https://bugs.kde.org/show_bug.cgi?id=525083
Bug ID: 525083
Summary: kwin_wayland fails to open DRM node for second
DisplayLink (evdi) output added during runtime, output
stays connected but blank
Classification: Plasma
Product: kwin
Version First unspecified
Reported In:
Platform: Other
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: core
Assignee: [email protected]
Reporter: [email protected]
Target Milestone: ---
I'm using a Targus docking station.
kwin_wayland fails to open DRM node for second DisplayLink (evdi) output added
during runtime, output stays connected but blank.
Setup:
- Ubuntu 26.04 (kernel 7.0.0-30-generic), Plasma/KWin 6.6.6, Wayland session
- Two external monitors connected via a single Targus USB3 DV4K DOCK w PD100W
(DisplayLink chipset, USB ID 17e9:6008), driven through evdi 1.15.0-193 +
displaylink-driver 6.3.0-48 (DisplayLinkManager userspace daemon)
- Internal GPU is Intel i915 (Raptor Lake-S UHD), with an NVIDIA RTX 4060 Max-Q
also present; the evdi devices are separate DRM cards (not backed by either
GPU)
Bug:
When DisplayLinkManager (re)creates its two evdi DRM devices — e.g. after
`systemctl restart displaylink-driver.service`, or after dock/USB
reconnection — kwin_wayland successfully opens and paints to the first evdi
card, but fails to open the second one:
kwin_core: Failed to open drm node: ""
kwin_scene_opengl: couldn't find dev node for drm device
This happens consistently, not intermittently — across multiple repeated
restarts in the same session, it is always the second card (in kwin's
processing order) that fails, never the first. After the failure:
- /sys/class/drm/cardN-DVI-I-X/status remains "connected"
- /sys/class/drm/cardN-DVI-I-X/enabled becomes/stays "disabled"
- kscreen-doctor -o still reports the output as "enabled" and "connected"
with correct geometry — i.e. kwin's own config/KScreen state is out of
sync with the real DRM enable state
- The monitor shows no signal and stays that way indefinitely (not a
transient race that self-heals)
kwin does not appear to retry opening the DRM node after this initial
failure. The only way found so far to recover the output is a full KWin
session restart (log out/in), which re-enumerates all DRM devices at cold
startup rather than reacting to a live runtime hotplug.
Workarounds tried that do NOT fix it:
- kscreen-doctor output.N.disable followed by output.N.enable (even after
waiting 8+ seconds for udev to fully settle) — this only toggles kwin's
config intent, the DRM `enabled` sysfs attribute stays "disabled"
- Restarting displaylink-driver.service again — re-triggers the exact same
race, can fail again on retry (confirmed failing 2 restarts in a row in
one session)
- Physically unplugging/replugging just the monitor's HDMI cable from the
dock — did not change the outcome
Steps to reproduce:
1. Connect two DisplayLink/evdi-backed monitors via a USB dock to a Wayland
Plasma session
2. Run `sudo systemctl restart displaylink-driver.service` (or physically
unplug/replug the dock) to force both evdi DRM devices to be destroyed
and recreated at once
3. Observe: one monitor gets a picture, the other shows connected-but-blank
4. Check journalctl -b for "kwin_core: Failed to open drm node" occurring
for the second device
5. Confirm via /sys/class/drm/cardN-.../enabled that the broken output is
"disabled" at the kernel level despite kscreen-doctor reporting it enabled
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