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    Subject[PATCH v8 resend 6/6] Documentation/extcon: porting guide for Android kernel switch driver.
    Date
    Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>
    Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
    ---
    Documentation/extcon/porting-android-switch-class | 124 +++++++++++++++++++++
    1 files changed, 124 insertions(+), 0 deletions(-)
    create mode 100644 Documentation/extcon/porting-android-switch-class

    diff --git a/Documentation/extcon/porting-android-switch-class b/Documentation/extcon/porting-android-switch-class
    new file mode 100644
    index 0000000..eb0fa5f
    --- /dev/null
    +++ b/Documentation/extcon/porting-android-switch-class
    @@ -0,0 +1,124 @@
    +
    + 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.
    --
    1.7.4.1


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