Documentation/extcon: porting guide for Android kernel switch driver.
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com> Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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MyungJoo Ham
Greg Kroah-Hartman
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Staging/Android Switch Class Porting Guide
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(linux/drivers/staging/android/switch)
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(c) Copyright 2012 Samsung Electronics
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AUTHORS
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MyungJoo Ham <myungjoo.ham@samsung.com>
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/*****************************************************************
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* CHAPTER 1. *
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* PORTING SWITCH CLASS DEVICE DRIVERS *
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*****************************************************************/
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****** STEP 1. Basic Functionality
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No extcon extended feature, but switch features only.
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- struct switch_dev (fed to switch_dev_register/unregister)
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@name: no change
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@dev: no change
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@index: drop (not used in switch device driver side anyway)
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@state: no change
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If you have used @state with magic numbers, keep it
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at this step.
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@print_name: no change but type change (switch_dev->extcon_dev)
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@print_state: no change but type change (switch_dev->extcon_dev)
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- switch_dev_register(sdev, dev)
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=> extcon_dev_register(edev, dev)
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: no change but type change (sdev->edev)
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- switch_dev_unregister(sdev)
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=> extcon_dev_unregister(edev)
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: no change but type change (sdev->edev)
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- switch_get_state(sdev)
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=> extcon_get_state(edev)
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: no change but type change (sdev->edev) and (return: int->u32)
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- switch_set_state(sdev, state)
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=> extcon_set_state(edev, state)
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: no change but type change (sdev->edev) and (state: int->u32)
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With this changes, the ex-switch extcon class device works as it once
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worked as switch class device. However, it will now have additional
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interfaces (both ABI and in-kernel API) and different ABI locations.
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However, if CONFIG_ANDROID is enabled without CONFIG_ANDROID_SWITCH,
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/sys/class/switch/* will be symbolically linked to /sys/class/extcon/
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so that they are still compatible with legacy userspace processes.
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****** STEP 2. Multistate (no more magic numbers in state value)
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Extcon's extended features for switch device drivers with
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complex features usually required magic numbers in state
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value of switch_dev. With extcon, such magic numbers that
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support multiple cables (
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1. Define cable names at edev->supported_cable.
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2. (Recommended) remove print_state callback.
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3. Use extcon_get_cable_state_(edev, index) or
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extcon_get_cable_state(edev, cable_name) instead of
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extcon_get_state(edev) if you intend to get a state of a specific
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cable. Same for set_state. This way, you can remove the usage of
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magic numbers in state value.
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4. Use extcon_update_state() if you are updating specific bits of
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the state value.
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Example: a switch device driver w/ magic numbers for two cables.
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"0x00": no cables connected.
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"0x01": cable 1 connected
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"0x02": cable 2 connected
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"0x03": cable 1 and 2 connected
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1. edev->supported_cable = {"1", "2", NULL};
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2. edev->print_state = NULL;
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3. extcon_get_cable_state_(edev, 0) shows cable 1's state.
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extcon_get_cable_state(edev, "1") shows cable 1's state.
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extcon_set_cable_state_(edev, 1) sets cable 2's state.
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extcon_set_cable_state(edev, "2") sets cable 2's state
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4. extcon_update_state(edev, 0x01, 0) sets the least bit's 0.
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****** STEP 3. Notify other device drivers
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You can notify others of the cable attach/detach events with
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notifier chains.
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At the side of other device drivers (the extcon device itself
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does not need to get notified of its own events), there are two
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methods to register notifier_block for cable events:
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(a) for a specific cable or (b) for every cable.
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(a) extcon_register_interest(obj, extcon_name, cable_name, nb)
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Example: want to get news of "MAX8997_MUIC"'s "USB" cable
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obj = kzalloc(sizeof(struct extcon_specific_cable_nb),
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GFP_KERNEL);
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nb->notifier_call = the_callback_to_handle_usb;
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extcon_register_intereset(obj, "MAX8997_MUIC", "USB", nb);
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(b) extcon_register_notifier(edev, nb)
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Call nb for any changes in edev.
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Please note that in order to properly behave with method (a),
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the extcon device driver should support multistate feature (STEP 2).
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****** STEP 4. Inter-cable relation (mutually exclusive)
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You can provide inter-cable mutually exclusiveness information
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for an extcon device. When cables A and B are declared to be mutually
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exclusive, the two cables cannot be in ATTACHED state simulteneously.
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/*****************************************************************
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* CHAPTER 2. *
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* PORTING USERSPACE w/ SWITCH CLASS DEVICE SUPPORT *
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*****************************************************************/
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****** ABI Location
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If "CONFIG_ANDROID" is enabled and "CONFIG_ANDROID_SWITCH" is
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disabled, /sys/class/switch/* are created as symbolic links to
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/sys/class/extcon/*. Because CONFIG_ANDROID_SWITCH creates
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/sys/class/switch directory, we disable symboling linking if
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CONFIG_ANDROID_SWITCH is enabled.
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The two files of switch class, name and state, are provided with
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extcon, too. When the multistate support (STEP 2 of CHAPTER 1.) is
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not enabled or print_state callback is supplied, the output of
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state ABI is same with switch class.
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