pc/usb: defer startup until config is available

The commit also adds lx_kit/initial_config.h as utility.

repos/dde_linux/src/include/lx_kit/initial_config.h

@@ -0,0 +1,56 @@
+/*
+ * \brief  Lx_kit initial config utility
+ * \author Christian Helmuth
+ * \author Norman Feske
+ * \date   2022-03-11
+ */
+
+/*
+ * Copyright (C) 2022 Genode Labs GmbH
+ *
+ * This file is distributed under the terms of the GNU General Public License
+ * version 2.
+ */
+
+#ifndef _LX_KIT__INITIAL_CONFIG_H_
+#define _LX_KIT__INITIAL_CONFIG_H_
+
+#include <base/env.h>
+#include <base/attached_rom_dataspace.h>
+
+namespace Lx_kit {
+	using namespace Genode;
+
+	struct Initial_config;
+}
+
+
+struct Lx_kit::Initial_config
+{
+	Attached_rom_dataspace rom;
+
+	void _handle_signal() { rom.update(); }
+
+	Initial_config(Genode::Env &env) : rom(env, "config")
+	{
+		/*
+		 * 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.
+		 */
+
+		Io_signal_handler<Initial_config> sigh {
+			env.ep(), *this, &Initial_config::_handle_signal };
+
+		rom.sigh(sigh);
+		_handle_signal();
+
+		while (rom.xml().type() != "config")
+			env.ep().wait_and_dispatch_one_io_signal();
+
+		rom.sigh(Signal_context_capability());
+	}
+};
+
+#endif /* _LX_KIT__INITIAL_CONFIG_H_ */

repos/pc/src/drivers/usb_host/pc/main.cc

@@ -11,7 +11,6 @@
  * version 2.
  */
 
-#include <base/attached_rom_dataspace.h>
 #include <base/component.h>
 #include <base/env.h>
 
@@ -19,6 +18,7 @@
 #include <lx_emul/usb.h>
 #include <lx_kit/env.h>
 #include <lx_kit/init.h>
+#include <lx_kit/initial_config.h>
 #include <lx_user/io.h>
 
 #include <genode_c_api/usb.h>
@@ -50,8 +50,6 @@ struct Main : private Entrypoint::Io_progress_handler
 	                                        &Main::handle_signal };
 	Sliced_heap            sliced_heap    { env.ram(), env.rm()  };
 
-	Attached_rom_dataspace config_rom { env, "config" };
-
 	/**
 	 * Entrypoint::Io_progress_handler
 	 */
@@ -68,7 +66,11 @@ struct Main : private Entrypoint::Io_progress_handler
 
 	Main(Env & env) : env(env)
 	{
-		_bios_handoff = config_rom.xml().attribute_value("bios_handoff", true);
+		{
+			Lx_kit::Initial_config config { env };
+
+			_bios_handoff = config.rom.xml().attribute_value("bios_handoff", true);
+		}
 
 		Lx_kit::initialize(env);