Recommit: Update libusb 1.0.9 to libusbx 1.0.16

third_party/libusb/src/libusb/os/darwin_usb.c

Index: third_party/libusb/src/libusb/os/darwin_usb.c
diff --git a/third_party/libusb/src/libusb/os/darwin_usb.c b/third_party/libusb/src/libusb/os/darwin_usb.c
index 8834d0f958b3693c94a62a9724f76dd1a2447b1b..a24558cb593b4ecdca032a45ddc8fcba7b0faabd 100644
--- a/third_party/libusb/src/libusb/os/darwin_usb.c
+++ b/third_party/libusb/src/libusb/os/darwin_usb.c
@@ -1,6 +1,7 @@
+/* -*- Mode: C; indent-tabs-mode:nil -*- */
 /*
- * darwin backend for libusb 1.0
- * Copyright (C) 2008-2011 Nathan Hjelm <hjelmn@users.sourceforge.net>
+ * darwin backend for libusbx 1.0
+ * Copyright © 2008-2013 Nathan Hjelm <hjelmn@users.sourceforge.net>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -17,19 +18,16 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
-#include <config.h>
+#include "config.h"
 #include <ctype.h>
-#include <dirent.h>
 #include <errno.h>
-#include <fcntl.h>
 #include <pthread.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <sys/ioctl.h>
-#include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
+#include <fcntl.h>
 #include <libkern/OSAtomic.h>
 
 #include <mach/clock.h>
@@ -42,22 +40,23 @@
   #include <objc/objc-auto.h>
 #endif
 
-#include <IOKit/IOCFBundle.h>
-#include <IOKit/usb/IOUSBLib.h>
-#include <IOKit/IOCFPlugIn.h>
-
 #include "darwin_usb.h"
 
 /* async event thread */
-static pthread_mutex_t libusb_darwin_at_mutex;
-static pthread_cond_t  libusb_darwin_at_cond;
+static pthread_mutex_t libusb_darwin_at_mutex = PTHREAD_MUTEX_INITIALIZER;
+static pthread_cond_t  libusb_darwin_at_cond = PTHREAD_COND_INITIALIZER;
 
 static clock_serv_t clock_realtime;
 static clock_serv_t clock_monotonic;
 
-static CFRunLoopRef libusb_darwin_acfl = NULL; /* async cf loop */
+static CFRunLoopRef libusb_darwin_acfl = NULL; /* event cf loop */
 static volatile int32_t initCount = 0;
 
+static usbi_mutex_t darwin_cached_devices_lock = PTHREAD_MUTEX_INITIALIZER;
+static struct list_head darwin_cached_devices = {&darwin_cached_devices, &darwin_cached_devices};
+
+#define DARWIN_CACHED_DEVICE(a) ((struct darwin_cached_device *) (((struct darwin_device_priv *)((a)->os_priv))->dev))
+
 /* async event thread */
 static pthread_t libusb_darwin_at;
 
@@ -67,6 +66,10 @@ static int darwin_release_interface(struct libusb_device_handle *dev_handle, int
 static int darwin_reset_device(struct libusb_device_handle *dev_handle);
 static void darwin_async_io_callback (void *refcon, IOReturn result, void *arg0);
 
+static int darwin_scan_devices(struct libusb_context *ctx);
+static int process_new_device (struct libusb_context *ctx, io_service_t service);
+
+#if defined(ENABLE_LOGGING)
 static const char *darwin_error_str (int result) {
   switch (result) {
   case kIOReturnSuccess:
@@ -95,10 +98,15 @@ static const char *darwin_error_str (int result) {
     return "data overrun";
   case kIOReturnCannotWire:
     return "physical memory can not be wired down";
+  case kIOReturnNoResources:
+    return "out of resources";
+  case kIOUSBHighSpeedSplitError:
+    return "high speed split error";
   default:
     return "unknown error";
   }
 }
+#endif
 
 static int darwin_to_libusb (int result) {
   switch (result) {
@@ -125,6 +133,21 @@ static int darwin_to_libusb (int result) {
   }
 }
 
+/* this function must be called with the darwin_cached_devices_lock held */
+static void darwin_deref_cached_device(struct darwin_cached_device *cached_dev) {
+  cached_dev->refcount--;
+  /* free the device and remove it from the cache */
+  if (0 == cached_dev->refcount) {
+    list_del(&cached_dev->list);
+
+    (*(cached_dev->device))->Release(cached_dev->device);
+    free (cached_dev);
+  }
+}
+
+static void darwin_ref_cached_device(struct darwin_cached_device *cached_dev) {
+  cached_dev->refcount++;
+}
 
 static int ep_to_pipeRef(struct libusb_device_handle *dev_handle, uint8_t ep, uint8_t *pipep, uint8_t *ifcp) {
   struct darwin_device_handle_priv *priv = (struct darwin_device_handle_priv *)dev_handle->os_priv;
@@ -134,19 +157,19 @@ static int ep_to_pipeRef(struct libusb_device_handle *dev_handle, uint8_t ep, ui
 
   int8_t i, iface;
 
-  usbi_info (HANDLE_CTX(dev_handle), "converting ep address 0x%02x to pipeRef and interface", ep);
+  usbi_dbg ("converting ep address 0x%02x to pipeRef and interface", ep);
 
   for (iface = 0 ; iface < USB_MAXINTERFACES ; iface++) {
     cInterface = &priv->interfaces[iface];
 
     if (dev_handle->claimed_interfaces & (1 << iface)) {
       for (i = 0 ; i < cInterface->num_endpoints ; i++) {
-	if (cInterface->endpoint_addrs[i] == ep) {
-	  *pipep = i + 1;
-	  *ifcp = iface;
-	  usbi_info (HANDLE_CTX(dev_handle), "pipe %d on interface %d matches", *pipep, *ifcp);
-	  return 0;
-	}
+        if (cInterface->endpoint_addrs[i] == ep) {
+          *pipep = i + 1;
+          *ifcp = iface;
+          usbi_dbg ("pipe %d on interface %d matches", *pipep, *ifcp);
+          return 0;
+        }
       }
     }
   }
@@ -157,7 +180,7 @@ static int ep_to_pipeRef(struct libusb_device_handle *dev_handle, uint8_t ep, ui
   return -1;
 }
 
-static int usb_setup_device_iterator (io_iterator_t *deviceIterator, long location) {
+static int usb_setup_device_iterator (io_iterator_t *deviceIterator, UInt32 location) {
   CFMutableDictionaryRef matchingDict = IOServiceMatching(kIOUSBDeviceClassName);
 
   if (!matchingDict)
@@ -169,7 +192,9 @@ static int usb_setup_device_iterator (io_iterator_t *deviceIterator, long locati
                                                                          &kCFTypeDictionaryValueCallBacks);
 
     if (propertyMatchDict) {
-      CFTypeRef locationCF = CFNumberCreate (NULL, kCFNumberLongType, &location);
+      /* there are no unsigned CFNumber types so treat the value as signed. the os seems to do this
+         internally (CFNumberType of locationID is 3) */
+      CFTypeRef locationCF = CFNumberCreate (NULL, kCFNumberSInt32Type, &location);
 
       CFDictionarySetValue (propertyMatchDict, CFSTR(kUSBDevicePropertyLocationID), locationCF);
       /* release our reference to the CFNumber (CFDictionarySetValue retains it) */
@@ -185,118 +210,92 @@ static int usb_setup_device_iterator (io_iterator_t *deviceIterator, long locati
   return IOServiceGetMatchingServices(kIOMasterPortDefault, matchingDict, deviceIterator);
 }
 
-static usb_device_t **usb_get_next_device (io_iterator_t deviceIterator, UInt32 *locationp) {
-  io_cf_plugin_ref_t *plugInInterface = NULL;
-  usb_device_t **device;
-  io_service_t usbDevice;
-  long result;
-  SInt32 score;
+static int get_ioregistry_value_number (io_service_t service, CFStringRef property, CFNumberType type, void *p) {
+  CFTypeRef cfNumber = IORegistryEntryCreateCFProperty (service, property, kCFAllocatorDefault, 0);
+  int ret = 0;
 
-  if (!IOIteratorIsValid (deviceIterator))
-    return NULL;
+  if (cfNumber) {
+    if (CFGetTypeID(cfNumber) == CFNumberGetTypeID()) {
+      ret = CFNumberGetValue(cfNumber, type, p);
+    }
 
+    CFRelease (cfNumber);
+  }
 
-  while ((usbDevice = IOIteratorNext(deviceIterator))) {
-    result = IOCreatePlugInInterfaceForService(usbDevice, kIOUSBDeviceUserClientTypeID,
-					       kIOCFPlugInInterfaceID, &plugInInterface,
-					       &score);
+  return ret;
+}
 
-    /* we are done with the usb_device_t */
-    (void)IOObjectRelease(usbDevice);
-    if (kIOReturnSuccess == result && plugInInterface)
-      break;
+static usb_device_t **darwin_device_from_service (io_service_t service)
+{
+  io_cf_plugin_ref_t *plugInInterface = NULL;
+  usb_device_t **device;
+  kern_return_t result;
+  SInt32 score;
 
-    usbi_dbg ("libusb/darwin.c usb_get_next_device: could not set up plugin for service: %s\n", darwin_error_str (result));
-  }
+  result = IOCreatePlugInInterfaceForService(service, kIOUSBDeviceUserClientTypeID,
+                                             kIOCFPlugInInterfaceID, &plugInInterface,
+                                             &score);
 
-  if (!usbDevice)
+  if (kIOReturnSuccess != result || !plugInInterface) {
+    usbi_dbg ("could not set up plugin for service: %s\n", darwin_error_str (result));
     return NULL;
+  }
 
   (void)(*plugInInterface)->QueryInterface(plugInInterface, CFUUIDGetUUIDBytes(DeviceInterfaceID),
-					   (LPVOID)&device);
-
-  (*plugInInterface)->Stop(plugInInterface);
-  IODestroyPlugInInterface (plugInInterface);
-
-  /* get the location from the device */
-  if (locationp)
-    (*(device))->GetLocationID(device, locationp);
+                                           (LPVOID)&device);
+  /* Use release instead of IODestroyPlugInInterface to avoid stopping IOServices associated with this device */
+  (*plugInInterface)->Release (plugInInterface);
 
   return device;
 }
 
-static kern_return_t darwin_get_device (uint32_t dev_location, usb_device_t ***darwin_device) {
-  kern_return_t kresult;
-  UInt32        location;
-  io_iterator_t deviceIterator;
+static void darwin_devices_attached (void *ptr, io_iterator_t add_devices) {
+  struct libusb_context *ctx;
+  io_service_t service;
 
-  kresult = usb_setup_device_iterator (&deviceIterator, dev_location);
-  if (kresult)
-    return kresult;
+  usbi_mutex_lock(&active_contexts_lock);
 
-  /* This port of libusb uses locations to keep track of devices. */
-  while ((*darwin_device = usb_get_next_device (deviceIterator, &location)) != NULL) {
-    if (location == dev_location)
-      break;
+  while ((service = IOIteratorNext(add_devices))) {
+    /* add this device to each active context's device list */
+    list_for_each_entry(ctx, &active_contexts_list, list, struct libusb_context) {
+      process_new_device (ctx, service);;
+    }
 
-    (**darwin_device)->Release(*darwin_device);
+    IOObjectRelease(service);
   }
 
-  IOObjectRelease (deviceIterator);
-
-  if (!(*darwin_device))
-    return kIOReturnNoDevice;
-
-  return kIOReturnSuccess;
+  usbi_mutex_unlock(&active_contexts_lock);
 }
 
-
-
 static void darwin_devices_detached (void *ptr, io_iterator_t rem_devices) {
-  struct libusb_context *ctx = (struct libusb_context *)ptr;
-  struct libusb_device_handle *handle;
-  struct darwin_device_priv *dpriv;
-  struct darwin_device_handle_priv *priv;
+  struct libusb_device *dev = NULL;
+  struct libusb_context *ctx;
 
   io_service_t device;
-  long location;
-  bool locationValid;
-  CFTypeRef locationCF;
-  UInt32 message;
-
-  usbi_info (ctx, "a device has been detached");
+  UInt64 session;
+  int ret;
 
   while ((device = IOIteratorNext (rem_devices)) != 0) {
     /* get the location from the i/o registry */
-    locationCF = IORegistryEntryCreateCFProperty (device, CFSTR(kUSBDevicePropertyLocationID), kCFAllocatorDefault, 0);
-
+    ret = get_ioregistry_value_number (device, CFSTR("sessionID"), kCFNumberSInt64Type, &session);
     IOObjectRelease (device);
-
-    if (!locationCF)
+    if (!ret)
       continue;
 
-    locationValid = CFGetTypeID(locationCF) == CFNumberGetTypeID() &&
-	    CFNumberGetValue(locationCF, kCFNumberLongType, &location);
-
-    CFRelease (locationCF);
-
-    if (!locationValid)
-      continue;
+    usbi_mutex_lock(&active_contexts_lock);
 
-    usbi_mutex_lock(&ctx->open_devs_lock);
-    list_for_each_entry(handle, &ctx->open_devs, list, struct libusb_device_handle) {
-      dpriv = (struct darwin_device_priv *)handle->dev->os_priv;
+    list_for_each_entry(ctx, &active_contexts_list, list, struct libusb_context) {
+      usbi_dbg ("notifying context %p of device disconnect", ctx);
 
-      /* the device may have been opened several times. write to each handle's event descriptor */
-      if (dpriv->location == location  && handle->os_priv) {
-	priv  = (struct darwin_device_handle_priv *)handle->os_priv;
-
-	message = MESSAGE_DEVICE_GONE;
-	write (priv->fds[1], &message, sizeof (message));
+      dev = usbi_get_device_by_session_id(ctx, session);
+      if (dev) {
+        /* signal the core that this device has been disconnected. the core will tear down this device
+           when the reference count reaches 0 */
+        usbi_disconnect_device(dev);
       }
     }
 
-    usbi_mutex_unlock(&ctx->open_devs_lock);
+    usbi_mutex_unlock(&active_contexts_lock);
   }
 }
 
@@ -307,7 +306,7 @@ static void darwin_clear_iterator (io_iterator_t iter) {
     IOObjectRelease (device);
 }
 
-static void *event_thread_main (void *arg0) {
+static void *darwin_event_thread_main (void *arg0) {
   IOReturn kresult;
   struct libusb_context *ctx = (struct libusb_context *)arg0;
   CFRunLoopRef runloop;
@@ -315,23 +314,22 @@ static void *event_thread_main (void *arg0) {
   /* Set this thread's name, so it can be seen in the debugger
      and crash reports. */
 #if MAC_OS_X_VERSION_MIN_REQUIRED >= 1060
-  pthread_setname_np ("org.libusb.device-detach");
-#endif
+  pthread_setname_np ("org.libusb.device-hotplug");
 
   /* Tell the Objective-C garbage collector about this thread.
      This is required because, unlike NSThreads, pthreads are
      not automatically registered. Although we don't use
      Objective-C, we use CoreFoundation, which does. */
-#if MAC_OS_X_VERSION_MIN_REQUIRED >= 1060
   objc_registerThreadWithCollector();
 #endif
 
-  /* hotplug (device removal) source */
+  /* hotplug (device arrival/removal) sources */
   CFRunLoopSourceRef     libusb_notification_cfsource;
   io_notification_port_t libusb_notification_port;
   io_iterator_t          libusb_rem_device_iterator;
+  io_iterator_t          libusb_add_device_iterator;
 
-  usbi_info (ctx, "creating hotplug event source");
+  usbi_dbg ("creating hotplug event source");
 
   runloop = CFRunLoopGetCurrent ();
   CFRetain (runloop);
@@ -343,9 +341,21 @@ static void *event_thread_main (void *arg0) {
 
   /* create notifications for removed devices */
   kresult = IOServiceAddMatchingNotification (libusb_notification_port, kIOTerminatedNotification,
-					      IOServiceMatching(kIOUSBDeviceClassName),
-					      (IOServiceMatchingCallback)darwin_devices_detached,
-					      (void *)ctx, &libusb_rem_device_iterator);
+                                              IOServiceMatching(kIOUSBDeviceClassName),
+                                              (IOServiceMatchingCallback)darwin_devices_detached,
+                                              (void *)ctx, &libusb_rem_device_iterator);
+
+  if (kresult != kIOReturnSuccess) {
+    usbi_err (ctx, "could not add hotplug event source: %s", darwin_error_str (kresult));
+
+    pthread_exit (NULL);
+  }
+
+  /* create notifications for attached devices */
+  kresult = IOServiceAddMatchingNotification(libusb_notification_port, kIOFirstMatchNotification,
+                                              IOServiceMatching(kIOUSBDeviceClassName),
+                                              (IOServiceMatchingCallback)darwin_devices_attached,
+                                              (void *)ctx, &libusb_add_device_iterator);
 
   if (kresult != kIOReturnSuccess) {
     usbi_err (ctx, "could not add hotplug event source: %s", darwin_error_str (kresult));
@@ -355,10 +365,11 @@ static void *event_thread_main (void *arg0) {
 
   /* arm notifiers */
   darwin_clear_iterator (libusb_rem_device_iterator);
+  darwin_clear_iterator (libusb_add_device_iterator);
 
-  usbi_info (ctx, "thread ready to receive events");
+  usbi_dbg ("darwin event thread ready to receive events");
 
-  /* signal the main thread that the async runloop has been created. */
+  /* signal the main thread that the hotplug runloop has been created. */
   pthread_mutex_lock (&libusb_darwin_at_mutex);
   libusb_darwin_acfl = runloop;
   pthread_cond_signal (&libusb_darwin_at_cond);
@@ -367,11 +378,17 @@ static void *event_thread_main (void *arg0) {
   /* run the runloop */
   CFRunLoopRun();
 
-  usbi_info (ctx, "thread exiting");
+  usbi_dbg ("darwin event thread exiting");
+
+  /* remove the notification cfsource */
+  CFRunLoopRemoveSource(runloop, libusb_notification_cfsource, kCFRunLoopDefaultMode);
 
   /* delete notification port */
   IONotificationPortDestroy (libusb_notification_port);
+
+  /* delete iterators */
   IOObjectRelease (libusb_rem_device_iterator);
+  IOObjectRelease (libusb_add_device_iterator);
 
   CFRelease (runloop);
 
@@ -380,21 +397,40 @@ static void *event_thread_main (void *arg0) {
   pthread_exit (NULL);
 }
 
+static void _darwin_finalize(void) {
+  struct darwin_cached_device *dev, *next;
+
+  usbi_mutex_lock(&darwin_cached_devices_lock);
+  list_for_each_entry_safe(dev, next, &darwin_cached_devices, list, struct darwin_cached_device) {
+    darwin_deref_cached_device(dev);
+  }
+  usbi_mutex_unlock(&darwin_cached_devices_lock);
+}
+
 static int darwin_init(struct libusb_context *ctx) {
   host_name_port_t host_self;
+  static int initted = 0;
+  int rc;
+
+  rc = darwin_scan_devices (ctx);
+  if (LIBUSB_SUCCESS != rc) {
+    return rc;
+  }
 
   if (OSAtomicIncrement32Barrier(&initCount) == 1) {
     /* create the clocks that will be used */
 
+    if (!initted) {
+      initted = 1;
+      atexit(_darwin_finalize);
+    }
+
     host_self = mach_host_self();
     host_get_clock_service(host_self, CALENDAR_CLOCK, &clock_realtime);
     host_get_clock_service(host_self, SYSTEM_CLOCK, &clock_monotonic);
     mach_port_deallocate(mach_task_self(), host_self);
 
-    pthread_mutex_init (&libusb_darwin_at_mutex, NULL);
-    pthread_cond_init (&libusb_darwin_at_cond, NULL);
-
-    pthread_create (&libusb_darwin_at, NULL, event_thread_main, (void *)ctx);
+    pthread_create (&libusb_darwin_at, NULL, darwin_event_thread_main, (void *)ctx);
 
     pthread_mutex_lock (&libusb_darwin_at_mutex);
     while (!libusb_darwin_acfl)
@@ -402,7 +438,7 @@ static int darwin_init(struct libusb_context *ctx) {
     pthread_mutex_unlock (&libusb_darwin_at_mutex);
   }
 
-  return 0;
+  return rc;
 }
 
 static void darwin_exit (void) {
@@ -410,14 +446,14 @@ static void darwin_exit (void) {
     mach_port_deallocate(mach_task_self(), clock_realtime);
     mach_port_deallocate(mach_task_self(), clock_monotonic);
 
-    /* stop the async runloop and wait for the thread to terminate. */
+    /* stop the event runloop and wait for the thread to terminate. */
     CFRunLoopStop (libusb_darwin_acfl);
     pthread_join (libusb_darwin_at, NULL);
   }
 }
 
 static int darwin_get_device_descriptor(struct libusb_device *dev, unsigned char *buffer, int *host_endian) {
-  struct darwin_device_priv *priv = (struct darwin_device_priv *)dev->os_priv;
+  struct darwin_cached_device *priv = DARWIN_CACHED_DEVICE(dev);
 
   /* return cached copy */
   memmove (buffer, &(priv->dev_descriptor), DEVICE_DESC_LENGTH);
@@ -428,7 +464,7 @@ static int darwin_get_device_descriptor(struct libusb_device *dev, unsigned char
 }
 
 static int get_configuration_index (struct libusb_device *dev, int config_value) {
-  struct darwin_device_priv *priv = (struct darwin_device_priv *)dev->os_priv;
+  struct darwin_cached_device *priv = DARWIN_CACHED_DEVICE(dev);
   UInt8 i, numConfig;
   IOUSBConfigurationDescriptorPtr desc;
   IOReturn kresult;
@@ -446,15 +482,15 @@ static int get_configuration_index (struct libusb_device *dev, int config_value)
   }
 
   /* configuration not found */
-  return LIBUSB_ERROR_OTHER;
+  return LIBUSB_ERROR_NOT_FOUND;
 }
 
 static int darwin_get_active_config_descriptor(struct libusb_device *dev, unsigned char *buffer, size_t len, int *host_endian) {
-  struct darwin_device_priv *priv = (struct darwin_device_priv *)dev->os_priv;
+  struct darwin_cached_device *priv = DARWIN_CACHED_DEVICE(dev);
   int config_index;
 
   if (0 == priv->active_config)
-    return LIBUSB_ERROR_INVALID_PARAM;
+    return LIBUSB_ERROR_NOT_FOUND;
 
   config_index = get_configuration_index (dev, priv->active_config);
   if (config_index < 0)
@@ -464,27 +500,15 @@ static int darwin_get_active_config_descriptor(struct libusb_device *dev, unsign
 }
 
 static int darwin_get_config_descriptor(struct libusb_device *dev, uint8_t config_index, unsigned char *buffer, size_t len, int *host_endian) {
-  struct darwin_device_priv *priv = (struct darwin_device_priv *)dev->os_priv;
+  struct darwin_cached_device *priv = DARWIN_CACHED_DEVICE(dev);
   IOUSBConfigurationDescriptorPtr desc;
   IOReturn kresult;
-  usb_device_t **device = NULL;
+  int ret;
 
-  if (!priv)
+  if (!priv || !priv->device)
     return LIBUSB_ERROR_OTHER;
 
-  if (!priv->device) {
-    kresult = darwin_get_device (priv->location, &device);
-    if (kresult || !device) {
-      usbi_err (DEVICE_CTX (dev), "could not find device: %s", darwin_error_str (kresult));
-
-      return darwin_to_libusb (kresult);
-    }
-
-    /* don't have to open the device to get a config descriptor */
-  } else
-    device = priv->device;
-
-  kresult = (*device)->GetConfigurationDescriptorPtr (device, config_index, &desc);
+  kresult = (*priv->device)->GetConfigurationDescriptorPtr (priv->device, config_index, &desc);
   if (kresult == kIOReturnSuccess) {
     /* copy descriptor */
     if (libusb_le16_to_cpu(desc->wTotalLength) < len)
@@ -496,15 +520,16 @@ static int darwin_get_config_descriptor(struct libusb_device *dev, uint8_t confi
     *host_endian = 0;
   }
 
-  if (!priv->device)
-    (*device)->Release (device);
+  ret = darwin_to_libusb (kresult);
+  if (ret != LIBUSB_SUCCESS)
+    return ret;
 
-  return darwin_to_libusb (kresult);
+  return len;
 }
 
 /* check whether the os has configured the device */
-static int darwin_check_configuration (struct libusb_context *ctx, struct libusb_device *dev, usb_device_t **darwin_device) {
-  struct darwin_device_priv *priv = (struct darwin_device_priv *)dev->os_priv;
+static int darwin_check_configuration (struct libusb_context *ctx, struct darwin_cached_device *dev) {
+  usb_device_t **darwin_device = dev->device;
 
   IOUSBConfigurationDescriptorPtr configDesc;
   IOUSBFindInterfaceRequest request;
@@ -512,14 +537,14 @@ static int darwin_check_configuration (struct libusb_context *ctx, struct libusb
   io_iterator_t             interface_iterator;
   io_service_t              firstInterface;
 
-  if (priv->dev_descriptor.bNumConfigurations < 1) {
+  if (dev->dev_descriptor.bNumConfigurations < 1) {
     usbi_err (ctx, "device has no configurations");
     return LIBUSB_ERROR_OTHER; /* no configurations at this speed so we can't use it */
   }
 
   /* find the first configuration */
   kresult = (*darwin_device)->GetConfigurationDescriptorPtr (darwin_device, 0, &configDesc);
-  priv->first_config = (kIOReturnSuccess == kresult) ? configDesc->bConfigurationValue : 1;
+  dev->first_config = (kIOReturnSuccess == kresult) ? configDesc->bConfigurationValue : 1;
 
   /* check if the device is already configured. there is probably a better way than iterating over the
      to accomplish this (the trick is we need to avoid a call to GetConfigurations since buggy devices
@@ -545,72 +570,83 @@ static int darwin_check_configuration (struct libusb_context *ctx, struct libusb
     IOObjectRelease (firstInterface);
 
     /* device is configured */
-    if (priv->dev_descriptor.bNumConfigurations == 1)
+    if (dev->dev_descriptor.bNumConfigurations == 1)
       /* to avoid problems with some devices get the configurations value from the configuration descriptor */
-      priv->active_config = priv->first_config;
+      dev->active_config = dev->first_config;
     else
       /* devices with more than one configuration should work with GetConfiguration */
-      (*darwin_device)->GetConfiguration (darwin_device, &priv->active_config);
+      (*darwin_device)->GetConfiguration (darwin_device, &dev->active_config);
   } else
     /* not configured */
-    priv->active_config = 0;
+    dev->active_config = 0;
   
-  usbi_info (ctx, "active config: %u, first config: %u", priv->active_config, priv->first_config);
+  usbi_dbg ("active config: %u, first config: %u", dev->active_config, dev->first_config);
 
   return 0;
 }
 
-static int darwin_cache_device_descriptor (struct libusb_context *ctx, struct libusb_device *dev, usb_device_t **device) {
-  struct darwin_device_priv *priv;
-  int retries = 5, delay = 30000;
+static int darwin_request_descriptor (usb_device_t **device, UInt8 desc, UInt8 desc_index, void *buffer, size_t buffer_size) {
+  IOUSBDevRequestTO req;
+
+  memset (buffer, 0, buffer_size);
+
+  /* Set up request for descriptor/ */
+  req.bmRequestType = USBmakebmRequestType(kUSBIn, kUSBStandard, kUSBDevice);
+  req.bRequest      = kUSBRqGetDescriptor;
+  req.wValue        = desc << 8;
+  req.wIndex        = desc_index;
+  req.wLength       = buffer_size;
+  req.pData         = buffer;
+  req.noDataTimeout = 20;
+  req.completionTimeout = 100;
+
+  return (*device)->DeviceRequestTO (device, &req);
+}
+
+static int darwin_cache_device_descriptor (struct libusb_context *ctx, struct darwin_cached_device *dev) {
+  usb_device_t **device = dev->device;
+  int retries = 1, delay = 30000;
   int unsuspended = 0, try_unsuspend = 1, try_reconfigure = 1;
   int is_open = 0;
   int ret = 0, ret2;
-  IOUSBDevRequest req;
   UInt8 bDeviceClass;
   UInt16 idProduct, idVendor;
 
+  dev->can_enumerate = 0;
+
   (*device)->GetDeviceClass (device, &bDeviceClass);
   (*device)->GetDeviceProduct (device, &idProduct);
   (*device)->GetDeviceVendor (device, &idVendor);
 
-  priv = (struct darwin_device_priv *)dev->os_priv;
-
-  /* try to open the device (we can usually continue even if this fails) */
+  /* According to Apple's documentation the device must be open for DeviceRequest but we may not be able to open some
+   * devices and Apple's USB Prober doesn't bother to open the device before issuing a descriptor request.  Still,
+   * to follow the spec as closely as possible, try opening the device */
   is_open = ((*device)->USBDeviceOpenSeize(device) == kIOReturnSuccess);
 
-  /**** retrieve device descriptor ****/
   do {
-    /* Set up request for device descriptor */
-    memset (&(priv->dev_descriptor), 0, sizeof(IOUSBDeviceDescriptor));
-    req.bmRequestType = USBmakebmRequestType(kUSBIn, kUSBStandard, kUSBDevice);
-    req.bRequest      = kUSBRqGetDescriptor;
-    req.wValue        = kUSBDeviceDesc << 8;
-    req.wIndex        = 0;
-    req.wLength       = sizeof(priv->dev_descriptor);
-    req.pData         = &(priv->dev_descriptor);
-
-    /* according to Apple's documentation the device must be open for DeviceRequest but we may not be able to open some
-     * devices and Apple's USB Prober doesn't bother to open the device before issuing a descriptor request.  Still,
-     * to follow the spec as closely as possible, try opening the device */
-
-    ret = (*(device))->DeviceRequest (device, &req);
-
-    if (kIOReturnOverrun == ret && kUSBDeviceDesc == priv->dev_descriptor.bDescriptorType)
+    /**** retrieve device descriptor ****/
+    ret = darwin_request_descriptor (device, kUSBDeviceDesc, 0, &dev->dev_descriptor, sizeof(dev->dev_descriptor));
+
+    if (kIOReturnOverrun == ret && kUSBDeviceDesc == dev->dev_descriptor.bDescriptorType)
       /* received an overrun error but we still received a device descriptor */
       ret = kIOReturnSuccess;
 
-    if (kIOReturnSuccess == ret && (0 == priv->dev_descriptor.bNumConfigurations ||
-				    0 == priv->dev_descriptor.bcdUSB)) {
+    if (kIOUSBVendorIDAppleComputer == idVendor) {
+      /* NTH: don't bother retrying or unsuspending Apple devices */
+      break;
+    }
+
+    if (kIOReturnSuccess == ret && (0 == dev->dev_descriptor.bNumConfigurations ||
+                                    0 == dev->dev_descriptor.bcdUSB)) {
       /* work around for incorrectly configured devices */
       if (try_reconfigure && is_open) {
-	usbi_dbg("descriptor appears to be invalid. resetting configuration before trying again...");
+        usbi_dbg("descriptor appears to be invalid. resetting configuration before trying again...");
 
-	/* set the first configuration */
-	(*device)->SetConfiguration(device, 1);
+        /* set the first configuration */
+        (*device)->SetConfiguration(device, 1);
 
-	/* don't try to reconfigure again */
-	try_reconfigure = 0;
+        /* don't try to reconfigure again */
+        try_reconfigure = 0;
       }
 
       ret = kIOUSBPipeStalled;
@@ -619,25 +655,27 @@ static int darwin_cache_device_descriptor (struct libusb_context *ctx, struct li
     if (kIOReturnSuccess != ret && is_open && try_unsuspend) {
       /* device may be suspended. unsuspend it and try again */
 #if DeviceVersion >= 320
-      UInt32 info;
+      UInt32 info = 0;
 
       /* IOUSBFamily 320+ provides a way to detect device suspension but earlier versions do not */
       (void)(*device)->GetUSBDeviceInformation (device, &info);
 
-      try_unsuspend = info & (1 << kUSBInformationDeviceIsSuspendedBit);
+      /* note that the device was suspended */
+      if (info & (1 << kUSBInformationDeviceIsSuspendedBit) || 0 == info)
+        try_unsuspend = 1;
 #endif
 
       if (try_unsuspend) {
-	/* resume the device */
-	ret2 = (*device)->USBDeviceSuspend (device, 0);
-	if (kIOReturnSuccess != ret2) {
-	  /* prevent log spew from poorly behaving devices.  this indicates the
-	     os actually had trouble communicating with the device */
-	  usbi_dbg("could not retrieve device descriptor. failed to unsuspend: %s",darwin_error_str(ret2));
-	} else
-	  unsuspended = 1;
-
-	try_unsuspend = 0;
+        /* try to unsuspend the device */
+        ret2 = (*device)->USBDeviceSuspend (device, 0);
+        if (kIOReturnSuccess != ret2) {
+          /* prevent log spew from poorly behaving devices.  this indicates the
+             os actually had trouble communicating with the device */
+          usbi_dbg("could not retrieve device descriptor. failed to unsuspend: %s",darwin_error_str(ret2));
+        } else
+          unsuspended = 1;
+
+        try_unsuspend = 0;
       }
     }
 
@@ -658,128 +696,223 @@ static int darwin_cache_device_descriptor (struct libusb_context *ctx, struct li
   if (ret != kIOReturnSuccess) {
     /* a debug message was already printed out for this error */
     if (LIBUSB_CLASS_HUB == bDeviceClass)
-      usbi_dbg ("could not retrieve device descriptor %.4x:%.4x: %s. skipping device", idVendor, idProduct, darwin_error_str (ret));
+      usbi_dbg ("could not retrieve device descriptor %.4x:%.4x: %s (%x). skipping device",
+                idVendor, idProduct, darwin_error_str (ret), ret);
     else
-      usbi_warn (ctx, "could not retrieve device descriptor %.4x:%.4x: %s. skipping device", idVendor, idProduct, darwin_error_str (ret));
-
+      usbi_warn (ctx, "could not retrieve device descriptor %.4x:%.4x: %s (%x). skipping device",
+                 idVendor, idProduct, darwin_error_str (ret), ret);
     return -1;
   }
 
-  usbi_dbg ("device descriptor:");
-  usbi_dbg (" bDescriptorType:    0x%02x", priv->dev_descriptor.bDescriptorType);
-  usbi_dbg (" bcdUSB:             0x%04x", priv->dev_descriptor.bcdUSB);
-  usbi_dbg (" bDeviceClass:       0x%02x", priv->dev_descriptor.bDeviceClass);
-  usbi_dbg (" bDeviceSubClass:    0x%02x", priv->dev_descriptor.bDeviceSubClass);
-  usbi_dbg (" bDeviceProtocol:    0x%02x", priv->dev_descriptor.bDeviceProtocol);
-  usbi_dbg (" bMaxPacketSize0:    0x%02x", priv->dev_descriptor.bMaxPacketSize0);
-  usbi_dbg (" idVendor:           0x%04x", priv->dev_descriptor.idVendor);
-  usbi_dbg (" idProduct:          0x%04x", priv->dev_descriptor.idProduct);
-  usbi_dbg (" bcdDevice:          0x%04x", priv->dev_descriptor.bcdDevice);
-  usbi_dbg (" iManufacturer:      0x%02x", priv->dev_descriptor.iManufacturer);
-  usbi_dbg (" iProduct:           0x%02x", priv->dev_descriptor.iProduct);
-  usbi_dbg (" iSerialNumber:      0x%02x", priv->dev_descriptor.iSerialNumber);
-  usbi_dbg (" bNumConfigurations: 0x%02x", priv->dev_descriptor.bNumConfigurations);
-
   /* catch buggy hubs (which appear to be virtual). Apple's own USB prober has problems with these devices. */
-  if (libusb_le16_to_cpu (priv->dev_descriptor.idProduct) != idProduct) {
+  if (libusb_le16_to_cpu (dev->dev_descriptor.idProduct) != idProduct) {
     /* not a valid device */
     usbi_warn (ctx, "idProduct from iokit (%04x) does not match idProduct in descriptor (%04x). skipping device",
-	       idProduct, libusb_le16_to_cpu (priv->dev_descriptor.idProduct));
+               idProduct, libusb_le16_to_cpu (dev->dev_descriptor.idProduct));
     return -1;
   }
 
+  usbi_dbg ("cached device descriptor:");
+  usbi_dbg ("  bDescriptorType:    0x%02x", dev->dev_descriptor.bDescriptorType);
+  usbi_dbg ("  bcdUSB:             0x%04x", dev->dev_descriptor.bcdUSB);
+  usbi_dbg ("  bDeviceClass:       0x%02x", dev->dev_descriptor.bDeviceClass);
+  usbi_dbg ("  bDeviceSubClass:    0x%02x", dev->dev_descriptor.bDeviceSubClass);
+  usbi_dbg ("  bDeviceProtocol:    0x%02x", dev->dev_descriptor.bDeviceProtocol);
+  usbi_dbg ("  bMaxPacketSize0:    0x%02x", dev->dev_descriptor.bMaxPacketSize0);
+  usbi_dbg ("  idVendor:           0x%04x", dev->dev_descriptor.idVendor);
+  usbi_dbg ("  idProduct:          0x%04x", dev->dev_descriptor.idProduct);
+  usbi_dbg ("  bcdDevice:          0x%04x", dev->dev_descriptor.bcdDevice);
+  usbi_dbg ("  iManufacturer:      0x%02x", dev->dev_descriptor.iManufacturer);
+  usbi_dbg ("  iProduct:           0x%02x", dev->dev_descriptor.iProduct);
+  usbi_dbg ("  iSerialNumber:      0x%02x", dev->dev_descriptor.iSerialNumber);
+  usbi_dbg ("  bNumConfigurations: 0x%02x", dev->dev_descriptor.bNumConfigurations);
+
+  dev->can_enumerate = 1;
+
   return 0;
 }
 
-static int process_new_device (struct libusb_context *ctx, usb_device_t **device, UInt32 locationID, struct discovered_devs **_discdevs) {
-  struct darwin_device_priv *priv;
-  struct libusb_device *dev;
-  struct discovered_devs *discdevs;
-  UInt16                address;
-  UInt8                 devSpeed;
-  int ret = 0, need_unref = 0;
+static int darwin_get_cached_device(struct libusb_context *ctx, io_service_t service,
+                                    struct darwin_cached_device **cached_out) {
+  struct darwin_cached_device *new_device;
+  UInt64 sessionID, parent_sessionID;
+  int ret = LIBUSB_SUCCESS;
+  usb_device_t **device;
+  io_service_t parent;
+  kern_return_t result;
+  UInt8 port = 0;
+
+  *cached_out = NULL;
+
+  /* get some info from the io registry */
+  (void) get_ioregistry_value_number (service, CFSTR("sessionID"), kCFNumberSInt64Type, &sessionID);
+  (void) get_ioregistry_value_number (service, CFSTR("PortNum"), kCFNumberSInt8Type, &port);
+
+  usbi_dbg("finding cached device for sessionID 0x\n" PRIx64, sessionID);
 
+  result = IORegistryEntryGetParentEntry (service, kIOUSBPlane, &parent);
+
+  if (kIOReturnSuccess == result) {
+    (void) get_ioregistry_value_number (parent, CFSTR("sessionID"), kCFNumberSInt64Type, &parent_sessionID);
+    IOObjectRelease(parent);
+  }
+
+  usbi_mutex_lock(&darwin_cached_devices_lock);
   do {
-    dev = usbi_get_device_by_session_id(ctx, locationID);
-    if (!dev) {
-      usbi_info (ctx, "allocating new device for location 0x%08x", locationID);
-      dev = usbi_alloc_device(ctx, locationID);
-      need_unref = 1;
-    } else
-      usbi_info (ctx, "using existing device for location 0x%08x", locationID);
+    list_for_each_entry(new_device, &darwin_cached_devices, list, struct darwin_cached_device) {
+      usbi_dbg("matching sessionID 0x%x against cached device with sessionID 0x%x", sessionID, new_device->session);
+      if (new_device->session == sessionID) {
+        usbi_dbg("using cached device for device");
+        *cached_out = new_device;
+        break;
+      }
+    }
 
-    if (!dev) {
+    if (*cached_out)
+      break;
+
+    usbi_dbg("caching new device with sessionID 0x%x\n", sessionID);
+
+    new_device = calloc (1, sizeof (*new_device));
+    if (!new_device) {
       ret = LIBUSB_ERROR_NO_MEM;
       break;
     }
 
-    priv = (struct darwin_device_priv *)dev->os_priv;
+    device = darwin_device_from_service (service);
+    if (!device) {
+      ret = LIBUSB_ERROR_NO_DEVICE;
+      free (new_device);
+      new_device = NULL;
+      break;
+    }
 
-    (*device)->GetDeviceAddress (device, (USBDeviceAddress *)&address);
+    /* add this device to the cached device list */
+    list_add(&new_device->list, &darwin_cached_devices);
 
-    ret = darwin_cache_device_descriptor (ctx, dev, device);
-    if (ret < 0)
+    (*device)->GetDeviceAddress (device, (USBDeviceAddress *)&new_device->address);
+
+    /* keep a reference to this device */
+    darwin_ref_cached_device(new_device);
+
+    new_device->device = device;
+    new_device->session = sessionID;
+    (*device)->GetLocationID (device, &new_device->location);
+    new_device->port = port;
+    new_device->parent_session = parent_sessionID;
+
+    /* cache the device descriptor */
+    ret = darwin_cache_device_descriptor(ctx, new_device);
+    if (ret)
       break;
 
-    /* check current active configuration (and cache the first configuration value-- which may be used by claim_interface) */
-    ret = darwin_check_configuration (ctx, dev, device);
-    if (ret < 0)
+    if (new_device->can_enumerate) {
+      snprintf(new_device->sys_path, 20, "%03i-%04x-%04x-%02x-%02x", new_device->address,
+               new_device->dev_descriptor.idVendor, new_device->dev_descriptor.idProduct,
+               new_device->dev_descriptor.bDeviceClass, new_device->dev_descriptor.bDeviceSubClass);
+    }
+  } while (0);
+
+  usbi_mutex_unlock(&darwin_cached_devices_lock);
+
+  /* keep track of devices regardless of if we successfully enumerate them to
+     prevent them from being enumerated multiple times */
+
+  *cached_out = new_device;
+
+  return ret;
+}
+
+static int process_new_device (struct libusb_context *ctx, io_service_t service) {
+  struct darwin_device_priv *priv;
+  struct libusb_device *dev = NULL;
+  struct darwin_cached_device *cached_device;
+  UInt8 devSpeed;
+  int ret = 0;
+
+  do {
+    ret = darwin_get_cached_device (ctx, service, &cached_device);
+
+    if (ret < 0 || (cached_device && !cached_device->can_enumerate)) {
+      return ret;
+    }
+
+    /* check current active configuration (and cache the first configuration value--
+       which may be used by claim_interface) */
+    ret = darwin_check_configuration (ctx, cached_device);
+    if (ret)
       break;
 
-    dev->bus_number     = locationID >> 24;
-    dev->device_address = address;
+    usbi_dbg ("allocating new device in context %p for with session 0x%08x",
+              ctx, cached_device->session);
 
-    (*device)->GetDeviceSpeed (device, &devSpeed);
+    dev = usbi_alloc_device(ctx, cached_device->session);
+    if (!dev) {
+      return LIBUSB_ERROR_NO_MEM;
+    }
+
+    priv = (struct darwin_device_priv *)dev->os_priv;
+
+    priv->dev = cached_device;
+    darwin_ref_cached_device (priv->dev);
+
+    if (cached_device->parent_session > 0) {
+      dev->parent_dev = usbi_get_device_by_session_id (ctx, cached_device->parent_session);
+    } else {
+      dev->parent_dev = NULL;
+    }
+    dev->port_number    = cached_device->port;
+    dev->bus_number     = cached_device->location >> 24;
+    dev->device_address = cached_device->address;
+
+    /* need to add a reference to the parent device */
+    if (dev->parent_dev) {
+      libusb_ref_device(dev->parent_dev);
+    }
+
+    (*(priv->dev->device))->GetDeviceSpeed (priv->dev->device, &devSpeed);
 
     switch (devSpeed) {
     case kUSBDeviceSpeedLow: dev->speed = LIBUSB_SPEED_LOW; break;
     case kUSBDeviceSpeedFull: dev->speed = LIBUSB_SPEED_FULL; break;
     case kUSBDeviceSpeedHigh: dev->speed = LIBUSB_SPEED_HIGH; break;
+#if DeviceVersion >= 500
+    case kUSBDeviceSpeedSuper: dev->speed = LIBUSB_SPEED_SUPER; break;
+#endif
     default:
       usbi_warn (ctx, "Got unknown device speed %d", devSpeed);
     }
 
-    /* save our location, we'll need this later */
-    priv->location = locationID;
-    snprintf(priv->sys_path, 20, "%03i-%04x-%04x-%02x-%02x", address, priv->dev_descriptor.idVendor, priv->dev_descriptor.idProduct,
-	     priv->dev_descriptor.bDeviceClass, priv->dev_descriptor.bDeviceSubClass);
-
     ret = usbi_sanitize_device (dev);
     if (ret < 0)
       break;
 
-    /* append the device to the list of discovered devices */
-    discdevs = discovered_devs_append(*_discdevs, dev);
-    if (!discdevs) {
-      ret = LIBUSB_ERROR_NO_MEM;
-      break;
-    }
-
-    *_discdevs = discdevs;
-
-    usbi_info (ctx, "found device with address %d at %s", dev->device_address, priv->sys_path);
+    usbi_dbg ("found device with address %d port = %d parent = %p at %p", dev->device_address,
+              dev->port_number, (void *) dev->parent_dev, priv->dev->sys_path);
   } while (0);
 
-  if (need_unref)
-    libusb_unref_device(dev);
+  if (0 == ret) {
+    usbi_connect_device (dev);
+  } else {
+    libusb_unref_device (dev);
+  }
 
   return ret;
 }
 
-static int darwin_get_device_list(struct libusb_context *ctx, struct discovered_devs **_discdevs) {
-  io_iterator_t        deviceIterator;
-  usb_device_t         **device;
-  kern_return_t        kresult;
-  UInt32               location;
+static int darwin_scan_devices(struct libusb_context *ctx) {
+  io_iterator_t deviceIterator;
+  io_service_t service;
+  kern_return_t kresult;
 
   kresult = usb_setup_device_iterator (&deviceIterator, 0);
   if (kresult != kIOReturnSuccess)
     return darwin_to_libusb (kresult);
 
-  while ((device = usb_get_next_device (deviceIterator, &location)) != NULL) {
-    (void) process_new_device (ctx, device, location, _discdevs);
+  while ((service = IOIteratorNext (deviceIterator))) {
+    (void) process_new_device (ctx, service);
 
-    (*(device))->Release(device);
+    IOObjectRelease(service);
   }
 
   IOObjectRelease(deviceIterator);
@@ -789,56 +922,43 @@ static int darwin_get_device_list(struct libusb_context *ctx, struct discovered_
 
 static int darwin_open (struct libusb_device_handle *dev_handle) {
   struct darwin_device_handle_priv *priv = (struct darwin_device_handle_priv *)dev_handle->os_priv;
-  struct darwin_device_priv *dpriv = (struct darwin_device_priv *)dev_handle->dev->os_priv;
-  usb_device_t  **darwin_device;
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(dev_handle->dev);
   IOReturn kresult;
 
   if (0 == dpriv->open_count) {
-    kresult = darwin_get_device (dpriv->location, &darwin_device);
-    if (kresult) {
-      usbi_err (HANDLE_CTX (dev_handle), "could not find device: %s", darwin_error_str (kresult));
-      return darwin_to_libusb (kresult);
-    }
-
-    dpriv->device = darwin_device;
-
     /* try to open the device */
     kresult = (*(dpriv->device))->USBDeviceOpenSeize (dpriv->device);
-
     if (kresult != kIOReturnSuccess) {
-      usbi_err (HANDLE_CTX (dev_handle), "USBDeviceOpen: %s", darwin_error_str(kresult));
-
-      switch (kresult) {
-      case kIOReturnExclusiveAccess:
-	/* it is possible to perform some actions on a device that is not open so do not return an error */
-	priv->is_open = 0;
-
-	break;
-      default:
-	(*(dpriv->device))->Release (dpriv->device);
-	dpriv->device = NULL;
-	return darwin_to_libusb (kresult);
+      usbi_warn (HANDLE_CTX (dev_handle), "USBDeviceOpen: %s", darwin_error_str(kresult));
+
+      if (kIOReturnExclusiveAccess != kresult) {
+        return darwin_to_libusb (kresult);
       }
+
+      /* it is possible to perform some actions on a device that is not open so do not return an error */
+      priv->is_open = 0;
     } else {
-      /* create async event source */
-      kresult = (*(dpriv->device))->CreateDeviceAsyncEventSource (dpriv->device, &priv->cfSource);
-      if (kresult != kIOReturnSuccess) {
-	usbi_err (HANDLE_CTX (dev_handle), "CreateDeviceAsyncEventSource: %s", darwin_error_str(kresult));
+      priv->is_open = 1;
+    }
 
-	(*(dpriv->device))->USBDeviceClose (dpriv->device);
-	(*(dpriv->device))->Release (dpriv->device);
+    /* create async event source */
+    kresult = (*(dpriv->device))->CreateDeviceAsyncEventSource (dpriv->device, &priv->cfSource);
+    if (kresult != kIOReturnSuccess) {
+      usbi_err (HANDLE_CTX (dev_handle), "CreateDeviceAsyncEventSource: %s", darwin_error_str(kresult));
 
-	dpriv->device = NULL;
-	return darwin_to_libusb (kresult);
+      if (priv->is_open) {
+        (*(dpriv->device))->USBDeviceClose (dpriv->device);
       }
 
-      priv->is_open = 1;
-
-      CFRetain (libusb_darwin_acfl);
+      priv->is_open = 0;
 
-      /* add the cfSource to the aync run loop */
-      CFRunLoopAddSource(libusb_darwin_acfl, priv->cfSource, kCFRunLoopCommonModes);
+      return darwin_to_libusb (kresult);
     }
+
+    CFRetain (libusb_darwin_acfl);
+
+    /* add the cfSource to the aync run loop */
+    CFRunLoopAddSource(libusb_darwin_acfl, priv->cfSource, kCFRunLoopCommonModes);
   }
 
   /* device opened successfully */
@@ -852,14 +972,14 @@ static int darwin_open (struct libusb_device_handle *dev_handle) {
 
   usbi_add_pollfd(HANDLE_CTX(dev_handle), priv->fds[0], POLLIN);
 
-  usbi_info (HANDLE_CTX (dev_handle), "device open for access");
+  usbi_dbg ("device open for access");
 
   return 0;
 }
 
 static void darwin_close (struct libusb_device_handle *dev_handle) {
   struct darwin_device_handle_priv *priv = (struct darwin_device_handle_priv *)dev_handle->os_priv;
-  struct darwin_device_priv *dpriv = (struct darwin_device_priv *)dev_handle->dev->os_priv;
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(dev_handle->dev);
   IOReturn kresult;
   int i;
 
@@ -877,30 +997,23 @@ static void darwin_close (struct libusb_device_handle *dev_handle) {
       libusb_release_interface (dev_handle, i);
 
   if (0 == dpriv->open_count) {
-    if (priv->is_open) {
-      /* delete the device's async event source */
-      if (priv->cfSource) {
-	CFRunLoopRemoveSource (libusb_darwin_acfl, priv->cfSource, kCFRunLoopDefaultMode);
-	CFRelease (priv->cfSource);
-      }
+    /* delete the device's async event source */
+    if (priv->cfSource) {
+      CFRunLoopRemoveSource (libusb_darwin_acfl, priv->cfSource, kCFRunLoopDefaultMode);
+      CFRelease (priv->cfSource);
+      priv->cfSource = NULL;
+      CFRelease (libusb_darwin_acfl);
+    }
 
+    if (priv->is_open) {
       /* close the device */
       kresult = (*(dpriv->device))->USBDeviceClose(dpriv->device);
       if (kresult) {
-	/* Log the fact that we had a problem closing the file, however failing a
-	 * close isn't really an error, so return success anyway */
-	usbi_err (HANDLE_CTX (dev_handle), "USBDeviceClose: %s", darwin_error_str(kresult));
+        /* Log the fact that we had a problem closing the file, however failing a
+         * close isn't really an error, so return success anyway */
+        usbi_warn (HANDLE_CTX (dev_handle), "USBDeviceClose: %s", darwin_error_str(kresult));
       }
     }
-
-    kresult = (*(dpriv->device))->Release(dpriv->device);
-    if (kresult) {
-      /* Log the fact that we had a problem closing the file, however failing a
-       * close isn't really an error, so return success anyway */
-      usbi_err (HANDLE_CTX (dev_handle), "Release: %s", darwin_error_str(kresult));
-    }
-
-    dpriv->device = NULL;
   }
 
   /* file descriptors are maintained per-instance */
@@ -912,7 +1025,7 @@ static void darwin_close (struct libusb_device_handle *dev_handle) {
 }
 
 static int darwin_get_configuration(struct libusb_device_handle *dev_handle, int *config) {
-  struct darwin_device_priv *dpriv = (struct darwin_device_priv *)dev_handle->dev->os_priv;
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(dev_handle->dev);
 
   *config = (int) dpriv->active_config;
 
@@ -920,7 +1033,7 @@ static int darwin_get_configuration(struct libusb_device_handle *dev_handle, int
 }
 
 static int darwin_set_configuration(struct libusb_device_handle *dev_handle, int config) {
-  struct darwin_device_priv *dpriv = (struct darwin_device_priv *)dev_handle->dev->os_priv;
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(dev_handle->dev);
   IOReturn kresult;
   int i;
 
@@ -946,9 +1059,10 @@ static int darwin_set_configuration(struct libusb_device_handle *dev_handle, int
 
 static int darwin_get_interface (usb_device_t **darwin_device, uint8_t ifc, io_service_t *usbInterfacep) {
   IOUSBFindInterfaceRequest request;
-  uint8_t                   current_interface;
   kern_return_t             kresult;
   io_iterator_t             interface_iterator;
+  UInt8                     bInterfaceNumber;
+  int                       ret;
 
   *usbInterfacep = IO_OBJECT_NULL;
 
@@ -962,10 +1076,16 @@ static int darwin_get_interface (usb_device_t **darwin_device, uint8_t ifc, io_s
   if (kresult)
     return kresult;
 
-  for ( current_interface = 0 ; current_interface <= ifc ; current_interface++ ) {
-    *usbInterfacep = IOIteratorNext(interface_iterator);
-    if (current_interface != ifc)
-      (void) IOObjectRelease (*usbInterfacep);
+  while ((*usbInterfacep = IOIteratorNext(interface_iterator))) {
+    /* find the interface number */
+    ret = get_ioregistry_value_number (*usbInterfacep, CFSTR("bInterfaceNumber"), kCFNumberSInt8Type,
+                                       &bInterfaceNumber);
+
+    if (ret && bInterfaceNumber == ifc) {
+      break;
+    }
+
+    (void) IOObjectRelease (*usbInterfacep);
   }
 
   /* done with the interface iterator */
@@ -987,7 +1107,7 @@ static int get_endpoints (struct libusb_device_handle *dev_handle, int iface) {
   u_int16_t dont_care2;
   int i;
 
-  usbi_info (HANDLE_CTX (dev_handle), "building table of endpoints.");
+  usbi_dbg ("building table of endpoints.");
 
   /* retrieve the total number of endpoints on this interface */
   kresult = (*(cInterface->interface))->GetNumEndpoints(cInterface->interface, &numep);
@@ -999,7 +1119,7 @@ static int get_endpoints (struct libusb_device_handle *dev_handle, int iface) {
   /* iterate through pipe references */
   for (i = 1 ; i <= numep ; i++) {
     kresult = (*(cInterface->interface))->GetPipeProperties(cInterface->interface, i, &direction, &number, &dont_care1,
-							    &dont_care2, &dont_care3);
+                                                            &dont_care2, &dont_care3);
 
     if (kresult != kIOReturnSuccess) {
       usbi_err (HANDLE_CTX (dev_handle), "error getting pipe information for pipe %d: %s", i, darwin_error_str(kresult));
@@ -1007,7 +1127,7 @@ static int get_endpoints (struct libusb_device_handle *dev_handle, int iface) {
       return darwin_to_libusb (kresult);
     }
 
-    usbi_info (HANDLE_CTX (dev_handle), "interface: %i pipe %i: dir: %i number: %i", iface, i, direction, number);
+    usbi_dbg ("interface: %i pipe %i: dir: %i number: %i", iface, i, direction, number);
 
     cInterface->endpoint_addrs[i - 1] = ((direction << 7 & LIBUSB_ENDPOINT_DIR_MASK) | (number & LIBUSB_ENDPOINT_ADDRESS_MASK));
   }
@@ -1018,7 +1138,7 @@ static int get_endpoints (struct libusb_device_handle *dev_handle, int iface) {
 }
 
 static int darwin_claim_interface(struct libusb_device_handle *dev_handle, int iface) {
-  struct darwin_device_priv *dpriv = (struct darwin_device_priv *)dev_handle->dev->os_priv;
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(dev_handle->dev);
   struct darwin_device_handle_priv *priv = (struct darwin_device_handle_priv *)dev_handle->os_priv;
   io_service_t          usbInterface = IO_OBJECT_NULL;
   IOReturn kresult;
@@ -1057,7 +1177,7 @@ static int darwin_claim_interface(struct libusb_device_handle *dev_handle, int i
 
   /* get an interface to the device's interface */
   kresult = IOCreatePlugInInterfaceForService (usbInterface, kIOUSBInterfaceUserClientTypeID,
-					       kIOCFPlugInInterfaceID, &plugInInterface, &score);
+                                               kIOCFPlugInInterfaceID, &plugInInterface, &score);
 
   /* ignore release error */
   (void)IOObjectRelease (usbInterface);
@@ -1074,10 +1194,11 @@ static int darwin_claim_interface(struct libusb_device_handle *dev_handle, int i
 
   /* Do the actual claim */
   kresult = (*plugInInterface)->QueryInterface(plugInInterface,
-					       CFUUIDGetUUIDBytes(kIOUSBInterfaceInterfaceID),
-					       (LPVOID)&cInterface->interface);
+                                               CFUUIDGetUUIDBytes(kIOUSBInterfaceInterfaceID),
+                                               (LPVOID)&cInterface->interface);
   /* We no longer need the intermediate plug-in */
-  IODestroyPlugInInterface (plugInInterface);
+  /* Use release instead of IODestroyPlugInInterface to avoid stopping IOServices associated with this device */
+  (*plugInInterface)->Release (plugInInterface);
   if (kresult || !cInterface->interface) {
     usbi_err (HANDLE_CTX (dev_handle), "QueryInterface: %s", darwin_error_str(kresult));
     return darwin_to_libusb (kresult);
@@ -1115,7 +1236,7 @@ static int darwin_claim_interface(struct libusb_device_handle *dev_handle, int i
   /* add the cfSource to the async thread's run loop */
   CFRunLoopAddSource(libusb_darwin_acfl, cInterface->cfSource, kCFRunLoopDefaultMode);
 
-  usbi_info (HANDLE_CTX (dev_handle), "interface opened");
+  usbi_dbg ("interface opened");
 
   return 0;
 }
@@ -1142,11 +1263,11 @@ static int darwin_release_interface(struct libusb_device_handle *dev_handle, int
 
   kresult = (*(cInterface->interface))->USBInterfaceClose(cInterface->interface);
   if (kresult)
-    usbi_err (HANDLE_CTX (dev_handle), "USBInterfaceClose: %s", darwin_error_str(kresult));
+    usbi_warn (HANDLE_CTX (dev_handle), "USBInterfaceClose: %s", darwin_error_str(kresult));
 
   kresult = (*(cInterface->interface))->Release(cInterface->interface);
   if (kresult != kIOReturnSuccess)
-    usbi_err (HANDLE_CTX (dev_handle), "Release: %s", darwin_error_str(kresult));
+    usbi_warn (HANDLE_CTX (dev_handle), "Release: %s", darwin_error_str(kresult));
 
   cInterface->interface = IO_OBJECT_NULL;
 
@@ -1196,31 +1317,68 @@ static int darwin_clear_halt(struct libusb_device_handle *dev_handle, unsigned c
 
   cInterface = &priv->interfaces[iface];
 
-#if (InterfaceVersion < 190)
-  kresult = (*(cInterface->interface))->ClearPipeStall(cInterface->interface, pipeRef);
-#else
   /* newer versions of darwin support clearing additional bits on the device's endpoint */
   kresult = (*(cInterface->interface))->ClearPipeStallBothEnds(cInterface->interface, pipeRef);
-#endif
   if (kresult)
-    usbi_err (HANDLE_CTX (dev_handle), "ClearPipeStall: %s", darwin_error_str (kresult));
+    usbi_warn (HANDLE_CTX (dev_handle), "ClearPipeStall: %s", darwin_error_str (kresult));
 
   return darwin_to_libusb (kresult);
 }
 
 static int darwin_reset_device(struct libusb_device_handle *dev_handle) {
-  struct darwin_device_priv *dpriv = (struct darwin_device_priv *)dev_handle->dev->os_priv;
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(dev_handle->dev);
+  IOUSBDeviceDescriptor descriptor;
+  IOUSBConfigurationDescriptorPtr cached_configuration;
+  IOUSBConfigurationDescriptor configuration;
+  bool reenumerate = false;
   IOReturn kresult;
+  int i;
 
   kresult = (*(dpriv->device))->ResetDevice (dpriv->device);
-  if (kresult)
+  if (kresult) {
     usbi_err (HANDLE_CTX (dev_handle), "ResetDevice: %s", darwin_error_str (kresult));
+    return darwin_to_libusb (kresult);
+  }
 
-  return darwin_to_libusb (kresult);
+  do {
+    usbi_dbg ("darwin/reset_device: checking if device descriptor changed");
+
+    /* ignore return code. if we can't get a descriptor it might be worthwhile re-enumerating anway */
+    (void) darwin_request_descriptor (dpriv->device, kUSBDeviceDesc, 0, &descriptor, sizeof (descriptor));
+
+    /* check if the device descriptor has changed */
+    if (0 != memcmp (&dpriv->dev_descriptor, &descriptor, sizeof (descriptor))) {
+      reenumerate = true;
+      break;
+    }
+
+    /* check if any configuration descriptor has changed */
+    for (i = 0 ; i < descriptor.bNumConfigurations ; ++i) {
+      usbi_dbg ("darwin/reset_device: checking if configuration descriptor %d changed", i);
+
+      (void) darwin_request_descriptor (dpriv->device, kUSBConfDesc, i, &configuration, sizeof (configuration));
+      (*(dpriv->device))->GetConfigurationDescriptorPtr (dpriv->device, i, &cached_configuration);
+
+      if (!cached_configuration || 0 != memcmp (cached_configuration, &configuration, sizeof (configuration))) {
+        reenumerate = true;
+        break;
+      }
+    }
+  } while (0);
+
+  if (reenumerate) {
+    usbi_dbg ("darwin/reset_device: device requires reenumeration");
+    (void) (*(dpriv->device))->USBDeviceReEnumerate (dpriv->device, 0);
+    return LIBUSB_ERROR_NOT_FOUND;
+  }
+
+  usbi_dbg ("darwin/reset_device: device reset complete");
+
+  return LIBUSB_SUCCESS;
 }
 
 static int darwin_kernel_driver_active(struct libusb_device_handle *dev_handle, int interface) {
-  struct darwin_device_priv *dpriv = (struct darwin_device_priv *)dev_handle->dev->os_priv;
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(dev_handle->dev);
   io_service_t usbInterface;
   CFTypeRef driver;
   IOReturn kresult;
@@ -1259,7 +1417,15 @@ static int darwin_detach_kernel_driver (struct libusb_device_handle *dev_handle,
 }
 
 static void darwin_destroy_device(struct libusb_device *dev) {
-  (void)dev;
+  struct darwin_device_priv *dpriv = (struct darwin_device_priv *) dev->os_priv;
+
+  if (dpriv->dev) {
+    /* need to hold the lock in case this is the last reference to the device */
+    usbi_mutex_lock(&darwin_cached_devices_lock);
+    darwin_deref_cached_device (dpriv->dev);
+    dpriv->dev = NULL;
+    usbi_mutex_unlock(&darwin_cached_devices_lock);
+  }
 }
 
 static int submit_bulk_transfer(struct usbi_transfer *itransfer) {
@@ -1268,15 +1434,12 @@ static int submit_bulk_transfer(struct usbi_transfer *itransfer) {
 
   IOReturn               ret;
   uint8_t                transferType;
-  /* None of the values below are used in libusb for bulk transfers */
+  /* None of the values below are used in libusbx for bulk transfers */
   uint8_t                direction, number, interval, pipeRef, iface;
   uint16_t               maxPacketSize;
 
   struct darwin_interface *cInterface;
 
-  if (IS_XFEROUT(transfer) && transfer->flags & LIBUSB_TRANSFER_ADD_ZERO_PACKET)
-    return LIBUSB_ERROR_NOT_SUPPORTED;
-
   if (ep_to_pipeRef (transfer->dev_handle, transfer->endpoint, &pipeRef, &iface) != 0) {
     usbi_err (TRANSFER_CTX (transfer), "endpoint not found on any open interface");
 
@@ -1286,33 +1449,38 @@ static int submit_bulk_transfer(struct usbi_transfer *itransfer) {
   cInterface = &priv->interfaces[iface];
 
   (*(cInterface->interface))->GetPipeProperties (cInterface->interface, pipeRef, &direction, &number,
-						 &transferType, &maxPacketSize, &interval);
+                                                 &transferType, &maxPacketSize, &interval);
+
+  if (0 != (transfer->length % maxPacketSize)) {
+    /* do not need a zero packet */
+    transfer->flags &= ~LIBUSB_TRANSFER_ADD_ZERO_PACKET;
+  }
 
   /* submit the request */
   /* timeouts are unavailable on interrupt endpoints */
   if (transferType == kUSBInterrupt) {
     if (IS_XFERIN(transfer))
       ret = (*(cInterface->interface))->ReadPipeAsync(cInterface->interface, pipeRef, transfer->buffer,
-						      transfer->length, darwin_async_io_callback, itransfer);
+                                                      transfer->length, darwin_async_io_callback, itransfer);
     else
       ret = (*(cInterface->interface))->WritePipeAsync(cInterface->interface, pipeRef, transfer->buffer,
-						       transfer->length, darwin_async_io_callback, itransfer);
+                                                       transfer->length, darwin_async_io_callback, itransfer);
   } else {
     itransfer->flags |= USBI_TRANSFER_OS_HANDLES_TIMEOUT;
 
     if (IS_XFERIN(transfer))
       ret = (*(cInterface->interface))->ReadPipeAsyncTO(cInterface->interface, pipeRef, transfer->buffer,
-							transfer->length, transfer->timeout, transfer->timeout,
-							darwin_async_io_callback, (void *)itransfer);
+                                                        transfer->length, transfer->timeout, transfer->timeout,
+                                                        darwin_async_io_callback, (void *)itransfer);
     else
       ret = (*(cInterface->interface))->WritePipeAsyncTO(cInterface->interface, pipeRef, transfer->buffer,
-							 transfer->length, transfer->timeout, transfer->timeout,
-							 darwin_async_io_callback, (void *)itransfer);
+                                                         transfer->length, transfer->timeout, transfer->timeout,
+                                                         darwin_async_io_callback, (void *)itransfer);
   }
 
   if (ret)
     usbi_err (TRANSFER_CTX (transfer), "bulk transfer failed (dir = %s): %s (code = 0x%08x)", IS_XFERIN(transfer) ? "In" : "Out",
-	       darwin_error_str(ret), ret);
+               darwin_error_str(ret), ret);
 
   return darwin_to_libusb (ret);
 }
@@ -1322,11 +1490,12 @@ static int submit_iso_transfer(struct usbi_transfer *itransfer) {
   struct darwin_transfer_priv *tpriv = usbi_transfer_get_os_priv(itransfer);
   struct darwin_device_handle_priv *priv = (struct darwin_device_handle_priv *)transfer->dev_handle->os_priv;
 
-  IOReturn                kresult;
-  uint8_t                 pipeRef, iface;
-  UInt64                  frame;
-  AbsoluteTime            atTime;
-  int                     i;
+  IOReturn kresult;
+  uint8_t direction, number, interval, pipeRef, iface, transferType;
+  uint16_t maxPacketSize;
+  UInt64 frame;
+  AbsoluteTime atTime;
+  int i;
 
   struct darwin_interface *cInterface;
 
@@ -1343,7 +1512,7 @@ static int submit_iso_transfer(struct usbi_transfer *itransfer) {
       return LIBUSB_ERROR_NO_MEM;
   }
 
-  /* copy the frame list from the libusb descriptor (the structures differ only is member order) */
+  /* copy the frame list from the libusbx descriptor (the structures differ only is member order) */
   for (i = 0 ; i < transfer->num_iso_packets ; i++)
     tpriv->isoc_framelist[i].frReqCount = transfer->iso_packet_desc[i].length;
 
@@ -1356,6 +1525,10 @@ static int submit_iso_transfer(struct usbi_transfer *itransfer) {
 
   cInterface = &priv->interfaces[iface];
 
+  /* determine the properties of this endpoint and the speed of the device */
+  (*(cInterface->interface))->GetPipeProperties (cInterface->interface, pipeRef, &direction, &number,
+                                                 &transferType, &maxPacketSize, &interval);
+
   /* Last but not least we need the bus frame number */
   kresult = (*(cInterface->interface))->GetBusFrameNumber(cInterface->interface, &frame, &atTime);
   if (kresult) {
@@ -1366,6 +1539,9 @@ static int submit_iso_transfer(struct usbi_transfer *itransfer) {
     return darwin_to_libusb (kresult);
   }
 
+  (*(cInterface->interface))->GetPipeProperties (cInterface->interface, pipeRef, &direction, &number,
+                                                 &transferType, &maxPacketSize, &interval);
+
   /* schedule for a frame a little in the future */
   frame += 4;
 
@@ -1375,18 +1551,23 @@ static int submit_iso_transfer(struct usbi_transfer *itransfer) {
   /* submit the request */
   if (IS_XFERIN(transfer))
     kresult = (*(cInterface->interface))->ReadIsochPipeAsync(cInterface->interface, pipeRef, transfer->buffer, frame,
-							     transfer->num_iso_packets, tpriv->isoc_framelist, darwin_async_io_callback,
-							     itransfer);
+                                                             transfer->num_iso_packets, tpriv->isoc_framelist, darwin_async_io_callback,
+                                                             itransfer);
   else
     kresult = (*(cInterface->interface))->WriteIsochPipeAsync(cInterface->interface, pipeRef, transfer->buffer, frame,
-							      transfer->num_iso_packets, tpriv->isoc_framelist, darwin_async_io_callback,
-							      itransfer);
+                                                              transfer->num_iso_packets, tpriv->isoc_framelist, darwin_async_io_callback,
+                                                              itransfer);
 
-  cInterface->frames[transfer->endpoint] = frame + transfer->num_iso_packets / 8;
+  if (LIBUSB_SPEED_FULL == transfer->dev_handle->dev->speed)
+    /* Full speed */
+    cInterface->frames[transfer->endpoint] = frame + transfer->num_iso_packets * (1 << (interval - 1));
+  else
+    /* High/super speed */
+    cInterface->frames[transfer->endpoint] = frame + transfer->num_iso_packets * (1 << (interval - 1)) / 8;
 
   if (kresult != kIOReturnSuccess) {
     usbi_err (TRANSFER_CTX (transfer), "isochronous transfer failed (dir: %s): %s", IS_XFERIN(transfer) ? "In" : "Out",
-	       darwin_error_str(kresult));
+               darwin_error_str(kresult));
     free (tpriv->isoc_framelist);
     tpriv->isoc_framelist = NULL;
   }
@@ -1397,7 +1578,7 @@ static int submit_iso_transfer(struct usbi_transfer *itransfer) {
 static int submit_control_transfer(struct usbi_transfer *itransfer) {
   struct libusb_transfer *transfer = USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
   struct libusb_control_setup *setup = (struct libusb_control_setup *) transfer->buffer;
-  struct darwin_device_priv *dpriv = (struct darwin_device_priv *)transfer->dev_handle->dev->os_priv;
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(transfer->dev_handle->dev);
   struct darwin_device_handle_priv *priv = (struct darwin_device_handle_priv *)transfer->dev_handle->os_priv;
   struct darwin_transfer_priv *tpriv = usbi_transfer_get_os_priv(itransfer);
 
@@ -1412,7 +1593,7 @@ static int submit_control_transfer(struct usbi_transfer *itransfer) {
   tpriv->req.wValue            = OSSwapLittleToHostInt16 (setup->wValue);
   tpriv->req.wIndex            = OSSwapLittleToHostInt16 (setup->wIndex);
   tpriv->req.wLength           = OSSwapLittleToHostInt16 (setup->wLength);
-  /* data is stored after the libusb control block */
+  /* data is stored after the libusbx control block */
   tpriv->req.pData             = transfer->buffer + LIBUSB_CONTROL_SETUP_SIZE;
   tpriv->req.completionTimeout = transfer->timeout;
   tpriv->req.noDataTimeout     = transfer->timeout;
@@ -1463,10 +1644,10 @@ static int darwin_submit_transfer(struct usbi_transfer *itransfer) {
 
 static int cancel_control_transfer(struct usbi_transfer *itransfer) {
   struct libusb_transfer *transfer = USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
-  struct darwin_device_priv *dpriv = (struct darwin_device_priv *)transfer->dev_handle->dev->os_priv;
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(transfer->dev_handle->dev);
   IOReturn kresult;
 
-  usbi_info (ITRANSFER_CTX (itransfer), "WARNING: aborting all transactions control pipe");
+  usbi_warn (ITRANSFER_CTX (itransfer), "aborting all transactions control pipe");
 
   if (!dpriv->device)
     return LIBUSB_ERROR_NO_DEVICE;
@@ -1478,7 +1659,7 @@ static int cancel_control_transfer(struct usbi_transfer *itransfer) {
 
 static int darwin_abort_transfers (struct usbi_transfer *itransfer) {
   struct libusb_transfer *transfer = USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
-  struct darwin_device_priv *dpriv = (struct darwin_device_priv *)transfer->dev_handle->dev->os_priv;
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(transfer->dev_handle->dev);
   struct darwin_device_handle_priv *priv = (struct darwin_device_handle_priv *)transfer->dev_handle->os_priv;
   struct darwin_interface *cInterface;
   uint8_t pipeRef, iface;
@@ -1495,20 +1676,15 @@ static int darwin_abort_transfers (struct usbi_transfer *itransfer) {
   if (!dpriv->device)
     return LIBUSB_ERROR_NO_DEVICE;
 
-  usbi_info (ITRANSFER_CTX (itransfer), "WARNING: aborting all transactions on interface %d pipe %d", iface, pipeRef);
+  usbi_warn (ITRANSFER_CTX (itransfer), "aborting all transactions on interface %d pipe %d", iface, pipeRef);
 
   /* abort transactions */
   (*(cInterface->interface))->AbortPipe (cInterface->interface, pipeRef);
 
-  usbi_info (ITRANSFER_CTX (itransfer), "calling clear pipe stall to clear the data toggle bit");
+  usbi_dbg ("calling clear pipe stall to clear the data toggle bit");
 
-  /* clear the data toggle bit */
-#if (InterfaceVersion < 190)
-  kresult = (*(cInterface->interface))->ClearPipeStall(cInterface->interface, pipeRef);
-#else
   /* newer versions of darwin support clearing additional bits on the device's endpoint */
   kresult = (*(cInterface->interface))->ClearPipeStallBothEnds(cInterface->interface, pipeRef);
-#endif
 
   return darwin_to_libusb (kresult);
 }
@@ -1543,18 +1719,24 @@ static void darwin_async_io_callback (void *refcon, IOReturn result, void *arg0)
   struct usbi_transfer *itransfer = (struct usbi_transfer *)refcon;
   struct libusb_transfer *transfer = USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
   struct darwin_device_handle_priv *priv = (struct darwin_device_handle_priv *)transfer->dev_handle->os_priv;
-  UInt32 message, size;
+  struct darwin_msg_async_io_complete message = {.itransfer = itransfer, .result = result,
+                                                 .size = (UInt32) (uintptr_t) arg0};
+
+  usbi_dbg ("an async io operation has completed");
+
+  /* if requested write a zero packet */
+  if (kIOReturnSuccess == result && IS_XFEROUT(transfer) && transfer->flags & LIBUSB_TRANSFER_ADD_ZERO_PACKET) {
+    struct darwin_interface *cInterface;
+    uint8_t iface, pipeRef;
 
-  usbi_info (ITRANSFER_CTX (itransfer), "an async io operation has completed");
+    (void) ep_to_pipeRef (transfer->dev_handle, transfer->endpoint, &pipeRef, &iface);
+    cInterface = &priv->interfaces[iface];
 
-  size = (UInt32) (uintptr_t) arg0;
+    (*(cInterface->interface))->WritePipe (cInterface->interface, pipeRef, transfer->buffer, 0);
+  }
 
   /* send a completion message to the device's file descriptor */
-  message = MESSAGE_ASYNC_IO_COMPLETE;
   write (priv->fds[1], &message, sizeof (message));
-  write (priv->fds[1], &itransfer, sizeof (itransfer));
-  write (priv->fds[1], &result, sizeof (IOReturn));
-  write (priv->fds[1], &size, sizeof (size));
 }
 
 static int darwin_transfer_status (struct usbi_transfer *itransfer, kern_return_t result) {
@@ -1568,17 +1750,17 @@ static int darwin_transfer_status (struct usbi_transfer *itransfer, kern_return_
   case kIOReturnAborted:
     return LIBUSB_TRANSFER_CANCELLED;
   case kIOUSBPipeStalled:
-    usbi_warn (ITRANSFER_CTX (itransfer), "transfer error: pipe is stalled");
+    usbi_dbg ("transfer error: pipe is stalled");
     return LIBUSB_TRANSFER_STALL;
   case kIOReturnOverrun:
-    usbi_err (ITRANSFER_CTX (itransfer), "transfer error: data overrun");
+    usbi_warn (ITRANSFER_CTX (itransfer), "transfer error: data overrun");
     return LIBUSB_TRANSFER_OVERFLOW;
   case kIOUSBTransactionTimeout:
-    usbi_err (ITRANSFER_CTX (itransfer), "transfer error: timed out");
+    usbi_warn (ITRANSFER_CTX (itransfer), "transfer error: timed out");
     itransfer->flags |= USBI_TRANSFER_TIMED_OUT;
     return LIBUSB_TRANSFER_TIMED_OUT;
   default:
-    usbi_err (ITRANSFER_CTX (itransfer), "transfer error: %s (value = 0x%08x)", darwin_error_str (result), result);
+    usbi_warn (ITRANSFER_CTX (itransfer), "transfer error: %s (value = 0x%08x)", darwin_error_str (result), result);
     return LIBUSB_TRANSFER_ERROR;
   }
 }
@@ -1597,17 +1779,17 @@ static void darwin_handle_callback (struct usbi_transfer *itransfer, kern_return
     return;
   }
 
-  usbi_info (ITRANSFER_CTX (itransfer), "handling %s completion with kernel status %d",
-	     isControl ? "control" : isBulk ? "bulk" : isIsoc ? "isoc" : "interrupt", result);
+  usbi_dbg ("handling %s completion with kernel status %d",
+             isControl ? "control" : isBulk ? "bulk" : isIsoc ? "isoc" : "interrupt", result);
 
   if (kIOReturnSuccess == result || kIOReturnUnderrun == result) {
     if (isIsoc && tpriv->isoc_framelist) {
       /* copy isochronous results back */
 
       for (i = 0; i < transfer->num_iso_packets ; i++) {
-	struct libusb_iso_packet_descriptor *lib_desc = &transfer->iso_packet_desc[i];
-	lib_desc->status = darwin_to_libusb (tpriv->isoc_framelist[i].frStatus);
-	lib_desc->actual_length = tpriv->isoc_framelist[i].frActCount;
+        struct libusb_iso_packet_descriptor *lib_desc = &transfer->iso_packet_desc[i];
+        lib_desc->status = darwin_to_libusb (tpriv->isoc_framelist[i].frStatus);
+        lib_desc->actual_length = tpriv->isoc_framelist[i].frActCount;
       }
     } else if (!isIsoc)
       itransfer->transferred += io_size;
@@ -1618,64 +1800,35 @@ static void darwin_handle_callback (struct usbi_transfer *itransfer, kern_return
 }
 
 static int op_handle_events(struct libusb_context *ctx, struct pollfd *fds, POLL_NFDS_TYPE nfds, int num_ready) {
-  struct usbi_transfer *itransfer;
-  UInt32 io_size;
-  IOReturn kresult;
+  struct darwin_msg_async_io_complete message;
   POLL_NFDS_TYPE i = 0;
   ssize_t ret;
-  UInt32 message;
 
   usbi_mutex_lock(&ctx->open_devs_lock);
+
   for (i = 0; i < nfds && num_ready > 0; i++) {
     struct pollfd *pollfd = &fds[i];
-    struct libusb_device_handle *handle;
-    struct darwin_device_handle_priv *hpriv = NULL;
 
-    usbi_info (ctx, "checking fd %i with revents = %x", fds[i], pollfd->revents);
+    usbi_dbg ("checking fd %i with revents = %x", pollfd->fd, pollfd->revents);
 
     if (!pollfd->revents)
       continue;
 
     num_ready--;
-    list_for_each_entry(handle, &ctx->open_devs, list, struct libusb_device_handle) {
-      hpriv =  (struct darwin_device_handle_priv *)handle->os_priv;
-      if (hpriv->fds[0] == pollfd->fd)
-	break;
-    }
-
-    if (!(pollfd->revents & POLLERR)) {
-      ret = read (hpriv->fds[0], &message, sizeof (message));
-      if (ret < (ssize_t)sizeof (message))
-	continue;
-    } else
-      /* could not poll the device-- response is to delete the device (this seems a little heavy-handed) */
-      message = MESSAGE_DEVICE_GONE;
-
-    switch (message) {
-    case MESSAGE_DEVICE_GONE:
-      /* remove the device's async port from the runloop */
-      if (hpriv->cfSource) {
-	if (libusb_darwin_acfl)
-	  CFRunLoopRemoveSource (libusb_darwin_acfl, hpriv->cfSource, kCFRunLoopDefaultMode);
-	CFRelease (hpriv->cfSource);
-	hpriv->cfSource = NULL;
-      }
 
-      usbi_remove_pollfd(HANDLE_CTX(handle), hpriv->fds[0]);
-      usbi_handle_disconnect(handle);
-
-      /* done with this device */
+    if (pollfd->revents & POLLERR) {
+      /* this probably will never happen so ignore the error an move on. */
       continue;
-    case MESSAGE_ASYNC_IO_COMPLETE:
-      read (hpriv->fds[0], &itransfer, sizeof (itransfer));
-      read (hpriv->fds[0], &kresult, sizeof (IOReturn));
-      read (hpriv->fds[0], &io_size, sizeof (UInt32));
+    }
 
-      darwin_handle_callback (itransfer, kresult, io_size);
-      break;
-    default:
-      usbi_err (ctx, "unknown message received from device pipe");
+    /* there is only one type of message */
+    ret = read (pollfd->fd, &message, sizeof (message));
+    if (ret < (ssize_t) sizeof (message)) {
+      usbi_dbg ("WARNING: short read on async io completion pipe\n");
+      continue;
     }
+
+    darwin_handle_callback (message.itransfer, message.result, message.size);
   }
 
   usbi_mutex_unlock(&ctx->open_devs_lock);
@@ -1709,42 +1862,42 @@ static int darwin_clock_gettime(int clk_id, struct timespec *tp) {
 }
 
 const struct usbi_os_backend darwin_backend = {
-	.name = "Darwin",
-	.init = darwin_init,
-	.exit = darwin_exit,
-	.get_device_list = darwin_get_device_list,
-	.get_device_descriptor = darwin_get_device_descriptor,
-	.get_active_config_descriptor = darwin_get_active_config_descriptor,
-	.get_config_descriptor = darwin_get_config_descriptor,
-
-	.open = darwin_open,
-	.close = darwin_close,
-	.get_configuration = darwin_get_configuration,
-	.set_configuration = darwin_set_configuration,
-	.claim_interface = darwin_claim_interface,
-	.release_interface = darwin_release_interface,
-
-	.set_interface_altsetting = darwin_set_interface_altsetting,
-	.clear_halt = darwin_clear_halt,
-	.reset_device = darwin_reset_device,
-
-	.kernel_driver_active = darwin_kernel_driver_active,
-	.detach_kernel_driver = darwin_detach_kernel_driver,
-	.attach_kernel_driver = darwin_attach_kernel_driver,
-
-	.destroy_device = darwin_destroy_device,
-
-	.submit_transfer = darwin_submit_transfer,
-	.cancel_transfer = darwin_cancel_transfer,
-	.clear_transfer_priv = darwin_clear_transfer_priv,
-
-	.handle_events = op_handle_events,
-
-	.clock_gettime = darwin_clock_gettime,
-
-	.device_priv_size = sizeof(struct darwin_device_priv),
-	.device_handle_priv_size = sizeof(struct darwin_device_handle_priv),
-	.transfer_priv_size = sizeof(struct darwin_transfer_priv),
-	.add_iso_packet_size = 0,
+        .name = "Darwin",
+        .caps = 0,
+        .init = darwin_init,
+        .exit = darwin_exit,
+        .get_device_list = NULL, /* not needed */
+        .get_device_descriptor = darwin_get_device_descriptor,
+        .get_active_config_descriptor = darwin_get_active_config_descriptor,
+        .get_config_descriptor = darwin_get_config_descriptor,
+
+        .open = darwin_open,
+        .close = darwin_close,
+        .get_configuration = darwin_get_configuration,
+        .set_configuration = darwin_set_configuration,
+        .claim_interface = darwin_claim_interface,
+        .release_interface = darwin_release_interface,
+
+        .set_interface_altsetting = darwin_set_interface_altsetting,
+        .clear_halt = darwin_clear_halt,
+        .reset_device = darwin_reset_device,
+
+        .kernel_driver_active = darwin_kernel_driver_active,
+        .detach_kernel_driver = darwin_detach_kernel_driver,
+        .attach_kernel_driver = darwin_attach_kernel_driver,
+
+        .destroy_device = darwin_destroy_device,
+
+        .submit_transfer = darwin_submit_transfer,
+        .cancel_transfer = darwin_cancel_transfer,
+        .clear_transfer_priv = darwin_clear_transfer_priv,
+
+        .handle_events = op_handle_events,
+
+        .clock_gettime = darwin_clock_gettime,
+
+        .device_priv_size = sizeof(struct darwin_device_priv),
+        .device_handle_priv_size = sizeof(struct darwin_device_handle_priv),
+        .transfer_priv_size = sizeof(struct darwin_transfer_priv),
+        .add_iso_packet_size = 0,
 };
-