Use timerfd for timeout handling

[platform/upstream/libusb.git] / libusb / libusbi.h

/*
 * Internal header for libusb
 * Copyright (C) 2007-2009 Daniel Drake <dsd@gentoo.org>
 * Copyright (c) 2001 Johannes Erdfelt <johannes@erdfelt.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
 */

#ifndef __LIBUSBI_H__
#define __LIBUSBI_H__

#include <config.h>

#include <poll.h>
#include <pthread.h>
#include <stddef.h>
#include <time.h>

#include <libusb.h>

#define DEVICE_DESC_LENGTH              18

#define USB_MAXENDPOINTS        32
#define USB_MAXINTERFACES       32
#define USB_MAXCONFIG           8

struct list_head {
        struct list_head *prev, *next;
};

/* Get an entry from the list 
 *      ptr - the address of this list_head element in "type" 
 *      type - the data type that contains "member"
 *      member - the list_head element in "type" 
 */
#define list_entry(ptr, type, member) \
        ((type *)((char *)(ptr) - (unsigned long)(&((type *)0L)->member)))

/* Get each entry from a list
 *      pos - A structure pointer has a "member" element
 *      head - list head
 *      member - the list_head element in "pos"
 */
#define list_for_each_entry(pos, head, member)                          \
        for (pos = list_entry((head)->next, typeof(*pos), member);      \
             &pos->member != (head);                                    \
             pos = list_entry(pos->member.next, typeof(*pos), member))

#define list_for_each_entry_safe(pos, n, head, member)                  \
        for (pos = list_entry((head)->next, typeof(*pos), member),      \
                n = list_entry(pos->member.next, typeof(*pos), member); \
             &pos->member != (head);                                    \
             pos = n, n = list_entry(n->member.next, typeof(*n), member))

#define list_empty(entry) ((entry)->next == (entry))

static inline void list_init(struct list_head *entry)
{
        entry->prev = entry->next = entry;
}

static inline void list_add(struct list_head *entry, struct list_head *head)
{
        entry->next = head->next;
        entry->prev = head;

        head->next->prev = entry;
        head->next = entry;
}

static inline void list_add_tail(struct list_head *entry,
        struct list_head *head)
{
        entry->next = head;
        entry->prev = head->prev;

        head->prev->next = entry;
        head->prev = entry;
}

static inline void list_del(struct list_head *entry)
{
        entry->next->prev = entry->prev;
        entry->prev->next = entry->next;
}

#define container_of(ptr, type, member) ({                      \
        const typeof( ((type *)0)->member ) *__mptr = (ptr);    \
        (type *)( (char *)__mptr - offsetof(type,member) );})

#define MIN(a, b)       ((a) < (b) ? (a) : (b))
#define MAX(a, b)       ((a) > (b) ? (a) : (b))

#define TIMESPEC_IS_SET(ts) ((ts)->tv_sec != 0 || (ts)->tv_nsec != 0)

enum usbi_log_level {
        LOG_LEVEL_DEBUG,
        LOG_LEVEL_INFO,
        LOG_LEVEL_WARNING,
        LOG_LEVEL_ERROR,
};

void usbi_log(struct libusb_context *ctx, enum usbi_log_level,
        const char *function, const char *format, ...);

#ifdef ENABLE_LOGGING
#define _usbi_log(ctx, level, fmt...) usbi_log(ctx, level, __FUNCTION__, fmt)
#else
#define _usbi_log(ctx, level, fmt...)
#endif

#ifdef ENABLE_DEBUG_LOGGING
#define usbi_dbg(fmt...) _usbi_log(NULL, LOG_LEVEL_DEBUG, fmt)
#else
#define usbi_dbg(fmt...)
#endif

#define usbi_info(ctx, fmt...) _usbi_log(ctx, LOG_LEVEL_INFO, fmt)
#define usbi_warn(ctx, fmt...) _usbi_log(ctx, LOG_LEVEL_WARNING, fmt)
#define usbi_err(ctx, fmt...) _usbi_log(ctx, LOG_LEVEL_ERROR, fmt)

#define USBI_GET_CONTEXT(ctx) if (!(ctx)) (ctx) = usbi_default_context
#define DEVICE_CTX(dev) ((dev)->ctx)
#define HANDLE_CTX(handle) (DEVICE_CTX((handle)->dev))
#define TRANSFER_CTX(transfer) (HANDLE_CTX((transfer)->dev_handle))
#define ITRANSFER_CTX(transfer) \
        (TRANSFER_CTX(__USBI_TRANSFER_TO_LIBUSB_TRANSFER(transfer)))

extern struct libusb_context *usbi_default_context;

struct libusb_context {
        int debug;
        int debug_fixed;

        /* internal control pipe, used for interrupting event handling when
         * something needs to modify poll fds. */
        int ctrl_pipe[2];

        struct list_head usb_devs;
        pthread_mutex_t usb_devs_lock;

        /* A list of open handles. Backends are free to traverse this if required.
         */
        struct list_head open_devs;
        pthread_mutex_t open_devs_lock;

        /* this is a list of in-flight transfer handles, sorted by timeout 
         * expiration. URBs to timeout the soonest are placed at the beginning of
         * the list, URBs that will time out later are placed after, and urbs with
         * infinite timeout are always placed at the very end. */
        struct list_head flying_transfers;
        pthread_mutex_t flying_transfers_lock;

        /* list of poll fds */
        struct list_head pollfds;
        pthread_mutex_t pollfds_lock;

        /* a counter that is set when we want to interrupt event handling, in order
         * to modify the poll fd set. and a lock to protect it. */
        unsigned int pollfd_modify;
        pthread_mutex_t pollfd_modify_lock;

        /* user callbacks for pollfd changes */
        libusb_pollfd_added_cb fd_added_cb;
        libusb_pollfd_removed_cb fd_removed_cb;
        void *fd_cb_user_data;

        /* ensures that only one thread is handling events at any one time */
        pthread_mutex_t events_lock;

        /* used to see if there is an active thread doing event handling */
        int event_handler_active;

        /* used to wait for event completion in threads other than the one that is
         * event handling */
        pthread_mutex_t event_waiters_lock;
        pthread_cond_t event_waiters_cond;

#ifdef USBI_TIMERFD_AVAILABLE
        /* used for timeout handling, if supported by OS.
         * this timerfd is maintained to trigger on the next pending timeout */
        int timerfd;
#endif
};

#ifdef USBI_TIMERFD_AVAILABLE
#define usbi_using_timerfd(ctx) ((ctx)->timerfd >= 0)
#else
#define usbi_using_timerfd(ctx) (0)
#endif

struct libusb_device {
        /* lock protects refcnt, everything else is finalized at initialization
         * time */
        pthread_mutex_t lock;
        int refcnt;

        struct libusb_context *ctx;

        uint8_t bus_number;
        uint8_t device_address;
        uint8_t num_configurations;

        struct list_head list;
        unsigned long session_data;
        unsigned char os_priv[0];
};

struct libusb_device_handle {
        /* lock protects claimed_interfaces */
        pthread_mutex_t lock;
        unsigned long claimed_interfaces;

        struct list_head list;
        struct libusb_device *dev;
        unsigned char os_priv[0];
};

#define USBI_TRANSFER_TIMED_OUT                         (1<<0)

enum {
  USBI_CLOCK_MONOTONIC,
  USBI_CLOCK_REALTIME
};

/* in-memory transfer layout:
 *
 * 1. struct usbi_transfer
 * 2. struct libusb_transfer (which includes iso packets) [variable size]
 * 3. os private data [variable size]
 *
 * from a libusb_transfer, you can get the usbi_transfer by rewinding the
 * appropriate number of bytes.
 * the usbi_transfer includes the number of allocated packets, so you can
 * determine the size of the transfer and hence the start and length of the
 * OS-private data.
 */

struct usbi_transfer {
        int num_iso_packets;
        struct list_head list;
        struct timeval timeout;
        int transferred;
        uint8_t flags;

        /* this lock is held during libusb_submit_transfer() and
         * libusb_cancel_transfer() (allowing the OS backend to prevent duplicate
         * cancellation, submission-during-cancellation, etc). the OS backend
         * should also take this lock in the handle_events path, to prevent the user
         * cancelling the transfer from another thread while you are processing
         * its completion (presumably there would be races within your OS backend
         * if this were possible). */
        pthread_mutex_t lock;
};

#define __USBI_TRANSFER_TO_LIBUSB_TRANSFER(transfer) \
        ((struct libusb_transfer *)(((void *)(transfer)) \
                + sizeof(struct usbi_transfer)))
#define __LIBUSB_TRANSFER_TO_USBI_TRANSFER(transfer) \
        ((struct usbi_transfer *)(((void *)(transfer)) \
                - sizeof(struct usbi_transfer)))

static inline void *usbi_transfer_get_os_priv(struct usbi_transfer *transfer)
{
        return ((void *)transfer) + sizeof(struct usbi_transfer)
                + sizeof(struct libusb_transfer)
                + (transfer->num_iso_packets
                        * sizeof(struct libusb_iso_packet_descriptor));
}

/* bus structures */

/* All standard descriptors have these 2 fields in common */
struct usb_descriptor_header {
        uint8_t  bLength;
        uint8_t  bDescriptorType;
};

/* shared data and functions */

int usbi_io_init(struct libusb_context *ctx);
void usbi_io_exit(struct libusb_context *ctx);

struct libusb_device *usbi_alloc_device(struct libusb_context *ctx,
        unsigned long session_id);
struct libusb_device *usbi_get_device_by_session_id(struct libusb_context *ctx,
        unsigned long session_id);
int usbi_sanitize_device(struct libusb_device *dev);
void usbi_handle_disconnect(struct libusb_device_handle *handle);

int usbi_handle_transfer_completion(struct usbi_transfer *itransfer,
        enum libusb_transfer_status status);
int usbi_handle_transfer_cancellation(struct usbi_transfer *transfer);

int usbi_parse_descriptor(unsigned char *source, char *descriptor, void *dest,
        int host_endian);
int usbi_get_config_index_by_value(struct libusb_device *dev,
        uint8_t bConfigurationValue, int *idx);

/* polling */

struct usbi_pollfd {
        /* must come first */
        struct libusb_pollfd pollfd;

        struct list_head list;
};

int usbi_add_pollfd(struct libusb_context *ctx, int fd, short events);
void usbi_remove_pollfd(struct libusb_context *ctx, int fd);

/* device discovery */

/* we traverse usbfs without knowing how many devices we are going to find.
 * so we create this discovered_devs model which is similar to a linked-list
 * which grows when required. it can be freed once discovery has completed,
 * eliminating the need for a list node in the libusb_device structure
 * itself. */
struct discovered_devs {
        size_t len;
        size_t capacity;
        struct libusb_device *devices[0];
};

struct discovered_devs *discovered_devs_append(
        struct discovered_devs *discdevs, struct libusb_device *dev);

/* OS abstraction */

/* This is the interface that OS backends need to implement.
 * All fields are mandatory, except ones explicitly noted as optional. */
struct usbi_os_backend {
        /* A human-readable name for your backend, e.g. "Linux usbfs" */
        const char *name;

        /* Perform initialization of your backend. You might use this function
         * to determine specific capabilities of the system, allocate required
         * data structures for later, etc.
         *
         * This function is called when a libusb user initializes the library
         * prior to use.
         *
         * Return 0 on success, or a LIBUSB_ERROR code on failure.
         */
        int (*init)(struct libusb_context *ctx);

        /* Deinitialization. Optional. This function should destroy anything
         * that was set up by init.
         *
         * This function is called when the user deinitializes the library.
         */
        void (*exit)(void);

        /* Enumerate all the USB devices on the system, returning them in a list
         * of discovered devices.
         *
         * Your implementation should enumerate all devices on the system,
         * regardless of whether they have been seen before or not.
         *
         * When you have found a device, compute a session ID for it. The session
         * ID should uniquely represent that particular device for that particular
         * connection session since boot (i.e. if you disconnect and reconnect a
         * device immediately after, it should be assigned a different session ID).
         * If your OS cannot provide a unique session ID as described above,
         * presenting a session ID of (bus_number << 8 | device_address) should
         * be sufficient. Bus numbers and device addresses wrap and get reused,
         * but that is an unlikely case.
         *
         * After computing a session ID for a device, call
         * usbi_get_device_by_session_id(). This function checks if libusb already
         * knows about the device, and if so, it provides you with a libusb_device
         * structure for it.
         *
         * If usbi_get_device_by_session_id() returns NULL, it is time to allocate
         * a new device structure for the device. Call usbi_alloc_device() to
         * obtain a new libusb_device structure with reference count 1. Populate
         * the bus_number and device_address attributes of the new device, and
         * perform any other internal backend initialization you need to do. At
         * this point, you should be ready to provide device descriptors and so
         * on through the get_*_descriptor functions. Finally, call
         * usbi_sanitize_device() to perform some final sanity checks on the
         * device. Assuming all of the above succeeded, we can now continue.
         * If any of the above failed, remember to unreference the device that
         * was returned by usbi_alloc_device().
         *
         * At this stage we have a populated libusb_device structure (either one
         * that was found earlier, or one that we have just allocated and
         * populated). This can now be added to the discovered devices list
         * using discovered_devs_append(). Note that discovered_devs_append()
         * may reallocate the list, returning a new location for it, and also
         * note that reallocation can fail. Your backend should handle these
         * error conditions appropriately.
         *
         * This function should not generate any bus I/O and should not block.
         * If I/O is required (e.g. reading the active configuration value), it is
         * OK to ignore these suggestions :)
         *
         * This function is executed when the user wishes to retrieve a list
         * of USB devices connected to the system.
         *
         * Return 0 on success, or a LIBUSB_ERROR code on failure.
         */
        int (*get_device_list)(struct libusb_context *ctx,
                struct discovered_devs **discdevs);

        /* Open a device for I/O and other USB operations. The device handle
         * is preallocated for you, you can retrieve the device in question
         * through handle->dev.
         *
         * Your backend should allocate any internal resources required for I/O
         * and other operations so that those operations can happen (hopefully)
         * without hiccup. This is also a good place to inform libusb that it
         * should monitor certain file descriptors related to this device -
         * see the usbi_add_pollfd() function.
         *
         * This function should not generate any bus I/O and should not block.
         *
         * This function is called when the user attempts to obtain a device
         * handle for a device.
         *
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_ACCESS if the user has insufficient permissions
         * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since
         *   discovery
         * - another LIBUSB_ERROR code on other failure
         *
         * Do not worry about freeing the handle on failed open, the upper layers
         * do this for you.
         */
        int (*open)(struct libusb_device_handle *handle);

        /* Close a device such that the handle cannot be used again. Your backend
         * should destroy any resources that were allocated in the open path.
         * This may also be a good place to call usbi_remove_pollfd() to inform
         * libusb of any file descriptors associated with this device that should
         * no longer be monitored.
         *
         * This function is called when the user closes a device handle.
         */
        void (*close)(struct libusb_device_handle *handle);

        /* Retrieve the device descriptor from a device.
         *
         * The descriptor should be retrieved from memory, NOT via bus I/O to the
         * device. This means that you may have to cache it in a private structure
         * during get_device_list enumeration. Alternatively, you may be able
         * to retrieve it from a kernel interface (some Linux setups can do this)
         * still without generating bus I/O.
         *
         * This function is expected to write DEVICE_DESC_LENGTH (18) bytes into
         * buffer, which is guaranteed to be big enough.
         *
         * This function is called when sanity-checking a device before adding
         * it to the list of discovered devices, and also when the user requests
         * to read the device descriptor.
         *
         * This function is expected to return the descriptor in bus-endian format
         * (LE). If it returns the multi-byte values in host-endian format,
         * set the host_endian output parameter to "1".
         *
         * Return 0 on success or a LIBUSB_ERROR code on failure.
         */
        int (*get_device_descriptor)(struct libusb_device *device,
                unsigned char *buffer, int *host_endian);

        /* Get the ACTIVE configuration descriptor for a device.
         *
         * The descriptor should be retrieved from memory, NOT via bus I/O to the
         * device. This means that you may have to cache it in a private structure
         * during get_device_list enumeration. You may also have to keep track
         * of which configuration is active when the user changes it.
         *
         * This function is expected to write len bytes of data into buffer, which
         * is guaranteed to be big enough. If you can only do a partial write,
         * return an error code.
         *
         * This function is expected to return the descriptor in bus-endian format
         * (LE). If it returns the multi-byte values in host-endian format,
         * set the host_endian output parameter to "1".
         *
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_NOT_FOUND if the device is in unconfigured state
         * - another LIBUSB_ERROR code on other failure
         */
        int (*get_active_config_descriptor)(struct libusb_device *device,
                unsigned char *buffer, size_t len, int *host_endian);

        /* Get a specific configuration descriptor for a device.
         *
         * The descriptor should be retrieved from memory, NOT via bus I/O to the
         * device. This means that you may have to cache it in a private structure
         * during get_device_list enumeration.
         *
         * The requested descriptor is expressed as a zero-based index (i.e. 0
         * indicates that we are requesting the first descriptor). The index does
         * not (necessarily) equal the bConfigurationValue of the configuration
         * being requested.
         *
         * This function is expected to write len bytes of data into buffer, which
         * is guaranteed to be big enough. If you can only do a partial write,
         * return an error code.
         *
         * This function is expected to return the descriptor in bus-endian format
         * (LE). If it returns the multi-byte values in host-endian format,
         * set the host_endian output parameter to "1".
         *
         * Return 0 on success or a LIBUSB_ERROR code on failure.
         */
        int (*get_config_descriptor)(struct libusb_device *device,
                uint8_t config_index, unsigned char *buffer, size_t len,
                int *host_endian);

        /* Get the bConfigurationValue for the active configuration for a device.
         * Optional. This should only be implemented if you can retrieve it from
         * cache (don't generate I/O).
         *
         * If you cannot retrieve this from cache, either do not implement this
         * function, or return LIBUSB_ERROR_NOT_SUPPORTED. This will cause
         * libusb to retrieve the information through a standard control transfer.
         *
         * This function must be non-blocking.
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
         *   was opened
         * - LIBUSB_ERROR_NOT_SUPPORTED if the value cannot be retrieved without
         *   blocking
         * - another LIBUSB_ERROR code on other failure.
         */
        int (*get_configuration)(struct libusb_device_handle *handle, int *config);

        /* Set the active configuration for a device.
         *
         * A configuration value of -1 should put the device in unconfigured state.
         *
         * This function can block.
         *
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_NOT_FOUND if the configuration does not exist
         * - LIBUSB_ERROR_BUSY if interfaces are currently claimed (and hence
         *   configuration cannot be changed)
         * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
         *   was opened
         * - another LIBUSB_ERROR code on other failure.
         */
        int (*set_configuration)(struct libusb_device_handle *handle, int config);

        /* Claim an interface. When claimed, the application can then perform
         * I/O to an interface's endpoints.
         *
         * This function should not generate any bus I/O and should not block.
         * Interface claiming is a logical operation that simply ensures that
         * no other drivers/applications are using the interface, and after
         * claiming, no other drivers/applicatiosn can use the interface because
         * we now "own" it.
         *
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_NOT_FOUND if the interface does not exist
         * - LIBUSB_ERROR_BUSY if the interface is in use by another driver/app
         * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
         *   was opened
         * - another LIBUSB_ERROR code on other failure
         */
        int (*claim_interface)(struct libusb_device_handle *handle, int iface);

        /* Release a previously claimed interface.
         *
         * This function should also generate a SET_INTERFACE control request,
         * resetting the alternate setting of that interface to 0. It's OK for
         * this function to block as a result.
         *
         * You will only ever be asked to release an interface which was
         * successfully claimed earlier.
         *
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
         *   was opened
         * - another LIBUSB_ERROR code on other failure
         */
        int (*release_interface)(struct libusb_device_handle *handle, int iface);

        /* Set the alternate setting for an interface.
         *
         * You will only ever be asked to set the alternate setting for an
         * interface which was successfully claimed earlier.
         *
         * It's OK for this function to block.
         *
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_NOT_FOUND if the alternate setting does not exist
         * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
         *   was opened
         * - another LIBUSB_ERROR code on other failure
         */
        int (*set_interface_altsetting)(struct libusb_device_handle *handle,
                int iface, int altsetting);

        /* Clear a halt/stall condition on an endpoint.
         *
         * It's OK for this function to block.
         *
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_NOT_FOUND if the endpoint does not exist
         * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
         *   was opened
         * - another LIBUSB_ERROR code on other failure
         */
        int (*clear_halt)(struct libusb_device_handle *handle,
                unsigned char endpoint);

        /* Perform a USB port reset to reinitialize a device.
         *
         * If possible, the handle should still be usable after the reset
         * completes, assuming that the device descriptors did not change during
         * reset and all previous interface state can be restored.
         *
         * If something changes, or you cannot easily locate/verify the resetted
         * device, return LIBUSB_ERROR_NOT_FOUND. This prompts the application
         * to close the old handle and re-enumerate the device.
         *
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_NOT_FOUND if re-enumeration is required, or if the device
         *   has been disconnected since it was opened
         * - another LIBUSB_ERROR code on other failure
         */
        int (*reset_device)(struct libusb_device_handle *handle);

        /* Determine if a kernel driver is active on an interface. Optional.
         *
         * The presence of a kernel driver on an interface indicates that any
         * calls to claim_interface would fail with the LIBUSB_ERROR_BUSY code.
         *
         * Return:
         * - 0 if no driver is active
         * - 1 if a driver is active
         * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
         *   was opened
         * - another LIBUSB_ERROR code on other failure
         */
        int (*kernel_driver_active)(struct libusb_device_handle *handle,
                int interface);
        
        /* Detach a kernel driver from an interface. Optional.
         *
         * After detaching a kernel driver, the interface should be available
         * for claim.
         *
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_NOT_FOUND if no kernel driver was active
         * - LIBUSB_ERROR_INVALID_PARAM if the interface does not exist
         * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
         *   was opened
         * - another LIBUSB_ERROR code on other failure
         */
        int (*detach_kernel_driver)(struct libusb_device_handle *handle,
                int interface);

        /* Attach a kernel driver to an interface. Optional.
         *
         * Reattach a kernel driver to the device.
         *
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_NOT_FOUND if no kernel driver was active
         * - LIBUSB_ERROR_INVALID_PARAM if the interface does not exist
         * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
         *   was opened
         * - LIBUSB_ERROR_BUSY if a program or driver has claimed the interface,
         *   preventing reattachment
         * - another LIBUSB_ERROR code on other failure
         */
        int (*attach_kernel_driver)(struct libusb_device_handle *handle,
                int interface);

        /* Destroy a device. Optional.
         *
         * This function is called when the last reference to a device is
         * destroyed. It should free any resources allocated in the get_device_list
         * path.
         */
        void (*destroy_device)(struct libusb_device *dev);

        /* Submit a transfer. Your implementation should take the transfer,
         * morph it into whatever form your platform requires, and submit it
         * asynchronously.
         *
         * This function must not block.
         *
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
         * - another LIBUSB_ERROR code on other failure
         */
        int (*submit_transfer)(struct usbi_transfer *itransfer);

        /* Cancel a previously submitted transfer.
         *
         * This function must not block. The transfer cancellation must complete
         * later, resulting in a call to usbi_handle_transfer_cancellation()
         * from the context of handle_events.
         */
        int (*cancel_transfer)(struct usbi_transfer *itransfer);

        /* Clear a transfer as if it has completed or cancelled, but do not
         * report any completion/cancellation to the library. You should free
         * all private data from the transfer as if you were just about to report
         * completion or cancellation.
         *
         * This function might seem a bit out of place. It is used when libusb
         * detects a disconnected device - it calls this function for all pending
         * transfers before reporting completion (with the disconnect code) to
         * the user. Maybe we can improve upon this internal interface in future.
         */
        void (*clear_transfer_priv)(struct usbi_transfer *itransfer);

        /* Handle any pending events. This involves monitoring any active
         * transfers and processing their completion or cancellation.
         *
         * The function is passed an array of pollfd structures (size nfds)
         * as a result of the poll() system call. The num_ready parameter
         * indicates the number of file descriptors that have reported events
         * (i.e. the poll() return value). This should be enough information
         * for you to determine which actions need to be taken on the currently
         * active transfers.
         *
         * For any cancelled transfers, call usbi_handle_transfer_cancellation().
         * For completed transfers, call usbi_handle_transfer_completion().
         * For control/bulk/interrupt transfers, populate the "transferred"
         * element of the appropriate usbi_transfer structure before calling the
         * above functions. For isochronous transfers, populate the status and
         * transferred fields of the iso packet descriptors of the transfer.
         *
         * This function should also be able to detect disconnection of the
         * device, reporting that situation with usbi_handle_disconnect().
         *
         * When processing an event related to a transfer, you probably want to
         * take usbi_transfer.lock to prevent races. See the documentation for
         * the usbi_transfer structure.
         *
         * Return 0 on success, or a LIBUSB_ERROR code on failure.
         */
        int (*handle_events)(struct libusb_context *ctx,
                struct pollfd *fds, nfds_t nfds, int num_ready);

        /* Get time from specified clock. At least two clocks must be implemented
           by the backend: USBI_CLOCK_REALTIME, and USBI_CLOCK_MONOTONIC.

           Description of clocks:
             USBI_CLOCK_REALTIME : clock returns time since system epoch.
             USBI_CLOCK_MONOTONIC: clock returns time since unspecified start
                                     time (usually boot).
         */
        int (*clock_gettime)(int clkid, struct timespec *tp);

#ifdef USBI_TIMERFD_AVAILABLE
        /* clock ID of the clock that should be used for timerfd */
        clockid_t (*get_timerfd_clockid)(void);
#endif

        /* Number of bytes to reserve for per-device private backend data.
         * This private data area is accessible through the "os_priv" field of
         * struct libusb_device. */
        size_t device_priv_size;

        /* Number of bytes to reserve for per-handle private backend data.
         * This private data area is accessible through the "os_priv" field of
         * struct libusb_device. */
        size_t device_handle_priv_size;

        /* Number of bytes to reserve for per-transfer private backend data.
         * This private data area is accessible by calling
         * usbi_transfer_get_os_priv() on the appropriate usbi_transfer instance.
         */
        size_t transfer_priv_size;

        /* Mumber of additional bytes for os_priv for each iso packet.
         * Can your backend use this? */
        /* FIXME: linux can't use this any more. if other OS's cannot either,
         * then remove this */
        size_t add_iso_packet_size;
};

extern const struct usbi_os_backend * const usbi_backend;

extern const struct usbi_os_backend linux_usbfs_backend;
extern const struct usbi_os_backend darwin_backend;

#endif