Public bug reported:
Regression from Ubuntu 24.04 (Noble).
In 24.04, bug #2063143 was fixed by adding systemd-udev-settle.service
as a startup dependency of SDDM (package sddm 0.20.0-2ubuntu4.2). This
fix is missing in the Ubuntu 26.04 package.
Problem:
On cold boot, SDDM starts before the AMDGPU DRM driver has finished
initializing. The greeter crashes with HELPER_TTY_ERROR and enters a
rapid restart loop, resulting in a black screen with lit backlight. This
happens on approximately 50-75% of cold boots.
System:
- OS: Kubuntu 26.04 (Resolute)
- Kernel: 7.0.0-15-generic
- SDDM: 0.21.0+git20250502.4fe234b-2ubuntu3
- Mesa: 26.0.3-1ubuntu1
- GPU: AMD Radeon
Evidence:
The 24.04 fix added these lines to sddm.service:
After=systemd-udev-settle.service
Wants=systemd-udev-settle.service
The 26.04 sddm.service does not contain these lines. Verified by
inspecting /usr/lib/systemd/system/sddm.service.
Workaround:
Adding a systemd override works around the race condition:
sudo mkdir -p /etc/systemd/system/sddm.service.d
sudo tee /etc/systemd/system/sddm.service.d/udev-settle.conf << 'EOF'
[Unit]
After=systemd-udev-settle.service
Wants=systemd-udev-settle.service
EOF
sudo systemctl daemon-reload
Expected behavior:
SDDM should wait for DRM drivers to be ready before starting, as it did
in Ubuntu 24.04.
Related bugs:
- #2063143 (24.04, Fix Released)
- #2066275 (24.04 SRU)
- sddm/sddm#1917 (upstream, still open)
** Affects: sddm (Ubuntu)
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/2152106
Title:
SDDM black screen on boot - missing systemd-udev-settle dependency
(regression from 24.04)
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