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.