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 for 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 68Drivers for radio transmitters normally implement an rfkill driver. 69 70Platform drivers might implement input devices if the rfkill button is just 71that, a button. If that button influences the hardware then you need to 72implement an rfkill driver instead. This also applies if the platform provides 73a way to turn on/off the transmitter(s). 74 75For some platforms, it is possible that the hardware state changes during 76suspend/hibernation, in which case it will be necessary to update the rfkill 77core with the current state at resume time. 78 79To create an rfkill driver, driver's Kconfig needs to have:: 80 81 depends on RFKILL || !RFKILL 82 83to ensure the driver cannot be built-in when rfkill is modular. The !RFKILL 84case allows the driver to be built when rfkill is not configured, in which 85case all rfkill API can still be used but will be provided by static inlines 86which compile to almost nothing. 87 88Calling rfkill_set_hw_state() when a state change happens is required from 89rfkill drivers that control devices that can be hard-blocked unless they also 90assign the poll_hw_block() callback (then the rfkill core will poll the 91device). Don't do this unless you cannot get the event in any other way. 92 93rfkill provides per-switch LED triggers, which can be used to drive LEDs 94according to the switch state (LED_FULL when blocked, LED_OFF otherwise). 95 96 97Userspace support 98================= 99 100The recommended userspace interface to use is /dev/rfkill, which is a misc 101character device that allows userspace to obtain and set the state of rfkill 102devices and sets of devices. It also notifies userspace about device addition 103and removal. The API is a simple read/write API that is defined in 104linux/rfkill.h, with one ioctl that allows turning off the deprecated input 105handler in the kernel for the transition period. 106 107Except for the one ioctl, communication with the kernel is done via read() 108and write() of instances of 'struct rfkill_event'. In this structure, the 109soft and hard block are properly separated (unlike sysfs, see below) and 110userspace is able to get a consistent snapshot of all rfkill devices in the 111system. Also, it is possible to switch all rfkill drivers (or all drivers of 112a specified type) into a state which also updates the default state for 113hotplugged devices. 114 115After an application opens /dev/rfkill, it can read the current state of all 116devices. Changes can be obtained by either polling the descriptor for 117hotplug or state change events or by listening for uevents emitted by the 118rfkill core framework. 119 120Additionally, each rfkill device is registered in sysfs and emits uevents. 121 122rfkill devices issue uevents (with an action of "change"), with the following 123environment variables set:: 124 125 RFKILL_NAME 126 RFKILL_STATE 127 RFKILL_TYPE 128 129The content of these variables corresponds to the "name", "state" and 130"type" sysfs files explained above. 131 132For further details consult Documentation/ABI/stable/sysfs-class-rfkill. 133