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+
+Device Interfaces
+
+Introduction
+~~~~~~~~~~~~
+
+Device interfaces are the logical interfaces of device classes that correlate
+directly to userspace interfaces, like device nodes. 
+   
+Each device class may have multiple interfaces through which you can 
+access the same device. An input device may support the mouse interface, 
+the 'evdev' interface, and the touchscreen interface. A SCSI disk would 
+support the disk interface, the SCSI generic interface, and possibly a raw 
+device interface. 
+
+Device interfaces are registered with the class they belong to. As devices
+are added to the class, they are added to each interface registered with
+the class. The interface is responsible for determining whether the device
+supports the interface or not. 
+
+
+Programming Interface
+~~~~~~~~~~~~~~~~~~~~~
+
+struct device_interface {
+        char                    * name;
+        rwlock_t                lock;
+        u32                     devnum;
+        struct device_class     * devclass;
+
+        struct list_head        node;
+        struct driver_dir_entry dir;
+
+        int (*add_device)(struct device *);
+        int (*add_device)(struct intf_data *);
+};
+
+int interface_register(struct device_interface *);
+void interface_unregister(struct device_interface *);
+
+
+An interface must specify the device class it belongs to. It is added
+to that class's list of interfaces on registration.
+
+
+Interfaces can be added to a device class at any time. Whenever it is
+added, each device in the class is passed to the interface's
+add_device callback. When an interface is removed, each device is
+removed from the interface.
+
+
+Devices
+~~~~~~~
+Once a device is added to a device class, it is added to each
+interface that is registered with the device class. The class
+is expected to place a class-specific data structure in 
+struct device::class_data. The interface can use that (along with
+other fields of struct device) to determine whether or not the driver
+and/or device support that particular interface.
+
+
+Data
+~~~~
+
+struct intf_data {
+        struct list_head        node;
+        struct device_interface * intf;
+        struct device           * dev;
+        u32                     intf_num;
+};
+
+int interface_add_data(struct interface_data *);
+
+The interface is responsible for allocating and initializing a struct 
+intf_data and calling interface_add_data() to add it to the device's list
+of interfaces it belongs to. This list will be iterated over when the device
+is removed from the class (instead of all possible interfaces for a class).
+This structure should probably be embedded in whatever per-device data 
+structure the interface is allocating anyway.
+   
+Devices are enumerated within the interface. This happens in interface_add_data()
+and the enumerated value is stored in the struct intf_data for that device. 
+
+sysfs
+~~~~~
+Each interface is given a directory in the directory of the device
+class it belongs to:
+
+Interfaces get a directory in the class's directory as well:
+
+   class/
+   `-- input
+       |-- devices
+       |-- drivers
+       |-- mouse
+       `-- evdev
+
+When a device is added to the interface, a symlink is created 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
+       |   `-- usb:usb_mouse -> ../../../bus/drivers/usb_mouse/
+       |-- mouse
+       |   `-- 1 -> ../../../root/pci0/00:1f.0/usb_bus/00:1f.2-1:0/
+       `-- evdev
+           `-- 1 -> ../../../root/pci0/00:1f.0/usb_bus/00:1f.2-1:0/
+
+
+Future Plans
+~~~~~~~~~~~~
+A device interface is correlated directly with a userspace interface
+for a device, specifically a device node. For instance, a SCSI disk
+exposes at least two interfaces to userspace: the standard SCSI disk
+interface and the SCSI generic interface. It might also export a raw
+device interface. 
+
+Many interfaces have a major number associated with them and each
+device gets a minor number. Or, multiple interfaces might share one
+major number, and each will receive a range of minor numbers (like in
+the case of input devices).
+
+These major and minor numbers could be stored in the interface
+structure. Major and minor allocations could happen when the interface
+is registered with the class, or via a helper function. 
+