Staging/Android Switch Class Porting Guide	(linux/drivers/staging/android/switch)	(c) Copyright 2012 Samsung Electronics AUTHORSMyungJoo 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 onceworked as switch class device. However, it will now have additionalinterfaces (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 withnotifier chains.   At the side of other device drivers (the extcon device itselfdoes not need to get notified of its own events), there are twomethods 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 informationfor an extcon device. When cables A and B are declared to be mutuallyexclusive, 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 createdas symbolic links to /sys/class/extcon/*.   The two files of switch class, name and state, are provided withextcon, too. When the multistate support (STEP 2 of CHAPTER 1.) isnot enabled or print_state callback is supplied, the output ofstate ABI is same with switch class.