[kwin] [Bug 524007] New: eDP-1 laptop panel brightness stuck at ~98% when slider is set to 100% (regression in 6.7.4)

https://bugs.kde.org/show_bug.cgi?id=524007

            Bug ID: 524007
           Summary: eDP-1 laptop panel brightness stuck at ~98% when
                    slider is set to 100% (regression in 6.7.4)
    Classification: Plasma
           Product: kwin
      Version First 6.7.4
       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 Plasma 6.7.4, the internal laptop panel (eDP-1) brightness is
physically dimmer when set to 100% than when set to 95-99%. Setting
the brightness slider to 100% results in a noticeably darker screen
(~10% visually darker) compared to any value below 100%.

This worked correctly before the update from Plasma 6.7.3 to 6.7.4
(system was updated on 2026-08-07). The issue appears immediately
after every boot/reboot, with the brightness slider showing 100%
but the display looking dimmer than at lower values.

STEPS TO REPRODUCE

1. On a laptop with an AMD GPU and an eDP internal panel, set the
   display brightness slider to 100%.
2. Observe the screen brightness.
3. Set the brightness slider to 95%.
4. Observe that the screen is now visibly brighter than at 100%.

OBSERVED RESULT

`kscreen-doctor -o` reports:
  Brightness control: supported, set to 98% and dimming to 100%
even when the slider/UI shows 100%.

EXPECTED RESULT

Setting brightness to 100% should result in the display at its
actual maximum brightness, matching what `kscreen-doctor -o`
reports.

ADDITIONAL INFORMATION

- Backlight device: /sys/class/backlight/amdgpu_bl1,
  max_brightness=65535
- Manually writing the maximum value directly to
  /sys/class/backlight/amdgpu_bl1/brightness (65535) does NOT
  change the physical screen brightness at all, which suggests the
  dimming happens in KWin's color/brightness pipeline rather than
  at the backlight/sysfs level.
- Already checked and ruled out:
  - AMD ABM: /sys/module/amdgpu/parameters/dcdebugmask = 0
  - /sys/class/drm/*-eDP-1/amdgpu/panel_power_savings = 0
  - HDR is disabled and reported as incapable on this panel
  - Allow EDR: never
  - Toggling "Color power preference" between
    prefer-efficiency and prefer-accuracy in System Settings has
    no effect on this issue
- Workaround: running `kscreen-doctor output.eDP-1.brightness.95`
  after login restores full physical brightness at 95%, confirming
  the panel itself is capable of full brightness — only the 100%
  value in the current pipeline is affected.
- Package versions: plasma-workspace, libkscreen, kwin, and
  kscreenlocker were all upgraded from 6.7.3-1 to 6.7.4-1 on
  2026-08-07, which is when this issue started.

SOFTWARE/OS VERSIONS
Manjaro Linux, KDE Plasma 6.7.4, Wayland session, AMD GPU

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