linux/Documentation/rfkill.txt
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   1===============================
   2rfkill - RF kill switch support
   3===============================
   4
   5
   6.. contents::
   7   :depth: 2
   8
   9Introduction
  10============
  11
  12The rfkill subsystem provides a generic interface to disabling any radio
  13transmitter in the system. When a transmitter is blocked, it shall not
  14radiate any power.
  15
  16The subsystem also provides the ability to react on button presses and
  17disable all transmitters of a certain type (or all). This is intended for
  18situations where transmitters need to be turned off, for example on
  19aircraft.
  20
  21The rfkill subsystem has a concept of "hard" and "soft" block, which
  22differ little in their meaning (block == transmitters off) but rather in
  23whether they can be changed or not:
  24
  25 - hard block
  26        read-only radio block that cannot be overridden by software
  27
  28 - soft block
  29        writable radio block (need not be readable) that is set by
  30        the system software.
  31
  32The rfkill subsystem has two parameters, rfkill.default_state and
  33rfkill.master_switch_mode, which are documented in
  34admin-guide/kernel-parameters.rst.
  35
  36
  37Implementation details
  38======================
  39
  40The rfkill subsystem is composed of three main components:
  41
  42 * the rfkill core,
  43 * the deprecated rfkill-input module (an input layer handler, being
  44   replaced by userspace policy code) and
  45 * the rfkill drivers.
  46
  47The rfkill core provides API for kernel drivers to register their radio
  48transmitter with the kernel, methods for turning it on and off and, letting
  49the system know about hardware-disabled states that may be implemented on
  50the device.
  51
  52The rfkill core code also notifies userspace of state changes, and provides
  53ways for userspace to query the current states. See the "Userspace support"
  54section below.
  55
  56When the device is hard-blocked (either by a call to rfkill_set_hw_state()
  57or from query_hw_block) set_block() will be invoked for additional software
  58block, but drivers can ignore the method call since they can use the return
  59value of the function rfkill_set_hw_state() to sync the software state
  60instead of keeping track of calls to set_block(). In fact, drivers should
  61use the return value of rfkill_set_hw_state() unless the hardware actually
  62keeps track of soft and hard block separately.
  63
  64
  65Kernel API
  66==========
  67
  68
  69Drivers for radio transmitters normally implement an rfkill driver.
  70
  71Platform drivers might implement input devices if the rfkill button is just
  72that, a button. If that button influences the hardware then you need to
  73implement an rfkill driver instead. This also applies if the platform provides
  74a way to turn on/off the transmitter(s).
  75
  76For some platforms, it is possible that the hardware state changes during
  77suspend/hibernation, in which case it will be necessary to update the rfkill
  78core with the current state is at resume time.
  79
  80To create an rfkill driver, driver's Kconfig needs to have::
  81
  82        depends on RFKILL || !RFKILL
  83
  84to ensure the driver cannot be built-in when rfkill is modular. The !RFKILL
  85case allows the driver to be built when rfkill is not configured, which
  86case all rfkill API can still be used but will be provided by static inlines
  87which compile to almost nothing.
  88
  89Calling rfkill_set_hw_state() when a state change happens is required from
  90rfkill drivers that control devices that can be hard-blocked unless they also
  91assign the poll_hw_block() callback (then the rfkill core will poll the
  92device). Don't do this unless you cannot get the event in any other way.
  93
  94RFKill provides per-switch LED triggers, which can be used to drive LEDs
  95according to the switch state (LED_FULL when blocked, LED_OFF otherwise).
  96
  97
  98Userspace support
  99=================
 100
 101The recommended userspace interface to use is /dev/rfkill, which is a misc
 102character device that allows userspace to obtain and set the state of rfkill
 103devices and sets of devices. It also notifies userspace about device addition
 104and removal. The API is a simple read/write API that is defined in
 105linux/rfkill.h, with one ioctl that allows turning off the deprecated input
 106handler in the kernel for the transition period.
 107
 108Except for the one ioctl, communication with the kernel is done via read()
 109and write() of instances of 'struct rfkill_event'. In this structure, the
 110soft and hard block are properly separated (unlike sysfs, see below) and
 111userspace is able to get a consistent snapshot of all rfkill devices in the
 112system. Also, it is possible to switch all rfkill drivers (or all drivers of
 113a specified type) into a state which also updates the default state for
 114hotplugged devices.
 115
 116After an application opens /dev/rfkill, it can read the current state of all
 117devices. Changes can be either obtained by either polling the descriptor for
 118hotplug or state change events or by listening for uevents emitted by the
 119rfkill core framework.
 120
 121Additionally, each rfkill device is registered in sysfs and emits uevents.
 122
 123rfkill devices issue uevents (with an action of "change"), with the following
 124environment variables set::
 125
 126        RFKILL_NAME
 127        RFKILL_STATE
 128        RFKILL_TYPE
 129
 130The contents of these variables corresponds to the "name", "state" and
 131"type" sysfs files explained above.
 132
 133
 134For further details consult Documentation/ABI/stable/sysfs-class-rfkill.
 135