A short post detailing the many trials and tribulations of setting brightness on multiple monitors in tandem.

## Background

As regular readers might know, I have a multi-screen setup, which accounts for having touch enabled on my primary laptop screen (detailed here). A failing of this setup was that I was not able to control the brightness of both monitors at the same time.

## Existing Stack

Since I use i3, my brightness control is simply done with bindsym lines as follows1:

bindsym XF86MonBrightnessDown exec light -U 10
bindsym XF86MonBrightnessUp exec light -A 10


Note that to get the right bindsym I use screenkey with the keysyms preference. My software of choice was Unfortunately, my existing setup (with light, since that is in the Arch community repo) did not actually allow dimming external screens arbitarily. To be more exact,

light -h

Usage:
light [OPTIONS] [VALUE]

Commands:
-H, -h      Show this help and exit
-V          Show program version and exit
-L          List available devices
-A          Increase brightness by value
-U          Decrease brightness by value
-T          Multiply brightness by value (can be a non-whole number, ignores raw mode)
-S          Set brightness to value
-G          Get brightness
-N          Set minimum brightness to value
-P          Get minimum brightness
-O          Save the current brightness
-I          Restore the previously saved brightness

Options:
-r          Interpret input and output values in raw mode (ignored for -T)
-s          Specify device target path to use, use -L to list available
-v          Specify the verbosity level (default 0)
0: Values only
1: Values, Errors.
2: Values, Errors, Warnings.
3: Values, Errors, Warnings, Notices.

Copyright (C) 2012 - 2018  Fredrik Haikarainen
This is free software, see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE


Clearly it is possible to target specific devices, but for arbitrary additions this is quite tough. Additionally, the project has been more or less been stuck in maintainence mode for a while now.

### Exposing Brightness

The first hurdle faced in o was actually getting the external monitor to expose the right controls. This is accomplished neatly with ddci-driver found here. As demonstrated (courtesy of the ArchWiki):

# Load module
sudo modprobe ddcci_backlight
# Check that it worked
sudo ddcutil capabilities | grep "Feature: 10"
sudo ddcutil getvcp 10
# Set brightness
sudo ddcutil setvcp 10 70


One of the obvious caveats of this technique is that sudo access or a dedicated polkit agent is required. My preferred method of loading this comes from a comment on the ddcci-driver-linux-dkms page of the AUR:

# Put in /etc/systemd/system/ddcci-backlight.service:
# https://aur.archlinux.org/packages/ddcci-driver-linux-dkms/
# leads to hang on boot with external (HDMI) monitor
# connected to the laptop, so we need to add the module later.

# And ddcci_backlight can't detect monitor during sddm.service startup.

[Unit]
After=multi-user.target
Before=sddm.service

[Service]
Type=oneshot
ExecStart=/usr/bin/modprobe ddcci_backlight
ExecStop=/usr/bin/modprobe --remove ddcci_backlight
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target


This is then activated with a standard systemctl enable ddcci_backlight.service command. At this point, the device interface should be exposed to most backlight controllers.

### Xrandr

This is the most obvious of all methods, and does not even require the ddcci-driver. We will simply tweak the brightness with xrandr.

# Get devices
xrandr | grep " connected"
# Tweak
xrandr --output DP1 --brightness 0.2


Note that this is an in-exact method, since it actually adjusts the gamma value instead, and it effectively tints your screen rather than modifying the brightness.

### Clight

clight is an excellent, highly performant alternative to redshift, but it tends to force the main screen brightness to zero. Nothing which can’t be configured away, but in practice, I work late and tend to turn off the tint anyway. This requires a daemon to be run, as well as needing to be turned on manually for i3. A very elegant additional feature gained by using clight is that external monitors are turned off automagically when lockscreens are activated.

### Brillo

brillo is one of the newer controllers, and is pretty actively developed. The interface is almost exactly like light, and unlike clight there is no need to use a daemon. It meshes perfectly with a key-press based system like i3 and also has controls for keyboard LEDS as well as for smoothed ramping up and down of the brightness. Most importantly, it features an -e flag which sets the brightness across all output devices. Essentially this means my configuration is simply modified to:

bindsym XF86MonBrightnessDown exec brillo -e -U 10
bindsym XF86MonBrightnessUp exec brillo -e -A 10


## Conclusions

tl;dr moving from light to brillo with ddcci-driver-linux-dkms was a fantastic idea.

1. My dotfiles are here ↩︎