path: root/libusb/os/linux_usbfs.c

/* -*- Mode: C; c-basic-offset:8 ; indent-tabs-mode:t -*- */
/*
 * Linux usbfs backend for libusb
 * Copyright © 2007-2009 Daniel Drake <dsd@gentoo.org>
 * Copyright © 2001 Johannes Erdfelt <johannes@erdfelt.com>
 * Copyright © 2013 Nathan Hjelm <hjelmn@mac.com>
 * Copyright © 2012-2013 Hans de Goede <hdegoede@redhat.com>
 * Copyright © 2020 Chris Dickens <christopher.a.dickens@gmail.com>
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

#include "libusbi.h"
#include "linux_usbfs.h"

#include <alloca.h>
#include <ctype.h>
#include <dirent.h>
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <sys/utsname.h>
#include <sys/vfs.h>
#include <unistd.h>

/* sysfs vs usbfs:
 * opening a usbfs node causes the device to be resumed, so we attempt to
 * avoid this during enumeration.
 *
 * sysfs allows us to read the kernel's in-memory copies of device descriptors
 * and so forth, avoiding the need to open the device:
 *  - The binary "descriptors" file contains all config descriptors since
 *    2.6.26, commit 217a9081d8e69026186067711131b77f0ce219ed
 *  - The binary "descriptors" file was added in 2.6.23, commit
 *    69d42a78f935d19384d1f6e4f94b65bb162b36df, but it only contains the
 *    active config descriptors
 *  - The "busnum" file was added in 2.6.22, commit
 *    83f7d958eab2fbc6b159ee92bf1493924e1d0f72
 *  - The "devnum" file has been present since pre-2.6.18
 *  - the "bConfigurationValue" file has been present since pre-2.6.18
 *
 * If we have bConfigurationValue, busnum, and devnum, then we can determine
 * the active configuration without having to open the usbfs node in RDWR mode.
 * The busnum file is important as that is the only way we can relate sysfs
 * devices to usbfs nodes.
 *
 * If we also have all descriptors, we can obtain the device descriptor and
 * configuration without touching usbfs at all.
 */

/* endianness for multi-byte fields:
 *
 * Descriptors exposed by usbfs have the multi-byte fields in the device
 * descriptor as host endian. Multi-byte fields in the other descriptors are
 * bus-endian. The kernel documentation says otherwise, but it is wrong.
 *
 * In sysfs all descriptors are bus-endian.
 */

#define USBDEV_PATH		"/dev"
#define USB_DEVTMPFS_PATH	"/dev/bus/usb"

/* use usbdev*.* device names in /dev instead of the usbfs bus directories */
static int usbdev_names = 0;

/* Linux has changed the maximum length of an individual isochronous packet
 * over time.  Initially this limit was 1,023 bytes, but Linux 2.6.18
 * (commit 3612242e527eb47ee4756b5350f8bdf791aa5ede) increased this value to
 * 8,192 bytes to support higher bandwidth devices.  Linux 3.10
 * (commit e2e2f0ea1c935edcf53feb4c4c8fdb4f86d57dd9) further increased this
 * value to 49,152 bytes to support super speed devices.  Linux 5.2
 * (commit 8a1dbc8d91d3d1602282c7e6b4222c7759c916fa) even further increased
 * this value to 98,304 bytes to support super speed plus devices.
 */
static unsigned int max_iso_packet_len = 0;

/* is sysfs available (mounted) ? */
static int sysfs_available = -1;

/* how many times have we initted (and not exited) ? */
static int init_count = 0;

/* have no authority to operate usb device directly */
static int no_enumeration = 0;

/* Serialize scan-devices, event-thread, and poll */
usbi_mutex_static_t linux_hotplug_lock = USBI_MUTEX_INITIALIZER;

static int linux_scan_devices(struct libusb_context *ctx);
static int detach_kernel_driver_and_claim(struct libusb_device_handle *, uint8_t);

#if !defined(HAVE_LIBUDEV)
static int linux_default_scan_devices(struct libusb_context *ctx);
#endif

struct kernel_version {
	int major;
	int minor;
	int sublevel;
};

struct config_descriptor {
	struct usbi_configuration_descriptor *desc;
	size_t actual_len;
};

struct linux_device_priv {
	char *sysfs_dir;
	void *descriptors;
	size_t descriptors_len;
	struct config_descriptor *config_descriptors;
	int active_config; /* cache val for !sysfs_available  */
};

struct linux_device_handle_priv {
	int fd;
	int fd_removed;
	int fd_keep;
	uint32_t caps;
};

enum reap_action {
	NORMAL = 0,
	/* submission failed after the first URB, so await cancellation/completion
	 * of all the others */
	SUBMIT_FAILED,

	/* cancelled by user or timeout */
	CANCELLED,

	/* completed multi-URB transfer in non-final URB */
	COMPLETED_EARLY,

	/* one or more urbs encountered a low-level error */
	ERROR,
};

struct linux_transfer_priv {
	union {
		struct usbfs_urb *urbs;
		struct usbfs_urb **iso_urbs;
	};

	enum reap_action reap_action;
	int num_urbs;
	int num_retired;
	enum libusb_transfer_status reap_status;

	/* next iso packet in user-supplied transfer to be populated */
	int iso_packet_offset;
};

static int dev_has_config0(struct libusb_device *dev)
{
	struct linux_device_priv *priv = usbi_get_device_priv(dev);
	struct config_descriptor *config;
	uint8_t idx;

	for (idx = 0; idx < dev->device_descriptor.bNumConfigurations; idx++) {
		config = &priv->config_descriptors[idx];
		if (config->desc->bConfigurationValue == 0)
			return 1;
	}

	return 0;
}

static int get_usbfs_fd(struct libusb_device *dev, mode_t mode, int silent)
{
	struct libusb_context *ctx = DEVICE_CTX(dev);
	char path[24];
	int fd;

	if (usbdev_names)
		snprintf(path, sizeof(path), USBDEV_PATH "/usbdev%u.%u",
			dev->bus_number, dev->device_address);
	else
		snprintf(path, sizeof(path), USB_DEVTMPFS_PATH "/%03u/%03u",
			dev->bus_number, dev->device_address);

	fd = open(path, mode | O_CLOEXEC);
	if (fd != -1)
		return fd; /* Success */

	if (errno == ENOENT) {
		const long delay_ms = 10L;
		const struct timespec delay_ts = { 0L, delay_ms * 1000L * 1000L };

		if (!silent)
			usbi_err(ctx, "File doesn't exist, wait %ld ms and try again", delay_ms);

		/* Wait 10ms for USB device path creation.*/
		nanosleep(&delay_ts, NULL);

		fd = open(path, mode | O_CLOEXEC);
		if (fd != -1)
			return fd; /* Success */
	}

	if (!silent) {
		usbi_err(ctx, "libusb couldn't open USB device %s, errno=%d", path, errno);
		if (errno == EACCES && mode == O_RDWR)
			usbi_err(ctx, "libusb requires write access to USB device nodes");
	}

	if (errno == EACCES)
		return LIBUSB_ERROR_ACCESS;
	if (errno == ENOENT)
		return LIBUSB_ERROR_NO_DEVICE;
	return LIBUSB_ERROR_IO;
}

/* check dirent for a /dev/usbdev%d.%d name
 * optionally return bus/device on success */
static int is_usbdev_entry(const char *name, uint8_t *bus_p, uint8_t *dev_p)
{
	int busnum, devnum;

	if (sscanf(name, "usbdev%d.%d", &busnum, &devnum) != 2)
		return 0;
	if (busnum < 0 || busnum > UINT8_MAX || devnum < 0 || devnum > UINT8_MAX) {
		usbi_dbg(NULL, "invalid usbdev format '%s'", name);
		return 0;
	}

	usbi_dbg(NULL, "found: %s", name);
	if (bus_p)
		*bus_p = (uint8_t)busnum;
	if (dev_p)
		*dev_p = (uint8_t)devnum;
	return 1;
}

static const char *find_usbfs_path(void)
{
	const char *path;
	DIR *dir;
	struct dirent *entry;

	path = USB_DEVTMPFS_PATH;
	dir = opendir(path);
	if (dir) {
		while ((entry = readdir(dir))) {
			if (entry->d_name[0] == '.')
				continue;

			/* We assume if we find any files that it must be the right place */
			break;
		}

		closedir(dir);

		if (entry)
			return path;
	}

	/* look for /dev/usbdev*.* if the normal place fails */
	path = USBDEV_PATH;
	dir = opendir(path);
	if (dir) {
		while ((entry = readdir(dir))) {
			if (entry->d_name[0] == '.')
				continue;

			if (is_usbdev_entry(entry->d_name, NULL, NULL)) {
				/* found one; that's enough */
				break;
			}
		}

		closedir(dir);

		if (entry) {
			usbdev_names = 1;
			return path;
		}
	}

/* On udev based systems without any usb-devices /dev/bus/usb will not
 * exist. So if we've not found anything and we're using udev for hotplug
 * simply assume /dev/bus/usb rather then making libusb_init fail.
 * Make the same assumption for Android where SELinux policies might block us
 * from reading /dev on newer devices. */
#if defined(HAVE_LIBUDEV) || defined(__ANDROID__)
	return USB_DEVTMPFS_PATH;
#else
	return NULL;
#endif
}

static int get_kernel_version(struct libusb_context *ctx,
	struct kernel_version *ver)
{
	struct utsname uts;
	int atoms;

	if (uname(&uts) < 0) {
		usbi_err(ctx, "uname failed, errno=%d", errno);
		return -1;
	}

	atoms = sscanf(uts.release, "%d.%d.%d", &ver->major, &ver->minor, &ver->sublevel);
	if (atoms < 2) {
		usbi_err(ctx, "failed to parse uname release '%s'", uts.release);
		return -1;
	}

	if (atoms < 3)
		ver->sublevel = -1;

	usbi_dbg(ctx, "reported kernel version is %s", uts.release);

	return 0;
}

static int kernel_version_ge(const struct kernel_version *ver,
	int major, int minor, int sublevel)
{
	if (ver->major > major)
		return 1;
	else if (ver->major < major)
		return 0;

	/* kmajor == major */
	if (ver->minor > minor)
		return 1;
	else if (ver->minor < minor)
		return 0;

	/* kminor == minor */
	if (ver->sublevel == -1)
		return sublevel == 0;

	return ver->sublevel >= sublevel;
}

static int op_init(struct libusb_context *ctx)
{
	struct kernel_version kversion;
	const char *usbfs_path;
	int r;

	if (get_kernel_version(ctx, &kversion) < 0)
		return LIBUSB_ERROR_OTHER;

	if (!kernel_version_ge(&kversion, 2, 6, 32)) {
		usbi_err(ctx, "kernel version is too old (reported as %d.%d.%d)",
			 kversion.major, kversion.minor,
			 kversion.sublevel != -1 ? kversion.sublevel : 0);
		return LIBUSB_ERROR_NOT_SUPPORTED;
	}

	usbfs_path = find_usbfs_path();
	if (!usbfs_path) {
		usbi_err(ctx, "could not find usbfs");
		return LIBUSB_ERROR_OTHER;
	}

	usbi_dbg(ctx, "found usbfs at %s", usbfs_path);

	if (!max_iso_packet_len) {
		if (kernel_version_ge(&kversion, 5, 2, 0))
			max_iso_packet_len = 98304;
		else if (kernel_version_ge(&kversion, 3, 10, 0))
			max_iso_packet_len = 49152;
		else
			max_iso_packet_len = 8192;
	}

	usbi_dbg(ctx, "max iso packet length is (likely) %u bytes", max_iso_packet_len);

	if (sysfs_available == -1) {
		struct statfs statfsbuf;

		r = statfs(SYSFS_MOUNT_PATH, &statfsbuf);
		if (r == 0 && statfsbuf.f_type == SYSFS_MAGIC) {
			usbi_dbg(ctx, "sysfs is available");
			sysfs_available = 1;
		} else {
			usbi_warn(ctx, "sysfs not mounted");
			sysfs_available = 0;
		}
	}

	if (no_enumeration) {
		return LIBUSB_SUCCESS;
	}

	r = LIBUSB_SUCCESS;
	if (init_count == 0) {
		/* start up hotplug event handler */
		r = linux_start_event_monitor();
	}
	if (r == LIBUSB_SUCCESS) {
		r = linux_scan_devices(ctx);
		if (r == LIBUSB_SUCCESS)
			init_count++;
		else if (init_count == 0)
			linux_stop_event_monitor();
	} else {
		usbi_err(ctx, "error starting hotplug event monitor");
	}

	return r;
}

static void op_exit(struct libusb_context *ctx)
{
	UNUSED(ctx);

	if (no_enumeration) {
		return;
	}

	assert(init_count != 0);
	if (!--init_count) {
		/* tear down event handler */
		linux_stop_event_monitor();
	}
}

static int op_set_option(struct libusb_context *ctx, enum libusb_option option, va_list ap)
{
	UNUSED(ctx);
	UNUSED(ap);

	if (option == LIBUSB_OPTION_NO_DEVICE_DISCOVERY) {
		usbi_dbg(ctx, "no enumeration will be performed");
		no_enumeration = 1;
		return LIBUSB_SUCCESS;
	}

	return LIBUSB_ERROR_NOT_SUPPORTED;
}

static int linux_scan_devices(struct libusb_context *ctx)
{
	int ret;

	usbi_mutex_static_lock(&linux_hotplug_lock);

#if defined(HAVE_LIBUDEV)
	ret = linux_udev_scan_devices(ctx);
#else
	ret = linux_default_scan_devices(ctx);
#endif

	usbi_mutex_static_unlock(&linux_hotplug_lock);

	return ret;
}

static void op_hotplug_poll(void)
{
	linux_hotplug_poll();
}

static int open_sysfs_attr(struct libusb_context *ctx,
	const char *sysfs_dir, const char *attr)
{
	char filename[256];
	int fd;

	snprintf(filename, sizeof(filename), SYSFS_DEVICE_PATH "/%s/%s", sysfs_dir, attr);
	fd = open(filename, O_RDONLY | O_CLOEXEC);
	if (fd < 0) {
		if (errno == ENOENT) {
			/* File doesn't exist. Assume the device has been
			   disconnected (see trac ticket #70). */
			return LIBUSB_ERROR_NO_DEVICE;
		}
		usbi_err(ctx, "open %s failed, errno=%d", filename, errno);
		return LIBUSB_ERROR_IO;
	}

	return fd;
}

/* Note only suitable for attributes which always read >= 0, < 0 is error */
static int read_sysfs_attr(struct libusb_context *ctx,
	const char *sysfs_dir, const char *attr, int max_value, int *value_p)
{
	char buf[20], *endptr;
	long value;
	ssize_t r;
	int fd;

	fd = open_sysfs_attr(ctx, sysfs_dir, attr);
	if (fd < 0)
		return fd;

	r = read(fd, buf, sizeof(buf) - 1);
	if (r < 0) {
		r = errno;
		close(fd);
		if (r == ENODEV)
			return LIBUSB_ERROR_NO_DEVICE;
		usbi_err(ctx, "attribute %s read failed, errno=%zd", attr, r);
		return LIBUSB_ERROR_IO;
	}
	close(fd);

	if (r == 0) {
		/* Certain attributes (e.g. bConfigurationValue) are not
		 * populated if the device is not configured. */
		*value_p = -1;
		return 0;
	}

	/* The kernel does *not* NUL-terminate the string, but every attribute
	 * should be terminated with a newline character. */
	if (!isdigit(buf[0])) {
		usbi_err(ctx, "attribute %s doesn't have numeric value?", attr);
		return LIBUSB_ERROR_IO;
	} else if (buf[r - 1] != '\n') {
		usbi_warn(ctx, "attribute %s doesn't end with newline?", attr);
	} else {
		/* Remove the terminating newline character */
		r--;
	}
	buf[r] = '\0';

	errno = 0;
	value = strtol(buf, &endptr, 10);
	if (value < 0 || value > (long)max_value || errno) {
		usbi_err(ctx, "attribute %s contains an invalid value: '%s'", attr, buf);
		return LIBUSB_ERROR_INVALID_PARAM;
	} else if (*endptr != '\0') {
		/* Consider the value to be valid if the remainder is a '.'
		 * character followed by numbers.  This occurs, for example,
		 * when reading the "speed" attribute for a low-speed device
		 * (e.g. "1.5") */
		if (*endptr == '.' && isdigit(*(endptr + 1))) {
			endptr++;
			while (isdigit(*endptr))
				endptr++;
		}
		if (*endptr != '\0') {
			usbi_err(ctx, "attribute %s contains an invalid value: '%s'", attr, buf);
			return LIBUSB_ERROR_INVALID_PARAM;
		}
	}

	*value_p = (int)value;
	return 0;
}

static int sysfs_scan_device(struct libusb_context *ctx, const char *devname)
{
	uint8_t busnum, devaddr;
	int ret;

	ret = linux_get_device_address(ctx, 0, &busnum, &devaddr, NULL, devname, -1);
	if (ret != LIBUSB_SUCCESS)
		return ret;

	return linux_enumerate_device(ctx, busnum, devaddr, devname);
}

/* read the bConfigurationValue for a device */
static int sysfs_get_active_config(struct libusb_device *dev, int *config)
{
	struct linux_device_priv *priv = usbi_get_device_priv(dev);

	return read_sysfs_attr(DEVICE_CTX(dev), priv->sysfs_dir, "bConfigurationValue",
			UINT8_MAX, config);
}

int linux_get_device_address(struct libusb_context *ctx, int detached,
	uint8_t *busnum, uint8_t *devaddr, const char *dev_node,
	const char *sys_name, int fd)
{
	int sysfs_val;
	int r;

	usbi_dbg(ctx, "getting address for device: %s detached: %d", sys_name, detached);
	/* can't use sysfs to read the bus and device number if the
	 * device has been detached */
	if (!sysfs_available || detached || !sys_name) {
		if (!dev_node && fd >= 0) {
			char *fd_path = alloca(PATH_MAX);
			char proc_path[32];

			/* try to retrieve the device node from fd */
			snprintf(proc_path, sizeof(proc_path), "/proc/self/fd/%d", fd);
			r = readlink(proc_path, fd_path, PATH_MAX - 1);
			if (r > 0) {
				fd_path[r] = '\0';
				dev_node = fd_path;
			}
		}

		if (!dev_node)
			return LIBUSB_ERROR_OTHER;

		/* will this work with all supported kernel versions? */
		if (!strncmp(dev_node, "/dev/bus/usb", 12))
			sscanf(dev_node, "/dev/bus/usb/%hhu/%hhu", busnum, devaddr);
		else
			return LIBUSB_ERROR_OTHER;

		return LIBUSB_SUCCESS;
	}

	usbi_dbg(ctx, "scan %s", sys_name);

	r = read_sysfs_attr(ctx, sys_name, "busnum", UINT8_MAX, &sysfs_val);
	if (r < 0)
		return r;
	*busnum = (uint8_t)sysfs_val;

	r = read_sysfs_attr(ctx, sys_name, "devnum", UINT8_MAX, &sysfs_val);
	if (r < 0)
		return r;
	*devaddr = (uint8_t)sysfs_val;

	usbi_dbg(ctx, "bus=%u dev=%u", *busnum, *devaddr);

	return LIBUSB_SUCCESS;
}

/* Return offset of the next config descriptor */
static int seek_to_next_config(struct libusb_context *ctx,
	uint8_t *buffer, size_t len)
{
	struct usbi_descriptor_header *header;
	int offset;

	/* Start seeking past the config descriptor */
	offset = LIBUSB_DT_CONFIG_SIZE;
	buffer += LIBUSB_DT_CONFIG_SIZE;
	len -= LIBUSB_DT_CONFIG_SIZE;

	while (len > 0) {
		if (len < 2) {
			usbi_err(ctx, "short descriptor read %zu/2", len);
			return LIBUSB_ERROR_IO;
		}

		header = (struct usbi_descriptor_header *)buffer;
		if (header->bDescriptorType == LIBUSB_DT_CONFIG)
			return offset;

		if (len < header->bLength) {
			usbi_err(ctx, "bLength overflow by %zu bytes",
				 (size_t)header->bLength - len);
			return LIBUSB_ERROR_IO;
		}

		offset += header->bLength;
		buffer += header->bLength;
		len -= header->bLength;
	}

	usbi_err(ctx, "config descriptor not found");
	return LIBUSB_ERROR_IO;
}

static int parse_config_descriptors(struct libusb_device *dev)
{
	struct libusb_context *ctx = DEVICE_CTX(dev);
	struct linux_device_priv *priv = usbi_get_device_priv(dev);
	struct usbi_device_descriptor *device_desc;
	uint8_t idx, num_configs;
	uint8_t *buffer;
	size_t remaining;

	device_desc = priv->descriptors;
	num_configs = device_desc->bNumConfigurations;

	if (num_configs == 0)
		return 0;	/* no configurations? */

	priv->config_descriptors = malloc(num_configs * sizeof(priv->config_descriptors[0]));
	if (!priv->config_descriptors)
		return LIBUSB_ERROR_NO_MEM;

	buffer = (uint8_t *)priv->descriptors + LIBUSB_DT_DEVICE_SIZE;
	remaining = priv->descriptors_len - LIBUSB_DT_DEVICE_SIZE;

	for (idx = 0; idx < num_configs; idx++) {
		struct usbi_configuration_descriptor *config_desc;
		uint16_t config_len;

		if (remaining < LIBUSB_DT_CONFIG_SIZE) {
			usbi_err(ctx, "short descriptor read %zu/%d",
				 remaining, LIBUSB_DT_CONFIG_SIZE);
			return LIBUSB_ERROR_IO;
		}

		config_desc = (struct usbi_configuration_descriptor *)buffer;
		if (config_desc->bDescriptorType != LIBUSB_DT_CONFIG) {
			usbi_err(ctx, "descriptor is not a config desc (type 0x%02x)",
				 config_desc->bDescriptorType);
			return LIBUSB_ERROR_IO;
		} else if (config_desc->bLength < LIBUSB_DT_CONFIG_SIZE) {
			usbi_err(ctx, "invalid descriptor bLength %u",
				 config_desc->bLength);
			return LIBUSB_ERROR_IO;
		}

		config_len = libusb_le16_to_cpu(config_desc->wTotalLength);
		if (config_len < LIBUSB_DT_CONFIG_SIZE) {
			usbi_err(ctx, "invalid wTotalLength %u", config_len);
			return LIBUSB_ERROR_IO;
		}

		if (priv->sysfs_dir) {
			/*
			 * In sysfs wTotalLength is ignored, instead the kernel returns a
			 * config descriptor with verified bLength fields, with descriptors
			 * with an invalid bLength removed.
			 */
			uint16_t sysfs_config_len;
			int offset;

			if (num_configs > 1 && idx < num_configs - 1) {
				offset = seek_to_next_config(ctx, buffer, remaining);
				if (offset < 0)
					return offset;
				sysfs_config_len = (uint16_t)offset;
			} else {
				sysfs_config_len = (uint16_t)remaining;
			}

			if (config_len != sysfs_config_len) {
				usbi_warn(ctx, "config length mismatch wTotalLength %u real %u",
					  config_len, sysfs_config_len);
				config_len = sysfs_config_len;
			}
		} else {
			/*
			 * In usbfs the config descriptors are wTotalLength bytes apart,
			 * with any short reads from the device appearing as holes in the file.
			 */
			if (config_len > remaining) {
				usbi_warn(ctx, "short descriptor read %zu/%u", remaining, config_len);
				config_len = (uint16_t)remaining;
			}
		}

		if (config_desc->bConfigurationValue == 0)
			usbi_warn(ctx, "device has configuration 0");

		priv->config_descriptors[idx].desc = config_desc;
		priv->config_descriptors[idx].actual_len = config_len;

		buffer += config_len;
		remaining -= config_len;
	}

	return LIBUSB_SUCCESS;
}

static int op_get_config_descriptor_by_value(struct libusb_device *dev,
	uint8_t value, void **buffer)
{
	struct linux_device_priv *priv = usbi_get_device_priv(dev);
	struct config_descriptor *config;
	uint8_t idx;

	for (idx = 0; idx < dev->device_descriptor.bNumConfigurations; idx++) {
		config = &priv->config_descriptors[idx];
		if (config->desc->bConfigurationValue == value) {
			*buffer = config->desc;
			return (int)config->actual_len;
		}
	}

	return LIBUSB_ERROR_NOT_FOUND;
}

static int op_get_active_config_descriptor(struct libusb_device *dev,
	void *buffer, size_t len)
{
	struct linux_device_priv *priv = usbi_get_device_priv(dev);
	void *config_desc;
	int active_config;
	int r;

	if (priv->sysfs_dir) {
		r = sysfs_get_active_config(dev, &active_config);
		if (r < 0)
			return r;
	} else {
		/* Use cached bConfigurationValue */
		active_config = priv->active_config;
	}

	if (active_config == -1) {
		usbi_err(DEVICE_CTX(dev), "device unconfigured");
		return LIBUSB_ERROR_NOT_FOUND;
	}

	r = op_get_config_descriptor_by_value(dev, (uint8_t)active_config, &config_desc);
	if (r < 0)
		return r;

	len = MIN(len, (size_t)r);
	memcpy(buffer, config_desc, len);
	return len;
}

static int op_get_config_descriptor(struct libusb_device *dev,
	uint8_t config_index, void *buffer, size_t len)
{
	struct linux_device_priv *priv = usbi_get_device_priv(dev);
	struct config_descriptor *config;

	if (config_index >= dev->device_descriptor.bNumConfigurations)
		return LIBUSB_ERROR_NOT_FOUND;

	config = &priv->config_descriptors[config_index];
	len = MIN(len, config->actual_len);
	memcpy(buffer, config->desc, len);
	return len;
}

/* send a control message to retrieve active configuration */
static int usbfs_get_active_config(struct libusb_device *dev, int fd)
{
	struct linux_device_priv *priv = usbi_get_device_priv(dev);
	uint8_t active_config = 0;
	int r;

	struct usbfs_ctrltransfer ctrl = {
		.bmRequestType = LIBUSB_ENDPOINT_IN,
		.bRequest = LIBUSB_REQUEST_GET_CONFIGURATION,
		.wValue = 0,
		.wIndex = 0,
		.wLength = 1,
		.timeout = 1000,
		.data = &active_config
	};

	r = ioctl(fd, IOCTL_USBFS_CONTROL, &ctrl);
	if (r < 0) {
		if (errno == ENODEV)
			return LIBUSB_ERROR_NO_DEVICE;

		/* we hit this error path frequently with buggy devices :( */
		usbi_warn(DEVICE_CTX(dev), "get configuration failed, errno=%d", errno);

		/* assume the current configuration is the first one if we have
		 * the configuration descriptors, otherwise treat the device
		 * as unconfigured. */
		if (priv->config_descriptors)
			priv->active_config = (int)priv->config_descriptors[0].desc->bConfigurationValue;
		else
			priv->active_config = -1;
	} else if (active_config == 0) {
		if (dev_has_config0(dev)) {
			/* some buggy devices have a configuration 0, but we're
			 * reaching into the corner of a corner case here. */
			priv->active_config = 0;
		} else {
			priv->active_config = -1;
		}
	} else {
		priv->active_config = (int)active_config;
	}

	return LIBUSB_SUCCESS;
}

static enum libusb_speed usbfs_get_speed(struct libusb_context *ctx, int fd)
{
	int r;

	r = ioctl(fd, IOCTL_USBFS_GET_SPEED, NULL);
	switch (r) {
	case USBFS_SPEED_UNKNOWN:	return LIBUSB_SPEED_UNKNOWN;
	case USBFS_SPEED_LOW:		return LIBUSB_SPEED_LOW;
	case USBFS_SPEED_FULL:		return LIBUSB_SPEED_FULL;
	case USBFS_SPEED_HIGH:		return LIBUSB_SPEED_HIGH;
	case USBFS_SPEED_WIRELESS:	return LIBUSB_SPEED_HIGH;
	case USBFS_SPEED_SUPER:		return LIBUSB_SPEED_SUPER;
	case USBFS_SPEED_SUPER_PLUS:	return LIBUSB_SPEED_SUPER_PLUS;
	default:
		usbi_warn(ctx, "Error getting device speed: %d", r);
	}

	return LIBUSB_SPEED_UNKNOWN;
}

static int initialize_device(struct libusb_device *dev, uint8_t busnum,
	uint8_t devaddr, const char *sysfs_dir, int wrapped_fd)
{
	struct linux_device_priv *priv = usbi_get_device_priv(dev);
	struct libusb_context *ctx = DEVICE_CTX(dev);
	size_t alloc_len;
	int fd, speed, r;
	ssize_t nb;

	dev->bus_number = busnum;
	dev->device_address = devaddr;

	if (sysfs_dir) {
		priv->sysfs_dir = strdup(sysfs_dir);
		if (!priv->sysfs_dir)
			return LIBUSB_ERROR_NO_MEM;

		/* Note speed can contain 1.5, in this case read_sysfs_attr()
		   will stop parsing at the '.' and return 1 */
		if (read_sysfs_attr(ctx, sysfs_dir, "speed", INT_MAX, &speed) == 0) {
			switch (speed) {
			case     1: dev->speed = LIBUSB_SPEED_LOW; break;
			case    12: dev->speed = LIBUSB_SPEED_FULL; break;
			case   480: dev->speed = LIBUSB_SPEED_HIGH; break;
			case  5000: dev->speed = LIBUSB_SPEED_SUPER; break;
			case 10000: dev->speed = LIBUSB_SPEED_SUPER_PLUS; break;
			default:
				usbi_warn(ctx, "unknown device speed: %d Mbps", speed);
			}
		}
	} else if (wrapped_fd >= 0) {
		dev->speed = usbfs_get_speed(ctx, wrapped_fd);
	}

	/* cache descriptors in memory */
	if (sysfs_dir) {
		fd = open_sysfs_attr(ctx, sysfs_dir, "descriptors");
	} else if (wrapped_fd < 0) {
		fd = get_usbfs_fd(dev, O_RDONLY, 0);
	} else {
		fd = wrapped_fd;
		r = lseek(fd, 0, SEEK_SET);
		if (r < 0) {
			usbi_err(ctx, "lseek failed, errno=%d", errno);
			return LIBUSB_ERROR_IO;
		}
	}
	if (fd < 0)
		return fd;

	alloc_len = 0;
	do {
		const size_t desc_read_length = 256;
		uint8_t *read_ptr;

		alloc_len += desc_read_length;
		priv->descriptors = usbi_reallocf(priv->descriptors, alloc_len);
		if (!priv->descriptors) {
			if (fd != wrapped_fd)
				close(fd);
			return LIBUSB_ERROR_NO_MEM;
		}
		read_ptr = (uint8_t *)priv->descriptors + priv->descriptors_len;
		/* usbfs has holes in the file */
		if (!sysfs_dir)
			memset(read_ptr, 0, desc_read_length);
		nb = read(fd, read_ptr, desc_read_length);
		if (nb < 0) {
			usbi_err(ctx, "read descriptor failed, errno=%d", errno);
			if (fd != wrapped_fd)
				close(fd);
			return LIBUSB_ERROR_IO;
		}
		priv->descriptors_len += (size_t)nb;
	} while (priv->descriptors_len == alloc_len);

	if (fd != wrapped_fd)
		close(fd);

	if (priv->descriptors_len < LIBUSB_DT_DEVICE_SIZE) {
		usbi_err(ctx, "short descriptor read (%zu)", priv->descriptors_len);
		return LIBUSB_ERROR_IO;
	}

	r = parse_config_descriptors(dev);
	if (r < 0)
		return r;

	memcpy(&dev->device_descriptor, priv->descriptors, LIBUSB_DT_DEVICE_SIZE);

	if (sysfs_dir) {
		/* sysfs descriptors are in bus-endian format */
		usbi_localize_device_descriptor(&dev->device_descriptor);
		return LIBUSB_SUCCESS;
	}

	/* cache active config */
	if (wrapped_fd < 0)
		fd = get_usbfs_fd(dev, O_RDWR, 1);
	else
		fd = wrapped_fd;
	if (fd < 0) {
		/* cannot send a control message to determine the active
		 * config. just assume the first one is active. */
		usbi_warn(ctx, "Missing rw usbfs access; cannot determine "
			       "active configuration descriptor");
		if (priv->config_descriptors)
			priv->active_config = (int)priv->config_descriptors[0].desc->bConfigurationValue;
		else
			priv->active_config = -1; /* No config dt */

		return LIBUSB_SUCCESS;
	}

	r = usbfs_get_active_config(dev, fd);
	if (fd != wrapped_fd)
		close(fd);

	return r;
}

static int linux_get_parent_info(struct libusb_device *dev, const char *sysfs_dir)
{
	struct libusb_context *ctx = DEVICE_CTX(dev);
	struct libusb_device *it;
	char *parent_sysfs_dir, *tmp;
	int ret, add_parent = 1;

	/* XXX -- can we figure out the topology when using usbfs? */
	if (!sysfs_dir || !strncmp(sysfs_dir, "usb", 3)) {
		/* either using usbfs or finding the parent of a root hub */
		return LIBUSB_SUCCESS;
	}

	parent_sysfs_dir = strdup(sysfs_dir);
	if (!parent_sysfs_dir)
		return LIBUSB_ERROR_NO_MEM;

	if ((tmp = strrchr(parent_sysfs_dir, '.')) ||
	    (tmp = strrchr(parent_sysfs_dir, '-'))) {
	        dev->port_number = atoi(tmp + 1);
		*tmp = '\0';
	} else {
		usbi_warn(ctx, "Can not parse sysfs_dir: %s, no parent info",
			  parent_sysfs_dir);
		free(parent_sysfs_dir);
		return LIBUSB_SUCCESS;
	}

	/* is the parent a root hub? */
	if (!strchr(parent_sysfs_dir, '-')) {
		tmp = parent_sysfs_dir;
		ret = asprintf(&parent_sysfs_dir, "usb%s", tmp);
		free(tmp);
		if (ret < 0)
			return LIBUSB_ERROR_NO_MEM;
	}

retry:
	/* find the parent in the context */
	usbi_mutex_lock(&ctx->usb_devs_lock);
	for_each_device(ctx, it) {
		struct linux_device_priv *priv = usbi_get_device_priv(it);

		if (priv->sysfs_dir) {
			if (!strcmp(priv->sysfs_dir, parent_sysfs_dir)) {
				dev->parent_dev = libusb_ref_device(it);
				break;
			}
		}
	}
	usbi_mutex_unlock(&ctx->usb_devs_lock);

	if (!dev->parent_dev && add_parent) {
		usbi_dbg(ctx, "parent_dev %s not enumerated yet, enumerating now",
			 parent_sysfs_dir);
		sysfs_scan_device(ctx, parent_sysfs_dir);
		add_parent = 0;
		goto retry;
	}

	usbi_dbg(ctx, "dev %p (%s) has parent %p (%s) port %u", dev, sysfs_dir,
		 dev->parent_dev, parent_sysfs_dir, dev->port_number);

	free(parent_sysfs_dir);

	return LIBUSB_SUCCESS;
}

int linux_enumerate_device(struct libusb_context *ctx,
	uint8_t busnum, uint8_t devaddr, const char *sysfs_dir)
{
	unsigned long session_id;
	struct libusb_device *dev;
	int r;

	/* FIXME: session ID is not guaranteed unique as addresses can wrap and
	 * will be reused. instead we should add a simple sysfs attribute with
	 * a session ID. */
	session_id = busnum << 8 | devaddr;
	usbi_dbg(ctx, "busnum %u devaddr %u session_id %lu", busnum, devaddr, session_id);

	dev = usbi_get_device_by_session_id(ctx, session_id);
	if (dev) {
		/* device already exists in the context */
		usbi_dbg(ctx, "session_id %lu already exists", session_id);
		libusb_unref_device(dev);
		return LIBUSB_SUCCESS;
	}

	usbi_dbg(ctx, "allocating new device for %u/%u (session %lu)",
		 busnum, devaddr, session_id);
	dev = usbi_alloc_device(ctx, session_id);
	if (!dev)
		return LIBUSB_ERROR_NO_MEM;

	r = initialize_device(dev, busnum, devaddr, sysfs_dir, -1);
	if (r < 0)
		goto out;
	r = usbi_sanitize_device(dev);
	if (r < 0)
		goto out;

	r = linux_get_parent_info(dev, sysfs_dir);
	if (r < 0)
		goto out;
out:
	if (r < 0)
		libusb_unref_device(dev);
	else
		usbi_connect_device(dev);

	return r;
}

void linux_hotplug_enumerate(uint8_t busnum, uint8_t devaddr, const char *sys_name)
{
	struct libusb_context *ctx;

	usbi_mutex_static_lock(&active_contexts_lock);
	for_each_context(ctx) {
		linux_enumerate_device(ctx, busnum, devaddr, sys_name);
	}
	usbi_mutex_static_unlock(&active_contexts_lock);
}

void linux_device_disconnected(uint8_t busnum, uint8_t devaddr)
{
	struct libusb_context *ctx;
	struct libusb_device *dev;
	unsigned long session_id = busnum << 8 | devaddr;

	usbi_mutex_static_lock(&active_contexts_lock);
	for_each_context(ctx) {
		dev = usbi_get_device_by_session_id(ctx, session_id);
		if (dev) {
			usbi_disconnect_device(dev);
			libusb_unref_device(dev);
		} else {
			usbi_dbg(ctx, "device not found for session %lx", session_id);
		}
	}
	usbi_mutex_static_unlock(&active_contexts_lock);
}

#if !defined(HAVE_LIBUDEV)
static int parse_u8(const char *str, uint8_t *val_p)
{
	char *endptr;
	long num;

	errno = 0;
	num = strtol(str, &endptr, 10);
	if (num < 0 || num > UINT8_MAX || errno)
		return 0;
	if (endptr == str || *endptr != '\0')
		return 0;

	*val_p = (uint8_t)num;
	return 1;
}

/* open a bus directory and adds all discovered devices to the context */
static int usbfs_scan_busdir(struct libusb_context *ctx, uint8_t busnum)
{
	DIR *dir;
	char dirpath[20];
	struct dirent *entry;
	int r = LIBUSB_ERROR_IO;

	snprintf(dirpath, sizeof(dirpath), USB_DEVTMPFS_PATH "/%03u", busnum);
	usbi_dbg(ctx, "%s", dirpath);
	dir = opendir(dirpath);
	if (!dir) {
		usbi_err(ctx, "opendir '%s' failed, errno=%d", dirpath, errno);
		/* FIXME: should handle valid race conditions like hub unplugged
		 * during directory iteration - this is not an error */
		return r;
	}

	while ((entry = readdir(dir))) {
		uint8_t devaddr;

		if (entry->d_name[0] == '.')
			continue;

		if (!parse_u8(entry->d_name, &devaddr)) {
			usbi_dbg(ctx, "unknown dir entry %s", entry->d_name);
			continue;
		}

		if (linux_enumerate_device(ctx, busnum, devaddr, NULL)) {
			usbi_dbg(ctx, "failed to enumerate dir entry %s", entry->d_name);
			continue;
		}

		r = 0;
	}

	closedir(dir);
	return r;
}

static int usbfs_get_device_list(struct libusb_context *ctx)
{
	struct dirent *entry;
	DIR *buses;
	uint8_t busnum, devaddr;
	int r = 0;

	if (usbdev_names)
		buses = opendir(USBDEV_PATH);
	else
		buses = opendir(USB_DEVTMPFS_PATH);

	if (!buses) {
		usbi_err(ctx, "opendir buses failed, errno=%d", errno);
		return LIBUSB_ERROR_IO;
	}

	while ((entry = readdir(buses))) {
		if (entry->d_name[0] == '.')
			continue;

		if (usbdev_names) {
			if (!is_usbdev_entry(entry->d_name, &busnum, &devaddr))
				continue;

			r = linux_enumerate_device(ctx, busnum, devaddr, NULL);
			if (r < 0) {
				usbi_dbg(ctx, "failed to enumerate dir entry %s", entry->d_name);
				continue;
			}
		} else {
			if (!parse_u8(entry->d_name, &busnum)) {
				usbi_dbg(ctx, "unknown dir entry %s", entry->d_name);
				continue;
			}

			r = usbfs_scan_busdir(ctx, busnum);
			if (r < 0)
				break;
		}
	}

	closedir(buses);
	return r;

}

static int sysfs_get_device_list(struct libusb_context *ctx)
{
	DIR *devices = opendir(SYSFS_DEVICE_PATH);
	struct dirent *entry;
	int num_devices = 0;
	int num_enumerated = 0;

	if (!devices) {
		usbi_err(ctx, "opendir devices failed, errno=%d", errno);
		return LIBUSB_ERROR_IO;
	}

	while ((entry = readdir(devices))) {
		if ((!isdigit(entry->d_name[0]) && strncmp(entry->d_name, "usb", 3))
		    || strchr(entry->d_name, ':'))
			continue;

		num_devices++;

		if (sysfs_scan_device(ctx, entry->d_name)) {
			usbi_dbg(ctx, "failed to enumerate dir entry %s", entry->d_name);
			continue;
		}

		num_enumerated++;
	}

	closedir(devices);

	/* successful if at least one device was enumerated or no devices were found */
	if (num_enumerated || !num_devices)
		return LIBUSB_SUCCESS;
	else
		return LIBUSB_ERROR_IO;
}

static int linux_default_scan_devices(struct libusb_context *ctx)
{
	/* we can retrieve device list and descriptors from sysfs or usbfs.
	 * sysfs is preferable, because if we use usbfs we end up resuming
	 * any autosuspended USB devices. however, sysfs is not available
	 * everywhere, so we need a usbfs fallback too.
	 */
	if (sysfs_available)
		return sysfs_get_device_list(ctx);
	else
		return usbfs_get_device_list(ctx);
}
#endif

static int initialize_handle(struct libusb_device_handle *handle, int fd)
{
	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
	int r;

	hpriv->fd = fd;

	r = ioctl(fd, IOCTL_USBFS_GET_CAPABILITIES, &hpriv->caps);
	if (r < 0) {
		if (errno == ENOTTY)
			usbi_dbg(HANDLE_CTX(handle), "getcap not available");
		else
			usbi_err(HANDLE_CTX(handle), "getcap failed, errno=%d", errno);
		hpriv->caps = USBFS_CAP_BULK_CONTINUATION;
	}

	return usbi_add_event_source(HANDLE_CTX(handle), hpriv->fd, POLLOUT);
}

static int op_wrap_sys_device(struct libusb_context *ctx,
	struct libusb_device_handle *handle, intptr_t sys_dev)
{
	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
	int fd = (int)sys_dev;
	uint8_t busnum, devaddr;
	struct usbfs_connectinfo ci;
	struct libusb_device *dev;
	int r;

	r = linux_get_device_address(ctx, 1, &busnum, &devaddr, NULL, NULL, fd);
	if (r < 0) {
		r = ioctl(fd, IOCTL_USBFS_CONNECTINFO, &ci);
		if (r < 0) {
			usbi_err(ctx, "connectinfo failed, errno=%d", errno);
			return LIBUSB_ERROR_IO;
		}
		/* There is no ioctl to get the bus number. We choose 0 here
		 * as linux starts numbering buses from 1. */
		busnum = 0;
		devaddr = ci.devnum;
	}

	/* Session id is unused as we do not add the device to the list of
	 * connected devices. */
	usbi_dbg(ctx, "allocating new device for fd %d", fd);
	dev = usbi_alloc_device(ctx, 0);
	if (!dev)
		return LIBUSB_ERROR_NO_MEM;

	r = initialize_device(dev, busnum, devaddr, NULL, fd);
	if (r < 0)
		goto out;
	r = usbi_sanitize_device(dev);
	if (r < 0)
		goto out;
	/* Consider the device as connected, but do not add it to the managed
	 * device list. */
	usbi_atomic_store(&dev->attached, 1);
	handle->dev = dev;

	r = initialize_handle(handle, fd);
	hpriv->fd_keep = 1;

out:
	if (r < 0)
		libusb_unref_device(dev);
	return r;
}

static int op_open(struct libusb_device_handle *handle)
{
	int fd, r;

	fd = get_usbfs_fd(handle->dev, O_RDWR, 0);
	if (fd < 0) {
		if (fd == LIBUSB_ERROR_NO_DEVICE) {
			/* device will still be marked as attached if hotplug monitor thread
			 * hasn't processed remove event yet */
			usbi_mutex_static_lock(&linux_hotplug_lock);
			if (usbi_atomic_load(&handle->dev->attached)) {
				usbi_dbg(HANDLE_CTX(handle), "open failed with no device, but device still attached");
				linux_device_disconnected(handle->dev->bus_number,
							  handle->dev->device_address);
			}
			usbi_mutex_static_unlock(&linux_hotplug_lock);
		}
		return fd;
	}

	r = initialize_handle(handle, fd);
	if (r < 0)
		close(fd);

	return r;
}

static void op_close(struct libusb_device_handle *dev_handle)
{
	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(dev_handle);

	/* fd may have already been removed by POLLERR condition in op_handle_events() */
	if (!hpriv->fd_removed)
		usbi_remove_event_source(HANDLE_CTX(dev_handle), hpriv->fd);
	if (!hpriv->fd_keep)
		close(hpriv->fd);
}

static int op_get_configuration(struct libusb_device_handle *handle,
	uint8_t *config)
{
	struct linux_device_priv *priv = usbi_get_device_priv(handle->dev);
	int active_config = -1; /* to please compiler */
	int r;

	if (priv->sysfs_dir) {
		r = sysfs_get_active_config(handle->dev, &active_config);
	} else {
		struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);

		r = usbfs_get_active_config(handle->dev, hpriv->fd);
		if (r == LIBUSB_SUCCESS)
			active_config = priv->active_config;
	}
	if (r < 0)
		return r;

	if (active_config == -1) {
		usbi_warn(HANDLE_CTX(handle), "device unconfigured");
		active_config = 0;
	}

	*config = (uint8_t)active_config;

	return 0;
}

static int op_set_configuration(struct libusb_device_handle *handle, int config)
{
	struct linux_device_priv *priv = usbi_get_device_priv(handle->dev);
	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
	int fd = hpriv->fd;
	int r = ioctl(fd, IOCTL_USBFS_SETCONFIGURATION, &config);

	if (r < 0) {
		if (errno == EINVAL)
			return LIBUSB_ERROR_NOT_FOUND;
		else if (errno == EBUSY)
			return LIBUSB_ERROR_BUSY;
		else if (errno == ENODEV)
			return LIBUSB_ERROR_NO_DEVICE;

		usbi_err(HANDLE_CTX(handle), "set configuration failed, errno=%d", errno);
		return LIBUSB_ERROR_OTHER;
	}

	/* if necessary, update our cached active config descriptor */
	if (!priv->sysfs_dir) {
		if (config == 0 && !dev_has_config0(handle->dev))
			config = -1;

		priv->active_config = config;
	}

	return LIBUSB_SUCCESS;
}

static int claim_interface(struct libusb_device_handle *handle, unsigned int iface)
{
	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
	int fd = hpriv->fd;
	int r = ioctl(fd, IOCTL_USBFS_CLAIMINTERFACE, &iface);

	if (r < 0) {
		if (errno == ENOENT)
			return LIBUSB_ERROR_NOT_FOUND;
		else if (errno == EBUSY)
			return LIBUSB_ERROR_BUSY;
		else if (errno == ENODEV)
			return LIBUSB_ERROR_NO_DEVICE;

		usbi_err(HANDLE_CTX(handle), "claim interface failed, errno=%d", errno);
		return LIBUSB_ERROR_OTHER;
	}
	return 0;
}

static int release_interface(struct libusb_device_handle *handle, unsigned int iface)
{
	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
	int fd = hpriv->fd;
	int r = ioctl(fd, IOCTL_USBFS_RELEASEINTERFACE, &iface);

	if (r < 0) {
		if (errno == ENODEV)
			return LIBUSB_ERROR_NO_DEVICE;

		usbi_err(HANDLE_CTX(handle), "release interface failed, errno=%d", errno);
		return LIBUSB_ERROR_OTHER;
	}
	return 0;
}

static int op_set_interface(struct libusb_device_handle *handle, uint8_t interface,
	uint8_t altsetting)
{
	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
	int fd = hpriv->fd;
	struct usbfs_setinterface setintf;
	int r;

	setintf.interface = interface;
	setintf.altsetting = altsetting;
	r = ioctl(fd, IOCTL_USBFS_SETINTERFACE, &setintf);
	if (r < 0) {
		if (errno == EINVAL)
			return LIBUSB_ERROR_NOT_FOUND;
		else if (errno == ENODEV)
			return LIBUSB_ERROR_NO_DEVICE;

		usbi_err(HANDLE_CTX(handle), "set interface failed, errno=%d", errno);
		return LIBUSB_ERROR_OTHER;
	}

	return 0;
}

static int op_clear_halt(struct libusb_device_handle *handle,
	unsigned char endpoint)
{
	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
	int fd = hpriv->fd;
	unsigned int _endpoint = endpoint;
	int r = ioctl(fd, IOCTL_USBFS_CLEAR_HALT, &_endpoint);

	if (r < 0) {
		if (errno == ENOENT)
			return LIBUSB_ERROR_NOT_FOUND;
		else if (errno == ENODEV)
			return LIBUSB_ERROR_NO_DEVICE;

		usbi_err(HANDLE_CTX(handle), "clear halt failed, errno=%d", errno);
		return LIBUSB_ERROR_OTHER;
	}

	return 0;
}

static int op_reset_device(struct libusb_device_handle *handle)
{
	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
	int fd = hpriv->fd;
	int r, ret = 0;
	uint8_t i;

	/* Doing a device reset will cause the usbfs driver to get unbound
	 * from any interfaces it is bound to. By voluntarily unbinding
	 * the usbfs driver ourself, we stop the kernel from rebinding
	 * the interface after reset (which would end up with the interface
	 * getting bound to the in kernel driver if any). */
	for (i = 0; i < USB_MAXINTERFACES; i++) {
		if (handle->claimed_interfaces & (1UL << i))
			release_interface(handle, i);
	}

	usbi_mutex_lock(&handle->lock);
	r = ioctl(fd, IOCTL_USBFS_RESET, NULL);
	if (r < 0) {
		if (errno == ENODEV) {
			ret = LIBUSB_ERROR_NOT_FOUND;
			goto out;
		}

		usbi_err(HANDLE_CTX(handle), "reset failed, errno=%d", errno);
		ret = LIBUSB_ERROR_OTHER;
		goto out;
	}

	/* And re-claim any interfaces which were claimed before the reset */
	for (i = 0; i < USB_MAXINTERFACES; i++) {
		if (!(handle->claimed_interfaces & (1UL << i)))
			continue;
		/*
		 * A driver may have completed modprobing during
		 * IOCTL_USBFS_RESET, and bound itself as soon as
		 * IOCTL_USBFS_RESET released the device lock
		 */
		r = detach_kernel_driver_and_claim(handle, i);
		if (r) {
			usbi_warn(HANDLE_CTX(handle), "failed to re-claim interface %u after reset: %s",
				  i, libusb_error_name(r));
			handle->claimed_interfaces &= ~(1UL << i);
			ret = LIBUSB_ERROR_NOT_FOUND;
		}
	}
out:
	usbi_mutex_unlock(&handle->lock);
	return ret;
}

static int do_streams_ioctl(struct libusb_device_handle *handle, long req,
	uint32_t num_streams, unsigned char *endpoints, int num_endpoints)
{
	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
	int r, fd = hpriv->fd;
	struct usbfs_streams *streams;

	if (num_endpoints > 30) /* Max 15 in + 15 out eps */
		return LIBUSB_ERROR_INVALID_PARAM;

	streams = malloc(sizeof(*streams) + num_endpoints);
	if (!streams)
		return LIBUSB_ERROR_NO_MEM;

	streams->num_streams = num_streams;
	streams->num_eps = num_endpoints;
	memcpy(streams->eps, endpoints, num_endpoints);

	r = ioctl(fd, req, streams);

	free(streams);

	if (r < 0) {
		if (errno == ENOTTY)
			return LIBUSB_ERROR_NOT_SUPPORTED;
		else if (errno == EINVAL)
			return LIBUSB_ERROR_INVALID_PARAM;
		else if (errno == ENODEV)
			return LIBUSB_ERROR_NO_DEVICE;

		usbi_err(HANDLE_CTX(handle), "streams-ioctl failed, errno=%d", errno);
		return LIBUSB_ERROR_OTHER;
	}
	return r;
}

static int op_alloc_streams(struct libusb_device_handle *handle,
	uint32_t num_streams, unsigned char *endpoints, int num_endpoints)
{
	return do_streams_ioctl(handle, IOCTL_USBFS_ALLOC_STREAMS,
				num_streams, endpoints, num_endpoints);
}

static int op_free_streams(struct libusb_device_handle *handle,
		unsigned char *endpoints, int num_endpoints)
{
	return do_streams_ioctl(handle, IOCTL_USBFS_FREE_STREAMS, 0,
				endpoints, num_endpoints);
}

static void *op_dev_mem_alloc(struct libusb_device_handle *handle, size_t len)
{
	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
	void *buffer;

	buffer = mmap(NULL, len, PROT_READ | PROT_WRITE, MAP_SHARED, hpriv->fd, 0);
	if (buffer == MAP_FAILED) {
		usbi_err(HANDLE_CTX(handle), "alloc dev mem failed, errno=%d", errno);
		return NULL;
	}
	return buffer;
}

static int op_dev_mem_free(struct libusb_device_handle *handle, void *buffer,
	size_t len)
{
	if (munmap(buffer, len) != 0) {
		usbi_err(HANDLE_CTX(handle), "free dev mem failed, errno=%d", errno);
		return LIBUSB_ERROR_OTHER;
	} else {
		return LIBUSB_SUCCESS;
	}
}

static int op_kernel_driver_active(struct libusb_device_handle *handle,
	uint8_t interface)
{
	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
	int fd = hpriv->fd;
	struct usbfs_getdriver getdrv;
	int r;

	getdrv.interface = interface;
	r = ioctl(fd, IOCTL_USBFS_GETDRIVER, &getdrv);
	if (r < 0) {
		if (errno == ENODATA)
			return 0;
		else if (errno == ENODEV)
			return LIBUSB_ERROR_NO_DEVICE;

		usbi_err(HANDLE_CTX(handle), "get driver failed, errno=%d", errno);
		return LIBUSB_ERROR_OTHER;
	}

	return strcmp(getdrv.driver, "usbfs") != 0;
}

static int op_detach_kernel_driver(struct libusb_device_handle *handle,
	uint8_t interface)
{
	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
	int fd = hpriv->fd;
	struct usbfs_ioctl command;
	struct usbfs_getdriver getdrv;
	int r;

	command.ifno = interface;
	command.ioctl_code = IOCTL_USBFS_DISCONNECT;
	command.data = NULL;

	getdrv.interface = interface;
	r = ioctl(fd, IOCTL_USBFS_GETDRIVER, &getdrv);
	if (r == 0 && !strcmp(getdrv.driver, "usbfs"))
		return LIBUSB_ERROR_NOT_FOUND;

	r = ioctl(fd, IOCTL_USBFS_IOCTL, &command);
	if (r < 0) {
		if (errno == ENODATA)
			return LIBUSB_ERROR_NOT_FOUND;
		else if (errno == EINVAL)
			return LIBUSB_ERROR_INVALID_PARAM;
		else if (errno == ENODEV)
			return LIBUSB_ERROR_NO_DEVICE;

		usbi_err(HANDLE_CTX(handle), "detach failed, errno=%d", errno);
		return LIBUSB_ERROR_OTHER;
	}

	return 0;
}

static int op_attach_kernel_driver(struct libusb_device_handle *handle,
	uint8_t interface)
{
	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
	int fd = hpriv->fd;
	struct usbfs_ioctl command;
	int r;

	command.ifno = interface;
	command.ioctl_code = IOCTL_USBFS_CONNECT;
	command.data = NULL;

	r = ioctl(fd, IOCTL_USBFS_IOCTL, &command);
	if (r < 0) {
		if (errno == ENODATA)
			return LIBUSB_ERROR_NOT_FOUND;
		else if (errno == EINVAL)
			return LIBUSB_ERROR_INVALID_PARAM;
		else if (errno == ENODEV)
			return LIBUSB_ERROR_NO_DEVICE;
		else if (errno == EBUSY)
			return LIBUSB_ERROR_BUSY;

		usbi_err(HANDLE_CTX(handle), "attach failed, errno=%d", errno);
		return LIBUSB_ERROR_OTHER;
	} else if (r == 0) {
		return LIBUSB_ERROR_NOT_FOUND;
	}

	return 0;
}

static int detach_kernel_driver_and_claim(struct libusb_device_handle *handle,
	uint8_t interface)
{
	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
	struct usbfs_disconnect_claim dc;
	int r, fd = hpriv->fd;

	dc.interface = interface;
	strcpy(dc.driver, "usbfs");
	dc.flags = USBFS_DISCONNECT_CLAIM_EXCEPT_DRIVER;
	r = ioctl(fd, IOCTL_USBFS_DISCONNECT_CLAIM, &dc);
	if (r == 0)
		return 0;
	switch (errno) {
	case ENOTTY:
		break;
	case EBUSY:
		return LIBUSB_ERROR_BUSY;
	case EINVAL:
		return LIBUSB_ERROR_INVALID_PARAM;
	case ENODEV:
		return LIBUSB_ERROR_NO_DEVICE;
	default:
		usbi_err(HANDLE_CTX(handle), "disconnect-and-claim failed, errno=%d", errno);
		return LIBUSB_ERROR_OTHER;
	}

	/* Fallback code for kernels which don't support the
	   disconnect-and-claim ioctl */
	r = op_detach_kernel_driver(handle, interface);
	if (r != 0 && r != LIBUSB_ERROR_NOT_FOUND)
		return r;

	return claim_interface(handle, interface);
}

static int op_claim_interface(struct libusb_device_handle *handle, uint8_t interface)
{
	if (handle->auto_detach_kernel_driver)
		return detach_kernel_driver_and_claim(handle, interface);
	else
		return claim_interface(handle, interface);
}

static int op_release_interface(struct libusb_device_handle *handle, uint8_t interface)
{
	int r;

	r = release_interface(handle, interface);
	if (r)
		return r;

	if (handle->auto_detach_kernel_driver)
		op_attach_kernel_driver(handle, interface);

	return 0;
}

static void op_destroy_device(struct libusb_device *dev)
{
	struct linux_device_priv *priv = usbi_get_device_priv(dev);

	free(priv->config_descriptors);
	free(priv->descriptors);
	free(priv->sysfs_dir);
}

/* URBs are discarded in reverse order of submission to avoid races. */
static int discard_urbs(struct usbi_transfer *itransfer, int first, int last_plus_one)
{
	struct libusb_transfer *transfer =
		USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
	struct linux_transfer_priv *tpriv = usbi_get_transfer_priv(itransfer);
	struct linux_device_handle_priv *hpriv =
		usbi_get_device_handle_priv(transfer->dev_handle);
	int i, ret = 0;
	struct usbfs_urb *urb;

	for (i = last_plus_one - 1; i >= first; i--) {
		if (transfer->type == LIBUSB_TRANSFER_TYPE_ISOCHRONOUS)
			urb = tpriv->iso_urbs[i];
		else
			urb = &tpriv->urbs[i];

		if (ioctl(hpriv->fd, IOCTL_USBFS_DISCARDURB, urb) == 0)
			continue;

		if (errno == EINVAL) {
			usbi_dbg(TRANSFER_CTX(transfer), "URB not found --> assuming ready to be reaped");
			if (i == (last_plus_one - 1))
				ret = LIBUSB_ERROR_NOT_FOUND;
		} else if (errno == ENODEV) {
			usbi_dbg(TRANSFER_CTX(transfer), "Device not found for URB --> assuming ready to be reaped");
			ret = LIBUSB_ERROR_NO_DEVICE;
		} else {
			usbi_warn(TRANSFER_CTX(transfer), "unrecognised discard errno %d", errno);
			ret = LIBUSB_ERROR_OTHER;
		}
	}
	return ret;
}

static void free_iso_urbs(struct linux_transfer_priv *tpriv)
{
	int i;

	for (i = 0; i < tpriv->num_urbs; i++) {
		struct usbfs_urb *urb = tpriv->iso_urbs[i];

		if (!urb)
			break;
		free(urb);
	}

	free(tpriv->iso_urbs);
	tpriv->iso_urbs = NULL;
}

static int submit_bulk_transfer(struct usbi_transfer *itransfer)
{
	struct libusb_transfer *transfer =
		USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
	struct linux_transfer_priv *tpriv = usbi_get_transfer_priv(itransfer);
	struct linux_device_handle_priv *hpriv =
		usbi_get_device_handle_priv(transfer->dev_handle);
	struct usbfs_urb *urbs;
	int is_out = IS_XFEROUT(transfer);
	int bulk_buffer_len, use_bulk_continuation;
	int num_urbs;
	int last_urb_partial = 0;
	int r;
	int i;

	/*
	 * Older versions of usbfs place a 16kb limit on bulk URBs. We work
	 * around this by splitting large transfers into 16k blocks, and then
	 * submit all urbs at once. it would be simpler to submit one urb at
	 * a time, but there is a big performance gain doing it this way.
	 *
	 * Newer versions lift the 16k limit (USBFS_CAP_NO_PACKET_SIZE_LIM),
	 * using arbitrary large transfers can still be a bad idea though, as
	 * the kernel needs to allocate physical contiguous memory for this,
	 * which may fail for large buffers.
	 *
	 * The kernel solves this problem by splitting the transfer into
	 * blocks itself when the host-controller is scatter-gather capable
	 * (USBFS_CAP_BULK_SCATTER_GATHER), which most controllers are.
	 *
	 * Last, there is the issue of short-transfers when splitting, for
	 * short split-transfers to work reliable USBFS_CAP_BULK_CONTINUATION
	 * is needed, but this is not always available.
	 */
	if (hpriv->caps & USBFS_CAP_BULK_SCATTER_GATHER) {
		/* Good! Just submit everything in one go */
		bulk_buffer_len = transfer->length ? transfer->length : 1;
		use_bulk_continuation = 0;
	} else if (hpriv->caps & USBFS_CAP_BULK_CONTINUATION) {
		/* Split the transfers and use bulk-continuation to
		   avoid issues with short-transfers */
		bulk_buffer_len = MAX_BULK_BUFFER_LENGTH;
		use_bulk_continuation = 1;
	} else if (hpriv->caps & USBFS_CAP_NO_PACKET_SIZE_LIM) {
		/* Don't split, assume the kernel can alloc the buffer
		   (otherwise the submit will fail with -ENOMEM) */
		bulk_buffer_len = transfer->length ? transfer->length : 1;
		use_bulk_continuation = 0;
	} else {
		/* Bad, splitting without bulk-continuation, short transfers
		   which end before the last urb will not work reliable! */
		/* Note we don't warn here as this is "normal" on kernels <
		   2.6.32 and not a problem for most applications */
		bulk_buffer_len = MAX_BULK_BUFFER_LENGTH;
		use_bulk_continuation = 0;
	}

	num_urbs = transfer->length / bulk_buffer_len;

	if (transfer->length == 0) {
		num_urbs = 1;
	} else if ((transfer->length % bulk_buffer_len) > 0) {
		last_urb_partial = 1;
		num_urbs++;
	}
	usbi_dbg(TRANSFER_CTX(transfer), "need %d urbs for new transfer with length %d", num_urbs, transfer->length);
	urbs = calloc(num_urbs, sizeof(*urbs));
	if (!urbs)
		return LIBUSB_ERROR_NO_MEM;
	tpriv->urbs = urbs;
	tpriv->num_urbs = num_urbs;
	tpriv->num_retired = 0;
	tpriv->reap_action = NORMAL;
	tpriv->reap_status = LIBUSB_TRANSFER_COMPLETED;

	for (i = 0; i < num_urbs; i++) {
		struct usbfs_urb *urb = &urbs[i];

		urb->usercontext = itransfer;
		switch (transfer->type) {
		case LIBUSB_TRANSFER_TYPE_BULK:
			urb->type = USBFS_URB_TYPE_BULK;
			urb->stream_id = 0;
			break;
		case LIBUSB_TRANSFER_TYPE_BULK_STREAM:
			urb->type = USBFS_URB_TYPE_BULK;
			urb->stream_id = itransfer->stream_id;
			break;
		case LIBUSB_TRANSFER_TYPE_INTERRUPT:
			urb->type = USBFS_URB_TYPE_INTERRUPT;
			break;
		}
		urb->endpoint = transfer->endpoint;
		urb->buffer = transfer->buffer + (i * bulk_buffer_len);

		/* don't set the short not ok flag for the last URB */
		if (use_bulk_continuation && !is_out && (i < num_urbs - 1))
			urb->flags = USBFS_URB_SHORT_NOT_OK;

		if (i == num_urbs - 1 && last_urb_partial)
			urb->buffer_length = transfer->length % bulk_buffer_len;
		else if (transfer->length == 0)
			urb->buffer_length = 0;
		else
			urb->buffer_length = bulk_buffer_len;

		if (i > 0 && use_bulk_continuation)
			urb->flags |= USBFS_URB_BULK_CONTINUATION;

		/* we have already checked that the flag is supported */
		if (is_out && i == num_urbs - 1 &&
		    (transfer->flags & LIBUSB_TRANSFER_ADD_ZERO_PACKET))
			urb->flags |= USBFS_URB_ZERO_PACKET;

		r = ioctl(hpriv->fd, IOCTL_USBFS_SUBMITURB, urb);
		if (r == 0)
			continue;

		if (errno == ENODEV) {
			r = LIBUSB_ERROR_NO_DEVICE;
		} else if (errno == ENOMEM) {
			r = LIBUSB_ERROR_NO_MEM;
		} else {
			usbi_err(TRANSFER_CTX(transfer), "submiturb failed, errno=%d", errno);
			r = LIBUSB_ERROR_IO;
		}

		/* if the first URB submission fails, we can simply free up and
		 * return failure immediately. */
		if (i == 0) {
			usbi_dbg(TRANSFER_CTX(transfer), "first URB failed, easy peasy");
			free(urbs);
			tpriv->urbs = NULL;
			return r;
		}

		/* if it's not the first URB that failed, the situation is a bit
		 * tricky. we may need to discard all previous URBs. there are
		 * complications:
		 *  - discarding is asynchronous - discarded urbs will be reaped
		 *    later. the user must not have freed the transfer when the
		 *    discarded URBs are reaped, otherwise libusb will be using
		 *    freed memory.
		 *  - the earlier URBs may have completed successfully and we do
		 *    not want to throw away any data.
		 *  - this URB failing may be no error; EREMOTEIO means that
		 *    this transfer simply didn't need all the URBs we submitted
		 * so, we report that the transfer was submitted successfully and
		 * in case of error we discard all previous URBs. later when
		 * the final reap completes we can report error to the user,
		 * or success if an earlier URB was completed successfully.
		 */
		tpriv->reap_action = errno == EREMOTEIO ? COMPLETED_EARLY : SUBMIT_FAILED;

		/* The URBs we haven't submitted yet we count as already
		 * retired. */
		tpriv->num_retired += num_urbs - i;

		/* If we completed short then don't try to discard. */
		if (tpriv->reap_action == COMPLETED_EARLY)
			return 0;

		discard_urbs(itransfer, 0, i);

		usbi_dbg(TRANSFER_CTX(transfer), "reporting successful submission but waiting for %d "
			 "discards before reporting error", i);
		return 0;
	}

	return 0;
}

static int submit_iso_transfer(struct usbi_transfer *itransfer)
{
	struct libusb_transfer *transfer =
		USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
	struct linux_transfer_priv *tpriv = usbi_get_transfer_priv(itransfer);
	struct linux_device_handle_priv *hpriv =
		usbi_get_device_handle_priv(transfer->dev_handle);
	struct usbfs_urb **urbs;
	int num_packets = transfer->num_iso_packets;
	int num_packets_remaining;
	int i, j;
	int num_urbs;
	unsigned int packet_len;
	unsigned int total_len = 0;
	unsigned char *urb_buffer = transfer->buffer;

	if (num_packets < 1)
		return LIBUSB_ERROR_INVALID_PARAM;

	/* usbfs places arbitrary limits on iso URBs. this limit has changed
	 * at least three times, but we attempt to detect this limit during
	 * init and check it here. if the kernel rejects the request due to
	 * its size, we return an error indicating such to the user.
	 */
	for (i = 0; i < num_packets; i++) {
		packet_len = transfer->iso_packet_desc[i].length;

		if (packet_len > max_iso_packet_len) {
			usbi_warn(TRANSFER_CTX(transfer),
				  "iso packet length of %u bytes exceeds maximum of %u bytes",
				  packet_len, max_iso_packet_len);
			return LIBUSB_ERROR_INVALID_PARAM;
		}

		total_len += packet_len;
	}

	if (transfer->length < (int)total_len)
		return LIBUSB_ERROR_INVALID_PARAM;

	/* usbfs limits the number of iso packets per URB */
	num_urbs = (num_packets + (MAX_ISO_PACKETS_PER_URB - 1)) / MAX_ISO_PACKETS_PER_URB;

	usbi_dbg(TRANSFER_CTX(transfer), "need %d urbs for new transfer with length %d", num_urbs, transfer->length);

	urbs = calloc(num_urbs, sizeof(*urbs));
	if (!urbs)
		return LIBUSB_ERROR_NO_MEM;

	tpriv->iso_urbs = urbs;
	tpriv->num_urbs = num_urbs;
	tpriv->num_retired = 0;
	tpriv->reap_action = NORMAL;
	tpriv->iso_packet_offset = 0;

	/* allocate + initialize each URB with the correct number of packets */
	num_packets_remaining = num_packets;
	for (i = 0, j = 0; i < num_urbs; i++) {
		int num_packets_in_urb = MIN(num_packets_remaining, MAX_ISO_PACKETS_PER_URB);
		struct usbfs_urb *urb;
		size_t alloc_size;
		int k;

		alloc_size = sizeof(*urb)
			+ (num_packets_in_urb * sizeof(struct usbfs_iso_packet_desc));
		urb = calloc(1, alloc_size);
		if (!urb) {
			free_iso_urbs(tpriv);
			return LIBUSB_ERROR_NO_MEM;
		}
		urbs[i] = urb;

		/* populate packet lengths */
		for (k = 0; k < num_packets_in_urb; j++, k++) {
			packet_len = transfer->iso_packet_desc[j].length;
			urb->buffer_length += packet_len;
			urb->iso_frame_desc[k].length = packet_len;
		}

		urb->usercontext = itransfer;
		urb->type = USBFS_URB_TYPE_ISO;
		/* FIXME: interface for non-ASAP data? */
		urb->flags = USBFS_URB_ISO_ASAP;
		urb->endpoint = transfer->endpoint;
		urb->number_of_packets = num_packets_in_urb;
		urb->buffer = urb_buffer;

		urb_buffer += urb->buffer_length;
		num_packets_remaining -= num_packets_in_urb;
	}

	/* submit URBs */
	for (i = 0; i < num_urbs; i++) {
		int r = ioctl(hpriv->fd, IOCTL_USBFS_SUBMITURB, urbs[i]);

		if (r == 0)
			continue;

		if (errno == ENODEV) {
			r = LIBUSB_ERROR_NO_DEVICE;
		} else if (errno == EINVAL) {
			usbi_warn(TRANSFER_CTX(transfer), "submiturb failed, transfer too large");
			r = LIBUSB_ERROR_INVALID_PARAM;
		} else if (errno == EMSGSIZE) {
			usbi_warn(TRANSFER_CTX(transfer), "submiturb failed, iso packet length too large");
			r = LIBUSB_ERROR_INVALID_PARAM;
		} else {
			usbi_err(TRANSFER_CTX(transfer), "submiturb failed, errno=%d", errno);
			r = LIBUSB_ERROR_IO;
		}

		/* if the first URB submission fails, we can simply free up and
		 * return failure immediately. */
		if (i == 0) {
			usbi_dbg(TRANSFER_CTX(transfer), "first URB failed, easy peasy");
			free_iso_urbs(tpriv);
			return r;
		}

		/* if it's not the first URB that failed, the situation is a bit
		 * tricky. we must discard all previous URBs. there are
		 * complications:
		 *  - discarding is asynchronous - discarded urbs will be reaped
		 *    later. the user must not have freed the transfer when the
		 *    discarded URBs are reaped, otherwise libusb will be using
		 *    freed memory.
		 *  - the earlier URBs may have completed successfully and we do
		 *    not want to throw away any data.
		 * so, in this case we discard all the previous URBs BUT we report
		 * that the transfer was submitted successfully. then later when
		 * the final discard completes we can report error to the user.
		 */
		tpriv->reap_action = SUBMIT_FAILED;

		/* The URBs we haven't submitted yet we count as already
		 * retired. */
		tpriv->num_retired = num_urbs - i;
		discard_urbs(itransfer, 0, i);

		usbi_dbg(TRANSFER_CTX(transfer), "reporting successful submission but waiting for %d "
			 "discards before reporting error", i);
		return 0;
	}

	return 0;
}

static int submit_control_transfer(struct usbi_transfer *itransfer)
{
	struct linux_transfer_priv *tpriv = usbi_get_transfer_priv(itransfer);
	struct libusb_transfer *transfer =
		USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
	struct linux_device_handle_priv *hpriv =
		usbi_get_device_handle_priv(transfer->dev_handle);
	struct usbfs_urb *urb;
	int r;

	if (transfer->length - LIBUSB_CONTROL_SETUP_SIZE > MAX_CTRL_BUFFER_LENGTH)
		return LIBUSB_ERROR_INVALID_PARAM;

	urb = calloc(1, sizeof(*urb));
	if (!urb)
		return LIBUSB_ERROR_NO_MEM;
	tpriv->urbs = urb;
	tpriv->num_urbs = 1;
	tpriv->reap_action = NORMAL;

	urb->usercontext = itransfer;
	urb->type = USBFS_URB_TYPE_CONTROL;
	urb->endpoint = transfer->endpoint;
	urb->buffer = transfer->buffer;
	urb->buffer_length = transfer->length;

	r = ioctl(hpriv->fd, IOCTL_USBFS_SUBMITURB, urb);
	if (r < 0) {
		free(urb);
		tpriv->urbs = NULL;
		if (errno == ENODEV)
			return LIBUSB_ERROR_NO_DEVICE;

		usbi_err(TRANSFER_CTX(transfer), "submiturb failed, errno=%d", errno);
		return LIBUSB_ERROR_IO;
	}
	return 0;
}

static int op_submit_transfer(struct usbi_transfer *itransfer)
{
	struct libusb_transfer *transfer =
		USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);

	switch (transfer->type) {
	case LIBUSB_TRANSFER_TYPE_CONTROL:
		return submit_control_transfer(itransfer);
	case LIBUSB_TRANSFER_TYPE_BULK:
	case LIBUSB_TRANSFER_TYPE_BULK_STREAM:
		return submit_bulk_transfer(itransfer);
	case LIBUSB_TRANSFER_TYPE_INTERRUPT:
		return submit_bulk_transfer(itransfer);
	case LIBUSB_TRANSFER_TYPE_ISOCHRONOUS:
		return submit_iso_transfer(itransfer);
	default:
		usbi_err(TRANSFER_CTX(transfer), "unknown transfer type %u", transfer->type);
		return LIBUSB_ERROR_INVALID_PARAM;
	}
}

static int op_cancel_transfer(struct usbi_transfer *itransfer)
{
	struct linux_transfer_priv *tpriv = usbi_get_transfer_priv(itransfer);
	struct libusb_transfer *transfer =
		USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
	int r;

	if (!tpriv->urbs)
		return LIBUSB_ERROR_NOT_FOUND;

	r = discard_urbs(itransfer, 0, tpriv->num_urbs);
	if (r != 0)
		return r;

	switch (transfer->type) {
	case LIBUSB_TRANSFER_TYPE_BULK:
	case LIBUSB_TRANSFER_TYPE_BULK_STREAM:
		if (tpriv->reap_action == ERROR)
			break;
		/* else, fall through */
	default:
		tpriv->reap_action = CANCELLED;
	}

	return 0;
}

static void op_clear_transfer_priv(struct usbi_transfer *itransfer)
{
	struct libusb_transfer *transfer =
		USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
	struct linux_transfer_priv *tpriv = usbi_get_transfer_priv(itransfer);

	switch (transfer->type) {
	case LIBUSB_TRANSFER_TYPE_CONTROL:
	case LIBUSB_TRANSFER_TYPE_BULK:
	case LIBUSB_TRANSFER_TYPE_BULK_STREAM:
	case LIBUSB_TRANSFER_TYPE_INTERRUPT:
		if (tpriv->urbs) {
			free(tpriv->urbs);
			tpriv->urbs = NULL;
		}
		break;
	case LIBUSB_TRANSFER_TYPE_ISOCHRONOUS:
		if (tpriv->iso_urbs) {
			free_iso_urbs(tpriv);
			tpriv->iso_urbs = NULL;
		}
		break;
	default:
		usbi_err(TRANSFER_CTX(transfer), "unknown transfer type %u", transfer->type);
	}
}

static int handle_bulk_completion(struct usbi_transfer *itransfer,
	struct usbfs_urb *urb)
{
	struct linux_transfer_priv *tpriv = usbi_get_transfer_priv(itransfer);
	struct libusb_transfer *transfer = USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
	int urb_idx = urb - tpriv->urbs;

	usbi_mutex_lock(&itransfer->lock);
	usbi_dbg(TRANSFER_CTX(transfer), "handling completion status %d of bulk urb %d/%d", urb->status,
		 urb_idx + 1, tpriv->num_urbs);

	tpriv->num_retired++;

	if (tpriv->reap_action != NORMAL) {
		/* cancelled, submit_fail, or completed early */
		usbi_dbg(TRANSFER_CTX(transfer), "abnormal reap: urb status %d", urb->status);

		/* even though we're in the process of cancelling, it's possible that
		 * we may receive some data in these URBs that we don't want to lose.
		 * examples:
		 * 1. while the kernel is cancelling all the packets that make up an
		 *    URB, a few of them might complete. so we get back a successful
		 *    cancellation *and* some data.
		 * 2. we receive a short URB which marks the early completion condition,
		 *    so we start cancelling the remaining URBs. however, we're too
		 *    slow and another URB completes (or at least completes partially).
		 *    (this can't happen since we always use BULK_CONTINUATION.)
		 *
		 * When this happens, our objectives are not to lose any "surplus" data,
		 * and also to stick it at the end of the previously-received data
		 * (closing any holes), so that libusb reports the total amount of
		 * transferred data and presents it in a contiguous chunk.
		 */
		if (urb->actual_length > 0) {
			unsigned char *target = transfer->buffer + itransfer->transferred;

			usbi_dbg(TRANSFER_CTX(transfer), "received %d bytes of surplus data", urb->actual_length);
			if (urb->buffer != target) {
				usbi_dbg(TRANSFER_CTX(transfer), "moving surplus data from offset %zu to offset %zu",
					 (unsigned char *)urb->buffer - transfer->buffer,
					 target - transfer->buffer);
				memmove(target, urb->buffer, urb->actual_length);
			}
			itransfer->transferred += urb->actual_length;
		}

		if (tpriv->num_retired == tpriv->num_urbs) {
			usbi_dbg(TRANSFER_CTX(transfer), "abnormal reap: last URB handled, reporting");
			if (tpriv->reap_action != COMPLETED_EARLY &&
			    tpriv->reap_status == LIBUSB_TRANSFER_COMPLETED)
				tpriv->reap_status = LIBUSB_TRANSFER_ERROR;
			goto completed;
		}
		goto out_unlock;
	}

	itransfer->transferred += urb->actual_length;

	/* Many of these errors can occur on *any* urb of a multi-urb
	 * transfer.  When they do, we tear down the rest of the transfer.
	 */
	switch (urb->status) {
	case 0:
		break;
	case -EREMOTEIO: /* short transfer */
		break;
	case -ENOENT: /* cancelled */
	case -ECONNRESET:
		break;
	case -ENODEV:
	case -ESHUTDOWN:
		usbi_dbg(TRANSFER_CTX(transfer), "device removed");
		tpriv->reap_status = LIBUSB_TRANSFER_NO_DEVICE;
		goto cancel_remaining;
	case -EPIPE:
		usbi_dbg(TRANSFER_CTX(transfer), "detected endpoint stall");
		if (tpriv->reap_status == LIBUSB_TRANSFER_COMPLETED)
			tpriv->reap_status = LIBUSB_TRANSFER_STALL;
		goto cancel_remaining;
	case -EOVERFLOW:
		/* overflow can only ever occur in the last urb */
		usbi_dbg(TRANSFER_CTX(transfer), "overflow, actual_length=%d", urb->actual_length);
		if (tpriv->reap_status == LIBUSB_TRANSFER_COMPLETED)
			tpriv->reap_status = LIBUSB_TRANSFER_OVERFLOW;
		goto completed;
	case -ETIME:
	case -EPROTO:
	case -EILSEQ:
	case -ECOMM:
	case -ENOSR:
		usbi_dbg(TRANSFER_CTX(transfer), "low-level bus error %d", urb->status);
		tpriv->reap_action = ERROR;
		goto cancel_remaining;
	default:
		usbi_warn(ITRANSFER_CTX(itransfer), "unrecognised urb status %d", urb->status);
		tpriv->reap_action = ERROR;
		goto cancel_remaining;
	}

	/* if we've reaped all urbs or we got less data than requested then we're
	 * done */
	if (tpriv->num_retired == tpriv->num_urbs) {
		usbi_dbg(TRANSFER_CTX(transfer), "all URBs in transfer reaped --> complete!");
		goto completed;
	} else if (urb->actual_length < urb->buffer_length) {
		usbi_dbg(TRANSFER_CTX(transfer), "short transfer %d/%d --> complete!",
			 urb->actual_length, urb->buffer_length);
		if (tpriv->reap_action == NORMAL)
			tpriv->reap_action = COMPLETED_EARLY;
	} else {
		goto out_unlock;
	}

cancel_remaining:
	if (tpriv->reap_action == ERROR && tpriv->reap_status == LIBUSB_TRANSFER_COMPLETED)
		tpriv->reap_status = LIBUSB_TRANSFER_ERROR;

	if (tpriv->num_retired == tpriv->num_urbs) /* nothing to cancel */
		goto completed;

	/* cancel remaining urbs and wait for their completion before
	 * reporting results */
	discard_urbs(itransfer, urb_idx + 1, tpriv->num_urbs);

out_unlock:
	usbi_mutex_unlock(&itransfer->lock);
	return 0;

completed:
	free(tpriv->urbs);
	tpriv->urbs = NULL;
	usbi_mutex_unlock(&itransfer->lock);
	return tpriv->reap_action == CANCELLED ?
		usbi_handle_transfer_cancellation(itransfer) :
		usbi_handle_transfer_completion(itransfer, tpriv->reap_status);
}

static int handle_iso_completion(struct usbi_transfer *itransfer,
	struct usbfs_urb *urb)
{
	struct libusb_transfer *transfer =
		USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
	struct linux_transfer_priv *tpriv = usbi_get_transfer_priv(itransfer);
	int num_urbs = tpriv->num_urbs;
	int urb_idx = 0;
	int i;
	enum libusb_transfer_status status = LIBUSB_TRANSFER_COMPLETED;

	usbi_mutex_lock(&itransfer->lock);
	for (i = 0; i < num_urbs; i++) {
		if (urb == tpriv->iso_urbs[i]) {
			urb_idx = i + 1;
			break;
		}
	}
	if (urb_idx == 0) {
		usbi_err(TRANSFER_CTX(transfer), "could not locate urb!");
		usbi_mutex_unlock(&itransfer->lock);
		return LIBUSB_ERROR_NOT_FOUND;
	}

	usbi_dbg(TRANSFER_CTX(transfer), "handling completion status %d of iso urb %d/%d", urb->status,
		 urb_idx, num_urbs);

	/* copy isochronous results back in */

	for (i = 0; i < urb->number_of_packets; i++) {
		struct usbfs_iso_packet_desc *urb_desc = &urb->iso_frame_desc[i];
		struct libusb_iso_packet_descriptor *lib_desc =
			&transfer->iso_packet_desc[tpriv->iso_packet_offset++];

		lib_desc->status = LIBUSB_TRANSFER_COMPLETED;
		switch (urb_desc->status) {
		case 0:
			break;
		case -ENOENT: /* cancelled */
		case -ECONNRESET:
			break;
		case -ENODEV:
		case -ESHUTDOWN:
			usbi_dbg(TRANSFER_CTX(transfer), "packet %d - device removed", i);
			lib_desc->status = LIBUSB_TRANSFER_NO_DEVICE;
			break;
		case -EPIPE:
			usbi_dbg(TRANSFER_CTX(transfer), "packet %d - detected endpoint stall", i);
			lib_desc->status = LIBUSB_TRANSFER_STALL;
			break;
		case -EOVERFLOW:
			usbi_dbg(TRANSFER_CTX(transfer), "packet %d - overflow error", i);
			lib_desc->status = LIBUSB_TRANSFER_OVERFLOW;
			break;
		case -ETIME:
		case -EPROTO:
		case -EILSEQ:
		case -ECOMM:
		case -ENOSR:
		case -EXDEV:
			usbi_dbg(TRANSFER_CTX(transfer), "packet %d - low-level USB error %d", i, urb_desc->status);
			lib_desc->status = LIBUSB_TRANSFER_ERROR;
			break;
		default:
			usbi_warn(TRANSFER_CTX(transfer), "packet %d - unrecognised urb status %d",
				  i, urb_desc->status);
			lib_desc->status = LIBUSB_TRANSFER_ERROR;
			break;
		}
		lib_desc->actual_length = urb_desc->actual_length;
	}

	tpriv->num_retired++;

	if (tpriv->reap_action != NORMAL) { /* cancelled or submit_fail */
		usbi_dbg(TRANSFER_CTX(transfer), "CANCEL: urb status %d", urb->status);

		if (tpriv->num_retired == num_urbs) {
			usbi_dbg(TRANSFER_CTX(transfer), "CANCEL: last URB handled, reporting");
			free_iso_urbs(tpriv);
			if (tpriv->reap_action == CANCELLED) {
				usbi_mutex_unlock(&itransfer->lock);
				return usbi_handle_transfer_cancellation(itransfer);
			} else {
				usbi_mutex_unlock(&itransfer->lock);
				return usbi_handle_transfer_completion(itransfer, LIBUSB_TRANSFER_ERROR);
			}
		}
		goto out;
	}

	switch (urb->status) {
	case 0:
		break;
	case -ENOENT: /* cancelled */
	case -ECONNRESET:
		break;
	case -ESHUTDOWN:
		usbi_dbg(TRANSFER_CTX(transfer), "device removed");
		status = LIBUSB_TRANSFER_NO_DEVICE;
		break;
	default:
		usbi_warn(TRANSFER_CTX(transfer), "unrecognised urb status %d", urb->status);
		status = LIBUSB_TRANSFER_ERROR;
		break;
	}

	/* if we've reaped all urbs then we're done */
	if (tpriv->num_retired == num_urbs) {
		usbi_dbg(TRANSFER_CTX(transfer), "all URBs in transfer reaped --> complete!");
		free_iso_urbs(tpriv);
		usbi_mutex_unlock(&itransfer->lock);
		return usbi_handle_transfer_completion(itransfer, status);
	}

out:
	usbi_mutex_unlock(&itransfer->lock);
	return 0;
}

static int handle_control_completion(struct usbi_transfer *itransfer,
	struct usbfs_urb *urb)
{
	struct linux_transfer_priv *tpriv = usbi_get_transfer_priv(itransfer);
	int status;

	usbi_mutex_lock(&itransfer->lock);
	usbi_dbg(ITRANSFER_CTX(itransfer), "handling completion status %d", urb->status);

	itransfer->transferred += urb->actual_length;

	if (tpriv->reap_action == CANCELLED) {
		if (urb->status && urb->status != -ENOENT)
			usbi_warn(ITRANSFER_CTX(itransfer), "cancel: unrecognised urb status %d",
				  urb->status);
		free(tpriv->urbs);
		tpriv->urbs = NULL;
		usbi_mutex_unlock(&itransfer->lock);
		return usbi_handle_transfer_cancellation(itransfer);
	}

	switch (urb->status) {
	case 0:
		status = LIBUSB_TRANSFER_COMPLETED;
		break;
	case -ENOENT: /* cancelled */
		status = LIBUSB_TRANSFER_CANCELLED;
		break;
	case -ENODEV:
	case -ESHUTDOWN:
		usbi_dbg(ITRANSFER_CTX(itransfer), "device removed");
		status = LIBUSB_TRANSFER_NO_DEVICE;
		break;
	case -EPIPE:
		usbi_dbg(ITRANSFER_CTX(itransfer), "unsupported control request");
		status = LIBUSB_TRANSFER_STALL;
		break;
	case -EOVERFLOW:
		usbi_dbg(ITRANSFER_CTX(itransfer), "overflow, actual_length=%d", urb->actual_length);
		status = LIBUSB_TRANSFER_OVERFLOW;
		break;
	case -ETIME:
	case -EPROTO:
	case -EILSEQ:
	case -ECOMM:
	case -ENOSR:
		usbi_dbg(ITRANSFER_CTX(itransfer), "low-level bus error %d", urb->status);
		status = LIBUSB_TRANSFER_ERROR;
		break;
	default:
		usbi_warn(ITRANSFER_CTX(itransfer), "unrecognised urb status %d", urb->status);
		status = LIBUSB_TRANSFER_ERROR;
		break;
	}

	free(tpriv->urbs);
	tpriv->urbs = NULL;
	usbi_mutex_unlock(&itransfer->lock);
	return usbi_handle_transfer_completion(itransfer, status);
}

static int reap_for_handle(struct libusb_device_handle *handle)
{
	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
	int r;
	struct usbfs_urb *urb = NULL;
	struct usbi_transfer *itransfer;
	struct libusb_transfer *transfer;

	r = ioctl(hpriv->fd, IOCTL_USBFS_REAPURBNDELAY, &urb);
	if (r < 0) {
		if (errno == EAGAIN)
			return 1;
		if (errno == ENODEV)
			return LIBUSB_ERROR_NO_DEVICE;

		usbi_err(HANDLE_CTX(handle), "reap failed, errno=%d", errno);
		return LIBUSB_ERROR_IO;
	}

	itransfer = urb->usercontext;
	transfer = USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);

	usbi_dbg(HANDLE_CTX(handle), "urb type=%u status=%d transferred=%d", urb->type, urb->status, urb->actual_length);

	switch (transfer->type) {
	case LIBUSB_TRANSFER_TYPE_ISOCHRONOUS:
		return handle_iso_completion(itransfer, urb);
	case LIBUSB_TRANSFER_TYPE_BULK:
	case LIBUSB_TRANSFER_TYPE_BULK_STREAM:
	case LIBUSB_TRANSFER_TYPE_INTERRUPT:
		return handle_bulk_completion(itransfer, urb);
	case LIBUSB_TRANSFER_TYPE_CONTROL:
		return handle_control_completion(itransfer, urb);
	default:
		usbi_err(HANDLE_CTX(handle), "unrecognised transfer type %u", transfer->type);
		return LIBUSB_ERROR_OTHER;
	}
}

static int op_handle_events(struct libusb_context *ctx,
	void *event_data, unsigned int count, unsigned int num_ready)
{
	struct pollfd *fds = event_data;
	unsigned int n;
	int r;

	usbi_mutex_lock(&ctx->open_devs_lock);
	for (n = 0; n < count && num_ready > 0; n++) {
		struct pollfd *pollfd = &fds[n];
		struct libusb_device_handle *handle;
		struct linux_device_handle_priv *hpriv = NULL;
		int reap_count;

		if (!pollfd->revents)
			continue;

		num_ready--;
		for_each_open_device(ctx, handle) {
			hpriv = usbi_get_device_handle_priv(handle);
			if (hpriv->fd == pollfd->fd)
				break;
		}

		if (!hpriv || hpriv->fd != pollfd->fd) {
			usbi_err(ctx, "cannot find handle for fd %d",
				 pollfd->fd);
			continue;
		}

		if (pollfd->revents & POLLERR) {
			/* remove the fd from the pollfd set so that it doesn't continuously
			 * trigger an event, and flag that it has been removed so op_close()
			 * doesn't try to remove it a second time */
			usbi_remove_event_source(HANDLE_CTX(handle), hpriv->fd);
			hpriv->fd_removed = 1;

			/* device will still be marked as attached if hotplug monitor thread
			 * hasn't processed remove event yet */
			usbi_mutex_static_lock(&linux_hotplug_lock);
			if (usbi_atomic_load(&handle->dev->attached))
				linux_device_disconnected(handle->dev->bus_number,
							  handle->dev->device_address);
			usbi_mutex_static_unlock(&linux_hotplug_lock);

			if (hpriv->caps & USBFS_CAP_REAP_AFTER_DISCONNECT) {
				do {
					r = reap_for_handle(handle);
				} while (r == 0);
			}

			usbi_handle_disconnect(handle);
			continue;
		}

		reap_count = 0;
		do {
			r = reap_for_handle(handle);
		} while (r == 0 && ++reap_count <= 25);

		if (r == 1 || r == LIBUSB_ERROR_NO_DEVICE)
			continue;
		else if (r < 0)
			goto out;
	}

	r = 0;
out:
	usbi_mutex_unlock(&ctx->open_devs_lock);
	return r;
}

const struct usbi_os_backend usbi_backend = {
	.name = "Linux usbfs",
	.caps = USBI_CAP_HAS_HID_ACCESS|USBI_CAP_SUPPORTS_DETACH_KERNEL_DRIVER,
	.init = op_init,
	.exit = op_exit,
	.set_option = op_set_option,
	.hotplug_poll = op_hotplug_poll,
	.get_active_config_descriptor = op_get_active_config_descriptor,
	.get_config_descriptor = op_get_config_descriptor,
	.get_config_descriptor_by_value = op_get_config_descriptor_by_value,

	.wrap_sys_device = op_wrap_sys_device,
	.open = op_open,
	.close = op_close,
	.get_configuration = op_get_configuration,
	.set_configuration = op_set_configuration,
	.claim_interface = op_claim_interface,
	.release_interface = op_release_interface,

	.set_interface_altsetting = op_set_interface,
	.clear_halt = op_clear_halt,
	.reset_device = op_reset_device,

	.alloc_streams = op_alloc_streams,
	.free_streams = op_free_streams,

	.dev_mem_alloc = op_dev_mem_alloc,
	.dev_mem_free = op_dev_mem_free,

	.kernel_driver_active = op_kernel_driver_active,
	.detach_kernel_driver = op_detach_kernel_driver,
	.attach_kernel_driver = op_attach_kernel_driver,

	.destroy_device = op_destroy_device,

	.submit_transfer = op_submit_transfer,
	.cancel_transfer = op_cancel_transfer,
	.clear_transfer_priv = op_clear_transfer_priv,

	.handle_events = op_handle_events,

	.device_priv_size = sizeof(struct linux_device_priv),
	.device_handle_priv_size = sizeof(struct linux_device_handle_priv),
	.transfer_priv_size = sizeof(struct linux_transfer_priv),
};