libusb: handle 'ack_avail' signals in a VFS plugin

Issue #4392

repos/libports/lib/mk/vfs_libusb.mk

@@ -0,0 +1,7 @@
+SRC_CC = vfs_libusb.cc
+
+vpath %.cc $(REP_DIR)/src/lib/vfs/libusb
+
+SHARED_LIB = yes
+
+CC_CXX_WARN_STRICT =

repos/libports/recipes/pkg/usb_webcam/archives

@@ -11,5 +11,5 @@ _/src/rom_filter
 _/src/report_rom
 _/src/stdcxx
 _/src/vfs
+_/src/vfs_libusb
 _/src/vfs_pipe
-

repos/libports/recipes/pkg/usb_webcam/runtime

@@ -6,6 +6,7 @@
 		<rom label="init"/>
 		<rom label="jpeg.lib.so"/>
 		<rom label="vfs.lib.so"/>
+		<rom label="vfs_libusb.lib.so"/>
 		<rom label="vfs_pipe.lib.so"/>
 		<rom label="libc.lib.so"/>
 		<rom label="libm.lib.so"/>

repos/libports/recipes/raw/usb_webcam/usb_webcam.config

@@ -75,7 +75,11 @@
 							<resource name="RAM" quantum="48M"/>
 							<config ld_verbose="no" enabled="yes" width="640" height="480" format="yuv" fps="15">
 								<vfs>
-									<dir name="dev"> <log/> <inline name="rtc">2018-01-01 00:01</inline> </dir>
+									<dir name="dev">
+										<log/>
+										<inline name="rtc">2018-01-01 00:01</inline>
+										<libusb/>
+									</dir>
 									<dir name="pipe"> <pipe/> </dir>
 								</vfs>
 								<libc stdout="/dev/log" stderr="/dev/log" rtc="/dev/rtc" pipe="/pipe"/>

repos/libports/recipes/src/vfs_libusb/content.mk

@@ -0,0 +1,9 @@
+MIRROR_FROM_REP_DIR := lib/mk/vfs_libusb.mk src/lib/vfs/libusb
+
+content: $(MIRROR_FROM_REP_DIR) LICENSE
+
+$(MIRROR_FROM_REP_DIR):
+	$(mirror_from_rep_dir)
+
+LICENSE:
+	cp $(GENODE_DIR)/LICENSE $@

repos/libports/recipes/src/vfs_libusb/hash

@@ -0,0 +1 @@
+2022-02-01 a4dec9e3717d7b06e20b057905b2c5e8c0608d93

repos/libports/recipes/src/vfs_libusb/used_apis

@@ -0,0 +1,5 @@
+base
+os
+so
+usb_session
+vfs

repos/libports/run/smartcard.run

@@ -26,6 +26,7 @@ set build_components {
 	core init timer
 	drivers/usb_host
 	test/smartcard
+	lib/vfs/libusb
 	lib/vfs/pipe
 }
 
@@ -91,7 +92,11 @@ append config {
 		<resource name="RAM" quantum="4M"/>
 		<config>
 			<vfs>
-				<dir name="dev"> <log/> <inline name="rtc">2018-01-01 00:01</inline> </dir>
+				<dir name="dev">
+					<log/>
+					<inline name="rtc">2018-01-01 00:01</inline>
+					<libusb/>
+				</dir>
 				<dir name="pipe"> <pipe/> </dir>
 				<dir name="ifd-ccid.bundle">
 					<dir name="Contents">
@@ -116,7 +121,7 @@ set boot_modules {
 	core init timer test-smartcard
 	ld.lib.so pcsc-lite.lib.so ccid.lib.so libusb.lib.so
 	libc.lib.so vfs.lib.so libm.lib.so posix.lib.so
-	Info.plist vfs_pipe.lib.so
+	Info.plist vfs_libusb.lib.so vfs_pipe.lib.so
 }
 
 lappend boot_modules [usb_host_drv_binary]

repos/libports/src/lib/libusb/README

@@ -11,3 +11,9 @@ the application using libusb:
 </config>
 
 See also the README file of the USB driver for additional policy attributes.
+
+The Genode USB connection object resides in a VFS plugin named 'vfs_libusb',
+which is necessary to handle the 'ack_avail' signal in the libc kernel context.
+So, an application using libusb needs to have a '<dir name="dev"><libusb/></dev>'
+node in its VFS configuration and the 'vfs_libusb.lib.so' file loadable at
+runtime.

repos/libports/src/lib/libusb/genode_usb_raw.cc

@@ -17,6 +17,7 @@
 #include <usb/usb.h>
 #include <usb_session/connection.h>
 
+#include <fcntl.h>
 #include <time.h>
 #include <libc/allocator.h>
 #include <libc-plugin/plugin.h>
@@ -36,66 +37,9 @@ static Genode::Env &genode_env()
 	abort();
 }
 
-
-struct Usb_ep
-{
-	Genode::Entrypoint _ep;
-	pthread_t          _pthread;
-
-	void _handle_pthread_registration()
-	{
-		Genode::Thread *myself = Genode::Thread::myself();
-		if (!myself || Libc::pthread_create_from_thread(&_pthread, *myself, &myself)) {
-			Genode::error("cannot register thread for pthread");
-			return;
-		}
-	}
-
-	Genode::Io_signal_handler<Usb_ep> _pthread_reg_sigh {
-		_ep, *this, &Usb_ep::_handle_pthread_registration };
-
-	Usb_ep(Genode::Env &env, size_t stack_size, char const *name,
-	       Genode::Affinity::Location location)
-	: _ep { env, stack_size, name, location }
-	{
-		Genode::Signal_transmitter(_pthread_reg_sigh).submit();
-	}
-
-	Genode::Entrypoint &ep() { return _ep; }
-};
-
-
-/*
- * Entrypoint for handling 'ack avail' signals from the USB driver.
- *
- * The entrypoint is needed because the main thread of an application
- * using libusb might be blocking on a pthread locking function, which
- * currently do not dispatch signals while blocking.
- */
-static Genode::Entrypoint &ep()
-{
-	static Usb_ep instance(genode_env(),
-	                       2*1024*sizeof(Genode::addr_t),
-	                       "usb_ack_ep",
-	                       Genode::Affinity::Location());
-	return instance.ep();
-}
-
-
 static Libc::Allocator libc_alloc { };
 
 
-/*
- * Prevent modification of packet allocator
- * by multiple threads.
- */
-static Genode::Mutex &usb_packet_allocator_mutex()
-{
-	static Genode::Mutex instance;
-	return instance;
-}
-
-
 struct Completion : Usb::Completion
 {
 	struct usbi_transfer *itransfer;
@@ -111,36 +55,95 @@ struct Usb_device
 {
 	private:
 
-		Genode::Allocator_avl _alloc { &libc_alloc };
-
-		Genode::Io_signal_handler<Usb_device> _state_changed_handler {
-			genode_env().ep(), *this, &Usb_device::_handle_state_changed };
-
 		unsigned _open { 0 };
 
-		void _handle_state_changed()
+	public:
+
+		Usb::Connection *usb_connection;
+
+		Usb::Device_descriptor  device_descriptor;
+		Usb::Config_descriptor  config_descriptor;
+		char                   *raw_config_descriptor = nullptr;
+
+		Usb_device(Usb::Connection *usb)
+		: usb_connection(usb)
 		{
-			/*
-			 * The handler is installed only to receive state-change signals
-			 * from the USB connection using the 'Usb_device' constructor.
-			 */
+			Genode::log("libusb: waiting until device is plugged...");
+			while (!usb_connection->plugged())
+				genode_env().ep().wait_and_dispatch_one_io_signal();
+			Genode::log("libusb: device is plugged");
+
+			usb_connection->config_descriptor(&device_descriptor, &config_descriptor);
+
+			raw_config_descriptor = (char*)malloc(config_descriptor.total_length);
+
+			Usb::Packet_descriptor p =
+				usb_connection->source()->alloc_packet(config_descriptor.total_length);
+
+			p.type                 = Usb::Packet_descriptor::CTRL;
+			p.control.request      = LIBUSB_REQUEST_GET_DESCRIPTOR;
+			p.control.request_type = LIBUSB_ENDPOINT_IN;
+			p.control.value        = (LIBUSB_DT_CONFIG << 8) | 0;
+			p.control.index        = 0;
+
+			usb_connection->source()->submit_packet(p);
+
+			while (!usb_connection->source()->ack_avail())
+				genode_env().ep().wait_and_dispatch_one_io_signal();
+
+			p = usb_connection->source()->get_acked_packet();
+
+			if (!p.succeded)
+				Genode::error(__PRETTY_FUNCTION__,
+				              ": could not read raw configuration descriptor");
+
+			if (p.control.actual_size != config_descriptor.total_length)
+				Genode::error(__PRETTY_FUNCTION__,
+				              ": received configuration descriptor of unexpected size");
+
+			char *packet_content = usb_connection->source()->packet_content(p);
+			Genode::memcpy(raw_config_descriptor, packet_content,
+			               config_descriptor.total_length);
+
+			usb_connection->source()->release_packet(p);
 		}
 
-		Genode::Io_signal_handler<Usb_device> _ack_avail_handler {
-			ep(), *this, &Usb_device::_handle_ack_avail };
+		~Usb_device()
+		{
+			free(raw_config_descriptor);
+		}
 
-		void _handle_ack_avail()
+		bool altsetting(int number, int alt_setting)
+		{
+			if (!usb_connection->source()->ready_to_submit())
+				return false;
+
+			Usb::Packet_descriptor p =
+				usb_connection->source()->alloc_packet(0);
+
+			p.type                  = Usb::Packet_descriptor::ALT_SETTING;
+			p.interface.number      = number;
+			p.interface.alt_setting = alt_setting;
+
+			usb_connection->source()->submit_packet(p);
+
+			return true;
+		}
+
+		void close() { _open--; }
+		void open()  { _open++; }
+
+		void handle_events()
 		{
 			struct libusb_context *ctx = nullptr;
 
-			while (usb_connection.source()->ack_avail()) {
+			while (usb_connection->source()->ack_avail()) {
 
 				Usb::Packet_descriptor p =
-					usb_connection.source()->get_acked_packet();
+					usb_connection->source()->get_acked_packet();
 
 				if (p.type == Usb::Packet_descriptor::ALT_SETTING) {
-					Genode::Mutex::Guard guard(usb_packet_allocator_mutex());
-					usb_connection.source()->release_packet(p);
+					usb_connection->source()->release_packet(p);
 					continue;
 				}
 
@@ -153,8 +156,7 @@ struct Usb_device
 				destroy(libc_alloc, completion);
 
 				if (_open == 0) {
-					Genode::Mutex::Guard guard(usb_packet_allocator_mutex());
-					usb_connection.source()->release_packet(p);
+					usb_connection->source()->release_packet(p);
 					continue;
 				}
 
@@ -162,15 +164,12 @@ struct Usb_device
 					if (!p.succeded)
 						Genode::error("USB transfer failed: ", (unsigned)p.type);
 					itransfer->transferred = 0;
-					{
-						Genode::Mutex::Guard guard(usb_packet_allocator_mutex());
-						usb_connection.source()->release_packet(p);
-					}
+					usb_connection->source()->release_packet(p);
 					usbi_signal_transfer_completion(itransfer);
 					continue;
 				}
 
-				char *packet_content = usb_connection.source()->packet_content(p);
+				char *packet_content = usb_connection->source()->packet_content(p);
 
 				struct libusb_transfer *transfer =
 					USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
@@ -239,15 +238,11 @@ struct Usb_device
 					default:
 						Genode::error(__PRETTY_FUNCTION__,
 						              ": unsupported transfer type");
-						Genode::Mutex::Guard guard(usb_packet_allocator_mutex());
-						usb_connection.source()->release_packet(p);
+						usb_connection->source()->release_packet(p);
 						continue;
 				}
 
-				{
-					Genode::Mutex::Guard guard(usb_packet_allocator_mutex());
-					usb_connection.source()->release_packet(p);
-				}
+				usb_connection->source()->release_packet(p);
 
 				usbi_signal_transfer_completion(itransfer);
 			}
@@ -255,100 +250,38 @@ struct Usb_device
 			if (ctx != nullptr)
 				usbi_signal_event(ctx);
 		}
-
-	public:
-
-		Usb::Connection usb_connection { genode_env(),
-		                                 &_alloc,
-		                                 "usb_device",
-		                                 1024*1024,
-		                                 _state_changed_handler };
-
-		Usb::Device_descriptor  device_descriptor;
-		Usb::Config_descriptor  config_descriptor;
-		char                   *raw_config_descriptor = nullptr;
-
-		Usb_device()
-		{
-			Genode::log("libusb: waiting until device is plugged...");
-			while (!usb_connection.plugged())
-				genode_env().ep().wait_and_dispatch_one_io_signal();
-			Genode::log("libusb: device is plugged");
-
-			usb_connection.config_descriptor(&device_descriptor, &config_descriptor);
-
-			raw_config_descriptor = (char*)malloc(config_descriptor.total_length);
-
-			Usb::Packet_descriptor p =
-				usb_connection.source()->alloc_packet(config_descriptor.total_length);
-
-			p.type                 = Usb::Packet_descriptor::CTRL;
-			p.control.request      = LIBUSB_REQUEST_GET_DESCRIPTOR;
-			p.control.request_type = LIBUSB_ENDPOINT_IN;
-			p.control.value        = (LIBUSB_DT_CONFIG << 8) | 0;
-			p.control.index        = 0;
-
-			usb_connection.source()->submit_packet(p);
-
-			while (!usb_connection.source()->ack_avail())
-				genode_env().ep().wait_and_dispatch_one_io_signal();
-
-			p = usb_connection.source()->get_acked_packet();
-
-			if (!p.succeded)
-				Genode::error(__PRETTY_FUNCTION__,
-				              ": could not read raw configuration descriptor");
-
-			if (p.control.actual_size != config_descriptor.total_length)
-				Genode::error(__PRETTY_FUNCTION__,
-				              ": received configuration descriptor of unexpected size");
-
-			char *packet_content = usb_connection.source()->packet_content(p);
-			Genode::memcpy(raw_config_descriptor, packet_content,
-			               config_descriptor.total_length);
-
-			usb_connection.source()->release_packet(p);
-
-			usb_connection.tx_channel()->sigh_ack_avail(_ack_avail_handler);
-		}
-
-		~Usb_device()
-		{
-			free(raw_config_descriptor);
-		}
-
-		bool altsetting(int number, int alt_setting)
-		{
-			Genode::Mutex::Guard guard(usb_packet_allocator_mutex());
-
-			if (!usb_connection.source()->ready_to_submit())
-				return false;
-
-			Usb::Packet_descriptor p =
-				usb_connection.source()->alloc_packet(0);
-
-			p.type                  = Usb::Packet_descriptor::ALT_SETTING;
-			p.interface.number      = number;
-			p.interface.alt_setting = alt_setting;
-
-			usb_connection.source()->submit_packet(p);
-
-			return true;
-		}
-
-		void close() { _open--; }
-		void open()  { _open++; }
 };
 
-static Usb_device *device_instance;
+
+static int vfs_libusb_fd { -1 };
+static Usb::Connection *usb_connection { nullptr };
+static Usb_device *device_instance { nullptr };
+
+
+/*
+ * This function is called by the VFS plugin on 'open()'
+ * to pass a pointer to the USB connection.
+ */
+void libusb_genode_usb_connection(Usb::Connection *usb)
+{
+	usb_connection = usb;
+}
+
 
 static int genode_init(struct libusb_context* ctx)
 {
 	if (!device_instance) {
-		device_instance = new (libc_alloc) Usb_device;
+		vfs_libusb_fd = open("/dev/libusb", O_RDONLY);
+		if (vfs_libusb_fd == -1) {
+			Genode::error("could not open /dev/libusb");
+			return LIBUSB_ERROR_OTHER;
+		}
+		device_instance = new (libc_alloc) Usb_device(usb_connection);
 	} else {
 		Genode::error("tried to init genode usb context twice");
+		return LIBUSB_ERROR_OTHER;
 	}
+
 	return LIBUSB_SUCCESS;
 }
 
@@ -358,6 +291,9 @@ static void genode_exit(void)
 	if (device_instance) {
 		destroy(libc_alloc, device_instance);
 		device_instance = nullptr;
+		close(vfs_libusb_fd);
+		usb_connection = nullptr;
+		vfs_libusb_fd = -1;
 	}
 }
 
@@ -436,7 +372,8 @@ static int genode_open(struct libusb_device_handle *dev_handle)
 	if (device_instance)
 		device_instance->open();
 
-	return LIBUSB_SUCCESS;
+	return usbi_add_pollfd(HANDLE_CTX(dev_handle), vfs_libusb_fd,
+	                       POLLIN);
 }
 
 
@@ -444,6 +381,8 @@ static void genode_close(struct libusb_device_handle *dev_handle)
 {
 	if (device_instance)
 		device_instance->close();
+
+	usbi_remove_pollfd(HANDLE_CTX(dev_handle), vfs_libusb_fd);
 }
 
 
@@ -511,7 +450,7 @@ static int genode_claim_interface(struct libusb_device_handle *dev_handle,
 	Usb_device *usb_device = *(Usb_device**)dev_handle->dev->os_priv;
 
 	try {
-		usb_device->usb_connection.claim_interface(interface_number);
+		usb_device->usb_connection->claim_interface(interface_number);
 	} catch (Usb::Session::Interface_not_found) {
 		Genode::error(__PRETTY_FUNCTION__, ": interface not found");
 		return LIBUSB_ERROR_NOT_FOUND;
@@ -533,7 +472,7 @@ static int genode_release_interface(struct libusb_device_handle *dev_handle,
 	Usb_device *usb_device = *(Usb_device**)dev_handle->dev->os_priv;
 
 	try {
-		usb_device->usb_connection.release_interface(interface_number);
+		usb_device->usb_connection->release_interface(interface_number);
 	} catch (Usb::Session::Interface_not_found) {
 		Genode::error(__PRETTY_FUNCTION__, ": interface not found");
 		return LIBUSB_ERROR_NOT_FOUND;
@@ -564,7 +503,7 @@ static int genode_submit_transfer(struct usbi_transfer * itransfer)
 
 	Usb_device *usb_device = *(Usb_device**)transfer->dev_handle->dev->os_priv;
 
-	if (!usb_device->usb_connection.source()->ready_to_submit())
+	if (!usb_device->usb_connection->source()->ready_to_submit())
 		return LIBUSB_ERROR_BUSY;
 
 	switch (transfer->type) {
@@ -577,8 +516,7 @@ static int genode_submit_transfer(struct usbi_transfer * itransfer)
 			Usb::Packet_descriptor p;
 
 			try {
-				Genode::Mutex::Guard guard(usb_packet_allocator_mutex());
-				p = usb_device->usb_connection.source()->alloc_packet(setup->wLength);
+				p = usb_device->usb_connection->source()->alloc_packet(setup->wLength);
 			} catch (Usb::Session::Tx::Source::Packet_alloc_failed) {
 				return LIBUSB_ERROR_BUSY;
 			}
@@ -596,14 +534,14 @@ static int genode_submit_transfer(struct usbi_transfer * itransfer)
 			    LIBUSB_ENDPOINT_OUT) {
 
 				char *packet_content =
-					usb_device->usb_connection.source()->packet_content(p);
+					usb_device->usb_connection->source()->packet_content(p);
 
 				Genode::memcpy(packet_content,
 				               transfer->buffer + LIBUSB_CONTROL_SETUP_SIZE,
 				               setup->wLength);
 			}
 
-			usb_device->usb_connection.source()->submit_packet(p);
+			usb_device->usb_connection->source()->submit_packet(p);
 
 			return LIBUSB_SUCCESS;
 		}
@@ -622,8 +560,7 @@ static int genode_submit_transfer(struct usbi_transfer * itransfer)
 			Usb::Packet_descriptor p;
 			
 			try {
-				Genode::Mutex::Guard guard(usb_packet_allocator_mutex());
-				p = usb_device->usb_connection.source()->alloc_packet(transfer->length);
+				p = usb_device->usb_connection->source()->alloc_packet(transfer->length);
 			} catch (Usb::Session::Tx::Source::Packet_alloc_failed) {
 				return LIBUSB_ERROR_BUSY;
 			}
@@ -640,12 +577,12 @@ static int genode_submit_transfer(struct usbi_transfer * itransfer)
 
 			if (IS_XFEROUT(transfer)) {
 				char *packet_content =
-					usb_device->usb_connection.source()->packet_content(p);
+					usb_device->usb_connection->source()->packet_content(p);
 				Genode::memcpy(packet_content, transfer->buffer,
 				               transfer->length);
 			}
 
-			usb_device->usb_connection.source()->submit_packet(p);
+			usb_device->usb_connection->source()->submit_packet(p);
 
 			return LIBUSB_SUCCESS;
 		}
@@ -659,8 +596,7 @@ static int genode_submit_transfer(struct usbi_transfer * itransfer)
 
 			Usb::Packet_descriptor p;
 			try {
-				Genode::Mutex::Guard guard(usb_packet_allocator_mutex());
-				p = usb_device->usb_connection.source()->alloc_packet(total_length);
+				p = usb_device->usb_connection->source()->alloc_packet(total_length);
 			} catch (Usb::Session::Tx::Source::Packet_alloc_failed) {
 				return LIBUSB_ERROR_BUSY;
 			}
@@ -679,12 +615,12 @@ static int genode_submit_transfer(struct usbi_transfer * itransfer)
 
 			if (IS_XFEROUT(transfer)) {
 				char *packet_content =
-					usb_device->usb_connection.source()->packet_content(p);
+					usb_device->usb_connection->source()->packet_content(p);
 				Genode::memcpy(packet_content, transfer->buffer,
 				               transfer->length);
 			}
 
-			usb_device->usb_connection.source()->submit_packet(p);
+			usb_device->usb_connection->source()->submit_packet(p);
 
 			return LIBUSB_SUCCESS;
 		}
@@ -706,6 +642,19 @@ static int genode_cancel_transfer(struct usbi_transfer * itransfer)
 static void genode_clear_transfer_priv(struct usbi_transfer * itransfer) { }
 
 
+static int genode_handle_events(struct libusb_context *,
+                            struct pollfd *,
+                            POLL_NFDS_TYPE, int)
+{
+	if (device_instance) {
+		device_instance->handle_events();
+		return LIBUSB_SUCCESS;
+	}
+
+	return LIBUSB_ERROR_NO_DEVICE;
+}
+
+
 static int genode_handle_transfer_completion(struct usbi_transfer * itransfer)
 {
 	enum libusb_transfer_status status = LIBUSB_TRANSFER_COMPLETED;
@@ -768,7 +717,7 @@ const struct usbi_os_backend genode_usb_raw_backend = {
 	/*.cancel_transfer =*/ genode_cancel_transfer,
 	/*.clear_transfer_priv =*/ genode_clear_transfer_priv,
 
-	/*.handle_events =*/ NULL,
+	/*.handle_events =*/ genode_handle_events,
 	/*.handle_transfer_completion =*/ genode_handle_transfer_completion,
 
 	/*.clock_gettime =*/ genode_clock_gettime,
@@ -808,3 +757,4 @@ void __attribute__((constructor)) init_libc_libusb(void)
 {
 	static Plugin plugin;
 }
+

repos/libports/src/lib/vfs/libusb/target.mk

@@ -0,0 +1,4 @@
+TARGET = dummy-vfs_libusb
+LIBS = vfs_libusb
+
+CC_CXX_WARN_STRICT =

repos/libports/src/lib/vfs/libusb/vfs_libusb.cc

@@ -0,0 +1,144 @@
+/*
+ * \brief  libusb file system
+ * \author Christian Prochaska
+ * \date   2022-01-31
+ */
+
+/*
+ * Copyright (C) 2022 Genode Labs GmbH
+ *
+ * This file is part of the Genode OS framework, which is distributed
+ * under the terms of the GNU Affero General Public License version 3.
+ */
+
+
+/* Genode includes */
+#include <base/allocator_avl.h>
+#include <usb_session/connection.h>
+#include <util/xml_node.h>
+#include <vfs/file_system_factory.h>
+#include <vfs/single_file_system.h>
+
+
+/*
+ * This function is implemented in the Genode backend
+ * of libusb.
+ */
+extern void libusb_genode_usb_connection(Usb::Connection *);
+
+class Libusb_file_system : public Vfs::Single_file_system
+{
+	private:
+
+		Vfs::Env &_env;
+
+		class Libusb_vfs_handle : public Single_vfs_handle
+		{
+			private:
+
+				Genode::Env           &_env;
+				Genode::Allocator_avl  _alloc_avl;
+				Usb::Connection        _usb_connection;
+
+				Genode::Io_signal_handler<Libusb_vfs_handle> _state_changed_handler {
+					_env.ep(), *this, &Libusb_vfs_handle::_handle_state_changed };
+
+				void _handle_state_changed()
+				{
+					/*
+					 * The handler is installed only to receive state-change
+					 * signals from the USB connection using the 'Usb_device'
+					 * constructor.
+					 */
+				}
+
+				Genode::Io_signal_handler<Libusb_vfs_handle> _ack_avail_handler {
+					_env.ep(), *this, &Libusb_vfs_handle::_handle_ack_avail };
+
+				void _handle_ack_avail()
+				{
+					io_progress_response();
+				}
+
+			public:
+
+				Libusb_vfs_handle(Directory_service &ds,
+				                  File_io_service   &fs,
+				                  Genode::Allocator &alloc,
+				                  Genode::Env &env)
+				: Single_vfs_handle(ds, fs, alloc, 0),
+				  _env(env), _alloc_avl(&alloc),
+				  _usb_connection(_env, &_alloc_avl,
+				                  "usb_device",
+				                  1024*1024,
+				                  _state_changed_handler)
+				{
+					_usb_connection.tx_channel()->sigh_ack_avail(_ack_avail_handler);
+					libusb_genode_usb_connection(&_usb_connection);
+				}
+
+				bool read_ready() override
+				{
+					return _usb_connection.source()->ack_avail();
+				}
+
+				Read_result read(char *dst, Vfs::file_size count,
+				                 Vfs::file_size &out_count) override
+				{
+					return READ_ERR_IO;
+				}
+
+				Write_result write(char const *src, Vfs::file_size count,
+				                   Vfs::file_size &out_count) override
+				{
+					return WRITE_ERR_IO;
+				}
+		};
+
+	public:
+
+		Libusb_file_system(Vfs::Env &env,
+		                   Genode::Xml_node config)
+		:
+			Single_file_system(Vfs::Node_type::CONTINUOUS_FILE, name(),
+			                   Vfs::Node_rwx::ro(), config),
+			_env(env) { }
+
+		~Libusb_file_system() { }
+
+		static char const *name()   { return "libusb"; }
+		char const *type() override { return "libusb"; }
+
+		/*********************************
+		 ** Directory service interface **
+		 *********************************/
+
+		Open_result open(char const         *path, unsigned,
+		                 Vfs::Vfs_handle   **out_handle,
+		                 Genode::Allocator  &alloc) override
+		{
+			if (!_single_file(path))
+				return OPEN_ERR_UNACCESSIBLE;
+
+			*out_handle = new (alloc)
+				Libusb_vfs_handle(*this, *this, alloc, _env.env());
+			return OPEN_OK;
+		}
+
+};
+
+
+struct Libusb_factory : Vfs::File_system_factory
+{
+	Vfs::File_system *create(Vfs::Env &env, Genode::Xml_node node) override
+	{
+		return new (env.alloc()) Libusb_file_system(env, node);
+	}
+};
+
+
+extern "C" Vfs::File_system_factory *vfs_file_system_factory(void)
+{
+	static Libusb_factory factory;
+	return &factory;
+}