Comment 12 for bug 1861925

I tried Ubuntu Mate 19.10 live on my testing system (AMD Ryzen 5 2500U with Radeon Vega Graphics - seems to be the same AMD CPU on your Acer), and brightness control works without any problems even though actual_brightness has a fixed and higher value.

The below is the latest function can be ftrace'd before setting brightness on my AMD platform.

https://github.com/torvalds/linux/blob/master/drivers/gpu/drm/amd/display/dc/dce/dce_abm.c#L418

See result of using kprobe-perf (needs to be installed) with writing max (255) to brightness

$ sudo kprobe-perf 'p:dce_abm_set_backlight_level_pwm $arg1 $arg2 $arg3 $arg4 $arg5 '
Tracing kprobe dce_abm_set_backlight_level_pwm. Ctrl-C to end.
             tee-2074 [001] .... 1009.569878: dce_abm_set_backlight_level_pwm: (dce_abm_set_backlight_level_pwm+0x0/0x410 [amdgpu]) arg1=0xffff9874e0928900 arg2=0xffff arg3=0x0 arg4=0x0 arg5=0x0

Note the arg2 = 0xffff (max brightness) and arg5 = 0 (no "use_smooth_brightness")

If you observe the same behaiours but different results, the Acer hardware may have something special that needs attention either from Acer or from AMD...