rpi4: Add option to turn on the ARM I2C bus (i2c1).

raspberry-pi/4/default.nix

@@ -4,6 +4,7 @@
   imports = [
     ./audio.nix
     ./dwc2.nix
+    ./i2c1.nix
     ./modesetting.nix
     ./poe-hat.nix
     ./tc358743.nix

raspberry-pi/4/i2c1.nix

@@ -0,0 +1,44 @@
+{ config, lib, pkgs, ... }:
+
+let
+  cfg = config.hardware.raspberry-pi."4".i2c1;
+in
+{
+  options.hardware = {
+    raspberry-pi."4".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.
+      '';
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    hardware = {
+      i2c.enable = true;
+      deviceTree = {
+        overlays = [
+          # Equivalent to dtparam=i2c1=on
+          {
+            name = "i2c1-on-overlay";
+            dtsText = ''
+            /dts-v1/;
+            /plugin/;
+            / {
+              compatible = "brcm,bcm2711";
+              fragment@0 {
+                target = <&i2c1>;
+                __overlay__ {
+                  #address-cells = <1>;
+                  #size-cells = <0>;
+                  status = "okay";
+                };
+              };
+            };
+          '';
+          }
+        ];
+      };
+    };
+  };
+}