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-
- 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, dev)
- : no change but type change (sdev->edev)
-- 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 (
-
- 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 and "CONFIG_ANDROID_SWITCH" is
-disabled, /sys/class/switch/* are created as symbolic links to
-/sys/class/extcon/*. Because CONFIG_ANDROID_SWITCH creates
-/sys/class/switch directory, we disable symboling linking if
-CONFIG_ANDROID_SWITCH is enabled.
-
- 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.