https://bugs.kde.org/show_bug.cgi?id=528378
Bug ID: 528378
Summary: Laptop with two built-in screens doesn't control
brightness correctly
Classification: Plasma
Product: plasmashell
Version First 6.8.2
Reported In:
Platform: Other
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: Power management & brightness
Assignee: [email protected]
Reporter: [email protected]
Target Milestone: 1.0
DESCRIPTION
On a laptop with two internal panels, each with its own backlight device,
Plasma's power management (PowerDevil, via org.kde.ScreenBrightness) writes the
same brightness to both backlights as if they were one "Built-in Screen". The
second panel is then exposed as a separate display with software-only (KWin)
dimming. Brightness keys adjust both displays, so the second panel is dimmed
twice (hardware x software) and gets darker faster than the first.
HARDWARE
ASUS Zenbook Duo 2025 (UX8406CA), Intel Arrow Lake-P iGPU, two internal eDP
panels:
- eDP-1 (top): /sys/class/backlight/intel_backlight (max 400), parent
card1-eDP-1
- eDP-2 (bottom): /sys/class/backlight/card1-eDP-2-backlight (max 400), parent
card1-eDP-2
Both backlights work independently: writing 150 to card1-eDP-2-backlight dims
only the bottom panel, and intel_backlight dims only the top panel.
STEPS TO REPRODUCE
1. Use both internal panels (on this model UX8406CA: detach the keyboard so
eDP-2 turns on).
2. org.kde.ScreenBrightness now lists two displays, both labelled "Built-in
Screen" (display0, display1).
3. Set each one to 50% with SetBrightness and read sysfs:
display1 -> 50%: intel_backlight 400 -> 200 AND card1-eDP-2-backlight 400 ->
200
display0 -> 50%: no backlight changes; KScreen reports eDP-2 brightness 0.5
(software dimming)
4. Press the brightness-down key a few times.
OBSERVED RESULT
- One PowerDevil display drives both hardware backlights together.
- The other display (eDP-2) gets KWin software dimming, and
~/.config/kwinoutputconfig.json has "allowSdrSoftwareBrightness": true for
eDP-2 (KWin restores it to true if set to false).
- Brightness keys move both displays by the same step, so at "50%" the top
panel is at 50% and the bottom panel is at about 25% (50% hardware x 50%
software).
EXPECTED RESULT
Each internal panel is controlled through its own backlight device:
intel_backlight for eDP-1 and card1-eDP-2-backlight for eDP-2 (matched via the
DRM connector the backlight device is parented to), with no software dimming on
eDP-2.
SOFTWARE/OS VERSIONS
Operating System: Fedora Linux 44 (KDE Plasma Desktop Edition)
KDE Plasma Version: 6.8.2 (powerdevil-6.8.2-1.fc44, kwin-6.8.2-3.fc44)
KDE Frameworks Version: 6.31.0
Kernel Version: 7.2.13-200.fc44.x86_64 (64-bit)
Graphics Platform: Wayland
ADDITIONAL INFORMATION
Workaround: resetting eDP-2's software dimming to 100% (kscreen-doctor
output.eDP-2.brightness.100) whenever brightness changes makes both panels
match, because the shared hardware write already dims eDP-2.
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