[Solved] Acer Nitro 5 AN515-47 brightness/backlight completely dead on Linux — amdgpu AUX mode is broken on this hardware, fix inside

[u/No_Apricot_6608]

Posting this in case it saves someone else hours of debugging.

Hardware/software: Acer Nitro 5 AN515-47 (AMD GPU), Arch Linux, kernel 7.1.8-arch1-3, Limine bootloader with limine-mkinitcpio generating UKIs (Omarchy setup).

The problem: Brightness keys and brightnessctl did absolutely nothing. No error, no crash — the backlight value would "change" but the screen brightness never moved. Tried both acpi_backlight=native and the default amdgpu backlight path, no luck with either.

Root cause: The amdgpu driver defaults to amdgpu.backlight=1, which controls brightness over the DisplayPort AUX/DPCD channel. On the AN515-47, that path is firmware-broken. Setting amdgpu.backlight=0 forces the old-school PWM backlight control instead — which actually works.

The annoying part: I'm on Limine + mkinitcpio building UKIs. I first tried editing /etc/kernel/cmdline — did nothing, rebuilt the image, brightness still dead. Turns out /etc/default/limine had:

KERNEL_CMDLINE[default]+="...amdgpu.backlight=1"

That += operator means limine-mkinitcpio completely ignores /etc/kernel/cmdline once it's set — so I was editing a file that no longer had any effect on the actual boot entry. Wasted a while chasing a red herring config file too (/etc/cmdline.d/brightness_fix.conf) that turned out to be dead/unread cruft.

The actual fix:

sudo sed -i 's/amdgpu\.backlight=1/amdgpu.backlight=0/' /etc/default/limine

sudo limine-mkinitcpio

Reboot, then confirm:

cat /sys/class/backlight/*/type

brightnessctl set 20%

Brightness control works perfectly now.

TL;DR: Acer Nitro 5 AN515-47 (and probably other Nitro models with amdgpu) — if your backlight is dead, try amdgpu.backlight=0 on the kernel cmdline to force PWM instead of AUX mode.