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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:02 EST.

1	The Linux Kernel Device Model
2	
3	Patrick Mochel	<mochel@digitalimplant.org>
4	
5	Drafted 26 August 2002
6	Updated 31 January 2006
7	
8	
9	Overview
10	~~~~~~~~
11	
12	The Linux Kernel Driver Model is a unification of all the disparate driver
13	models that were previously used in the kernel. It is intended to augment the
14	bus-specific drivers for bridges and devices by consolidating a set of data
15	and operations into globally accessible data structures.
16	
17	Traditional driver models implemented some sort of tree-like structure
18	(sometimes just a list) for the devices they control. There wasn't any
19	uniformity across the different bus types.
20	
21	The current driver model provides a common, uniform data model for describing
22	a bus and the devices that can appear under the bus. The unified bus
23	model includes a set of common attributes which all busses carry, and a set
24	of common callbacks, such as device discovery during bus probing, bus
25	shutdown, bus power management, etc.
26	
27	The common device and bridge interface reflects the goals of the modern
28	computer: namely the ability to do seamless device "plug and play", power
29	management, and hot plug. In particular, the model dictated by Intel and
30	Microsoft (namely ACPI) ensures that almost every device on almost any bus
31	on an x86-compatible system can work within this paradigm.  Of course,
32	not every bus is able to support all such operations, although most
33	buses support most of those operations.
34	
35	
36	Downstream Access
37	~~~~~~~~~~~~~~~~~
38	
39	Common data fields have been moved out of individual bus layers into a common
40	data structure. These fields must still be accessed by the bus layers,
41	and sometimes by the device-specific drivers.
42	
43	Other bus layers are encouraged to do what has been done for the PCI layer.
44	struct pci_dev now looks like this:
45	
46	struct pci_dev {
47		...
48	
49		struct device dev;     /* Generic device interface */
50		...
51	};
52	
53	Note first that the struct device dev within the struct pci_dev is
54	statically allocated. This means only one allocation on device discovery.
55	
56	Note also that that struct device dev is not necessarily defined at the
57	front of the pci_dev structure.  This is to make people think about what
58	they're doing when switching between the bus driver and the global driver,
59	and to discourage meaningless and incorrect casts between the two.
60	
61	The PCI bus layer freely accesses the fields of struct device. It knows about
62	the structure of struct pci_dev, and it should know the structure of struct
63	device. Individual PCI device drivers that have been converted to the current
64	driver model generally do not and should not touch the fields of struct device,
65	unless there is a compelling reason to do so.
66	
67	The above abstraction prevents unnecessary pain during transitional phases.
68	If it were not done this way, then when a field was renamed or removed, every
69	downstream driver would break.  On the other hand, if only the bus layer
70	(and not the device layer) accesses the struct device, it is only the bus
71	layer that needs to change.
72	
73	
74	User Interface
75	~~~~~~~~~~~~~~
76	
77	By virtue of having a complete hierarchical view of all the devices in the
78	system, exporting a complete hierarchical view to userspace becomes relatively
79	easy. This has been accomplished by implementing a special purpose virtual
80	file system named sysfs.
81	
82	Almost all mainstream Linux distros mount this filesystem automatically; you
83	can see some variation of the following in the output of the "mount" command:
84	
85	$ mount
86	...
87	none on /sys type sysfs (rw,noexec,nosuid,nodev)
88	...
89	$
90	
91	The auto-mounting of sysfs is typically accomplished by an entry similar to
92	the following in the /etc/fstab file:
93	
94	none     	/sys	sysfs    defaults	  	0 0
95	
96	or something similar in the /lib/init/fstab file on Debian-based systems:
97	
98	none            /sys    sysfs    nodev,noexec,nosuid    0 0
99	
100	If sysfs is not automatically mounted, you can always do it manually with:
101	
102	# mount -t sysfs sysfs /sys
103	
104	Whenever a device is inserted into the tree, a directory is created for it.
105	This directory may be populated at each layer of discovery - the global layer,
106	the bus layer, or the device layer.
107	
108	The global layer currently creates two files - 'name' and 'power'. The
109	former only reports the name of the device. The latter reports the
110	current power state of the device. It will also be used to set the current
111	power state. 
112	
113	The bus layer may also create files for the devices it finds while probing the
114	bus. For example, the PCI layer currently creates 'irq' and 'resource' files
115	for each PCI device.
116	
117	A device-specific driver may also export files in its directory to expose
118	device-specific data or tunable interfaces.
119	
120	More information about the sysfs directory layout can be found in
121	the other documents in this directory and in the file 
122	Documentation/filesystems/sysfs.txt.
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