driver core: prune docs about device_interface

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. 

 *

 * CPUs are exported via sysfs in the class/cpu/devices/

 * directory. 

 *

 * Per-cpu interfaces can be implemented using a struct device_interface. 

 * See the following for how to do this: 

 * - drivers/base/intf.c 

 * - Documentation/driver-model/interface.txt

 */

#ifndef _LINUX_CPU_H_

#define _LINUX_CPU_H_

 *

 * Nodes are exported via driverfs in the class/node/devices/

 * directory. 

 *

 * Per-node interfaces can be implemented using a struct device_interface. 

 * See the following for how to do this: 

 * - drivers/base/intf.c 

 * - Documentation/driver-model/interface.txt

 */

#ifndef _LINUX_NODE_H_

#define _LINUX_NODE_H_