ROG Z13 / AMD OLED Brightness Fix

One-line kernel workaround for the amdgpu "custom brightness curve" regression (kernel 6.15+): on AMD OLED / AUX-backlight laptops the last few slider steps near 100% invert — 98% is full bright, 99% is dimmer, 100% is very dark.

Fix: set amdgpu.dcdebugmask=0x40000 (DC_DISABLE_CUSTOM_BRIGHTNESS_CURVE), bypassing the broken firmware brightness curve and restoring linear mapping.

Quick Install

git clone ssh://git@forgejo.fifthdread.com:223/Fifthdread/rog-z13-brightness-fix.git
cd rog-z13-brightness-fix
sudo ./install.sh        # --dry-run to preview without changing anything

Then reboot.

Verify

journalctl -k -b | grep -i 'brightness curve'    # no output = fix active
cat /sys/module/amdgpu/parameters/dcdebugmask     # 0x40000

Slide brightness 90% → 100%: it must increase monotonically and 100% must be full bright.

Uninstall

sudo ./uninstall.sh      # --dry-run to preview

Why this happens

Since kernel 6.15, amdgpu applies a firmware-supplied (ATIF/ACPI) "custom brightness curve" to map the slider to raw panel brightness. The curve data is authored in 8-bit space but applied to 16-bit values and is non-monotonic at the top, so the top slider positions map to lower raw brightness. Kernel log: amdgpu: [drm] Using custom brightness curve. The 0x40000 debug mask disables that curve; AMD added it upstream for exactly this class of problem.

Note: at 100% you now get the panel's true maximum brightness. If the old 98% seemed slightly brighter, that was the broken curve over-driving the panel past its calibrated range — not usable brightness.

How it works / bootloader compatibility

The script writes options amdgpu dcdebugmask=0x40000 to /etc/modprobe.d/amdgpu-brightness-fix.conf and rebuilds the initramfs. Module options apply when the amdgpu driver loads, so this is independent of bootloader — works with GRUB, systemd-boot, and EFISTUB alike (mkinitcpio bundles /etc/modprobe.d/ into the initramfs; dracut and update-initramfs are detected as fallbacks).

Manual alternatives (if you prefer editing the cmdline)

• systemd-boot: append amdgpu.dcdebugmask=0x40000 to the options line in /boot/loader/entries/*.conf
• GRUB: add it to GRUB_CMDLINE_LINUX_DEFAULT in /etc/default/grub, then sudo grub-mkconfig -o /boot/grub/grub.cfg

Affected hardware

AMD laptops with amdgpu where the firmware provides a custom brightness curve — most commonly OLED panels with AUX (DPCD) backlight (/sys/class/backlight/amdgpu_bl* with scale=non-linear, max_brightness=65535). Verified on ASUS ROG Flow Z13 (2025, GZ302, Strix Halo / Radeon 8060S).

References

• drm/amd/display: Add a new dcdebugmask to allow turning off brightness curve
• rohan.ga — Laptop brightness kernel oddity
• Manjaro forum: non-linear backlight since kernel 6.15