linux/Documentation/driver-model/interface.txt

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.