usb_drv: wait for first valid config

This patch defers the initialization of the USB driver until its
configuration is valid.

repos/dde_linux/src/drivers/usb/main.cc

@@ -12,14 +12,86 @@
  */
 
 #include <base/component.h>
+#include <base/attached_rom_dataspace.h>
 
 extern void start_usb_driver(Genode::Env &env);
 
 
+namespace Usb_driver {
+
+	using namespace Genode;
+
+	struct Driver_starter { virtual void start_driver() = 0; };
+	struct Main;
+}
+
+
+struct Usb_driver::Main : Driver_starter
+{
+	Env &_env;
+
+	/*
+	 * Defer the startup of the USB driver until the first configuration
+	 * becomes available. This is needed in scenarios where the configuration
+	 * is dynamically generated and supplied to the USB driver via the
+	 * report-ROM service.
+	 */
+	struct Initial_config_handler
+	{
+		Driver_starter &_driver_starter;
+
+		Attached_rom_dataspace _config;
+
+		Signal_handler<Initial_config_handler> _config_handler;
+
+		void _handle_config()
+		{
+			_config.update();
+
+			if (_config.xml().type() == "config")
+				_driver_starter.start_driver();
+		}
+
+		Initial_config_handler(Env &env, Driver_starter &driver_starter)
+		:
+			_driver_starter(driver_starter),
+			_config(env, "config"),
+			_config_handler(env.ep(), *this, &Initial_config_handler::_handle_config)
+		{
+			_config.sigh(_config_handler);
+			_handle_config();
+		}
+	};
+
+	void _handle_start()
+	{
+		if (_initial_config_handler.constructed()) {
+			_initial_config_handler.destruct();
+			start_usb_driver(_env);
+		}
+	}
+
+	Signal_handler<Main> _start_handler {
+		_env.ep(), *this, &Main::_handle_start };
+
+	Reconstructible<Initial_config_handler> _initial_config_handler { _env, *this };
+
+	/*
+	 * Called from 'Initial_config_handler'
+	 */
+	void start_driver() override
+	{
+		Signal_transmitter(_start_handler).submit();
+	}
+
+	Main(Env &env) : _env(env) { }
+};
+
+
 void Component::construct(Genode::Env &env)
 {
 	/* XXX execute constructors of global statics */
 	env.exec_static_constructors();
 
-	start_usb_driver(env);
+	static Usb_driver::Main main(env);
 }