Arch Linux

icar

Member

Registered: 2020-07-31

Posts: 562

Asus ZenBook | No backlight persistance across reboots

FYI: Model is UM431D.
---
So, I've been digging for an hour now. I started reading the Wiki and found this. I'm using amdgpu, so it looked promising. It exactly happens as it says:

de set. 02 17:44:20 asus-arch-icar systemd[1]: Starting Load/Save Screen Backlight Brightness of backlight:amdgpu_bl0...
de set. 02 17:44:20 asus-arch-icar systemd-backlight[10528]: amdgpu_bl0: Saved brightness 6520 too high; decreasing to 255.
de set. 02 17:44:20 asus-arch-icar systemd[1]: Finished Load/Save Screen Backlight Brightness of backlight:amdgpu_bl0.

I've created the script and the service unit but it doesn't work. I have nothing in /var/lib/systemd/backlight besides platform-asus-nb-wmi:leds:asus::kbd_backlight which is basically the keyboard backlight.

So I started digging inside /sys/devices/ and found /sys/devices/pci0000\:00/0000\:00\:08.1/0000\:03\:00.0/backlight/amdgpu_bl0/brightness which is relevant because not only it reports the screen brightness in an 8-bit digit, which is what systemd needs as explained in the bug the Wiki talks about, but also if echo-ed with a value between 0 and 255 the screen brightness changes accordingly.
[There's also, in the same directory, "actual_brightness" which reports a 16-bit digit and probably what systemd wrongly reads].

My next course of action was to directly edit the systemd service (/lib/systemd/systemd-backlight) to make it read from the file reporting an 8-bit number, but I found out that file is compiled and not plain text...

What would you do in my place? Report upstream? Try to recompile that one service? Maybe create a couple custom services that save the state at poweroff and reads+sets it at boot? I feel that last one a bit cumbersome as the systemd-backlight service is still working.

Last edited by icar (2020-09-02 18:37:04)