feat: add backlight support for rpi4

raspberry-pi/4/backlight.nix

@@ -0,0 +1,50 @@
+{ config, lib, ... }:
+
+let
+  cfg = config.hardware.raspberry-pi."4".backlight;
+in
+{
+  options.hardware = {
+    raspberry-pi."4".backlight = {
+      enable = lib.mkEnableOption ''
+        Enable the backlight support for the Raspberry Pi official Touch Display
+      '';
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    hardware.deviceTree = {
+      overlays = [
+        # This overlay was originally taken from:
+        # https://github.com/raspberrypi/linux/blob/rpi-5.15.y/arch/arm/boot/dts/overlays/rpi-backlight-overlay.dts
+        # The only modification made was to change the compatible field to bcm2711
+        {
+          name = "rpi-backlight-overlay";
+          dtsText = ''
+            /*
+             * Devicetree overlay for mailbox-driven Raspberry Pi DSI Display
+             * backlight controller
+             */
+            /dts-v1/;
+            /plugin/;
+
+            / {
+              compatible = "brcm,bcm2711";
+
+              fragment@0 {
+                target-path = "/";
+                __overlay__ {
+                  rpi_backlight: rpi_backlight {
+                    compatible = "raspberrypi,rpi-backlight";
+                    firmware = <&firmware>;
+                    status = "okay";
+                  };
+                };
+              };
+            };
+          '';
+        }
+      ];
+    };
+  };
+}

raspberry-pi/4/default.nix

@@ -1,8 +1,9 @@
-{ lib, pkgs, ...}:
+{ lib, pkgs, ... }:
 
 {
   imports = [
     ./audio.nix
+    ./backlight.nix
     ./cpu-revision.nix
     ./dwc2.nix
     ./i2c.nix
@@ -22,6 +23,7 @@
       "vc4"
       "pcie_brcmstb"      # required for the pcie bus to work
       "reset-raspberrypi" # required for vl805 firmware to load
+      "rpi_backlight"     # required for backlight support
     ];
 
     loader = {