1
  2	Staging/Android Switch Class Porting Guide
  3	(linux/drivers/staging/android/switch)
  4	(c) Copyright 2012 Samsung Electronics
  5
  6AUTHORS
  7MyungJoo Ham <myungjoo.ham@samsung.com>
  8
  9/*****************************************************************
 10 * CHAPTER 1.                                                    *
 11 * PORTING SWITCH CLASS DEVICE DRIVERS                           *
 12 *****************************************************************/
 13
 14****** STEP 1. Basic Functionality
 15	No extcon extended feature, but switch features only.
 16
 17- struct switch_dev (fed to switch_dev_register/unregister)
 18    @name: no change
 19    @dev: no change
 20    @index: drop (not used in switch device driver side anyway)
 21    @state: no change
 22	If you have used @state with magic numbers, keep it
 23	at this step.
 24    @print_name: no change but type change (switch_dev->extcon_dev)
 25    @print_state: no change but type change (switch_dev->extcon_dev)
 26
 27- switch_dev_register(sdev, dev)
 28	=> extcon_dev_register(edev)
 29	: type change (sdev->edev)
 30	: remove second param('dev'). if edev has parent device, should store
 31	  'dev' to 'edev.dev.parent' before registering extcon device
 32- switch_dev_unregister(sdev)
 33	=> extcon_dev_unregister(edev)
 34	: no change but type change (sdev->edev)
 35- switch_get_state(sdev)
 36	=> extcon_get_state(edev)
 37	: no change but type change (sdev->edev) and (return: int->u32)
 38- switch_set_state(sdev, state)
 39	=> extcon_set_state(edev, state)
 40	: no change but type change (sdev->edev) and (state: int->u32)
 41
 42With this changes, the ex-switch extcon class device works as it once
 43worked as switch class device. However, it will now have additional
 44interfaces (both ABI and in-kernel API) and different ABI locations.
 45However, if CONFIG_ANDROID is enabled without CONFIG_ANDROID_SWITCH,
 46/sys/class/switch/* will be symbolically linked to /sys/class/extcon/
 47so that they are still compatible with legacy userspace processes.
 48
 49****** STEP 2. Multistate (no more magic numbers in state value)
 50	Extcon's extended features for switch device drivers with
 51	complex features usually required magic numbers in state
 52	value of switch_dev. With extcon, such magic numbers that
 53	support multiple cables are no more required or supported.
 54
 55  1. Define cable names at edev->supported_cable.
 56  2. (Recommended) remove print_state callback.
 57  3. Use extcon_get_cable_state_(edev, index) or
 58   extcon_get_cable_state(edev, cable_name) instead of
 59   extcon_get_state(edev) if you intend to get a state of a specific
 60   cable. Same for set_state. This way, you can remove the usage of
 61   magic numbers in state value.
 62  4. Use extcon_update_state() if you are updating specific bits of
 63   the state value.
 64
 65Example: a switch device driver w/ magic numbers for two cables.
 66	"0x00": no cables connected.
 67	"0x01": cable 1 connected
 68	"0x02": cable 2 connected
 69	"0x03": cable 1 and 2 connected
 70  1. edev->supported_cable = {"1", "2", NULL};
 71  2. edev->print_state = NULL;
 72  3. extcon_get_cable_state_(edev, 0) shows cable 1's state.
 73     extcon_get_cable_state(edev, "1") shows cable 1's state.
 74     extcon_set_cable_state_(edev, 1) sets cable 2's state.
 75     extcon_set_cable_state(edev, "2") sets cable 2's state
 76  4. extcon_update_state(edev, 0x01, 0) sets the least bit's 0.
 77
 78****** STEP 3. Notify other device drivers
 79
 80  You can notify others of the cable attach/detach events with
 81notifier chains.
 82
 83  At the side of other device drivers (the extcon device itself
 84does not need to get notified of its own events), there are two
 85methods to register notifier_block for cable events:
 86(a) for a specific cable or (b) for every cable.
 87
 88  (a) extcon_register_interest(obj, extcon_name, cable_name, nb)
 89	Example: want to get news of "MAX8997_MUIC"'s "USB" cable
 90
 91	obj = kzalloc(sizeof(struct extcon_specific_cable_nb),
 92		      GFP_KERNEL);
 93	nb->notifier_call = the_callback_to_handle_usb;
 94
 95	extcon_register_intereset(obj, "MAX8997_MUIC", "USB", nb);
 96
 97  (b) extcon_register_notifier(edev, nb)
 98	Call nb for any changes in edev.
 99
100  Please note that in order to properly behave with method (a),
101the extcon device driver should support multistate feature (STEP 2).
102
103****** STEP 4. Inter-cable relation (mutually exclusive)
104
105  You can provide inter-cable mutually exclusiveness information
106for an extcon device. When cables A and B are declared to be mutually
107exclusive, the two cables cannot be in ATTACHED state simulteneously.
108
109
110/*****************************************************************
111 * CHAPTER 2.                                                    *
112 * PORTING USERSPACE w/ SWITCH CLASS DEVICE SUPPORT              *
113 *****************************************************************/
114
115****** ABI Location
116
117  If "CONFIG_ANDROID" is enabled, /sys/class/switch/* are created
118as symbolic links to /sys/class/extcon/*.
119
120  The two files of switch class, name and state, are provided with
121extcon, too. When the multistate support (STEP 2 of CHAPTER 1.) is
122not enabled or print_state callback is supplied, the output of
123state ABI is same with switch class.