linux-libre/Documentation/extcon/porting-android-switch-class

	Staging/Android Switch Class Porting Guide
	(linux/drivers/staging/android/switch)
	(c) Copyright 2012 Samsung Electronics

AUTHORS
MyungJoo Ham <myungjoo.ham@samsung.com>

/*****************************************************************
 * CHAPTER 1.                                                    *
 * PORTING SWITCH CLASS DEVICE DRIVERS                           *
 *****************************************************************/

****** STEP 1. Basic Functionality
	No extcon extended feature, but switch features only.

- struct switch_dev (fed to switch_dev_register/unregister)
    @name: no change
    @dev: no change
    @index: drop (not used in switch device driver side anyway)
    @state: no change
	If you have used @state with magic numbers, keep it
	at this step.
    @print_name: no change but type change (switch_dev->extcon_dev)
    @print_state: no change but type change (switch_dev->extcon_dev)

- switch_dev_register(sdev, dev)
	=> extcon_dev_register(edev)
	: type change (sdev->edev)
	: remove second param('dev'). if edev has parent device, should store
	  'dev' to 'edev.dev.parent' before registering extcon device
- switch_dev_unregister(sdev)
	=> extcon_dev_unregister(edev)
	: no change but type change (sdev->edev)
- switch_get_state(sdev)
	=> extcon_get_state(edev)
	: no change but type change (sdev->edev) and (return: int->u32)
- switch_set_state(sdev, state)
	=> extcon_set_state(edev, state)
	: no change but type change (sdev->edev) and (state: int->u32)

With this changes, the ex-switch extcon class device works as it once
worked as switch class device. However, it will now have additional
interfaces (both ABI and in-kernel API) and different ABI locations.
However, if CONFIG_ANDROID is enabled without CONFIG_ANDROID_SWITCH,
/sys/class/switch/* will be symbolically linked to /sys/class/extcon/
so that they are still compatible with legacy userspace processes.

****** STEP 2. Multistate (no more magic numbers in state value)
	Extcon's extended features for switch device drivers with
	complex features usually required magic numbers in state
	value of switch_dev. With extcon, such magic numbers that
	support multiple cables are no more required or supported.

  1. Define cable names at edev->supported_cable.
  2. (Recommended) remove print_state callback.
  3. Use extcon_get_cable_state_(edev, index) or
   extcon_get_cable_state(edev, cable_name) instead of
   extcon_get_state(edev) if you intend to get a state of a specific
   cable. Same for set_state. This way, you can remove the usage of
   magic numbers in state value.
  4. Use extcon_update_state() if you are updating specific bits of
   the state value.

Example: a switch device driver w/ magic numbers for two cables.
	"0x00": no cables connected.
	"0x01": cable 1 connected
	"0x02": cable 2 connected
	"0x03": cable 1 and 2 connected
  1. edev->supported_cable = {"1", "2", NULL};
  2. edev->print_state = NULL;
  3. extcon_get_cable_state_(edev, 0) shows cable 1's state.
     extcon_get_cable_state(edev, "1") shows cable 1's state.
     extcon_set_cable_state_(edev, 1) sets cable 2's state.
     extcon_set_cable_state(edev, "2") sets cable 2's state
  4. extcon_update_state(edev, 0x01, 0) sets the least bit's 0.

****** STEP 3. Notify other device drivers

  You can notify others of the cable attach/detach events with
notifier chains.

  At the side of other device drivers (the extcon device itself
does not need to get notified of its own events), there are two
methods to register notifier_block for cable events:
(a) for a specific cable or (b) for every cable.

  (a) extcon_register_interest(obj, extcon_name, cable_name, nb)
	Example: want to get news of "MAX8997_MUIC"'s "USB" cable

	obj = kzalloc(sizeof(struct extcon_specific_cable_nb),
		      GFP_KERNEL);
	nb->notifier_call = the_callback_to_handle_usb;

	extcon_register_intereset(obj, "MAX8997_MUIC", "USB", nb);

  (b) extcon_register_notifier(edev, nb)
	Call nb for any changes in edev.

  Please note that in order to properly behave with method (a),
the extcon device driver should support multistate feature (STEP 2).

****** STEP 4. Inter-cable relation (mutually exclusive)

  You can provide inter-cable mutually exclusiveness information
for an extcon device. When cables A and B are declared to be mutually
exclusive, the two cables cannot be in ATTACHED state simulteneously.


/*****************************************************************
 * CHAPTER 2.                                                    *
 * PORTING USERSPACE w/ SWITCH CLASS DEVICE SUPPORT              *
 *****************************************************************/

****** ABI Location

  If "CONFIG_ANDROID" is enabled, /sys/class/switch/* are created
as symbolic links to /sys/class/extcon/*.

  The two files of switch class, name and state, are provided with
extcon, too. When the multistate support (STEP 2 of CHAPTER 1.) is
not enabled or print_state callback is supplied, the output of
state ABI is same with switch class.