linux/Documentation/usb/callbacks.txt
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   1What callbacks will usbcore do?
   2===============================
   3
   4Usbcore will call into a driver through callbacks defined in the driver
   5structure and through the completion handler of URBs a driver submits.
   6Only the former are in the scope of this document. These two kinds of
   7callbacks are completely independent of each other. Information on the
   8completion callback can be found in Documentation/usb/URB.txt.
   9
  10The callbacks defined in the driver structure are:
  11
  121. Hotplugging callbacks:
  13
  14 * @probe: Called to see if the driver is willing to manage a particular
  15 *      interface on a device.
  16 * @disconnect: Called when the interface is no longer accessible, usually
  17 *      because its device has been (or is being) disconnected or the
  18 *      driver module is being unloaded.
  19
  202. Odd backdoor through usbfs:
  21
  22 * @ioctl: Used for drivers that want to talk to userspace through
  23 *      the "usbfs" filesystem.  This lets devices provide ways to
  24 *      expose information to user space regardless of where they
  25 *      do (or don't) show up otherwise in the filesystem.
  26
  273. Power management (PM) callbacks:
  28
  29 * @suspend: Called when the device is going to be suspended.
  30 * @resume: Called when the device is being resumed.
  31 * @reset_resume: Called when the suspended device has been reset instead
  32 *      of being resumed.
  33
  344. Device level operations:
  35
  36 * @pre_reset: Called when the device is about to be reset.
  37 * @post_reset: Called after the device has been reset
  38
  39The ioctl interface (2) should be used only if you have a very good
  40reason. Sysfs is preferred these days. The PM callbacks are covered
  41separately in Documentation/usb/power-management.txt.
  42
  43Calling conventions
  44===================
  45
  46All callbacks are mutually exclusive. There's no need for locking
  47against other USB callbacks. All callbacks are called from a task
  48context. You may sleep. However, it is important that all sleeps have a
  49small fixed upper limit in time. In particular you must not call out to
  50user space and await results.
  51
  52Hotplugging callbacks
  53=====================
  54
  55These callbacks are intended to associate and disassociate a driver with
  56an interface. A driver's bond to an interface is exclusive.
  57
  58The probe() callback
  59--------------------
  60
  61int (*probe) (struct usb_interface *intf,
  62                const struct usb_device_id *id);
  63
  64Accept or decline an interface. If you accept the device return 0,
  65otherwise -ENODEV or -ENXIO. Other error codes should be used only if a
  66genuine error occurred during initialisation which prevented a driver
  67from accepting a device that would else have been accepted.
  68You are strongly encouraged to use usbcore's facility,
  69usb_set_intfdata(), to associate a data structure with an interface, so
  70that you know which internal state and identity you associate with a
  71particular interface. The device will not be suspended and you may do IO
  72to the interface you are called for and endpoint 0 of the device. Device
  73initialisation that doesn't take too long is a good idea here.
  74
  75The disconnect() callback
  76-------------------------
  77
  78void (*disconnect) (struct usb_interface *intf);
  79
  80This callback is a signal to break any connection with an interface.
  81You are not allowed any IO to a device after returning from this
  82callback. You also may not do any other operation that may interfere
  83with another driver bound the interface, eg. a power management
  84operation.
  85If you are called due to a physical disconnection, all your URBs will be
  86killed by usbcore. Note that in this case disconnect will be called some
  87time after the physical disconnection. Thus your driver must be prepared
  88to deal with failing IO even prior to the callback.
  89
  90Device level callbacks
  91======================
  92
  93pre_reset
  94---------
  95
  96int (*pre_reset)(struct usb_interface *intf);
  97
  98A driver or user space is triggering a reset on the device which
  99contains the interface passed as an argument. Cease IO, wait for all
 100outstanding URBs to complete, and save any device state you need to
 101restore.  No more URBs may be submitted until the post_reset method
 102is called.
 103
 104If you need to allocate memory here, use GFP_NOIO or GFP_ATOMIC, if you
 105are in atomic context.
 106
 107post_reset
 108----------
 109
 110int (*post_reset)(struct usb_interface *intf);
 111
 112The reset has completed.  Restore any saved device state and begin
 113using the device again.
 114
 115If you need to allocate memory here, use GFP_NOIO or GFP_ATOMIC, if you
 116are in atomic context.
 117
 118Call sequences
 119==============
 120
 121No callbacks other than probe will be invoked for an interface
 122that isn't bound to your driver.
 123
 124Probe will never be called for an interface bound to a driver.
 125Hence following a successful probe, disconnect will be called
 126before there is another probe for the same interface.
 127
 128Once your driver is bound to an interface, disconnect can be
 129called at any time except in between pre_reset and post_reset.
 130pre_reset is always followed by post_reset, even if the reset
 131failed or the device has been unplugged.
 132
 133suspend is always followed by one of: resume, reset_resume, or
 134disconnect.
 135
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