Hardware: ASUSTeK COMPUTER INC. ROG Strix G512LV_G512LV/G512LV, BIOS G512LV.314 (04/27/2021)
Keyboard: USB ID 0b05:1866 "ASUSTeK Computer, Inc. N-KEY Device" (the per-key
RGB keyboard controller used on several ROG Strix G-series laptops).

Since a recent kernel update, I can no longer increase/decrease the keyboard
backlight brightness (Fn+F3/F4, and `asusctl -k/-n/-p`). This is specifically
the brightness-*level* control — the separate Aura per-key RGB interface
(colours/effects, and the keyboard-LEDs-on/off power-state toggle via
`asusctl led-pow-1 -a`) still works fine, since that goes through a
different HID path that initializes successfully. It's only the simple
backlight brightness stepping that's broken.

The in-kernel hid-asus driver fails to initialize that backlight at boot:

    asus 0003:0B05:1866.0006: using HID for asus::kbd_backlight
    asus 0003:0B05:1866.0006: Asus failed to request functions: -75
    asus 0003:0B05:1866.0006: Failed to initialize backlight.