Root/Documentation/rfkill.txt

1rfkill - RF kill switch support
2===============================
3
41. Introduction
52. Implementation details
63. Kernel API
74. Userspace support
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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.
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54
553. Kernel API
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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
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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
103all devices, and afterwards can poll the descriptor for hotplug or state
104change events.
105
106Applications must ignore operations (the "op" field) they do not handle,
107this allows the API to be extended in the future.
108
109Additionally, each rfkill device is registered in sysfs and there has the
110following attributes:
111
112    name: Name assigned by driver to this key (interface or driver name).
113    type: Driver type string ("wlan", "bluetooth", etc).
114    persistent: Whether the soft blocked state is initialised from
115                non-volatile storage at startup.
116    state: Current state of the transmitter
117        0: RFKILL_STATE_SOFT_BLOCKED
118            transmitter is turned off by software
119        1: RFKILL_STATE_UNBLOCKED
120            transmitter is (potentially) active
121        2: RFKILL_STATE_HARD_BLOCKED
122            transmitter is forced off by something outside of
123            the driver's control.
124           This file is deprecated because it can only properly show
125           three of the four possible states, soft-and-hard-blocked is
126           missing.
127    claim: 0: Kernel handles events
128           This file is deprecated because there no longer is a way to
129           claim just control over a single rfkill instance.
130
131rfkill devices also issue uevents (with an action of "change"), with the
132following environment variables set:
133
134RFKILL_NAME
135RFKILL_STATE
136RFKILL_TYPE
137
138The contents of these variables corresponds to the "name", "state" and
139"type" sysfs files explained above.
140

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