[RFKILL]: Add rfkill documentation

rfkill - RF switch subsystem support

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1 Implementation details

2 Driver support

3 Userspace support

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1: Implementation details

The rfkill switch subsystem offers support for keys often found on laptops

to enable wireless devices like WiFi and Bluetooth.

This is done by providing the user 3 possibilities:

 1 - The rfkill system handles all events; userspace is not aware of events.

 2 - The rfkill system handles all events; userspace is informed about the events.

 3 - The rfkill system does not handle events; userspace handles all events.

The buttons to enable and disable the wireless radios are important in

situations where the user is for example using his laptop on a location where

wireless radios _must_ be disabled (e.g. airplanes).

Because of this requirement, userspace support for the keys should not be

made mandatory. Because userspace might want to perform some additional smarter

tasks when the key is pressed, rfkill still provides userspace the possibility

to take over the task to handle the key events.

The system inside the kernel has been split into 2 separate sections:

	1 - RFKILL

	2 - RFKILL_INPUT

The first option enables rfkill support and will make sure userspace will

be notified of any events through the input device. It also creates several

sysfs entries which can be used by userspace. See section "Userspace support".

The second option provides an rfkill input handler. This handler will

listen to all rfkill key events and will toggle the radio accordingly.

With this option enabled userspace could either do nothing or simply

perform monitoring tasks.

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2: Driver support

To build a driver with rfkill subsystem support, the driver should

depend on the Kconfig symbol RFKILL; it should _not_ depend on

RKFILL_INPUT.

Unless key events trigger an interrupt to which the driver listens, polling

will be required to determine the key state changes. For this the input

layer providers the input-polldev handler.

A driver should implement a few steps to correctly make use of the

rfkill subsystem. First for non-polling drivers:

	- rfkill_allocate()

	- input_allocate_device()

	- rfkill_register()

	- input_register_device()

For polling drivers:

	- rfkill_allocate()

	- input_allocate_polled_device()

	- rfkill_register()

	- input_register_polled_device()

When a key event has been detected, the correct event should be

sent over the input device which has been registered by the driver.

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3: Userspace support

For each key an input device will be created which will send out the correct

key event when the rfkill key has been pressed.

The following sysfs entries will be created:

	name: Name assigned by driver to this key (interface or driver name).

	type: Name of the key type ("wlan", "bluetooth", etc).

	state: Current state of the key. 1: On, 0: Off.

	claim: 1: Userspace handles events, 0: Kernel handles events

Both the "state" and "claim" entries are also writable. For the "state" entry

this means that when 1 or 0 is written all radios, not yet in the requested

state, will be will be toggled accordingly.

For the "claim" entry writing 1 to it means that the kernel no longer handles

key events even though RFKILL_INPUT input was enabled. When "claim" has been

set to 0, userspace should make sure that it listens for the input events or

check the sysfs "state" entry regularly to correctly perform the required

tasks when the rkfill key is pressed.