209rnrb281yaypi033djq5gc646dyv0z-desktops/xterm.desktop"
Feb 15 09:47:36 nixos sddm-greeter[860]: inotify_add_watch("/usr/share/wayland-sessions") failed: "No such file or directory"
Feb 15 09:47:36 nixos sddm-greeter[860]: Connected to the daemon.
Feb 15 09:47:36 nixos sddm-greeter[860]: Loading file:///run/current-system/sw/share/sddm/themes/breeze/Main.qml...
Feb 15 09:47:37 nixos sddm-greeter[860]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:47:37 nixos sddm-greeter[860]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:47:37 nixos sddm-greeter[860]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:47:37 nixos sddm-greeter[860]: QObject: Cannot create children for a parent that is in a different thread.
Feb 15 09:47:37 nixos sddm-greeter[860]: QObject::installEventFilter(): Cannot filter events for objects in a different thread.
Feb 15 09:47:37 nixos sddm-greeter[860]: QObject: Cannot create children for a parent that is in a different thread.

I did some research on the error message and found that people have run into this with different versions of KDE but I wasn't able to determine a common root cause. So far I haven't had any luck in fixing it.

Related PR: #34726

[bkchr]

Could you please delete the sddm qml cache dir?
sudo rm -fr /var/lib/sddm/.cache/sddm-greeter/qmlcache

If that fixes it, this belongs to:
#32577

[mspaulding06]

Yes, removing the cache fixed the problem for me.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/sddm-fails-to-start-x11-session-for-non-root-user-in-kde-plasma-using-nvidia-after-upgrading-to-22-11/24139/1