raspberry-pi/4: Add dwc2 option

raspberry-pi/4/default.nix

@@ -2,6 +2,7 @@
 
 {
   imports = [
+    ./dwc2.nix
     ./modesetting.nix
   ];
 

raspberry-pi/4/dwc2.nix

@@ -0,0 +1,77 @@
+{ config, lib, pkgs, ... }:
+
+let
+  cfg = config.hardware.raspberry-pi."4".dwc2;
+in
+{
+  options.hardware = {
+    raspberry-pi."4".dwc2 = {
+      enable = lib.mkEnableOption ''
+        Enable the UDC controller to support USB OTG gadget functions.
+
+        In order to verify that this works, connect the Raspberry Pi with
+        another computer via the USB C cable, and then do one of:
+
+        - `modprobe g_serial`
+        - `modprobe g_mass_storage file=/path/to/some/iso-file.iso`
+
+        On the Raspberry Pi, `dmesg` should then show success-indicating output
+        that is related to the dwc2 and g_serial/g_mass_storage modules.
+        On the other computer, a serial/mass-storage device should pop up in
+        the system logs.
+
+        For more information about what gadget functions exist along with handy
+        guides on how to test them, please refer to:
+        https://www.kernel.org/doc/Documentation/usb/gadget-testing.txt
+      '';
+      dr_mode = lib.mkOption {
+        type = lib.types.enum [ "host" "peripheral" "otg" ];
+        default = "otg";
+        description = ''
+          Dual role mode setting for the dwc2 USB controller driver.
+        '';
+      };
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    # Configure for modesetting in the device tree
+    hardware.deviceTree = {
+      overlays = [
+        # this *should* be equivalent to (which doesn't work):
+        # https://github.com/raspberrypi/linux/blob/rpi-5.10.y/arch/arm/boot/dts/overlays/dwc2-overlay.dts
+        # but actually it's obtained using
+        # dtc -I dtb -O dts ${config.hardware.deviceTree.kernelPackage}/dtbs/overlays/dwc2.dtbo
+        # (changes: modified top-level "compatible" field)
+        # which is slightly different and works
+        {
+          name = "dwc2-overlay";
+          dtsText = ''
+            /dts-v1/;
+            /plugin/;
+
+            / {
+              compatible = "brcm,bcm2711";
+
+              fragment@0 {
+                target = <&usb>;
+                #address-cells = <0x01>;
+                #size-cells = <0x01>;
+
+                __overlay__ {
+                  compatible = "brcm,bcm2835-usb";
+                  dr_mode = "${cfg.dr_mode}";
+                  g-np-tx-fifo-size = <0x20>;
+                  g-rx-fifo-size = <0x22e>;
+                  g-tx-fifo-size = <0x200 0x200 0x200 0x200 0x200 0x100 0x100>;
+                  status = "okay";
+                  phandle = <0x01>;
+                };
+              };
+            };
+          '';
+        }
+      ];
+    };
+  };
+}