nixos/modules/hardware/firmware/default.nix

{ config, lib, ... }:
let
  cfg = config.my.hardware.firmware;
in
{
  options.my.hardware.firmware = with lib; {
    enable = mkEnableOption "firmware configuration";

    cpuFlavor = mkOption {
      type =
        with types;
        nullOr (enum [
          "intel"
          "amd"
        ]);
      default = null;
      example = "intel";
      description = "Which kind of CPU to activate micro-code updates";
    };
  };

  config = lib.mkIf cfg.enable (
    lib.mkMerge [
      {
        hardware = {
          enableRedistributableFirmware = true;
        };
      }

      # Intel CPU
      (lib.mkIf (cfg.cpuFlavor == "intel") {
        hardware = {
          cpu.intel.updateMicrocode = true;
        };
      })

      # AMD CPU
      (lib.mkIf (cfg.cpuFlavor == "amd") {
        hardware = {
          cpu.amd.updateMicrocode = true;
        };
      })
    ]
  );
}