1USB core callbacks
2~~~~~~~~~~~~~~~~~~
3
4What callbacks will usbcore do?
5===============================
6
7Usbcore will call into a driver through callbacks defined in the driver
8structure and through the completion handler of URBs a driver submits.
9Only the former are in the scope of this document. These two kinds of
10callbacks are completely independent of each other. Information on the
11completion callback can be found in :ref:`usb-urb`.
12
13The callbacks defined in the driver structure are:
14
151. Hotplugging callbacks:
16
17 - @probe:
18	Called to see if the driver is willing to manage a particular
19	interface on a device.
20
21 - @disconnect:
22	Called when the interface is no longer accessible, usually
23	because its device has been (or is being) disconnected or the
24	driver module is being unloaded.
25
262. Odd backdoor through usbfs:
27
28 - @ioctl:
29	Used for drivers that want to talk to userspace through
30	the "usbfs" filesystem.  This lets devices provide ways to
31	expose information to user space regardless of where they
32	do (or don't) show up otherwise in the filesystem.
33
343. Power management (PM) callbacks:
35
36 - @suspend:
37	Called when the device is going to be suspended.
38
39 - @resume:
40	Called when the device is being resumed.
41
42 - @reset_resume:
43	Called when the suspended device has been reset instead
44	of being resumed.
45
464. Device level operations:
47
48 - @pre_reset:
49	Called when the device is about to be reset.
50
51 - @post_reset:
52	Called after the device has been reset
53
54The ioctl interface (2) should be used only if you have a very good
55reason. Sysfs is preferred these days. The PM callbacks are covered
56separately in :ref:`usb-power-management`.
57
58Calling conventions
59===================
60
61All callbacks are mutually exclusive. There's no need for locking
62against other USB callbacks. All callbacks are called from a task
63context. You may sleep. However, it is important that all sleeps have a
64small fixed upper limit in time. In particular you must not call out to
65user space and await results.
66
67Hotplugging callbacks
68=====================
69
70These callbacks are intended to associate and disassociate a driver with
71an interface. A driver's bond to an interface is exclusive.
72
73The probe() callback
74--------------------
75
76::
77
78  int (*probe) (struct usb_interface *intf,
79		const struct usb_device_id *id);
80
81Accept or decline an interface. If you accept the device return 0,
82otherwise -ENODEV or -ENXIO. Other error codes should be used only if a
83genuine error occurred during initialisation which prevented a driver
84from accepting a device that would else have been accepted.
85You are strongly encouraged to use usbcore's facility,
86usb_set_intfdata(), to associate a data structure with an interface, so
87that you know which internal state and identity you associate with a
88particular interface. The device will not be suspended and you may do IO
89to the interface you are called for and endpoint 0 of the device. Device
90initialisation that doesn't take too long is a good idea here.
91
92The disconnect() callback
93-------------------------
94
95::
96
97  void (*disconnect) (struct usb_interface *intf);
98
99This callback is a signal to break any connection with an interface.
100You are not allowed any IO to a device after returning from this
101callback. You also may not do any other operation that may interfere
102with another driver bound the interface, eg. a power management
103operation.
104If you are called due to a physical disconnection, all your URBs will be
105killed by usbcore. Note that in this case disconnect will be called some
106time after the physical disconnection. Thus your driver must be prepared
107to deal with failing IO even prior to the callback.
108
109Device level callbacks
110======================
111
112pre_reset
113---------
114
115::
116
117  int (*pre_reset)(struct usb_interface *intf);
118
119A driver or user space is triggering a reset on the device which
120contains the interface passed as an argument. Cease IO, wait for all
121outstanding URBs to complete, and save any device state you need to
122restore.  No more URBs may be submitted until the post_reset method
123is called.
124
125If you need to allocate memory here, use GFP_NOIO or GFP_ATOMIC, if you
126are in atomic context.
127
128post_reset
129----------
130
131::
132
133  int (*post_reset)(struct usb_interface *intf);
134
135The reset has completed.  Restore any saved device state and begin
136using the device again.
137
138If you need to allocate memory here, use GFP_NOIO or GFP_ATOMIC, if you
139are in atomic context.
140
141Call sequences
142==============
143
144No callbacks other than probe will be invoked for an interface
145that isn't bound to your driver.
146
147Probe will never be called for an interface bound to a driver.
148Hence following a successful probe, disconnect will be called
149before there is another probe for the same interface.
150
151Once your driver is bound to an interface, disconnect can be
152called at any time except in between pre_reset and post_reset.
153pre_reset is always followed by post_reset, even if the reset
154failed or the device has been unplugged.
155
156suspend is always followed by one of: resume, reset_resume, or
157disconnect.
158