diff --git a/tools/lib/atusb.c b/tools/lib/atusb.c
index 5f7dc49..5efe1fb 100644
--- a/tools/lib/atusb.c
+++ b/tools/lib/atusb.c
@@ -54,6 +54,8 @@ static void *atusb_open(const char *arg)
 {
 	usb_dev_handle *dev;
 
+	if (arg)
+		restrict_usb_path(arg);
 	dev = open_usb(USB_VENDOR, USB_PRODUCT);
 	if (dev) {
 		error = 0;
diff --git a/tools/lib/usbopen.c b/tools/lib/usbopen.c
index 8561ebf..830cda9 100644
--- a/tools/lib/usbopen.c
+++ b/tools/lib/usbopen.c
@@ -1,8 +1,8 @@
 /*
  * lib/usbopen.c - Common USB device lookup and open code
  *
- * Written 2008-2010 by Werner Almesberger
- * Copyright 2008-2010 Werner Almesberger
+ * Written 2008-2011 by Werner Almesberger
+ * Copyright 2008-2011 Werner Almesberger
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -12,16 +12,37 @@
 
 
 
+#include <stdarg.h>
 #include <stdint.h>
 #include <stdlib.h>
 #include <stdio.h>
+#include <string.h>
 #include <usb.h>
 
 #include "usbopen.h"
 
 
+#define	USB_PATH_ROOT	"/sys/bus/usb/devices/"
+#define	USB_BUS_LEAF	"/busnum"
+#define	USB_DEV_LEAF	"/devnum"
+
 static uint16_t vendor = 0;
 static uint16_t product = 0;
+static const struct usb_device *restricted_path = NULL;
+
+
+static void initialize(void)
+{
+	static int initialized = 0;
+
+	if (initialized)
+		return;
+	initialized = 1;
+
+	usb_init();
+	usb_find_busses();
+	usb_find_devices();
+}
 
 
 usb_dev_handle *open_usb(uint16_t default_vendor, uint16_t default_product)
@@ -33,12 +54,12 @@ usb_dev_handle *open_usb(uint16_t default_vendor, uint16_t default_product)
 	int res;
 #endif
 
-	usb_init();
-	usb_find_busses();
-	usb_find_devices();
+	initialize();
 
 	for (bus = usb_get_busses(); bus; bus = bus->next)
 		for (dev = bus->devices; dev; dev = dev->next) {
+			if (restricted_path && restricted_path != dev)
+				continue;
 			if (dev->descriptor.idVendor !=
 			    (vendor ? vendor : default_vendor))
 				continue;
@@ -99,3 +120,111 @@ void parse_usb_id(const char *id)
 		bad_id(id);
 	product = tmp;
 }
+
+
+static void restrict_usb_dev(int bus_num, int dev_num)
+{
+	const struct usb_bus *bus;
+	const struct usb_device *dev;
+
+	initialize();
+
+	for (bus = usb_busses; bus; bus = bus->next) {
+		if (atoi(bus->dirname) != bus_num)
+			continue;
+		for (dev = bus->devices; dev; dev = dev->next)
+			if (dev->devnum == dev_num) {
+				restricted_path = dev;
+				return;
+			}
+	}
+	fprintf(stderr, "no device %d/%d\n", bus_num, dev_num);
+	exit(1);
+}
+
+
+static void restrict_usb_by_dev(const char *path)
+{
+	int bus_num, dev_num;
+
+	if (sscanf(path, "%d/%d", &bus_num, &dev_num) != 2) {
+		fprintf(stderr, "invalid device syntax \"%s\"\n", path);
+		exit(1);
+	}
+	restrict_usb_dev(bus_num, dev_num);
+}
+
+
+static int read_num(const char *fmt, ...)
+{
+	va_list ap;
+	FILE *file;
+	char *buf;
+	int n, num;
+
+	va_start(ap, fmt);
+	n = vsnprintf(NULL, 0, fmt, ap);
+	va_end(ap);
+
+	buf = malloc(n+1);
+	if (!buf) {
+		perror("malloc");
+		exit(1);
+	}
+
+	va_start(ap, fmt);
+	vsprintf(buf, fmt, ap);
+	va_end(ap);
+
+	file = fopen(buf, "r");
+	if (!file) {
+		perror(buf);
+		exit(1);
+	}
+
+	n = fscanf(file, "%d", &num);
+	if (n <0) {
+		perror(buf);
+		exit(1);
+	}
+	if (n != 1) {
+		fprintf(stderr, "%s: can't read number\n", buf);
+		exit(1);
+	}
+
+	fclose(file);
+	free(buf);
+
+	return num;
+}
+
+
+static void restrict_usb_by_port(const char *path)
+{
+	const char *p;
+	int bus_num, dev_num;
+
+	/*
+	 * We sanitize the path, in case we're part of a program running with
+	 * suid.
+	 */
+	for (p = path; *p; p++)
+		if (!strchr("0123456789-.", *p)) {
+			fprintf(stderr,
+			    "invalid character \'%c\' in USB path\n", *p);
+			exit(1);
+		}
+
+	bus_num = read_num("%s%s%s", USB_PATH_ROOT, path, USB_BUS_LEAF);
+	dev_num = read_num("%s%s%s", USB_PATH_ROOT, path, USB_DEV_LEAF);
+	restrict_usb_dev(bus_num, dev_num);
+}
+
+
+void restrict_usb_path(const char *path)
+{
+	if (strchr(path, '/'))
+		restrict_usb_by_dev(path);
+	else
+		restrict_usb_by_port(path);
+}
diff --git a/tools/lib/usbopen.h b/tools/lib/usbopen.h
index f784229..53a5e31 100644
--- a/tools/lib/usbopen.h
+++ b/tools/lib/usbopen.h
@@ -1,8 +1,8 @@
 /*
  * lib/usbopen.h - Common USB device lookup and open code
  *
- * Written 2008-2010 by Werner Almesberger
- * Copyright 2008-2010 Werner Almesberger
+ * Written 2008-2011 by Werner Almesberger
+ * Copyright 2008-2011 Werner Almesberger
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -20,5 +20,6 @@
 
 usb_dev_handle *open_usb(uint16_t default_vendor, uint16_t default_product);
 void parse_usb_id(const char *id);
+void restrict_usb_path(const char *path);
 
 #endif /* !USB_OPEN_H */