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rfkill - RF kill switch support
===============================
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.. contents::
   :depth: 2
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Introduction
============
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The rfkill subsystem provides a generic interface for disabling any radio
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transmitter in the system. When a transmitter is blocked, it shall not
radiate any power.
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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.
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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:
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 - hard block
	read-only radio block that cannot be overridden by software

 - soft block
	writable radio block (need not be readable) that is set by
        the system software.
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The rfkill subsystem has two parameters, rfkill.default_state and
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rfkill.master_switch_mode, which are documented in
admin-guide/kernel-parameters.rst.
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Implementation details
======================
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The rfkill subsystem is composed of three main components:
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 * the rfkill core,
 * the deprecated rfkill-input module (an input layer handler, being
   replaced by userspace policy code) and
 * the rfkill drivers.
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The rfkill core provides API for kernel drivers to register their radio
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transmitter with the kernel, methods for turning it on and off, and letting
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the system know about hardware-disabled states that may be implemented on
the device.
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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.
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When the device is hard-blocked (either by a call to rfkill_set_hw_state()
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or from query_hw_block), set_block() will be invoked for additional software
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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.
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Kernel API
==========
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Drivers for radio transmitters normally implement an rfkill driver.
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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
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implement an rfkill driver instead. This also applies if the platform provides
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a way to turn on/off the transmitter(s).
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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
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core with the current state at resume time.
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To create an rfkill driver, driver's Kconfig needs to have::
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	depends on RFKILL || !RFKILL
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to ensure the driver cannot be built-in when rfkill is modular. The !RFKILL
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case allows the driver to be built when rfkill is not configured, in which
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case all rfkill API can still be used but will be provided by static inlines
which compile to almost nothing.
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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.
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rfkill provides per-switch LED triggers, which can be used to drive LEDs
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according to the switch state (LED_FULL when blocked, LED_OFF otherwise).
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Userspace support
=================
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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.

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After an application opens /dev/rfkill, it can read the current state of all
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devices. Changes can be obtained by either polling the descriptor for
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hotplug or state change events or by listening for uevents emitted by the
rfkill core framework.

Additionally, each rfkill device is registered in sysfs and emits uevents.

rfkill devices issue uevents (with an action of "change"), with the following
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environment variables set::
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	RFKILL_NAME
	RFKILL_STATE
	RFKILL_TYPE
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The content of these variables corresponds to the "name", "state" and
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"type" sysfs files explained above.
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For further details consult Documentation/ABI/stable/sysfs-class-rfkill.