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+
+Device Classes
+
+
+Introduction
+~~~~~~~~~~~~
+A device class describes a type of device, like an audio or network
+device. The following device classes have been identified:
+
+<Insert List of Device Classes Here>
+
+
+Each device class defines a set of semantics and a programming interface
+that devices of that class adhere to. Device drivers are the
+implemention of that programming interface for a particular device on
+a particular bus. 
+
+Device classes are agnostic with respect to what bus a device resides
+on. 
+
+
+Programming Interface
+~~~~~~~~~~~~~~~~~~~~~
+The device class structure looks like: 
+
+
+typedef int (*devclass_add)(struct device *);
+typedef void (*devclass_remove)(struct device *);
+
+struct device_class {
+        char                    * name;
+        rwlock_t                lock;
+        u32                     devnum;
+        struct list_head        node;
+
+        struct list_head        drivers;
+        struct list_head        intf_list;
+
+        struct driver_dir_entry dir;
+        struct driver_dir_entry device_dir;
+        struct driver_dir_entry driver_dir;
+
+        devclass_add            add_device;
+        devclass_remove         remove_device;
+};
+
+A typical device class definition would look like: 
+
+struct device_class input_devclass = {
+        .name           = "input",
+        .add_device     = input_add_device,
+        .remove_device  = input_remove_device,
+};
+
+Each device class structure should be exported in a header file so it
+can be used by drivers, extensions and interfaces.
+
+Device classes are registered and unregistered with the core using: 
+
+int devclass_register(struct device_class * cls);
+void devclass_unregister(struct device_class * cls);
+
+
+Devices
+~~~~~~~
+As devices are bound to drivers, they are added to the device class
+that the driver belongs to. Before the driver model core, this would
+typically happen during the driver's probe() callback, once the device
+has been initialized. It now happens after the probe() callback
+finishes from the core. 
+
+The device is enumerated in the class. Each time a device is added to
+the class, the class's devnum field is incremented and assigned to the
+device. The field is never decremented, so if the device is removed
+from the class and re-added, it will receive a different enumerated
+value. 
+
+The class is allowed to create a class-specific structure for the
+device and store it in the device's class_data pointer. 
+
+There is no list of devices in the device class. Each driver has a
+list of devices that it supports. The device class has a list of
+drivers of that particular class. To access all of the devices in the
+class, iterate over the device lists of each driver in the class.
+
+
+Device Drivers
+~~~~~~~~~~~~~~
+Device drivers are added to device classes when they are registered
+with the core. A driver specifies the class it belongs to by setting
+the struct device_driver::devclass field. 
+
+
+sysfs directory structure
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+There is a top-level sysfs directory named 'class'. 
+
+Each class gets a directory in the class directory, along with two
+default subdirectories:
+
+        class/
+        `-- input
+            |-- devices
+            `-- drivers
+
+
+Drivers registered with the class get a symlink in the drivers/ directory 
+that points to the driver's directory (under its bus directory):
+
+   class/
+   `-- input
+       |-- devices
+       `-- drivers
+           `-- usb:usb_mouse -> ../../../bus/drivers/usb_mouse/
+
+
+Each device gets a symlink in the devices/ directory that points to the 
+device's directory in the physical hierarchy:
+
+   class/
+   `-- input
+       |-- devices
+       |   `-- 1 -> ../../../root/pci0/00:1f.0/usb_bus/00:1f.2-1:0/
+       `-- drivers
+
+
+Exporting Attributes
+~~~~~~~~~~~~~~~~~~~~
+struct devclass_attribute {
+        struct attribute        attr;
+        ssize_t (*show)(struct device_class *, char * buf, size_t count, loff_t off);
+        ssize_t (*store)(struct device_class *, const char * buf, size_t count, loff_t off);
+};
+
+Class drivers can export attributes using the DEVCLASS_ATTR macro that works
+similarly to the DEVICE_ATTR macro for devices. For example, a definition 
+like this:
+
+static DEVCLASS_ATTR(debug,0644,show_debug,store_debug);
+
+is equivalent to declaring:
+
+static devclass_attribute devclass_attr_debug;
+
+The bus driver can add and remove the attribute from the class's
+sysfs directory using:
+
+int devclass_create_file(struct device_class *, struct devclass_attribute *);
+void devclass_remove_file(struct device_class *, struct devclass_attribute *);
+
+In the example above, the file will be named 'debug' in placed in the
+class's directory in sysfs. 
+
+
+Interfaces
+~~~~~~~~~~
+There may exist multiple mechanisms for accessing the same device of a
+particular class type. Device interfaces describe these mechanisms. 
+
+When a device is added to a device class, the core attempts to add it
+to every interface that is registered with the device class.
+