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authorOliver Neukum <oliver@neukum.org>2008-04-16 09:46:37 -0400
committerGreg Kroah-Hartman <gregkh@suse.de>2008-04-25 00:16:53 -0400
commit08177e12b7b4c3d59060f829e5c151d06f9a08d6 (patch)
treeddee6dca14ac6c51ec01cc97f0449cf2d3e37a1d /Documentation/usb
parente872154921a6b5256a3c412dd69158ac0b135176 (diff)
USB: add documentation about callbacks
Add Documentation about callbacks in USB. Signed-off-by: Oliver Neukum <oneukum@suse.de> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
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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'sfacility,
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
98Another driver or user space is triggering a reset on the device which
99contains the interface passed as an argument. Cease IO and save any
100device state you need to restore.
101
102If you need to allocate memory here, use GFP_NOIO or GFP_ATOMIC, if you
103are in atomic context.
104
105post_reset
106----------
107
108int (*post_reset)(struct usb_interface *intf);
109
110The reset has completed. Restore any saved device state and begin
111using the device again.
112
113If you need to allocate memory here, use GFP_NOIO or GFP_ATOMIC, if you
114are in atomic context.
115
116Call sequences
117==============
118
119No callbacks other than probe will be invoked for an interface
120that isn't bound to your driver.
121
122Probe will never be called for an interface bound to a driver.
123Hence following a successful probe, disconnect will be called
124before there is another probe for the same interface.
125
126Once your driver is bound to an interface, disconnect can be
127called at any time except in between pre_reset and post_reset.
128pre_reset is always followed by post_reset, even if the reset
129failed or the device has been unplugged.
130
131suspend is always followed by one of: resume, reset_resume, or
132disconnect.