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