https://bugs.kde.org/show_bug.cgi?id=526519
Bug ID: 526519
Summary: External monitor on NVIDIA (secondary GPU, DP-MST via
TB3 dock) stays black after hotplug; disable/enable
output fixes it
Classification: Plasma
Product: kwin
Version First unspecified
Reported In:
Platform: Other
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: platform-drm
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Target Milestone: ---
Product: kwin / Component: platform-drm
## Environment
- KWin / Plasma 6.6.6, Wayland session, Kubuntu (Ubuntu 26.04.1 LTS), kernel
7.0.0-34-generic
- Hybrid graphics (Lenovo ThinkPad): Intel CometLake-H GT2 [8086:9bc4] (card1,
renders/composites, eDP-1)
+ NVIDIA Quadro RTX 3000 Mobile [10de:1f36] (card2), driver 595.91.07 (open
kernel module)
- Thunderbolt ports are wired to the NVIDIA GPU. Monitor: DELL U3417W
(3440x1440@60) via ThinkPad Thunderbolt 3 Dock (DP-MST, NVIDIA reports it as
DP-0.1, DRM connector card2-DP-5)
- KWin compositing on Intel (OpenGL renderer: Mesa Intel UHD Graphics), so DP-5
is a multi-GPU output
## Steps to reproduce
1. Boot without the dock connected.
2. Log in to Plasma Wayland.
3. Connect the dock (monitor already attached to dock).
## Actual result
- The monitor stays black (it receives a signal but shows no picture).
- Everything reports success: /sys/class/drm/card2-DP-5 is
"connected"/"enabled",
`kscreen-doctor -o` shows DP-5 enabled at [email protected], KWin
supportInformation lists DP-5 Enabled with correct geometry,
nvidia-smi shows display_active=Enabled, dGPU runtime PM status "active".
- No error is logged by KWin or the kernel (only `nvidia-modeset: Got unknown
DPCD revision 0.0 for DELL U3417W (DP-0.1)`, which also appears on boots where
the monitor works).
## Expected result
Picture appears on the external monitor, as it does when the dock is connected
at boot.
## Workaround / key evidence
`kscreen-doctor output.DP-5.disable && kscreen-doctor output.DP-5.enable`
immediately makes the picture appear.
So the first modeset KWin performs on hotplug of the MST output on the
secondary (NVIDIA) GPU leaves the output dark, and a second modeset succeeds.
Side effect: re-enabling resets the output position (both outputs end up at
0,0, i.e. visually mirrored).
## Notes
- Reproducible on several boots (journal: dock connected minutes after boot →
black; dock connected at boot → works).
- Happy to provide `KWIN_DRM_DEBUG`/drm.debug logs if helpful.
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