rfkill - RF kill switch support
  ===============================

  1. Introduction
  2. Implementation details

  3. Kernel API
  4. Userspace support

  1. Introduction

  The rfkill subsystem provides a generic interface to disabling any radio
  transmitter in the system. When a transmitter is blocked, it shall not
  radiate any power.

  The subsystem also provides the ability to react on button presses and
  disable all transmitters of a certain type (or all). This is intended for
  situations where transmitters need to be turned off, for example on
  aircraft.

  The rfkill subsystem has a concept of "hard" and "soft" block, which
  differ little in their meaning (block == transmitters off) but rather in
  whether they can be changed or not:
   - hard block: read-only radio block that cannot be overriden by software
   - soft block: writable radio block (need not be readable) that is set by
                 the system software.

  2. Implementation details

  The rfkill subsystem is composed of three main components:
   * the rfkill core,
   * the deprecated rfkill-input module (an input layer handler, being
     replaced by userspace policy code) and
   * the rfkill drivers.

  The rfkill core provides API for kernel drivers to register their radio
  transmitter with the kernel, methods for turning it on and off and, letting
  the system know about hardware-disabled states that may be implemented on
  the device.

  The rfkill core code also notifies userspace of state changes, and provides
  ways for userspace to query the current states. See the "Userspace support"
  section below.

  When the device is hard-blocked (either by a call to rfkill_set_hw_state()

  or from query_hw_block) set_block() will be invoked for additional software
  block, but drivers can ignore the method call since they can use the return
  value of the function rfkill_set_hw_state() to sync the software state
  instead of keeping track of calls to set_block(). In fact, drivers should
  use the return value of rfkill_set_hw_state() unless the hardware actually
  keeps track of soft and hard block separately.

  3. Kernel API

  Drivers for radio transmitters normally implement an rfkill driver.

  Platform drivers might implement input devices if the rfkill button is just
  that, a button. If that button influences the hardware then you need to

  implement an rfkill driver instead. This also applies if the platform provides

  a way to turn on/off the transmitter(s).

  For some platforms, it is possible that the hardware state changes during
  suspend/hibernation, in which case it will be necessary to update the rfkill
  core with the current state is at resume time.

  To create an rfkill driver, driver's Kconfig needs to have

  	depends on RFKILL || !RFKILL

  to ensure the driver cannot be built-in when rfkill is modular. The !RFKILL
  case allows the driver to be built when rfkill is not configured, which which
  case all rfkill API can still be used but will be provided by static inlines
  which compile to almost nothing.

  Calling rfkill_set_hw_state() when a state change happens is required from
  rfkill drivers that control devices that can be hard-blocked unless they also
  assign the poll_hw_block() callback (then the rfkill core will poll the
  device). Don't do this unless you cannot get the event in any other way.

  5. Userspace support

  The recommended userspace interface to use is /dev/rfkill, which is a misc
  character device that allows userspace to obtain and set the state of rfkill
  devices and sets of devices. It also notifies userspace about device addition
  and removal. The API is a simple read/write API that is defined in
  linux/rfkill.h, with one ioctl that allows turning off the deprecated input
  handler in the kernel for the transition period.
  
  Except for the one ioctl, communication with the kernel is done via read()
  and write() of instances of 'struct rfkill_event'. In this structure, the
  soft and hard block are properly separated (unlike sysfs, see below) and
  userspace is able to get a consistent snapshot of all rfkill devices in the
  system. Also, it is possible to switch all rfkill drivers (or all drivers of
  a specified type) into a state which also updates the default state for
  hotplugged devices.
  
  After an application opens /dev/rfkill, it can read the current state of
  all devices, and afterwards can poll the descriptor for hotplug or state
  change events.
  
  Applications must ignore operations (the "op" field) they do not handle,
  this allows the API to be extended in the future.
  
  Additionally, each rfkill device is registered in sysfs and there has the
  following attributes:

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

  	type: Driver type string ("wlan", "bluetooth", etc).

  	persistent: Whether the soft blocked state is initialised from
  	            non-volatile storage at startup.

  	state: Current state of the transmitter
  		0: RFKILL_STATE_SOFT_BLOCKED

  			transmitter is turned off by software

  		1: RFKILL_STATE_UNBLOCKED

  			transmitter is (potentially) active

  		2: RFKILL_STATE_HARD_BLOCKED
  			transmitter is forced off by something outside of

  			the driver's control.

  	       This file is deprecated because it can only properly show
  	       three of the four possible states, soft-and-hard-blocked is
  	       missing.
  	claim: 0: Kernel handles events
  	       This file is deprecated because there no longer is a way to
  	       claim just control over a single rfkill instance.

  
  rfkill devices also issue uevents (with an action of "change"), with the
  following environment variables set:
  
  RFKILL_NAME
  RFKILL_STATE
  RFKILL_TYPE
  
  The contents of these variables corresponds to the "name", "state" and
  "type" sysfs files explained above.