1
2Device Classes
3
4
5Introduction
6~~~~~~~~~~~~
7A device class describes a type of device, like an audio or network
8device. The following device classes have been identified:
9
10<Insert List of Device Classes Here>
11
12
13Each device class defines a set of semantics and a programming interface
14that devices of that class adhere to. Device drivers are the
15implementation of that programming interface for a particular device on
16a particular bus. 
17
18Device classes are agnostic with respect to what bus a device resides
19on. 
20
21
22Programming Interface
23~~~~~~~~~~~~~~~~~~~~~
24The device class structure looks like: 
25
26
27typedef int (*devclass_add)(struct device *);
28typedef void (*devclass_remove)(struct device *);
29
30See the kerneldoc for the struct class.
31
32A typical device class definition would look like: 
33
34struct device_class input_devclass = {
35        .name		= "input",
36        .add_device	= input_add_device,
37	.remove_device	= input_remove_device,
38};
39
40Each device class structure should be exported in a header file so it
41can be used by drivers, extensions and interfaces.
42
43Device classes are registered and unregistered with the core using: 
44
45int devclass_register(struct device_class * cls);
46void devclass_unregister(struct device_class * cls);
47
48
49Devices
50~~~~~~~
51As devices are bound to drivers, they are added to the device class
52that the driver belongs to. Before the driver model core, this would
53typically happen during the driver's probe() callback, once the device
54has been initialized. It now happens after the probe() callback
55finishes from the core. 
56
57The device is enumerated in the class. Each time a device is added to
58the class, the class's devnum field is incremented and assigned to the
59device. The field is never decremented, so if the device is removed
60from the class and re-added, it will receive a different enumerated
61value. 
62
63The class is allowed to create a class-specific structure for the
64device and store it in the device's class_data pointer. 
65
66There is no list of devices in the device class. Each driver has a
67list of devices that it supports. The device class has a list of
68drivers of that particular class. To access all of the devices in the
69class, iterate over the device lists of each driver in the class.
70
71
72Device Drivers
73~~~~~~~~~~~~~~
74Device drivers are added to device classes when they are registered
75with the core. A driver specifies the class it belongs to by setting
76the struct device_driver::devclass field. 
77
78
79sysfs directory structure
80~~~~~~~~~~~~~~~~~~~~~~~~~~~~
81There is a top-level sysfs directory named 'class'. 
82
83Each class gets a directory in the class directory, along with two
84default subdirectories:
85
86        class/
87        `-- input
88            |-- devices
89            `-- drivers
90
91
92Drivers registered with the class get a symlink in the drivers/ directory 
93that points to the driver's directory (under its bus directory):
94
95   class/
96   `-- input
97       |-- devices
98       `-- drivers
99           `-- usb:usb_mouse -> ../../../bus/drivers/usb_mouse/
100
101
102Each device gets a symlink in the devices/ directory that points to the 
103device's directory in the physical hierarchy:
104
105   class/
106   `-- input
107       |-- devices
108       |   `-- 1 -> ../../../root/pci0/00:1f.0/usb_bus/00:1f.2-1:0/
109       `-- drivers
110
111
112Exporting Attributes
113~~~~~~~~~~~~~~~~~~~~
114struct devclass_attribute {
115        struct attribute        attr;
116        ssize_t (*show)(struct device_class *, char * buf, size_t count, loff_t off);
117        ssize_t (*store)(struct device_class *, const char * buf, size_t count, loff_t off);
118};
119
120Class drivers can export attributes using the DEVCLASS_ATTR macro that works
121similarly to the DEVICE_ATTR macro for devices. For example, a definition 
122like this:
123
124static DEVCLASS_ATTR(debug,0644,show_debug,store_debug);
125
126is equivalent to declaring:
127
128static devclass_attribute devclass_attr_debug;
129
130The bus driver can add and remove the attribute from the class's
131sysfs directory using:
132
133int devclass_create_file(struct device_class *, struct devclass_attribute *);
134void devclass_remove_file(struct device_class *, struct devclass_attribute *);
135
136In the example above, the file will be named 'debug' in placed in the
137class's directory in sysfs. 
138
139
140Interfaces
141~~~~~~~~~~
142There may exist multiple mechanisms for accessing the same device of a
143particular class type. Device interfaces describe these mechanisms. 
144
145When a device is added to a device class, the core attempts to add it
146to every interface that is registered with the device class.
147
148