https://bugs.kde.org/show_bug.cgi?id=528425
Bug ID: 528425
Summary: Stale/ghosted frames on Intel iGPU monitor when NVIDIA
dGPU is primary render device in a dual-GPU setup
(regression 6.6.5 → 6.8.2)
Classification: Plasma
Product: kwin
Version First 6.8.2
Reported In:
Platform: CachyOS
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: platform-drm
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Target Milestone: ---
STEPS TO REPRODUCE
Dual-GPU system with two monitors: one on the Intel UHD 770 iGPU, one on the
NVIDIA dGPU. Default KWin device selection, no KWIN_DRM_DEVICES set.
Log into Plasma Wayland.
Open any window (e.g. System Settings) on the Intel-connected monitor and
interact with it, or move it around.
OBSERVED RESULT
On the Intel-connected monitor, a stale, offset copy of the window contents
stays on screen, with horizontal line corruption across the affected area
(photo attached). The monitor on the NVIDIA output renders correctly. Opening
Spectacle immediately clears the corruption until it builds up again, which
suggests stale buffers that only a full repaint flushes.
EXPECTED RESULT
Clean rendering on both monitors, as in 6.6.5.
ADDITIONAL INFORMATION
Started right after upgrading from kwin 6.6.5 to 6.8.2. The same update also
brought in Mesa 26.2.6, Qt 6.12.0, and kernel 7.3.1.
The NVIDIA card enumerates first (card1, renderD128) and the Intel card second
(card2, renderD129), so KWin picks NVIDIA as its primary render device and
copies frames to the Intel output.
Workaround: KWIN_DRM_DEVICES=/dev/dri/card2:/dev/dri/card1 (Intel primary,
NVIDIA secondary) fixes it, and both monitors work correctly. Intel only
(KWIN_DRM_DEVICES=/dev/dri/card2) also fixes it, but disables the NVIDIA
monitor.
The bug therefore occurs only when NVIDIA is the primary renderer and frames
are copied to the Intel output.
Not caused by the kernel: it reproduces on both 7.3.1-cachyos and
6.18.55-cachyos-lts.
Not caused by Mesa: Sway on the same Mesa 26.2.6 renders cleanly on the Intel
monitor.
These did not help: KWIN_DRM_NO_DIRECT_SCANOUT=1, KWIN_FORCE_SW_CURSOR=1,
KWIN_USE_OVERLAYS=0, KWIN_DRM_PREFER_COLOR_DEPTH=24, i915.enable_fbc=0.
Side note: KWIN_DRM_DEVICES splits on :, so stable paths like
/dev/dri/by-path/pci-0000:00:02.0-card can’t be used. KWin tries to open the
fragments, fails, and the session comes up black. A udev symlink without colons
works around it.
SOFTWARE/OS VERSIONS
Distro: CachyOS
KDE Plasma: 6.8.2
KDE Frameworks: 6.31.0
Qt: 6.12.0
Kernel: 7.3.1-cachyos (also tested 6.18.55-cachyos-lts)
Mesa: 26.2.6
NVIDIA driver: 615.78.08 (nvidia-open)
Graphics platform: Wayland
iGPU: Intel UHD 770, Alder Lake-S (8086:4680), i915 driver
dGPU: NVIDIA Corporation GB203 [GeForce RTX 5070 Ti] (rev a1)
Monitors: 2560x1080 on Intel output; 3440x1440 on NVIDIA output
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