Backlight

This page documents methods for controlling backlight (aka screen brightness) and tips to control it via hotkeys.

Kernel native

Some laptops, using a recent enough kernel, will automatically handle increasing and decreasing the backlight using the hot keys. The following tools will allow scripting or controlling the backlight using other means, if desired.

Desktop Environment native

Some desktop environments will handle querying and setting the backlight, including configuring the backlight keys. These include at least: Plasma (KDE) and XFCE. It may be needed to configure or start some desktop environment-specific services.

xbacklight

xbacklight uses X to change the light settings. This can be inconvenient in some situations, e.g. for use with the services.actkbd service, which doesn't know about the X session. It, though, has an history of being more compatible with different hardware, especially newer hardware^{[citation needed]}

To install xbacklight globally, add this to your configuration.nix.

  environment.systemPackages = with pkgs; [ xorg.xbacklight ];

Alternatively, use nix-env -iA nixos.xorg.xbacklight to install it to your user profile.

light

light does not use X to change the light settings. This can be used in situations where the X service isn't available. While it does not use X, it will need some privileges to work.

To enable the use of light, add this to your configuration.nix and make sure that your user is a member of the video group.

  programs.light.enable = true;

The following commands will allow you to test light:

• light -U 30 — the screen should become darker.
• light -A 30 — the screen should become brighter.

Be careful using light -U, as you might turn your backlight completely off! You will not be able to see what you're typing anymore.

brightnessctl

brightnessctl (homepage) is another option, which, like light, would work even without X or on Wayland.

You can use it by simply installing the package. Since brightnessctl supports the systemd-logind API it should work out of the box (i.e. without installing any udev rules or using a setuid wrapper).

The following commands will allow you to test brightnessctl:

• brightnessctl set 5%- - the screen should become darker.
• brightnessctl set 5%+ - the screen should become brighter.

If you get an error like Failed to set brightness: Protocol error, check that you are using the right device. You can get a list of the devices with running brightnessctl -l and then specify the device with -d deviceName.

Example: brightnessctl set 5%- -d intel_backlight

External Monitors

The DDC/CI (I2C) interface may be available to control the brightness of your external monitor. There are two ways to do this:

Via ddcutil

$ sudo modprobe i2c-dev
$ nix-shell -p ddcutil --command "sudo ddcutil --bus=MYBUSNUM setvcp 10 MYBRIGHTNESSPCT"

You can find the correct bus number by examining the paths in /sys/bus/i2c/devices. e.g. to control the DP4 device below use device 17.

$ ls -l /sys/bus/i2c/devices/
lrwxrwxrwx - root  2 Nov 12:34 i2c-13 -> ../../../devices/pci0000:00/0000:00:02.0/drm/card1/card1-eDP-1/i2c-13
lrwxrwxrwx - root  2 Nov 12:34 i2c-14 -> ../../../devices/pci0000:00/0000:00:02.0/drm/card1/card1-DP-1/i2c-14
lrwxrwxrwx - root  2 Nov 12:34 i2c-15 -> ../../../devices/pci0000:00/0000:00:02.0/drm/card1/card1-DP-2/i2c-15
lrwxrwxrwx - root  2 Nov 12:34 i2c-16 -> ../../../devices/pci0000:00/0000:00:02.0/drm/card1/card1-DP-3/i2c-16
lrwxrwxrwx - root  2 Nov 12:34 i2c-17 -> ../../../devices/pci0000:00/0000:00:02.0/drm/card1/card1-DP-4/i2c-17

All together:

nix-shell -p ddcutil --command "sudo ddcutil --bus=17 setvcp 10 65"

You can then bind this to a hotkey or similar. sudo may be avoidable if your user is in the correct group, or udev rules have set the appropriate /sys permissions.

Add i2c-dev to boot.kernelModules and ddcutil to your system packages to make permanent.

Via ddcci-driver

A driver exists which exposes the ddcci control as a more standard backlight device, allowing it to be controlled by standard utilities such as brightnessctl. e.g. when setup:

$ find /sys/class/backlight
/sys/class/backlight
/sys/class/backlight/ddcci17
/sys/class/backlight/intel_backlight

$ brightnessctl --device=ddcci17 set 100%
Updated device 'ddcci17':
Device 'ddcci17' of class 'backlight':
	Current brightness: 100 (100%)
	Max brightness: 100

Get the i2c name:

$ cat /sys/bus/i2c/devices/i2c-17/name                     
AUX USBC4/DDI TC4/PHY TC4

boot.extraModulePackages = with config.boot.kernelPackages; [ ddcci-driver ];
boot.kernelModules = [ "ddcci-backlight" ];
services.udev.extraRules = let
      bash = "${pkgs.bash}/bin/bash";
      ddcciDev = "AUX USBC4/DDI TC4/PHY TC4";                                                                                                      
      ddcciNode = "/sys/bus/i2c/devices/i2c-17/new_device";
    in ''
      SUBSYSTEM=="i2c", ACTION=="add", ATTR{name}=="${ddcciDev}", RUN+="${bash} -c 'sleep 30; printf ddcci\ 0x37 > ${ddcciNode}'"
    '';

Device autodetection is tricky with i2c so we need to trigger it manually with the 0x37 code. The sleep is often unfortunately necessary.

The driver has had some issues with kernels > 6.8, the below overlay should allow it to build if you encounter issues (source).

  nixpkgs.overlays = [
      (self: super: {
        linuxPackages_latest = super.linuxPackages_latest.extend (lpself: lpsuper: {
          ddcci-driver = super.linuxPackages_latest.ddcci-driver.overrideAttrs (oldAttrs: {
            version = super.linuxPackages_latest.ddcci-driver.version + "-FIXED";
            src = pkgs.fetchFromGitLab {
              owner = "ddcci-driver-linux";
              repo = "ddcci-driver-linux";
              rev = "0233e1ee5eddb4b8a706464f3097bad5620b65f4";
              hash = "sha256-Osvojt8UE+cenOuMoSY+T+sODTAAKkvY/XmBa5bQX88=";
            };
            patches = [
              (pkgs.fetchpatch {
                name = "ddcci-e0605c9cdff7bf3fe9587434614473ba8b7e5f63.patch";
                url = "https://gitlab.com/nullbytepl/ddcci-driver-linux/-/commit/e0605c9cdff7bf3fe9587434614473ba8b7e5f63.patch";
                hash = "sha256-sTq03HtWQBd7Wy4o1XbdmMjXQE2dG+1jajx4HtwBHjM=";                                                                      
              })
            ];
          });
        });
      })
    ];

/sys/class/backlight/...

The /sys/class/backlight/*/brightness files are a built-in way to set brightness. Use them e.g. with:

  sudo tee /sys/class/backlight/intel_backlight/brightness <<< 300

to set the brightness to 300, where the maximum is stored in brightness_max. You can set file permissions e.g. with a udev rule, if you don't want to use sudo. Here is an example udev rule, where you will likely have to replace intel_backlight, with the name in your /sys/class/backlight/:

  services.udev.extraRules = ''
    ACTION=="add", SUBSYSTEM=="backlight", KERNEL=="intel_backlight", MODE="0666", RUN+="${pkgs.coreutils}/bin/chmod a+w /sys/class/backlight/%k/brightness"
  '';

Tips

Key mapping

While controlling the backlight via the command line is useful, it would be preferable to control it using key bindings. This is especially true considering most laptops have backlight control keys.

There are two main choices to add key bindings, using a system-level service like actkbd or using an X session tool, either provided by your Desktop environment, Window manager or a tool like xbindkeys.

Depending on the tools that work for controlling the backlight you will be able to choose one of those options.

	light	xbacklight
System service	Yes	With hacks
X session	Yes	Yes

Follows, an example mapping for use with actkbd:

Note: This was verified to work with

• NixOS 18.03 on a Lenovo T440 and
• NixOS 19.09pre173166.373488e6f4c on a Lenovo X240

it may work on other models, but is unconfirmed. See actkbd for details on finding out the proper key bindings. Also check if the path to the light binary is correct.

  programs.light.enable = true;
  services.actkbd = {
    enable = true;
    bindings = [
      { keys = [ 224 ]; events = [ "key" ]; command = "/run/current-system/sw/bin/light -A 10"; }
      { keys = [ 225 ]; events = [ "key" ]; command = "/run/current-system/sw/bin/light -U 10"; }
    ];
  };

OLED Screens

OLED screens do not have a backlight, but their perceived brightness can be changed via xrandr:

• xrandr --output <output> --brightness .5 - dim to 50%
• xrandr --output <output> --brightness 1 - no dimming

See also

• Arch Linux wiki page about the backlight