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/sysfs-class-rfkill. 121