Created attachment 194459 [details]
bug-kwin-journal

kwin_wayland crashes with SIGSEGV inside
 KWin::DrmOutput::testPresentation
   (atomic test-commit null dereference) on an NVIDIA GeForce RTX 4070.
   Triggers: (1) at Plasma Wayland session startup (initial composite), and
   (2) when a fullscreen OpenGL/Vulkan app (Godot 4.7.1) goes fullscreen —
   the client then loses its display ("X connection to :0 broken").

   ENVIRONMENT
   - OS: Ubuntu 26.04 LTS (Kubuntu), kernel 7.0.0-28-generic
   - KWin: 6.6.5  (package 4:6.6.5-0ubuntu0.1)
   - Plasma: 6.6.5   KDE Frameworks: 6.24.0   Qt: 6.10.2
   - kwin_wayland build-id: c59b9504d168ac869b5482cabd80553bbe764cc6
   - GPU: NVIDIA GeForce RTX 4070 (AD104), single monitor DP-1 @ 1920x1080
   - Driver: 610.43.02, OPEN kernel modules (license: Dual MIT/GPL)
   - libdrm 2.4.131, mesa 26.0.3
   - Kernel cmdline: nvidia-drm.modeset=1
   - SDDM greeter runs Wayland (kwin_wayland --drm)

   REPRODUCIBLE WITH BOTH proprietary AND open NVIDIA 610.43.02 modules
   (identical backtrace) -> not a specific module regression.

   STEPS TO REPRODUCE
   1. Boot into Plasma (Wayland) on RTX 4070.
   2. kwin_wayland crashes during startup (before/at first composite).
      OR
   2'. Log in, launch Godot 4.7.1 (Vulkan), press fullscreen.
   3. kwin_wayland crashes; XWayland client loses display.

   ACTUAL RESULTS
   kwin_wayland SIGSEGV; session restarts or drops to display loss.

   EXPECTED RESULTS
   Stable Wayland session; fullscreen games should not crash kwin.

   BACKTRACE (symbolicated)
   Thread 1 "RenderLoop::frameRequested":
    #12
 KWin::DrmOutput::testPresentation(std::shared_ptr<KWin::OutputFrame>
 const&)  (libkwin.so.6 + 0x53a75f)
    #13 KWin::Compositor::composite(KWin::RenderLoop*)
 (libkwin.so.6 + 0x20a608)
    #15 KWin::RenderLoop::frameRequested(KWin::RenderLoop*)
 (libkwin.so.6 + 0x239e6a)
   SEGV occurs inside the atomic commit path:
    #6..#11 libkwin.so.6 (+0x50ff23 / 0x51586d / 0x543d41 / 0x545db4 /
 0x546099 / 0x5466c9)
      (DrmPipeline::present / atomic test commit)

   WORKAROUND THAT WORKS
   Logging into the Plasma (X11) session is stable (kwin_x11 unaffected).

   NOTES
   - KWIN_DRM_NO_AMS=1 -> blackscreen after login (legacy mode broken on this GPU).
   - KWIN_DRM_USE_MODIFIERS=0 -> blackscreen after login.
   - Attachments: journalctl -b (kwin/drm errors), nvidia-smi -q, drkonqi
 metadata.