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.
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...
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 ' 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=0xffff9874 e0928900 arg2=0xffff arg3=0x0 arg4=0x0 arg5=0x0
Tracing kprobe dce_abm_
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...