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