1rfkill - RF kill switch support
  2===============================
  3
  41. Introduction
  52. Implementation details
  63. Kernel API
  74. Userspace support
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  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.