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 overridden by software 25 - soft block: writable radio block (need not be readable) that is set by 26 the system software. 27 28The rfkill subsystem has two parameters, rfkill.default_state and 29rfkill.master_switch_mode, which are documented in kernel-parameters.txt. 30 31 322. Implementation details 33 34The 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 40The rfkill core provides API for kernel drivers to register their radio 41transmitter with the kernel, methods for turning it on and off and, letting 42the system know about hardware-disabled states that may be implemented on 43the device. 44 45The rfkill core code also notifies userspace of state changes, and provides 46ways for userspace to query the current states. See the "Userspace support" 47section below. 48 49When the device is hard-blocked (either by a call to rfkill_set_hw_state() 50or from query_hw_block) set_block() will be invoked for additional software 51block, but drivers can ignore the method call since they can use the return 52value of the function rfkill_set_hw_state() to sync the software state 53instead of keeping track of calls to set_block(). In fact, drivers should 54use the return value of rfkill_set_hw_state() unless the hardware actually 55keeps track of soft and hard block separately. 56 57 583. Kernel API 59 60 61Drivers for radio transmitters normally implement an rfkill driver. 62 63Platform drivers might implement input devices if the rfkill button is just 64that, a button. If that button influences the hardware then you need to 65implement an rfkill driver instead. This also applies if the platform provides 66a way to turn on/off the transmitter(s). 67 68For some platforms, it is possible that the hardware state changes during 69suspend/hibernation, in which case it will be necessary to update the rfkill 70core with the current state is at resume time. 71 72To create an rfkill driver, driver's Kconfig needs to have 73 74 depends on RFKILL || !RFKILL 75 76to ensure the driver cannot be built-in when rfkill is modular. The !RFKILL 77case allows the driver to be built when rfkill is not configured, which 78case all rfkill API can still be used but will be provided by static inlines 79which compile to almost nothing. 80 81Calling rfkill_set_hw_state() when a state change happens is required from 82rfkill drivers that control devices that can be hard-blocked unless they also 83assign the poll_hw_block() callback (then the rfkill core will poll the 84device). Don't do this unless you cannot get the event in any other way. 85 86RFKill provides per-switch LED triggers, which can be used to drive LEDs 87according to the switch state (LED_FULL when blocked, LED_OFF otherwise). 88 89 905. Userspace support 91 92The recommended userspace interface to use is /dev/rfkill, which is a misc 93character device that allows userspace to obtain and set the state of rfkill 94devices and sets of devices. It also notifies userspace about device addition 95and removal. The API is a simple read/write API that is defined in 96linux/rfkill.h, with one ioctl that allows turning off the deprecated input 97handler in the kernel for the transition period. 98 99Except for the one ioctl, communication with the kernel is done via read() 100and write() of instances of 'struct rfkill_event'. In this structure, the 101soft and hard block are properly separated (unlike sysfs, see below) and 102userspace is able to get a consistent snapshot of all rfkill devices in the 103system. Also, it is possible to switch all rfkill drivers (or all drivers of 104a specified type) into a state which also updates the default state for 105hotplugged devices. 106 107After an application opens /dev/rfkill, it can read the current state of all 108devices. Changes can be either obtained by either polling the descriptor for 109hotplug or state change events or by listening for uevents emitted by the 110rfkill core framework. 111 112Additionally, each rfkill device is registered in sysfs and emits uevents. 113 114rfkill devices issue uevents (with an action of "change"), with the following 115environment variables set: 116 117RFKILL_NAME 118RFKILL_STATE 119RFKILL_TYPE 120 121The contents of these variables corresponds to the "name", "state" and 122"type" sysfs files explained above. 123 124 125For further details consult Documentation/ABI/stable/sysfs-class-rfkill. 126