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Documentation / driver-model / binding.txt




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Based on kernel version 3.13. Page generated on 2014-01-20 22:02 EST.

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