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 27 typedef int (*devclass_add)(struct device *); 28 typedef void (*devclass_remove)(struct device *); 29 30See the kerneldoc for the struct class. 31 32A typical device class definition would look like:: 33 34 struct 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 45 int devclass_register(struct device_class * cls); 46 void 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~~~~~~~~~~~~~~~~~~~~ 114 115:: 116 117 struct devclass_attribute { 118 struct attribute attr; 119 ssize_t (*show)(struct device_class *, char * buf, size_t count, loff_t off); 120 ssize_t (*store)(struct device_class *, const char * buf, size_t count, loff_t off); 121 }; 122 123Class drivers can export attributes using the DEVCLASS_ATTR macro that works 124similarly to the DEVICE_ATTR macro for devices. For example, a definition 125like this:: 126 127 static DEVCLASS_ATTR(debug,0644,show_debug,store_debug); 128 129is equivalent to declaring:: 130 131 static devclass_attribute devclass_attr_debug; 132 133The bus driver can add and remove the attribute from the class's 134sysfs directory using:: 135 136 int devclass_create_file(struct device_class *, struct devclass_attribute *); 137 void devclass_remove_file(struct device_class *, struct devclass_attribute *); 138 139In the example above, the file will be named 'debug' in placed in the 140class's directory in sysfs. 141 142 143Interfaces 144~~~~~~~~~~ 145There may exist multiple mechanisms for accessing the same device of a 146particular class type. Device interfaces describe these mechanisms. 147 148When a device is added to a device class, the core attempts to add it 149to every interface that is registered with the device class. 150