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

1. Introduction
2. Implementation details
3. Kernel driver guidelines
4. Kernel API
5. 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.



2. Implementation details

The rfkill subsystem is composed of various components: the rfkill class, the
rfkill-input module (an input layer handler), and some specific input layer
events.

The rfkill class is provided for kernel drivers to register their radio
transmitter with the kernel, provide methods for turning it on and off and,
optionally, letting the system know about hardware-disabled states that may
be implemented on the device. This code is enabled with the CONFIG_RFKILL
Kconfig option, which drivers can "select".

The rfkill class code also notifies userspace of state changes, this is
achieved via uevents. It also provides some sysfs files for userspace to
check the status of radio transmitters. See the "Userspace support" section
below.


The rfkill-input code implements a basic response to rfkill buttons -- it
implements turning on/off all devices of a certain class (or all).

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 but drivers can well
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().


The entire functionality is spread over more than one subsystem:

 * The kernel input layer generates KEY_WWAN, KEY_WLAN etc. and
   SW_RFKILL_ALL -- when the user presses a button. Drivers for radio
   transmitters generally do not register to the input layer, unless the
   device really provides an input device (i.e. a button that has no
   effect other than generating a button press event)

 * The rfkill-input code hooks up to these events and switches the soft-block
   of the various radio transmitters, depending on the button type.

 * The rfkill drivers turn off/on their transmitters as requested.

 * The rfkill class will generate userspace notifications (uevents) to tell
   userspace what the current state is.



3. Kernel driver guidelines


Drivers for radio transmitters normally implement only the rfkill class.
These drivers may not unblock the transmitter based on own decisions, they
should act on information provided by the rfkill class only.

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 class instead. This also applies if the platform provides
a way to turn on/off the transmitter(s).

During suspend/hibernation, transmitters should only be left enabled when
wake-on wlan or similar functionality requires it and the device wasn't
blocked before suspend/hibernate. Note that it may be necessary to update
the rfkill subsystem's idea of what the current state is at resume time if
the state may have changed over suspend.



4. Kernel API

To build a driver with rfkill subsystem support, the driver should depend on
(or select) the Kconfig symbol RFKILL.

The hardware the driver talks to may be write-only (where the current state
of the hardware is unknown), or read-write (where the hardware can be queried
about its current state).

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 following sysfs entries exist for every rfkill device:

	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 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.
	claim: 0: Kernel handles events (currently always reads that value)

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.

An alternative userspace interface exists as a misc device /dev/rfkill,
which 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.