1 2Driver Binding 3 4Driver binding is the process of associating a device with a device 5driver that can control it. Bus drivers have typically handled this 6because there have been bus-specific structures to represent the 7devices and the drivers. With generic device and device driver 8structures, most of the binding can take place using common code. 9 10 11Bus 12~~~ 13 14The bus type structure contains a list of all devices that are on that bus 15type in the system. When device_register is called for a device, it is 16inserted into the end of this list. The bus object also contains a 17list of all drivers of that bus type. When driver_register is called 18for a driver, it is inserted at the end of this list. These are the 19two events which trigger driver binding. 20 21 22device_register 23~~~~~~~~~~~~~~~ 24 25When a new device is added, the bus's list of drivers is iterated over 26to find one that supports it. In order to determine that, the device 27ID of the device must match one of the device IDs that the driver 28supports. The format and semantics for comparing IDs is bus-specific. 29Instead of trying to derive a complex state machine and matching 30algorithm, it is up to the bus driver to provide a callback to compare 31a device against the IDs of a driver. The bus returns 1 if a match was 32found; 0 otherwise. 33 34int match(struct device * dev, struct device_driver * drv); 35 36If a match is found, the device's driver field is set to the driver 37and the driver's probe callback is called. This gives the driver a 38chance to verify that it really does support the hardware, and that 39it's in a working state. 40 41Device Class 42~~~~~~~~~~~~ 43 44Upon the successful completion of probe, the device is registered with 45the class to which it belongs. Device drivers belong to one and only one 46class, and that is set in the driver's devclass field. 47devclass_add_device is called to enumerate the device within the class 48and actually register it with the class, which happens with the 49class's register_dev callback. 50 51NOTE: The device class structures and core routines to manipulate them 52are not in the mainline kernel, so the discussion is still a bit 53speculative. 54 55 56Driver 57~~~~~~ 58 59When a driver is attached to a device, the device is inserted into the 60driver's list of devices. 61 62 63sysfs 64~~~~~ 65 66A symlink is created in the bus's 'devices' directory that points to 67the device's directory in the physical hierarchy. 68 69A symlink is created in the driver's 'devices' directory that points 70to the device's directory in the physical hierarchy. 71 72A directory for the device is created in the class's directory. A 73symlink is created in that directory that points to the device's 74physical location in the sysfs tree. 75 76A symlink can be created (though this isn't done yet) in the device's 77physical directory to either its class directory, or the class's 78top-level directory. One can also be created to point to its driver's 79directory also. 80 81 82driver_register 83~~~~~~~~~~~~~~~ 84 85The process is almost identical for when a new driver is added. 86The bus's list of devices is iterated over to find a match. Devices 87that already have a driver are skipped. All the devices are iterated 88over, to bind as many devices as possible to the driver. 89 90 91Removal 92~~~~~~~ 93 94When a device is removed, the reference count for it will eventually 95go to 0. When it does, the remove callback of the driver is called. It 96is removed from the driver's list of devices and the reference count 97of the driver is decremented. All symlinks between the two are removed. 98 99When a driver is removed, the list of devices that it supports is 100iterated over, and the driver's remove callback is called for each 101one. The device is removed from that list and the symlinks removed. 102 103