https://bugs.kde.org/show_bug.cgi?id=521668
Chrystal M. <[email protected]> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |[email protected] --- Comment #4 from Chrystal M. <[email protected]> --- Date: 2026-06-18 / 2026-06-19 System: Arch Linux, kernel 7.0.12-arch1-1, KDE Plasma (Wayland), AMD GPU (amdgpu), Dell D6000 USB dock (DisplayLink, USB VID 17e9), displaylink 6.2-1, evdi-dkms 1.14.15-1 --- Symptom Following a routine system update that brought in KWin 6.7.0-1, the Dell D6000 dock became non-functional under Plasma Wayland. At login, kwin_wayland entered a crash loop: 10+ restarts over approximately 28 seconds, each terminated by a SIGSEGV in the llvmpipe-3 thread (segfault at c30). Once the crash loop subsided, the dock monitors displayed the SDDM login screen rather than the Plasma desktop. kscreen-doctor -o showed only the internal display (eDP-1); the dock outputs were absent. --- Diagnosis Journal analysis (journalctl -b) confirmed repeated kwin_wayland exits with signal 11, followed by SDDM spawning a new Xorg process (/usr/bin/Xorg :1 -seat seat0 vt2) on each restart. By the time KWin stopped crashing, SDDM's Xorg held DRM master on the evdi devices, so KWin could no longer open them — hence the dock showing the SDDM session instead of Plasma. Root cause: KWin 6.7.0 GpuManager regression KWin 6.7.0 introduced a new GpuManager class (src/core/gpumanager.cpp) that separates GPU roles into "render devices" (cards with renderD* DRM nodes) and "display devices" (KMS/DRM outputs). When pairing a display device with a render device, GpuManager::findCompatibleRenderDevice() searches for a render device on the same bus. evdi (the DisplayLink kernel module) is a DRM_BUS_PLATFORM device and has no renderD* node. The AMD GPU is DRM_BUS_PCI. Because no platform-bus render device exists to pair with evdi, findCompatibleRenderDevice() returns nullptr. This causes the evdi DrmGpu's renderDevice() to be null, which triggers KWin to fall back to software rendering via llvmpipe for the DisplayLink outputs. llvmpipe then crashes during pipeline initialization for those outputs. KWin 6.6.x does not have GpuManager; the regression is entirely contained within 6.7.0. What was ruled out: - options evdi initial_device_count=4 in /etc/modprobe.d/evdi.conf — ensures evdi DRM nodes exist at boot but does not fix the null render device pairing; crash persisted. - SDDM/kwayland configuration — the SDDM-on-dock-displays symptom was a secondary consequence of the crash loop, not an independent cause. --- Fix Downgraded kwin from 6.7.0-1 to 6.6.5-4 using the Arch Linux package archive: `sudo pacman -U https://archive.archlinux.org/packages/k/kwin/kwin-6.6.5-4-x86_64.pkg.tar.zst` Only kwin itself required downgrading. KWin 6.6.5 links against libkdecorations3.so.6 and libKF6*.so.6 sonames that are unchanged in the 6.7.x Plasma packages, so the rest of the Plasma suite remained at 6.7.x without ABI conflict. To prevent the package manager from silently re-upgrading kwin on the next pacman -Syu, kwin was added to IgnorePkg in /etc/pacman.conf: IgnorePkg = kwin After relog, kwin_wayland --version confirmed 6.6.5. All dock displays came up correctly under Plasma Wayland. --- Resolution System is stable with kwin 6.6.5-4 pinned. The upstream bug is tracked as KDE Bugzilla #521668 ("kwin_wayland SIGSEGV when DisplayLink display is connected"). IgnorePkg should be cleared and kwin upgraded once that bug is resolved in a future release. -- You are receiving this mail because: You are watching all bug changes.
