521333 2026-06-11 12:14:09 +0000 KWin does not properly fallback to drmModeAddFB when drmModeAddFB2 fails 2026-06-19 02:55:23 +0000 1 1 4 Plasma kwin platform-drm unspecified Other Linux RESOLVED FIXED NOR normal --- 1 pavel.ondracka kwin-bugs-null nate xaver.hugl https://invent.kde.org/plasma/kwin/-/commit/ee01169f593151ae7743143227260973c3b1df8b 6.6.6 0 oldest_to_newest 2522341 0 pavel.ondracka 2026-06-11 12:14:09 +0000 DESCRIPTION When drmModeAddFB2 fails, KWin has a fallback path to drmModeAddFB, but that one is guarded by ret == EOPNOTSUPP https://github.com/KDE/kwin/blob/master/src/backends/drm/drm_gpu.cpp#L996, while the kernel returns -EONOTSUPP. This is problematic for some drm drivers on big endian (like for radeon driver) since kernel returns -EOPNOTSUPP for everything unless quirk_addfb_prefer_host_byte_order is set (and radeon does not). #ifdef __BIG_ENDIAN if (!dev->mode_config.quirk_addfb_prefer_host_byte_order) return -EOPNOTSUPP; #endif So the check should probably be more robust, possibly something like: if (ret < 0 && (ret == -EOPNOTSUPP || errno == EOPNOTSUPP)) STEPS TO REPRODUCE 1. Start kwin on a big endian machine with some driver without modifiers that does not have modifiers and quirk_addfb_prefer_host_byte_order. Or hack the kernel to always return -EOPNOTSUPP from drmModeAddFB2 OBSERVED RESULT kwin fails to start since it cannot ge t a framebuffer. EXPECTED RESULT kwin starts SOFTWARE/OS VERSIONS Operating System: linux kwin-6.6.5 2522406 1 bug-janitor 2026-06-11 15:55:07 +0000 A possibly relevant merge request was started @ https://invent.kde.org/plasma/kwin/-/merge_requests/9373 2523446 2 xaver.hugl 2026-06-16 11:39:02 +0000 Git commit 85fa4ee22b57e834505864a20ac13de0a97a1161 by Xaver Hugl. Committed on 16/06/2026 at 11:18. Pushed by zamundaaa into branch 'master'. backends/drm: fix addFB2 fallback Returned error codes are negative, not the actual value of the define. M +1 -1 src/backends/drm/drm_gpu.cpp https://invent.kde.org/plasma/kwin/-/commit/85fa4ee22b57e834505864a20ac13de0a97a1161 2523454 3 xaver.hugl 2026-06-16 12:32:23 +0000 Git commit 9f27237b14432624ee2798dc6866748e96a705dd by Xaver Hugl. Committed on 16/06/2026 at 11:49. Pushed by zamundaaa into branch 'Plasma/6.7'. backends/drm: fix addFB2 fallback Returned error codes are negative, not the actual value of the define. (cherry picked from commit 85fa4ee22b57e834505864a20ac13de0a97a1161) Co-authored-by: Xaver Hugl <xaver.hugl@kde.org> M +1 -1 src/backends/drm/drm_gpu.cpp https://invent.kde.org/plasma/kwin/-/commit/9f27237b14432624ee2798dc6866748e96a705dd 2523697 4 vlad.zahorodnii 2026-06-17 07:13:36 +0000 Git commit ee01169f593151ae7743143227260973c3b1df8b by Vlad Zahorodnii, on behalf of Xaver Hugl. Committed on 17/06/2026 at 06:37. Pushed by vladz into branch 'Plasma/6.6'. backends/drm: fix addFB2 fallback Returned error codes are negative, not the actual value of the define. (cherry picked from commit 85fa4ee22b57e834505864a20ac13de0a97a1161) Co-authored-by: Xaver Hugl <xaver.hugl@kde.org> M +1 -1 src/backends/drm/drm_gpu.cpp https://invent.kde.org/plasma/kwin/-/commit/ee01169f593151ae7743143227260973c3b1df8b