ddutils on Nixos • thein3rovert

Installation and Configuration

Search nix search for ddcutil package or simple add this command to your nix-configuration pkg config.

 environment.systemPackages = with pkgs; [
  ddcutil
];

Add it to configuration.nix and sudo rebuild

sudo rebuild switch

Verify it its installed by running ddcutil detect command, if you get an error like unable to find /ddc/dev/12c. directory.

sudo ddcutil detect

You should get something like this:

   I2C bus:  /dev/i2c-4
   DRM connector:           card1-HDMI-A-1
   EDID synopsis:
      Mfg id:               SEM - Samsung Electronics Company Ltd
      Model:                DM700A-H
      Product code:         804  (0x0324)
      Serial number:
      Binary serial number: 0 (0x00000000)
      Manufacture year:     2012,  Week: 0

Invalid display
   I2C bus:  /dev/i2c-12
   DRM connector:           card1-eDP-1
   EDID synopsis:
      Mfg id:               BOE - BOE
      Model:
      Product code:         2449  (0x0991)
      Serial number:
      Binary serial number: 0 (0x00000000)
      Manufacture year:     2020,  Week: 33
   This is a laptop display.  Laptop displays support DDC/CI

If you get that error that says you dont have the i2c kernel modules ,we need to load it manually by using the following command

sudo modprobe i2c-dev

you should get an i2c kernel modules 1 - 16 .

You can verify your i2c by usng this command this will list all the i2c needed.

`lc /dev/12c-*`

Then run the i2c detect again, you should get information about all the monitor you’re connected to.

Before setting the brightness, make sure that you know the current brightness you are currently i so make sure you run the command below, this will show you the current brightness if your monitors.

sudo ddcutil getvpc 10`

After that then try increase and decrease the brightness by running the

sudo ddcutil setvcp 10 +100 #Higher number

for increased brightness.

Then

sudo ddcutil setvcp 10 +10 # lower number

or increase brightness.

You can also verify if your monitor supports brightness control by running the

sudo ddcutil capabilities | grep "Feature: 10"

The next thing to DO is to create a udev rules, this gives i2c groups the read and writer permissions so non-root users can access and control the I2C devices and using the tools like ddcutil without having to use sudo everytime. We can do that by using this command.

sudo cp /usr/share/ddcutil/data/60-ddcutil-i2c.rules /etc/udev/rules.d

If you encounter an error in this process it because nixos has a different way of handling file systems and package management so we need to find the share directory. Basically it cannot find the share folder that contains the data we want to copy so we have to find the share folder. After a successful research..ha foufn out the if the share folder is not in the /usr/share/ then it can be found in the /run/current-system/sw/share/ddcutil son then we can run this command again with the correct path.

sudo cp /run/current-system/sw/share/ddcutil/data/60-ddcutil-i2c.rules /etc/udev/rules.d

On other linux distro this approach will work but as we all know as a nix user every thing got to be declarative so what we will have to do in other to solve the error.

sudo cp /run/current-system/sw/share/ddcutil/data/60-ddcutil-i2c.rules /etc/udev/rules.d
cp: cannot create regular file '/etc/udev/rules.d/60-ddcutil-i2c.rules': Read-only file system

We need to add the following line to our configuration.nix files.

services.udev.packages = [ (pkgs.runCommand "custom-udev-rules" { buildInputs = [ pkgs.coreutils ]; } '' mkdir -p $out/lib/udev/rules.d cp ${pkgs.ddcutil}/share/ddcutil/data/60-ddcutil-i2c.rules $out/lib/udev/rules.d/ '') ];

This will add the 60-ddcutil-i2c.rules directly to our udev rule to your NixOS system. After doing that save the changes and run the following command:

sudo nixos-rebuild switch

After rebuilding in other to apply the changes we need to reboot our system but there is a better way to reload the i2c without rebooting out system.

sudo groupadd --system i2c

sudo usermod $USER -aG i2c

You might be wondering why its important to creating i2c group or groups in general, this is because creating groups are important steps that should be taken for managing permissions perfectly on devices and their resources. In relation to the i2c, some devices require group permission in other to access i2c devices, ddcutil needed access to i/dev/i2c-* devices in other to interact with monitor settings.

Now that we have created a group for ic2 we need to verify the group we can do that use the command:

groups $USER

You can also check the i2c group file to see if the user is listed in the group:

grep i2c /etc/group

You should get a result like this:

i2c:x:544:$USER

Next we need to also make sure we can load the i2c-dev automatically, we can do that using this command:

sudo touch /etc/modules-load.d/i2c.conf

sudo sh -c 'echo "i2c-dev" >> /etc/modules-load.d/i2c.conf'

Then we reboot for the changes to take full effect

sudo reboot

Resources

https://www.ddcutil.com/i2c_permissions_using_group_i2c/ https://discourse.nixos.org/t/proper-way-to-access-share-folder/20495 https://github.com/daitj/gnome-display-brightness-ddcutil/blob/master/README.md https://search.nixos.org/packages?channel=24.05&show=gnomeExtensions.brightness-control-using-ddcutil&from=0&size=50&sort=relevance&type=packages&query=ddcutil https://github.com/NixOS/nixpkgs/issues/292049