Documentation/rfkill.txt

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