Different product_id

[hkoster1]

The executable lguf_brightness compiled OK (Debian Bullseye) but gave no output.

The lsusb command showed my 23.7-inch LG 4K UltraFine Display to have product_id 0x9a63 for the Display Controls,
while main.cpp has the value 0x9a40 hardcoded in. After correcting this and running the compile again, the executable
worked as expected.

[feiyax]

I think first of all is to see if the monitor is using USB to control brightness, since this project is USB only.

Some monitor uses USB HID monitor control for brightness. Some monitor uses VESA Monitor Control Command Set, whose signal goes through DDC (which is I2C).

A quick way to tell if it's using the VESA MCCS VCP is ddcutil getvcp for detecting a monitor's VCP capability.

I'd suggest to try that out. If it does support VCP, then things like ddcci-driver-linux should work