Persistend naming of iGPU in sway wrapper script

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1

In my laptop I have 2 GPUs (dedicated NVIDIA GPU and integrated AMD GPU). If I don’t need high performance or to have a second monitor, I generally prefer to launch Sway with the integrated gpu to save power. This is accomplished by specifying the WLR_DRM_DEVICES environmental variable to the path of the desired card. For instance, to launch sway with the iGPU:

WLR_DRM_DEVICES=/dev/dri/card1 sway

Being /dev/dri/card1 my iGPU.

To launch Sway with the iGPU using greetd, I have this in my configuration.nix:

  services.greetd = let
    sway-igpu = pkgs.writeShellScriptBin "sway-igpu" ''
      export WLR_DRM_DEVICES=/dev/dri/card1 && exec ${pkgs.sway}/bin/sway
    '';
  in {
    enable = true;
    settings = {
      default_session = {
        command = "${pkgs.greetd.tuigreet}/bin/tuigreet --time --cmd ${sway-igpu}/bin/sway-igpu";
        user = "sh4k0";
      };
    };
  };

Which works as intended… Most of the time. Since the /dev/dri/card* names are non-persistent, it occasionally happens that they change and my script no longer works. A workaround for this would be to use /dev/dri/by-path/* as mapping, which however is not possible since wlroots will interpret the colon symbols as separators (with no way of escaping them).

To fix this, I would like to declaratively specify a symlink, say, from /dev/dri/by-path/my-iGPU:000:01 to, say, /tmp/iGPU, and use that as value for WLR_DRM_DEVICES. How could I do that?
I didn’t fine many resources online on how to declare symlinks in NixOS. Could also well be a xy problem, so feel free to prompt for different solutions.

2

You can write a udev rule to give it a predictable name. This one should work: https://github.com/TLATER/dotfiles/blob/master/nixos-modules/nvidia/prime.nix#L41

3

@TLATER, thank you for your reply.
I extracted the relevant lines and added them to my configuration (I manually added the GPU paths):

  services.udev.packages = lib.singleton (
    pkgs.writeTextDir "lib/udev/rules.d/61-gpu-offload.rules" ''
      SYMLINK=="/dev/dri/by-path/pci-0000:01:00.0-card", SYMLINK+="dri/igpu1"
      SYMLINK=="/dev/dri/by-path/pci-0000:06:00.0-card", SYMLINK+="dri/dgpu1"
    ''
  );

After rebuilding, a corresponding package now appeared in my nix store, containing the above udev rules.
However, the symlinks do not appear anywhere in my system. systemd-udevd is (obviously) up and running, and the logs are clean. Is there something wrong with my snippet above?

4

The path is explicitly and deliberately relative in the rule.

5

Yes, that was my guess but I wanted to make sure. But unfortunately find /dev -name "igpu1" gives no results. Is there something else that needs to be done to enable specified udev rules?

6

Apparently udev isn’t applying the rule for some reason. journalctl -u systemd-udevd:

Jul 11 10:20:29 erebus (udev-worker)[788]: nvidia: Process '/nix/store/00zrahbb32nzawrmv9sjxn36h7qk9vrs-bash-5.2p37/bin/bash -c 'mknod -m 666 /dev/nvidiactl c 195 255'' failed with exit code 1.
Jul 11 10:20:30 erebus (udev-worker)[788]: nvidia: Process '/nix/store/00zrahbb32nzawrmv9sjxn36h7qk9vrs-bash-5.2p37/bin/bash -c 'for i in $(cat /proc/driver/nvidia/gpus/*/information | grep Minor | cut -d \  -f 4); do mknod -m 666 /dev/nvidia${i} c 195 ${i}; done'' failed with exit code 1.

I gen easily get rid of the errors by restarting systemd-udevd: in this case logs are clean, but I still don’t see the symlinks in /dev/.
My full NVIDIA config:

{ config, lib, pkgs, ... }:

{
  services.xserver.videoDrivers = [ 
    "amdgpu"
    "nvidia"
  ];
  
  hardware = {
    graphics = {
      enable = true;
    };
    nvidia = {
      open = true;
      modesetting.enable = true;
      powerManagement = {
        enable = false;
        finegrained = false;
      };
      prime = {
        offload = {
          enable = true;
          enableOffloadCmd = true;
        };
        amdgpuBusId = "PCI:1:0:0";
        nvidiaBusId = "PCI:6:0:0";
      };
    };
  };
  services.udev = {
    enable = true;
    packages = lib.singleton (
      pkgs.writeTextDir "lib/udev/rules.d/61-gpu-offload.rules" ''
        SYMLINK=="/dev/dri/by-path/pci-0000:01:00.0-card", SYMLINK+="dri/igpu1"
        SYMLINK=="/dev/dri/by-path/pci-0000:06:00.0-card", SYMLINK+="dri/dgpu1"
      ''
    );
  };
}

(I disabled finegrained powermanagement because enabling it seemingly prevents me from launching sway with the integrated gpu).

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