Long story short. I wanted to control my monitor brightness because reading documents and books in white pages at late night is insufferable. Except, FreeBSD doesn't have ddcutil. There's the i2c command, but good luck remembering all the ports and settings you want, without bricking your monitor by some obscure bug in its firmware (Been known to happen, yes). You can also use the buttons on your monitor, but that's tedious.

This tool wraps around the iicbus driver of the FreeBSD kernel to provide a simple tool to control your monitor brightness on FreeBSD using DDC/I2C. To run it, clone this repo and run make. (You probably need to be root to run it, as it needs to open and read /dev/iic* device nodes)

Usage is simple - Pass the device file to control the brightness, give a value between 0-255 (In many monitors, it may be 0-100 actually. If you don't notice a difference after going from 100 to 180 for example, you have such a monitor) and it should immediately change. Device files are /dev/iic<n> files (There may be multiple!). On my system, /dev/iic2 works. In yours, it may be another. Just run the tool in all the files as root and you should find the one which works.

If you have a laptop, look into backlight(8) instead. This tool works mostly on desktops which aren't usually supported by backlight.

Probably doesn't work on Linux. Try ddcutil instead.