https://bugs.kde.org/show_bug.cgi?id=522890
Bug ID: 522890
Summary: SDDM Wayland greeter freezes when a Lenovo USB-C dock
with two external monitors is connected. KWin
repeatedly fails DRM atomic modesetting with ENOSPC.
Switching only SDDM to X11 resolves the problem.
Classification: Plasma
Product: kwin
Version First 6.6.4
Reported In:
Platform: Other
OS: Linux
Status: REPORTED
Severity: major
Priority: NOR
Component: multi-screen
Assignee: [email protected]
Reporter: [email protected]
Target Milestone: ---
Created attachment 194169
--> https://bugs.kde.org/attachment.cgi?id=194169&action=edit
console output of the following command: journalctl -b --no-pager | grep -iE
'sddm-helper-start-wayland.*(atomic|failed to open drm|output configuration|no
outputs)'
**Summary**
SDDM Wayland greeter freezes when a Lenovo USB-C dock with two external
monitors is connected. KWin repeatedly fails DRM atomic modesetting with
`ENOSPC`. Switching only SDDM to X11 resolves the problem.
**System**
* Distribution: Kubuntu 26.04 LTS
* KDE Plasma: 6.6.4
* KWin: 6.6.4
* Graphics platform of the user session: Wayland
* Kernel: 7.0.0-27-generic
* Laptop: Lenovo ThinkPad, model 20Y0S3JS00
* GPU: Intel Tiger Lake integrated graphics, device 9a49
* Kernel driver: i915
* Dock: Lenovo USB-C dock
* Displays: laptop internal eDP display plus two external monitors connected
through the dock
**Steps to reproduce**
1. Boot the laptop with the USB-C dock disconnected.
2. Wait until the SDDM Wayland login screen appears.
3. Connect the Lenovo USB-C dock with both external monitors attached.
4. Initially the mouse cursor can still move.
5. Click or move into the password-entry field.
6. The cursor and greeter display then freeze.
7. Disconnecting the dock makes the internal display responsive again.
The issue also occurs when booting with the dock already connected. In one
occurrence, disconnecting the dock did not recover the greeter and a forced
power-off was required.
**Observed result**
SDDM’s `kwin_wayland` enters a rapid DRM atomic-modeset failure loop:
```
kwin_wayland_drm: Failed to open drm device
kwin_wayland_drm: Atomic modeset test failed! No space left on device
kwin_core: Applying output configuration failed!
```
These messages are repeated hundreds of times while the greeter appears frozen.
The USB keyboard and other dock USB devices are correctly enumerated by the
kernel, so this does not appear to be an input-device failure.
The kernel also reports:
```
i915 0000:00:02.0: [drm] Selective fetch area calculation failed in pipe A
```
**Expected result**
Connecting the dock at the SDDM login screen should activate the external
displays without freezing the greeter or the internal display.
**Tests and workarounds**
* Removing `/var/lib/sddm/.config/kwinoutputconfig.json` did not fix the
problem.
* Setting `KWIN_DRM_USE_MODIFIERS=0` did not fix the SDDM Wayland greeter.
* The normal Plasma Wayland session generally handles the dock after login.
* Switching only the SDDM display server to X11 completely resolves the
problem:
```
[General]
DisplayServer=x11
[X11]
ServerPath=/usr/lib/xorg/Xorg
```
With the X11 SDDM greeter, booting and connecting the dock at the login screen
work normally. The logged-in Plasma session remains Wayland.
**Possibly related reports**
* KDE bug 511490: External monitor receives no input and causes laptop screen
to freeze
* KDE bug 511216: Black screen on Wayland / implicit modifier fallback
Those reports were considered fixed in KWin 6.5.2. This system is running KWin
6.6.4, and the previous `KWIN_DRM_USE_MODIFIERS=0` workaround does not resolve
the SDDM greeter issue, so this may be a regression or a separate
multi-monitor/SDDM variant.
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