hydrophonitor/targets/raspberry-pi-3/i2c.nix

{
  config,
  lib,
  ...
}: let
  cfg = config.hardware.raspberry-pi."3";
  optionalProperty = name: value: lib.optionalString (value != null) "${name} = <${builtins.toString value}>;";
  simple-overlay = {
    target,
    status,
    frequency,
  }: {
    name = "${target}-${status}-overlay";
    dtsText = ''
      /dts-v1/;
      /plugin/;
      / {
        compatible = "brcm,bcm2711";
        fragment@0 {
          target = <&${target}>;
          __overlay__ {
            status = "${status}";
            ${optionalProperty "clock-frequency" frequency}
          };
        };
      };
    '';
  };
in {
  options.hardware.raspberry-pi."3" = {
    i2c0 = {
      enable = lib.mkEnableOption ''
        Turn on the VideoCore I2C bus (maps to /dev/i2c-22) and enable access from the i2c group.
        After a reboot, i2c-tools (e.g. i2cdetect -F 22) should work for root or any user in i2c.
      '';
      frequency = lib.mkOption {
        type = lib.types.nullOr lib.types.int;
        default = null;
        description = ''
          interface clock-frequency
        '';
      };
    };
    i2c1 = {
      enable = lib.mkEnableOption ''
        Turn on the ARM I2C bus (/dev/i2c-1 on GPIO pins 3 and 5) and enable access from the i2c group.
        After a reboot, i2c-tools (e.g. i2cdetect -F 1) should work for root or any user in i2c.
      '';
      frequency = lib.mkOption {
        type = lib.types.nullOr lib.types.int;
        default = null;
        description = ''
          interface clock-frequency
        '';
      };
    };
  };
  config.hardware = lib.mkMerge [
    (lib.mkIf cfg.i2c0.enable {
      i2c.enable = lib.mkDefault true;
      deviceTree = {
        overlays = [
          (simple-overlay {
            target = "i2c0if";
            status = "okay";
            inherit (cfg.i2c0) frequency;
          })
        ];
      };
    })
    (lib.mkIf cfg.i2c1.enable {
      i2c.enable = lib.mkDefault true;
      deviceTree = {
        overlays = [
          (simple-overlay {
            target = "i2c1";
            status = "okay";
            inherit (cfg.i2c1) frequency;
          })
        ];
      };
    })
  ];
}