linux驱动移植-usb驱动基础

/**
 * struct usb_host_endpoint - host-side endpoint descriptor and queue
 * @desc: descriptor for this endpoint, wMaxPacketSize in native byteorder
 * @ss_ep_comp: SuperSpeed companion descriptor for this endpoint
 * @ssp_isoc_ep_comp: SuperSpeedPlus isoc companion descriptor for this endpoint
 * @urb_list: urbs queued to this endpoint; maintained by usbcore
 * @hcpriv: for use by HCD; typically holds hardware dma queue head (QH)
 *      with one or more transfer descriptors (TDs) per urb
 * @ep_dev: ep_device for sysfs info
 * @extra: descriptors following this endpoint in the configuration
 * @extralen: how many bytes of "extra" are valid
 * @enabled: URBs may be submitted to this endpoint
 * @streams: number of USB-3 streams allocated on the endpoint
 *
 * USB requests are always queued to a given endpoint, identified by a
 * descriptor within an active interface in a given USB configuration.
 */
struct usb_host_endpoint {
        struct usb_endpoint_descriptor          desc;
        struct usb_ss_ep_comp_descriptor        ss_ep_comp;
        struct usb_ssp_isoc_ep_comp_descriptor  ssp_isoc_ep_comp;
        struct list_head                urb_list;
        void                            *hcpriv;
        struct ep_device                *ep_dev;        /* For sysfs info */

        unsigned char *extra;   /* Extra descriptors */
        int extralen;
        int enabled;
        int streams;
};

/* USB_DT_ENDPOINT: Endpoint descriptor */
struct usb_endpoint_descriptor {
        __u8  bLength;
        __u8  bDescriptorType;

        __u8  bEndpointAddress;
        __u8  bmAttributes;
        __le16 wMaxPacketSize;
        __u8  bInterval;

        /* NOTE:  these two are _only_ in audio endpoints. */
        /* use USB_DT_ENDPOINT*_SIZE in bLength, not sizeof. */
        __u8  bRefresh;
        __u8  bSynchAddress;
} __attribute__ ((packed));

/**
 * struct usb_interface - what usb device drivers talk to
 * @altsetting: array of interface structures, one for each alternate
 *      setting that may be selected.  Each one includes a set of
 *      endpoint configurations.  They will be in no particular order.
 * @cur_altsetting: the current altsetting.
 * @num_altsetting: number of altsettings defined.
 * @intf_assoc: interface association descriptor
 * @minor: the minor number assigned to this interface, if this
 *      interface is bound to a driver that uses the USB major number.
 *      If this interface does not use the USB major, this field should
 *      be unused.  The driver should set this value in the probe()
 *      function of the driver, after it has been assigned a minor
 *      number from the USB core by calling usb_register_dev().
 * @condition: binding state of the interface: not bound, binding
 *      (in probe()), bound to a driver, or unbinding (in disconnect())
 * @sysfs_files_created: sysfs attributes exist
 * @ep_devs_created: endpoint child pseudo-devices exist
 * @unregistering: flag set when the interface is being unregistered
 * @needs_remote_wakeup: flag set when the driver requires remote-wakeup
 *      capability during autosuspend.
 * @needs_altsetting0: flag set when a set-interface request for altsetting 0
 *      has been deferred.
 * @needs_binding: flag set when the driver should be re-probed or unbound
 *      following a reset or suspend operation it doesn't support.
 * @authorized: This allows to (de)authorize individual interfaces instead
 *      a whole device in contrast to the device authorization.
 * @dev: driver model's view of this device
 * @usb_dev: if an interface is bound to the USB major, this will point
 *      to the sysfs representation for that device.
 * @reset_ws: Used for scheduling resets from atomic context.
 * @resetting_device: USB core reset the device, so use alt setting 0 as
 *      current; needs bandwidth alloc after reset.
 *
 * USB device drivers attach to interfaces on a physical device.  Each
 * interface encapsulates a single high level function, such as feeding
 * an audio stream to a speaker or reporting a change in a volume control.
 * Many USB devices only have one interface.  The protocol used to talk to
 * an interface's endpoints can be defined in a usb "class" specification,
 * or by a product's vendor.  The (default) control endpoint is part of
 * every interface, but is never listed among the interface's descriptors.
 *
 * The driver that is bound to the interface can use standard driver model
 * calls such as dev_get_drvdata() on the dev member of this structure.
 *
 * Each interface may have alternate settings.  The initial configuration
 * of a device sets altsetting 0, but the device driver can change
 * that setting using usb_set_interface().  Alternate settings are often
 * used to control the use of periodic endpoints, such as by having
 * different endpoints use different amounts of reserved USB bandwidth.
 * All standards-conformant USB devices that use isochronous endpoints
 * will use them in non-default settings.
 *
 * The USB specification says that alternate setting numbers must run from
 * 0 to one less than the total number of alternate settings.  But some
 * devices manage to mess this up, and the structures aren't necessarily
 * stored in numerical order anyhow.  Use usb_altnum_to_altsetting() to
 * look up an alternate setting in the altsetting array based on its number.
 */
struct usb_interface {
        /* array of alternate settings for this interface,
         * stored in no particular order */
        struct usb_host_interface *altsetting;

        struct usb_host_interface *cur_altsetting;      /* the currently
                                         * active alternate setting */
        unsigned num_altsetting;        /* number of alternate settings */

        /* If there is an interface association descriptor then it will list
         * the associated interfaces */
        struct usb_interface_assoc_descriptor *intf_assoc;

        int minor;                      /* minor number this interface is
                                         * bound to */
        enum usb_interface_condition condition;         /* state of binding */
        unsigned sysfs_files_created:1; /* the sysfs attributes exist */
        unsigned ep_devs_created:1;     /* endpoint "devices" exist */
        unsigned unregistering:1;       /* unregistration is in progress */
        unsigned needs_remote_wakeup:1; /* driver requires remote wakeup */
        unsigned needs_altsetting0:1;   /* switch to altsetting 0 is pending */
        unsigned needs_binding:1;       /* needs delayed unbind/rebind */
        unsigned resetting_device:1;    /* true: bandwidth alloc after reset */
        unsigned authorized:1;          /* used for interface authorization */


        struct device dev;              /* interface specific device info */
        struct device *usb_dev;
        struct work_struct reset_ws;    /* for resets in atomic context */
};

/* host-side wrapper for one interface setting's parsed descriptors */
struct usb_host_interface {
        struct usb_interface_descriptor desc;

        int extralen;
        unsigned char *extra;   /* Extra descriptors */

        /* array of desc.bNumEndpoints endpoints associated with this
         * interface setting.  these will be in no particular order.
         */
        struct usb_host_endpoint *endpoint;

        char *string;           /* iInterface string, if present */
};

/* USB_DT_INTERFACE: Interface descriptor */
struct usb_interface_descriptor {
        __u8  bLength;
        __u8  bDescriptorType;

        __u8  bInterfaceNumber;
        __u8  bAlternateSetting;
        __u8  bNumEndpoints;
        __u8  bInterfaceClass;
        __u8  bInterfaceSubClass;
        __u8  bInterfaceProtocol;
        __u8  iInterface;
} __attribute__ ((packed));

/**
 * struct usb_host_config - representation of a device's configuration
 * @desc: the device's configuration descriptor.
 * @string: pointer to the cached version of the iConfiguration string, if
 *      present for this configuration.
 * @intf_assoc: list of any interface association descriptors in this config
 * @interface: array of pointers to usb_interface structures, one for each
 *      interface in the configuration.  The number of interfaces is stored
 *      in desc.bNumInterfaces.  These pointers are valid only while the
 *      the configuration is active.
 * @intf_cache: array of pointers to usb_interface_cache structures, one
 *      for each interface in the configuration.  These structures exist
 *      for the entire life of the device.
 * @extra: pointer to buffer containing all extra descriptors associated
 *      with this configuration (those preceding the first interface
 *      descriptor).
 * @extralen: length of the extra descriptors buffer.
 *
 * USB devices may have multiple configurations, but only one can be active
 * at any time.  Each encapsulates a different operational environment;
 * for example, a dual-speed device would have separate configurations for
 * full-speed and high-speed operation.  The number of configurations
 * available is stored in the device descriptor as bNumConfigurations.
 *
 * A configuration can contain multiple interfaces.  Each corresponds to
 * a different function of the USB device, and all are available whenever
 * the configuration is active.  The USB standard says that interfaces
 * are supposed to be numbered from 0 to desc.bNumInterfaces-1, but a lot
 * of devices get this wrong.  In addition, the interface array is not
 * guaranteed to be sorted in numerical order.  Use usb_ifnum_to_if() to
 * look up an interface entry based on its number.
 *
 * Device drivers should not attempt to activate configurations.  The choice
 * of which configuration to install is a policy decision based on such
 * considerations as available power, functionality provided, and the user's
 * desires (expressed through userspace tools).  However, drivers can call
 * usb_reset_configuration() to reinitialize the current configuration and
 * all its interfaces.
 */
struct usb_host_config {
        struct usb_config_descriptor    desc;

        char *string;           /* iConfiguration string, if present */

        /* List of any Interface Association Descriptors in this
         * configuration. */
        struct usb_interface_assoc_descriptor *intf_assoc[USB_MAXIADS];

        /* the interfaces associated with this configuration,
         * stored in no particular order */
        struct usb_interface *interface[USB_MAXINTERFACES];

        /* Interface information available even when this is not the
         * active configuration */
        struct usb_interface_cache *intf_cache[USB_MAXINTERFACES];

        unsigned char *extra;   /* Extra descriptors */
        int extralen;
};

/* USB_DT_CONFIG: Configuration descriptor information.
 *
 * USB_DT_OTHER_SPEED_CONFIG is the same descriptor, except that the
 * descriptor type is different.  Highspeed-capable devices can look
 * different depending on what speed they're currently running.  Only
 * devices with a USB_DT_DEVICE_QUALIFIER have any OTHER_SPEED_CONFIG
 * descriptors.
 */
struct usb_config_descriptor {
        __u8  bLength;
        __u8  bDescriptorType;

        __le16 wTotalLength;
        __u8  bNumInterfaces;
        __u8  bConfigurationValue;
        __u8  iConfiguration;
        __u8  bmAttributes;
        __u8  bMaxPower;
} __attribute__ ((packed));

/**
 * struct usb_device - kernel's representation of a USB device
 * @devnum: device number; address on a USB bus
 * @devpath: device ID string for use in messages (e.g., /port/...)
 * @route: tree topology hex string for use with xHCI
 * @state: device state: configured, not attached, etc.
 * @speed: device speed: high/full/low (or error)
 * @rx_lanes: number of rx lanes in use, USB 3.2 adds dual-lane support
 * @tx_lanes: number of tx lanes in use, USB 3.2 adds dual-lane support
 * @tt: Transaction Translator info; used with low/full speed dev, highspeed hub
 * @ttport: device port on that tt hub
 * @toggle: one bit for each endpoint, with ([0] = IN, [1] = OUT) endpoints
 * @parent: our hub, unless we're the root
 * @bus: bus we're part of
 * @ep0: endpoint 0 data (default control pipe)
 * @dev: generic device interface
 * @descriptor: USB device descriptor
 * @bos: USB device BOS descriptor set
 * @config: all of the device's configs
 * @actconfig: the active configuration
 * @ep_in: array of IN endpoints
 * @ep_out: array of OUT endpoints
 * @rawdescriptors: raw descriptors for each config
 * @bus_mA: Current available from the bus
 * @portnum: parent port number (origin 1)
 * @level: number of USB hub ancestors
 * @can_submit: URBs may be submitted
 * @persist_enabled:  USB_PERSIST enabled for this device
 * @have_langid: whether string_langid is valid
 * @authorized: policy has said we can use it;
 *      (user space) policy determines if we authorize this device to be
 *      used or not. By default, wired USB devices are authorized.
 *      WUSB devices are not, until we authorize them from user space.
 *      FIXME -- complete doc
 * @authenticated: Crypto authentication passed
 * @wusb: device is Wireless USB
 * @lpm_capable: device supports LPM
 * @usb2_hw_lpm_capable: device can perform USB2 hardware LPM
 * @usb2_hw_lpm_besl_capable: device can perform USB2 hardware BESL LPM
 * @usb2_hw_lpm_enabled: USB2 hardware LPM is enabled
 * @usb2_hw_lpm_allowed: Userspace allows USB 2.0 LPM to be enabled
 * @usb3_lpm_u1_enabled: USB3 hardware U1 LPM enabled
 * @usb3_lpm_u2_enabled: USB3 hardware U2 LPM enabled
 * @string_langid: language ID for strings
 * @product: iProduct string, if present (static)
 * @manufacturer: iManufacturer string, if present (static)
 * @serial: iSerialNumber string, if present (static)
 * @filelist: usbfs files that are open to this device
 * @maxchild: number of ports if hub
 * @quirks: quirks of the whole device
 * @urbnum: number of URBs submitted for the whole device
 * @active_duration: total time device is not suspended
 * @connect_time: time device was first connected
 * @do_remote_wakeup:  remote wakeup should be enabled
 * @reset_resume: needs reset instead of resume
 * @port_is_suspended: the upstream port is suspended (L2 or U3)
 * @wusb_dev: if this is a Wireless USB device, link to the WUSB
 *      specific data for the device.
 * @slot_id: Slot ID assigned by xHCI
 * @removable: Device can be physically removed from this port
 * @l1_params: best effor service latency for USB2 L1 LPM state, and L1 timeout.
 * @u1_params: exit latencies for USB3 U1 LPM state, and hub-initiated timeout.
 * @u2_params: exit latencies for USB3 U2 LPM state, and hub-initiated timeout.
 * @lpm_disable_count: Ref count used by usb_disable_lpm() and usb_enable_lpm()
 *      to keep track of the number of functions that require USB 3.0 Link Power
 *      Management to be disabled for this usb_device.  This count should only
 *      be manipulated by those functions, with the bandwidth_mutex is held.
 * @hub_delay: cached value consisting of:
 *              parent->hub_delay + wHubDelay + tTPTransmissionDelay (40ns)
 *
 *      Will be used as wValue for SetIsochDelay requests.
 *
 * Notes:
 * Usbcore drivers should not set usbdev->state directly.  Instead use
 * usb_set_device_state().
 */
struct usb_device {
        int             devnum;
        char            devpath[16];
        u32             route;
        enum usb_device_state   state;
        enum usb_device_speed   speed;
        unsigned int            rx_lanes;
        unsigned int            tx_lanes;

        struct usb_tt   *tt;
        int             ttport;

        unsigned int toggle[2];

        struct usb_device *parent;
        struct usb_bus *bus;
        struct usb_host_endpoint ep0;

        struct device dev;

        struct usb_device_descriptor descriptor;
        struct usb_host_bos *bos;
        struct usb_host_config *config;

        struct usb_host_config *actconfig;
        struct usb_host_endpoint *ep_in[16];
        struct usb_host_endpoint *ep_out[16];

        char **rawdescriptors;

        unsigned short bus_mA;
        u8 portnum;
        u8 level;

        unsigned can_submit:1;
        unsigned persist_enabled:1;
        unsigned have_langid:1;
        unsigned authorized:1;
        unsigned authenticated:1;
        unsigned wusb:1;
        unsigned lpm_capable:1;
        unsigned usb2_hw_lpm_capable:1;
        unsigned usb2_hw_lpm_besl_capable:1;
        unsigned usb2_hw_lpm_enabled:1;
        unsigned usb2_hw_lpm_allowed:1;
        unsigned usb3_lpm_u1_enabled:1;
        unsigned usb3_lpm_u2_enabled:1;
        int string_langid;

        /* static strings from the device */
        char *product;
        char *manufacturer;
        char *serial;

        struct list_head filelist;

        int maxchild;

        u32 quirks;
        atomic_t urbnum;

        unsigned long active_duration;

#ifdef CONFIG_PM
        unsigned long connect_time;

        unsigned do_remote_wakeup:1;
        unsigned reset_resume:1;
        unsigned port_is_suspended:1;
#endif
        struct wusb_dev *wusb_dev;
        int slot_id;
        enum usb_device_removable removable;
        struct usb2_lpm_parameters l1_params;
        struct usb3_lpm_parameters u1_params;
        struct usb3_lpm_parameters u2_params;
        unsigned lpm_disable_count;

        u16 hub_delay;
};

/* USB_DT_DEVICE: Device descriptor */
struct usb_device_descriptor {
        __u8  bLength;
        __u8  bDescriptorType;

        __le16 bcdUSB;
        __u8  bDeviceClass;
        __u8  bDeviceSubClass;
        __u8  bDeviceProtocol;
        __u8  bMaxPacketSize0;
        __le16 idVendor;
        __le16 idProduct;
        __le16 bcdDevice;
        __u8  iManufacturer;
        __u8  iProduct;
        __u8  iSerialNumber;
        __u8  bNumConfigurations;
} __attribute__ ((packed));

/**
 * struct urb - USB Request Block
 * @urb_list: For use by current owner of the URB.
 * @anchor_list: membership in the list of an anchor
 * @anchor: to anchor URBs to a common mooring
 * @ep: Points to the endpoint's data structure.  Will eventually
 *      replace @pipe.
 * @pipe: Holds endpoint number, direction, type, and more.
 *      Create these values with the eight macros available;
 *      usb_{snd,rcv}TYPEpipe(dev,endpoint), where the TYPE is "ctrl"
 *      (control), "bulk", "int" (interrupt), or "iso" (isochronous).
 *      For example usb_sndbulkpipe() or usb_rcvintpipe().  Endpoint
 *      numbers range from zero to fifteen.  Note that "in" endpoint two
 *      is a different endpoint (and pipe) from "out" endpoint two.
 *      The current configuration controls the existence, type, and
 *      maximum packet size of any given endpoint.
 * @stream_id: the endpoint's stream ID for bulk streams
 * @dev: Identifies the USB device to perform the request.
 * @status: This is read in non-iso completion functions to get the
 *      status of the particular request.  ISO requests only use it
 *      to tell whether the URB was unlinked; detailed status for
 *      each frame is in the fields of the iso_frame-desc.
 * @transfer_flags: A variety of flags may be used to affect how URB
 *      submission, unlinking, or operation are handled.  Different
 *      kinds of URB can use different flags.
 * @transfer_buffer:  This identifies the buffer to (or from) which the I/O
 *      request will be performed unless URB_NO_TRANSFER_DMA_MAP is set
 *      (however, do not leave garbage in transfer_buffer even then).
 *      This buffer must be suitable for DMA; allocate it with
 *      kmalloc() or equivalent.  For transfers to "in" endpoints, contents
 *      of this buffer will be modified.  This buffer is used for the data
 *      stage of control transfers.
 * @transfer_dma: When transfer_flags includes URB_NO_TRANSFER_DMA_MAP,
 *      the device driver is saying that it provided this DMA address,
 *      which the host controller driver should use in preference to the
 *      transfer_buffer.
 * @sg: scatter gather buffer list, the buffer size of each element in
 *      the list (except the last) must be divisible by the endpoint's
 *      max packet size if no_sg_constraint isn't set in 'struct usb_bus'
 * @num_mapped_sgs: (internal) number of mapped sg entries
 * @num_sgs: number of entries in the sg list
 * @transfer_buffer_length: How big is transfer_buffer.  The transfer may
 *      be broken up into chunks according to the current maximum packet
 *      size for the endpoint, which is a function of the configuration
 *      and is encoded in the pipe.  When the length is zero, neither
 *      transfer_buffer nor transfer_dma is used.
 * @actual_length: This is read in non-iso completion functions, and
 *      it tells how many bytes (out of transfer_buffer_length) were
 *      transferred.  It will normally be the same as requested, unless
 *      either an error was reported or a short read was performed.
 *      The URB_SHORT_NOT_OK transfer flag may be used to make such
 *      short reads be reported as errors.
 * @setup_packet: Only used for control transfers, this points to eight bytes
 *      of setup data.  Control transfers always start by sending this data
 *      to the device.  Then transfer_buffer is read or written, if needed.
 * @setup_dma: DMA pointer for the setup packet.  The caller must not use
 *      this field; setup_packet must point to a valid buffer.
 * @start_frame: Returns the initial frame for isochronous transfers.
 * @number_of_packets: Lists the number of ISO transfer buffers.
 * @interval: Specifies the polling interval for interrupt or isochronous
 *      transfers.  The units are frames (milliseconds) for full and low
 *      speed devices, and microframes (1/8 millisecond) for highspeed
 *      and SuperSpeed devices.
 * @error_count: Returns the number of ISO transfers that reported errors.
 * @context: For use in completion functions.  This normally points to
 *      request-specific driver context.
 * @complete: Completion handler. This URB is passed as the parameter to the
 *      completion function.  The completion function may then do what
 *      it likes with the URB, including resubmitting or freeing it.
 * @iso_frame_desc: Used to provide arrays of ISO transfer buffers and to
  collect the transfer status for each buffer.
 *
 * This structure identifies USB transfer requests.  URBs must be allocated by
 * calling usb_alloc_urb() and freed with a call to usb_free_urb().
 * Initialization may be done using various usb_fill_*_urb() functions.  URBs
 * are submitted using usb_submit_urb(), and pending requests may be canceled
 * using usb_unlink_urb() or usb_kill_urb().
 *
 * Data Transfer Buffers:
 *
 * Normally drivers provide I/O buffers allocated with kmalloc() or otherwise
 * taken from the general page pool.  That is provided by transfer_buffer
 * (control requests also use setup_packet), and host controller drivers
 * perform a dma mapping (and unmapping) for each buffer transferred.  Those
 * mapping operations can be expensive on some platforms (perhaps using a dma
 * bounce buffer or talking to an IOMMU),
 * although they're cheap on commodity x86 and ppc hardware.
 *
 * Alternatively, drivers may pass the URB_NO_TRANSFER_DMA_MAP transfer flag,
 * which tells the host controller driver that no such mapping is needed for
 * the transfer_buffer since
 * the device driver is DMA-aware.  For example, a device driver might
 * allocate a DMA buffer with usb_alloc_coherent() or call usb_buffer_map().
 * When this transfer flag is provided, host controller drivers will
 * attempt to use the dma address found in the transfer_dma
 * field rather than determining a dma address themselves.
 *
 * Note that transfer_buffer must still be set if the controller
 * does not support DMA (as indicated by bus.uses_dma) and when talking
 * to root hub. If you have to trasfer between highmem zone and the device
 * on such controller, create a bounce buffer or bail out with an error.
 * If transfer_buffer cannot be set (is in highmem) and the controller is DMA
 * capable, assign NULL to it, so that usbmon knows not to use the value.
 * The setup_packet must always be set, so it cannot be located in highmem.
 *
 * Initialization:
 *
 * All URBs submitted must initialize the dev, pipe, transfer_flags (may be
 * zero), and complete fields.  All URBs must also initialize
 * transfer_buffer and transfer_buffer_length.  They may provide the
 * URB_SHORT_NOT_OK transfer flag, indicating that short reads are
 * to be treated as errors; that flag is invalid for write requests.
 *
 * Bulk URBs may
 * use the URB_ZERO_PACKET transfer flag, indicating that bulk OUT transfers
 * should always terminate with a short packet, even if it means adding an
 * extra zero length packet.
 *
 * Control URBs must provide a valid pointer in the setup_packet field.
 * Unlike the transfer_buffer, the setup_packet may not be mapped for DMA
 * beforehand.
 *
 * Interrupt URBs must provide an interval, saying how often (in milliseconds
 * or, for highspeed devices, 125 microsecond units)
 * to poll for transfers.  After the URB has been submitted, the interval
 * field reflects how the transfer was actually scheduled.
 * The polling interval may be more frequent than requested.
 * For example, some controllers have a maximum interval of 32 milliseconds,
 * while others support intervals of up to 1024 milliseconds.
 * Isochronous URBs also have transfer intervals.  (Note that for isochronous
 * endpoints, as well as high speed interrupt endpoints, the encoding of
 * the transfer interval in the endpoint descriptor is logarithmic.
 * Device drivers must convert that value to linear units themselves.)
*
 * If an isochronous endpoint queue isn't already running, the host
 * controller will schedule a new URB to start as soon as bandwidth
 * utilization allows.  If the queue is running then a new URB will be
 * scheduled to start in the first transfer slot following the end of the
 * preceding URB, if that slot has not already expired.  If the slot has
 * expired (which can happen when IRQ delivery is delayed for a long time),
 * the scheduling behavior depends on the URB_ISO_ASAP flag.  If the flag
 * is clear then the URB will be scheduled to start in the expired slot,
 * implying that some of its packets will not be transferred; if the flag
 * is set then the URB will be scheduled in the first unexpired slot,
 * breaking the queue's synchronization.  Upon URB completion, the
 * start_frame field will be set to the (micro)frame number in which the
 * transfer was scheduled.  Ranges for frame counter values are HC-specific
 * and can go from as low as 256 to as high as 65536 frames.
 *
 * Isochronous URBs have a different data transfer model, in part because
 * the quality of service is only "best effort".  Callers provide specially
 * allocated URBs, with number_of_packets worth of iso_frame_desc structures
 * at the end.  Each such packet is an individual ISO transfer.  Isochronous
 * URBs are normally queued, submitted by drivers to arrange that
 * transfers are at least double buffered, and then explicitly resubmitted
 * in completion handlers, so
 * that data (such as audio or video) streams at as constant a rate as the
 * host controller scheduler can support.
 *
 * Completion Callbacks:
 *
 * The completion callback is made in_interrupt(), and one of the first
 * things that a completion handler should do is check the status field.
 * The status field is provided for all URBs.  It is used to report
 * unlinked URBs, and status for all non-ISO transfers.  It should not
 * be examined before the URB is returned to the completion handler.
 *
 * The context field is normally used to link URBs back to the relevant
 * driver or request state.
 *
 * When the completion callback is invoked for non-isochronous URBs, the
 * actual_length field tells how many bytes were transferred.  This field
 * is updated even when the URB terminated with an error or was unlinked.
 *
 * ISO transfer status is reported in the status and actual_length fields
 * of the iso_frame_desc array, and the number of errors is reported in
 * error_count.  Completion callbacks for ISO transfers will normally
 * (re)submit URBs to ensure a constant transfer rate.
 *
 * Note that even fields marked "public" should not be touched by the driver
 * when the urb is owned by the hcd, that is, since the call to
 * usb_submit_urb() till the entry into the completion routine.
 */
struct urb {
        /* private: usb core and host controller only fields in the urb */
        struct kref kref;               /* reference count of the URB */
        int unlinked;                   /* unlink error code */
        void *hcpriv;                   /* private data for host controller */
        atomic_t use_count;             /* concurrent submissions counter */
        atomic_t reject;                /* submissions will fail */

        /* public: documented fields in the urb that can be used by drivers */
        struct list_head urb_list;      /* list head for use by the urb's
                                         * current owner */
        struct list_head anchor_list;   /* the URB may be anchored */
        struct usb_anchor *anchor;
        struct usb_device *dev;         /* (in) pointer to associated device */
        struct usb_host_endpoint *ep;   /* (internal) pointer to endpoint */
        unsigned int pipe;              /* (in) pipe information */
        unsigned int stream_id;         /* (in) stream ID */
        int status;                     /* (return) non-ISO status */
        unsigned int transfer_flags;    /* (in) URB_SHORT_NOT_OK | ...*/
        void *transfer_buffer;          /* (in) associated data buffer */
        dma_addr_t transfer_dma;        /* (in) dma addr for transfer_buffer */
        struct scatterlist *sg;         /* (in) scatter gather buffer list */
        int num_mapped_sgs;             /* (internal) mapped sg entries */
        int num_sgs;                    /* (in) number of entries in the sg list */
        u32 transfer_buffer_length;     /* (in) data buffer length */
        u32 actual_length;              /* (return) actual transfer length */
        unsigned char *setup_packet;    /* (in) setup packet (control only) */
        dma_addr_t setup_dma;           /* (in) dma addr for setup_packet */
        int start_frame;                /* (modify) start frame (ISO) */
        int number_of_packets;          /* (in) number of ISO packets */
        int interval;                   /* (modify) transfer interval
                                         * (INT/ISO) */
        int error_count;                /* (return) number of ISO errors */
        void *context;                  /* (in) context for completion */
        usb_complete_t complete;        /* (in) completion routine */
        struct usb_iso_packet_descriptor iso_frame_desc[0];
                                        /* (in) ISO ONLY */
};

usb 1-1: new low-speed USB device number 2 using s3c2410-ohci