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