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1	rfkill - RF kill switch support
2	===============================
3
4	1. Introduction
5	2. Implementation details
6	3. Kernel API
7	4. Userspace support
8
9
10	1. Introduction
11
12	The rfkill subsystem provides a generic interface to disabling any radio
13	transmitter in the system. When a transmitter is blocked, it shall not
14	radiate any power.
15
16	The subsystem also provides the ability to react on button presses and
17	disable all transmitters of a certain type (or all). This is intended for
18	situations where transmitters need to be turned off, for example on
19	aircraft.
20
21	The rfkill subsystem has a concept of "hard" and "soft" block, which
22	differ little in their meaning (block == transmitters off) but rather in
23	whether 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
28	The rfkill subsystem has two parameters, rfkill.default_state and
29	rfkill.master_switch_mode, which are documented in kernel-parameters.txt.
30
31
32	2. Implementation details
33
34	The 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
40	The rfkill core provides API for kernel drivers to register their radio
41	transmitter with the kernel, methods for turning it on and off and, letting
42	the system know about hardware-disabled states that may be implemented on
43	the device.
44
45	The rfkill core code also notifies userspace of state changes, and provides
46	ways for userspace to query the current states. See the "Userspace support"
47	section below.
48
49	When the device is hard-blocked (either by a call to rfkill_set_hw_state()
50	or from query_hw_block) set_block() will be invoked for additional software
51	block, but drivers can ignore the method call since they can use the return
52	value of the function rfkill_set_hw_state() to sync the software state
53	instead of keeping track of calls to set_block(). In fact, drivers should
54	use the return value of rfkill_set_hw_state() unless the hardware actually
55	keeps track of soft and hard block separately.
56
57
58	3. Kernel API
59
60
61	Drivers for radio transmitters normally implement an rfkill driver.
62
63	Platform drivers might implement input devices if the rfkill button is just
64	that, a button. If that button influences the hardware then you need to
65	implement an rfkill driver instead. This also applies if the platform provides
66	a way to turn on/off the transmitter(s).
67
68	For some platforms, it is possible that the hardware state changes during
69	suspend/hibernation, in which case it will be necessary to update the rfkill
70	core with the current state is at resume time.
71
72	To create an rfkill driver, driver's Kconfig needs to have
73
74	depends on RFKILL || !RFKILL
75
76	to ensure the driver cannot be built-in when rfkill is modular. The !RFKILL
77	case allows the driver to be built when rfkill is not configured, which
78	case all rfkill API can still be used but will be provided by static inlines
79	which compile to almost nothing.
80
81	Calling rfkill_set_hw_state() when a state change happens is required from
82	rfkill drivers that control devices that can be hard-blocked unless they also
83	assign the poll_hw_block() callback (then the rfkill core will poll the
84	device). Don't do this unless you cannot get the event in any other way.
85
86	RFKill provides per-switch LED triggers, which can be used to drive LEDs
87	according to the switch state (LED_FULL when blocked, LED_OFF otherwise).
88
89
90	5. Userspace support
91
92	The recommended userspace interface to use is /dev/rfkill, which is a misc
93	character device that allows userspace to obtain and set the state of rfkill
94	devices and sets of devices. It also notifies userspace about device addition
95	and removal. The API is a simple read/write API that is defined in
96	linux/rfkill.h, with one ioctl that allows turning off the deprecated input
97	handler in the kernel for the transition period.
98
99	Except for the one ioctl, communication with the kernel is done via read()
100	and write() of instances of 'struct rfkill_event'. In this structure, the
101	soft and hard block are properly separated (unlike sysfs, see below) and
102	userspace is able to get a consistent snapshot of all rfkill devices in the
103	system. Also, it is possible to switch all rfkill drivers (or all drivers of
104	a specified type) into a state which also updates the default state for
105	hotplugged devices.
106
107	After an application opens /dev/rfkill, it can read the current state of all
108	devices. Changes can be either obtained by either polling the descriptor for
109	hotplug or state change events or by listening for uevents emitted by the
110	rfkill core framework.
111
112	Additionally, each rfkill device is registered in sysfs and emits uevents.
113
114	rfkill devices issue uevents (with an action of "change"), with the following
115	environment variables set:
116
117	RFKILL_NAME
118	RFKILL_STATE
119	RFKILL_TYPE
120
121	The contents of these variables corresponds to the "name", "state" and
122	"type" sysfs files explained above.
123
124
125	For further details consult Documentation/ABI/stable/sysfs-class-rfkill.
126