https://bugs.kde.org/show_bug.cgi?id=526345
Bug ID: 526345
Summary: Backlight jumps to 100% after disabling HDR on a
laptop panel while the brightness slider keeps its old
value
Classification: Plasma
Product: kwin
Version First 6.7.5
Reported In:
Platform: Other
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: colour-management
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Target Milestone: ---
SUMMARY
On a laptop's built-in panel, disabling HDR leaves the hardware backlight at
maximum, while the brightness slider keeps its previous value (30%). The
backlight
stays at maximum until the brightness is changed again.
STEPS TO REPRODUCE
1. Laptop built-in panel (eDP) with a hardware backlight (amdgpu_bl1), HDR off,
brightness at 30% (slider 3000/10000, backlight 19661/65535).
2. Enable HDR for the built-in screen (System Settings > Display & Monitor >
Built-in Screen, or `kscreen-doctor output.eDP-1.hdr.enable`).
3. Disable HDR again (System Settings, or `kscreen-doctor
output.eDP-1.hdr.disable`).
OBSERVED RESULT
The panel is at full brightness. The slider still shows 30%. Changing the
brightness by any amount (applet, or SetBrightness over D-Bus) immediately sets
the backlight back to the slider's level.
EXPECTED RESULT
After leaving HDR, the hardware backlight is set to the level the slider shows.
ADDITIONAL INFORMATION
Measured values (slider = org.kde.ScreenBrightness /display0
Brightness/MaxBrightness,
backlight = /sys/class/backlight/amdgpu_bl1/brightness, max 65535):
step slider backlight
0 HDR off (initial) 3000/10000 19661
1 HDR enabled 3000/10000 65535 (expected: HDR dims in
software)
2 HDR disabled, +3 s 3000/10000 65535 <- should be 19661
3 HDR disabled, +10 s 3000/10000 65535
4 HDR disabled, ~20 s 3000/10000 65535
5 after SetBrightness 3100->3000 3000/10000 19661 (correct again)
So on the HDR -> SDR transition KWin does not re-apply the hardware backlight;
it stays at the HDR maximum until the next brightness change.
The machine also exposes a second backlight device, nvidia_0, from the NVIDIA
dGPU (whose own eDP connector is disconnected; the panel is driven by the AMD
iGPU in hybrid mode), in case that affects which backlight KWin drives.
SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch Linux (rolling), kernel 7.2.7-arch1-1
KDE Plasma Version: 6.7.5 (kwin 6.7.5, powerdevil 6.7.5, kscreen 6.7.5)
KDE Frameworks Version: 6.30.0
Qt Version: 6.11.2
Graphics Platform: Wayland
ADDITIONAL HARDWARE INFO
PCSpecialist Recoil 16 AMD (Tongfang X6FR57TY), Ryzen 9 9955HX3D with Radeon
iGPU
(amdgpu, Mesa 26.2.3) driving the internal BOE NE160QDM-NZL panel (2560x1600,
10-bit, 240 Hz); NVIDIA RTX 5070 Ti Laptop (nvidia-open 615.71.09) as a
PRIME render-offload dGPU.
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