Display Data Channel... it's the signal that your video card uses to communicate with yours monitors to do things like adjust brightness.

https://en.wikipedia.org/wiki/Display_Data_Channel

Does there exist any hardware knob that you can put between display and displayport cable to adjust brightness? I hate the fiddly menus in monitors and in linux no ddc utils work for me (some "leds" that should include monitor brightness only has keyboard leds)

If you can't control the brightness from Linux when using proper software tools, then it's likely your monitor simply doesn't support the brightness control commands and inserting another device between your GPU and monitor won't achieve anything.

> your monitor simply doesn't support the brightness control commands

Or it does but the state of DDC/CI (and HDMI-CEC) is very sad as implementation is inconsistent and subpar to say the least (from order of commands to timing to subtle difference in required payloads that would make the display reject commands).

A dedicated device would cut the GPU out of the equation, but most likely patching ddc tools can be equally fruitful.

I have an Aten KVM switch in-between, maybe that one is doing something wrong, but it's a good KVM switch otherwise imho

Apologies. I hate when people do that as well.

In addition to the other links posted, ddcutil.org has some more good info: https://www.ddcutil.com/#introduction

Display data channel