Background

My built-in monitor in laptop has a 400 nits max brightness, but it still looks very dark.

Observation

I am using KDE desktop environment, the fn + f5/f6 key means XF86MonBrightnessUp and XF86MonBrightnessDown.
When XF86MonBrightness{Up/Down} keys is received by the linux os, the KDE pops up a tip says the brightness of Built-in Screen is changed.
However, the physical brightness of my monitor changed nothing.

Solution

The xbacklight says the value is only 48%, I have to use xbacklight = 100 to set the brightness to its max.

Edit /etc/default/grub to add acpi_backlight=native as kernel parameter.
Run grub-mkconfig -o /boot/grub/grub.cfg to re-generate a new grub configuration.
Run lsmod | grep asus to check if asus_nb_wmi is loaded.
Reboot the laptop, and now XF86MonBrightness{Up/Down} should work.