initrd-usb: Implement gadgetfs usb gadget

This has only been tested with google-walleye, with gsi.rndis.

modules/initrd-usb.nix

@@ -3,6 +3,7 @@
 with lib;
 
 let
+  device_info = config.mobile.device.info;
   cfg = config.mobile.boot.stage-1;
   device_name = device_config.name;
   device_config = config.mobile.device;
@@ -27,6 +28,20 @@ in
       '';
     };
   };
+  options.mobile.usb = {
+    idVendor = mkOption {
+      type = types.str;
+      description = ''
+        USB vendor ID for the USB gadget.
+      '';
+    };
+    idProduct = mkOption {
+      type = types.str;
+      description = ''
+        USB product ID for the USB gadget.
+      '';
+    };
+  };
 
   config = lib.mkIf cfg.usb.enable {
     boot.specialFileSystems = {
@@ -48,9 +63,16 @@ in
         ++ optional cfg.networking.enable "rndis"
       ;
       tasks = [
+        ./stage-1/tasks/usb-gadget-task.rb
       ];
       bootConfig = {
         boot.usb.features = cfg.usb.features;
+        boot.usb.functions = builtins.listToAttrs (
+          builtins.map (feature: { name = feature; value = device_info.gadgetfs.functions."${feature}"; }) cfg.usb.features
+        );
+        usb = {
+          inherit (config.mobile.usb) idVendor idProduct;
+        };
       };
     };
   };

modules/stage-1/tasks/usb-gadget-task.rb

@@ -0,0 +1,147 @@
+module System::ConfigFSUSB
+  CONFIGFS     = "/sys/kernel/config"
+  CONFIGFS_USB = File.join(CONFIGFS, "usb_gadget")
+
+  module Quirks
+    def self.gsi_rndis()
+      # Activate the IPA stuff... ugh.
+      File.write("/dev/ipa", 1)
+    end
+  end
+
+  # This is a bit underdocumented in the configfs and gadgetfs docs, but this
+  # would be used to translate strings in multiple languages.
+  # In practice, it never happens and is hardcoded to en-US.
+  # https://docs.microsoft.com/en-us/windows/win32/intl/language-identifier-constants-and-strings
+  # https://docs.microsoft.com/en-us/windows/win32/msi/localizing-the-error-and-actiontext-tables
+  # Yes, the language identifiers are the Microsoft ones. I haven't found a
+  # detailed explanation for this, but it's part of the USB spec.
+  STRINGS_SUFFIX = "strings/0x409" # en-US ¯\_(ツ)_/¯
+
+  class Gadget
+    attr_reader :name
+    attr_accessor :features
+
+    # Initializes a USB gadget
+    # The name is actually arbitrary, but it is customary to use `gn` where n
+    # is an incrementing number.
+    def initialize(name)
+      @name = name
+      FileUtils.mkdir_p(File.join(path_prefix, STRINGS_SUFFIX))
+    end
+
+    def path_prefix()
+      File.join(CONFIGFS_USB, name)
+    end
+
+    def id_vendor=(value)
+      set_id("idVendor", value)
+    end
+
+    def id_product=(value)
+      set_id("idProduct", value)
+    end
+
+    def manufacturer=(value)
+      set_string("manufacturer", value)
+    end
+
+    def product=(value)
+      set_string("product", value)
+    end
+
+    def serial_number=(value)
+      set_string("serialnumber", value)
+    end
+
+    def set_id(kind, value)
+      value = ["0", value.sub(/^0x/, "")].join("x")
+      File.write(File.join(path_prefix, kind), value)
+    end
+
+    def set_string(name, value)
+      File.write(File.join(path_prefix, STRINGS_SUFFIX, name), value)
+    end
+
+    def activate!()
+      # TODO: explore more than one "config" in a gadget. (c.1)
+      # First, "document" features.
+      config_dir = File.join(path_prefix, "configs/c.1")
+      FileUtils.mkdir_p(File.join(config_dir, STRINGS_SUFFIX))
+      File.write(File.join(config_dir, STRINGS_SUFFIX, "configuration"), features.join(","))
+
+      # Then activate features.
+      features.each do |feature|
+        # We're using a "feature name -> function name" mapping to make a better
+        # end-user UX. They don't care *how* rndis is enabled, only that rndis
+        # has to be enabled.
+        # Though here we need to know *how* to enable e.g. rndis.
+        # This is why `function_name` exists, it's the gadgetfs function name
+        # for the required "logical" feature.
+        function_name = Configuration["boot"]["usb"]["functions"][feature]
+        function_dir = File.join(path_prefix, "functions", function_name)
+        feature_dir = File.join(config_dir, feature)
+        FileUtils.mkdir_p(function_dir)
+        File.symlink(function_dir, feature_dir)
+
+        quirk_name = function_name.gsub(/\./, "_").to_sym
+        Quirks.send(quirk_name) if Quirks.respond_to?(quirk_name)
+      end
+
+      # Then, attach to the USB driver.
+      if features.length > 0 then
+        # Equivalent to:
+        # (cd /sys/class/udc; echo *) > g1/UDC
+        # The idea is to give a controller name taken from the filesystem.
+        File.write(
+          File.join(path_prefix, "UDC"),
+          Dir.children("/sys/class/udc").first
+        )
+      end
+    end
+
+  end
+end
+
+# This task detects which gadget mode to use, and sets it up.
+class Tasks::SetupGadgetMode < SingletonTask
+  ANDROID_USB  = "/sys/class/android_usb"
+
+  def initialize()
+    add_dependency(:Mount, "/sys")
+    add_dependency(:Mount, System::ConfigFSUSB::CONFIGFS)
+    # If there's a `/vendor` mount point, it's likely that it's highly possible
+    # that it's going to be required for firmwares.
+    if Configuration["nixos"]["boot"]["specialFileSystems"]["/vendor"]
+      add_dependency(:Mount, "/vendor")
+    end
+    Targets[:SwitchRoot].add_dependency(:Task, self)
+  end
+
+  def run()
+    if File.exist?(System::ConfigFSUSB::CONFIGFS_USB)
+      run_configfs_usb
+    elsif File.exist?(ANDROID_USB)
+      run_androidusb
+    else
+      log("No way to configure USB Gadget found.")
+    end
+  end
+
+  def run_configfs_usb()
+    log("Configuring CONFIGFS USB Gadget.")
+    gadget = System::ConfigFSUSB::Gadget.new("g1")
+    gadget.id_vendor = Configuration["usb"]["idVendor"]
+    gadget.id_product = Configuration["usb"]["idProduct"]
+    gadget.product = Configuration["device"]["name"]
+    # FIXME : could this cause issues?
+    gadget.manufacturer = "Mobile NixOS"
+    gadget.serial_number = "0123456789"
+    gadget.features = Configuration["boot"]["usb"]["features"]
+    gadget.activate!
+  end
+
+  def run_android_usb()
+    log("Configuring ANDROID_USB Gadget.")
+  end
+end