Ghetto Brightness Control

This is the second post in the series where I’m trying to switch away from the mac for a while.

One of the things I miss is the ability to adjust brightness of my monitor with the multimedia keyboard keys. When at home, my mac is usually hooked up to a 22" LG UltraFine, which controls its brightness automatically based on the ambient light sensor on the laptop, or I could adjust it by holding ctrl and using the brigthness keys on my MX Keys keyboard. If I plug the laptop into my Dell (which is usually hooked to the Linux desktop), I then use a tiny tool called MonitorControl to control the brightness with the multimedia keys. I can’t do that on the Linux desktop.

I did a bit of research online and the general consensus seems to be that you need a specific kernel extension to make it work for your particular setup. Screw that - there’s gotta be another way. Digging a bit further reveals the way MonitorControl works on the mac is by talking to the monitor with a protocol called DDC/CI, or Display Data Channel / Command Interface. It turns out ddcutil for Linux does just that.

It has a super simple command line interface. If you have more than one screen attached to your computer, you will need to work out which monitor’s brightness you’ll want to control:

$ ddcutil detect

Next run

$ ddcutil capabilities

and make a note of the number of the feature you’d like to control. On my machine brightness is 10, so to change it, the command is simply:

$ ddcutil setvcp 10 50

where the argument after setvcp is the code for the brightness VCP feature, and the next one is the desired brightness value between 0-100. Easy!

Ok, so if there’s a way to run this command every time I hit one of the brightness keys, then my all my troubles are over. I could set up a standard Ubuntu keyboard key assignment, e.g. pressing Meta + + would increase brightness, but I want to use the media keys for that.

If you read carefully, I did mention that my brightness multimedia keys are already moving the brightness slider back and forth. Luckily Pop_OS! stores this slider’s value in a file

$ cat /sys/class/backlight/acpi_video0/brightness

That’s right, try changing the slider either with your mouse or with the keyboard and watch the contents of the file change. Another file stores the maximum value: /sys/class/backlight/acpi_video0/max_brightness. Nice! Let’s put that in a script we can call once we have set our brightness to the desired level.

#!/bin/env ruby
slider_current = File.read('/sys/class/backlight/acpi_video0/brightness')
slider_max = File.read('/sys/class/backlight/acpi_video0/max_brightness')

brightness_max = 100

ratio = Float(slider_current) / Float(slider_max)

brightness = (ratio * brightness_max).to_i

`ddcutil setvcp 10 #{brightness}`

Move the slider and run your script. Sweet! Now all we need is something that will run our script everytime the slider changes. I’ll use a Rust program called watchexec which does exactly what you’d think - it watches a file for changes and runs a command everytime it does. Time to put all that in a systemd service so it runs on the background at boot. Let’s create a new service:

[Unit]
Description=Ghetto brightness control
[Service]
Type=simple
Restart=on-failure
ExecStart=~/.cargo/bin/watchexec -w /sys/class/backlight/acpi_video0/brightness /usr/local/bin/ghetto-brightness-control
[Install]
WantedBy=default.target

You might need to expand ~ to the full home path and obviously substitute with the correct path to your script, but the rest should be more or less the same. Save that under ~/.local/share/systemd/user/ghetto-brightness-control.service and then enable it:

$ sudo systemctl --user enable ghetto-brightness-control.service

Well done! Now you can control the brightness of the monitor either by dragging the slider or tapping the multimedia keys.