Allow configuration of the Nixpkgs instance on a per-system basis

[ndam-hexagon]

Description

This PR aims to allow setting some of the attribute of the config attribute set that is normally passed as argument when importing Nixpkgs.

An example that is currently not supported (but would be after this PR):

{
  pkgs = import nixpkgs {
    inherit system;
    overlays = [ ... ];
    config = {
      allowUnfree = true;
      allowBroken = true;
      cudaSupport = true;
      cudaCapabilities = [
        "7.5"
        "8.6"
        "8.9"
      ];
    };
  };
}

Context

We have a project where we work with CUDA and override various packages (e.g. OpenCV) to have CUDA support enabled. Currently, we are using devenv with the flake integration, but would like to now migrate to using the devenv command and devenv.yaml/devenv.nix to benefit from faster load times.

We also have the added complexity of supporting 2 architectures: x86 and aarch64 with incompatible sets of CUDA capabilities. We therefore needed to patch devenv in order to allow us to configure those settings on a per-system basis.

Changes in a nutshell

This PR modifies the config structure of the devenv.yaml file in the following ways:

• Add a top-level element to enable/disable CUDA support for nixpkgs globally: cudaSupport
• Add a new top-level element: config that can be used to configure on a per-system basis how devenv later configures its Nixpkgs instance

Here's an example of this new format for the config file:

# ... usual config

config:
  x86_64-linux:
    allowUnfree: true
    allowBroken: true
    cudaSupport: true
    cudaCapabilities:
      - "7.5"
      - "8.6"
      - "8.9"
  aarch64-linux:
    allowUnfree: true
    allowBroken: true
    cudaSupport: true
    cudaCapabilities:
      - "8.7"

The devenv/src/flake.tmpl.nix was modified in such a way that the config.<system>.<keys> take precedence over <keys> if present, but should not otherwise have an impact on existing configurations.

[domenkozar]

Hey! I'd change a bit the way the config is structured:

nixpkgs:
  allowUnfree: true
  allowBroken: false
  
  per-platform:
    x86_64-linux:
      allowUnfree: true
      allowBroken: true
    aarch64-darwin:
      allowUnfree: false

[ndam-hexagon]

Hey! I'd change a bit the way the config is structured:

nixpkgs:
  allowUnfree: true
  allowBroken: false
  
  per-platform:
    x86_64-linux:
      allowUnfree: true
      allowBroken: true
    aarch64-darwin:
      allowUnfree: false

Sure. I'll make those changes and push a new commit. Are you otherwise ok with how I structured the changes?

[domenkozar]

Hey! I'd change a bit the way the config is structured:

nixpkgs:
  allowUnfree: true
  allowBroken: false
  
  per-platform:
    x86_64-linux:
      allowUnfree: true
      allowBroken: true
    aarch64-darwin:
      allowUnfree: false

Sure. I'll make those changes and push a new commit. Are you otherwise ok with how I structured the changes?

Yeah, really good stuff :)

[ndam-hexagon]

Given the changes you are proposing, should the old syntax still be accepted?

allowUnfree: true

Or should the only way to configure be:

nixpkgs:
    allowUnfree: true

[domenkozar]

Yeah, we should still support the old way. And change docs for the nixpkgs :)

[ndam-hexagon]

Now that think about it, the followings would be introduced with this PR at the top-level:

• cudaSupport
• cudaCapabilities

Do you want those to only be allowed within nixpkgs?

[domenkozar]

Yes, thank you!

[domenkozar]

Note that jsonschema will get automatically generated, see https://github.com/cachix/devenv/actions/runs/15467562476/job/43544279281#step:15:23

[ndam-hexagon]

Note that jsonschema will get automatically generated, see https://github.com/cachix/devenv/actions/runs/15467562476/job/43544279281#step:15:23

Ah neat. I will remove those changes then.