https://bugs.kde.org/show_bug.cgi?id=527895
Bug ID: 527895
Summary: [QtQuickRendererSettings] SceneGraphBackend in
kdeglobals still forces software rendering on Plasma
6, with no UI to see or change it
Classification: Plasma
Product: plasma-integration
Version First 6.8.1
Reported In:
Platform: Arch Linux
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: general
Assignee: [email protected]
Reporter: [email protected]
Target Milestone: ---
Arch, Plasma 6.8.1, qt6-base 6.12.0-3 / qt6-declarative 6.12.0-2,
plasma-integration 6.8.1, nvidia 615.78.08, RTX 5070 Ti, Wayland.
SYMPTOM
plasma-systemmonitor shows: "Fell back to software rendering because hardware
acceleration is not available, and visual glitches may appear. Please check
your graphics drivers."
ROOT CAUSE
~/.config/kdeglobals contained
[QtQuickRendererSettings]
SceneGraphBackend=software
KDEPlasmaPlatformTheme6 contains PlasmaQtQuickSettings::RendererSettings and
calls QQuickWindow::setGraphicsApi(), so the platform integration forces the
software scene graph for every Qt Quick app, overriding the environment.
Removing the section fixed it (verified: launcher path, stock Plasma platform
theme still loaded, libEGL_nvidia mapped, 105 nvidia mappings in
/proc/<pid>/maps, no banner).
WHY IT IS HARD TO FIND
- The setting has no UI on Plasma 6. Plasma 5 had kcmshell5 qtquicksettings
("Rendering backend"); there is no equivalent KCM now, but the key is still
honoured and survives config migration.
- The banner sends users to driver debugging. The driver stack here was fine:
eglinfo on the Wayland platform returns 4.6.0 NVIDIA 615.78.08, vulkaninfo
lists the device, /usr/share/glvnd/egl_vendor.d/10_nvidia.json is present,
device ACLs and app-unit device policy are normal.
- Bisecting the environment makes it look like the Plasma platform theme is
broken: with QT_QPA_PLATFORMTHEME unset the app is software-rendered, with
qt6ct or gtk3 it is GPU-rendered. That is only because the Plasma theme plugin
is the only thing that runs the code applying the key.
- QT_QUICK_BACKEND=opengl / =vulkan have no effect, because the override
happens in C++ after the environment is read.
SEE ALSO
bugs.kde.org #466804 described the same forcing behaviour in 2023 and was
closed with "On Plasma 6 we don't do this anymore" - that refers to Plasma
setting the value automatically. The setting itself is still applied on Plasma
6.
PROPOSAL
Either drop [QtQuickRendererSettings] during config migration / ignore it when
the user did not set it in the current session, or expose the control again.
Separately, the System Monitor banner should name the Qt Quick renderer setting
instead of pointing at the graphics driver when software rendering was
explicitly requested.
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