rfkill.txt 4.99 KB
Newer Older
Johannes Berg's avatar
Johannes Berg committed
1 2
rfkill - RF kill switch support
===============================
3

Johannes Berg's avatar
Johannes Berg committed
4 5
1. Introduction
2. Implementation details
Johannes Berg's avatar
Johannes Berg committed
6 7
3. Kernel API
4. Userspace support
8

9

Johannes Berg's avatar
Johannes Berg committed
10
1. Introduction
11

Johannes Berg's avatar
Johannes Berg committed
12 13 14
The rfkill subsystem provides a generic interface to disabling any radio
transmitter in the system. When a transmitter is blocked, it shall not
radiate any power.
15

Johannes Berg's avatar
Johannes Berg committed
16 17 18 19
The subsystem also provides the ability to react on button presses and
disable all transmitters of a certain type (or all). This is intended for
situations where transmitters need to be turned off, for example on
aircraft.
20

Johannes Berg's avatar
Johannes Berg committed
21 22 23
The rfkill subsystem has a concept of "hard" and "soft" block, which
differ little in their meaning (block == transmitters off) but rather in
whether they can be changed or not:
24
 - hard block: read-only radio block that cannot be overridden by software
Johannes Berg's avatar
Johannes Berg committed
25 26
 - soft block: writable radio block (need not be readable) that is set by
               the system software.
27

28 29 30
The rfkill subsystem has two parameters, rfkill.default_state and
rfkill.master_switch_mode, which are documented in kernel-parameters.txt.

31

Johannes Berg's avatar
Johannes Berg committed
32
2. Implementation details
33

Johannes Berg's avatar
Johannes Berg committed
34 35 36 37 38
The rfkill subsystem is composed of three main components:
 * the rfkill core,
 * the deprecated rfkill-input module (an input layer handler, being
   replaced by userspace policy code) and
 * the rfkill drivers.
39

Johannes Berg's avatar
Johannes Berg committed
40 41 42 43
The rfkill core provides API for kernel drivers to register their radio
transmitter with the kernel, methods for turning it on and off and, letting
the system know about hardware-disabled states that may be implemented on
the device.
44

Johannes Berg's avatar
Johannes Berg committed
45 46 47
The rfkill core code also notifies userspace of state changes, and provides
ways for userspace to query the current states. See the "Userspace support"
section below.
48

Johannes Berg's avatar
Johannes Berg committed
49
When the device is hard-blocked (either by a call to rfkill_set_hw_state()
Johannes Berg's avatar
Johannes Berg committed
50 51 52 53 54 55
or from query_hw_block) set_block() will be invoked for additional software
block, but drivers can ignore the method call since they can use the return
value of the function rfkill_set_hw_state() to sync the software state
instead of keeping track of calls to set_block(). In fact, drivers should
use the return value of rfkill_set_hw_state() unless the hardware actually
keeps track of soft and hard block separately.
56 57


Johannes Berg's avatar
Johannes Berg committed
58
3. Kernel API
59 60


Johannes Berg's avatar
Johannes Berg committed
61
Drivers for radio transmitters normally implement an rfkill driver.
62

Johannes Berg's avatar
Johannes Berg committed
63 64
Platform drivers might implement input devices if the rfkill button is just
that, a button. If that button influences the hardware then you need to
Johannes Berg's avatar
Johannes Berg committed
65
implement an rfkill driver instead. This also applies if the platform provides
Johannes Berg's avatar
Johannes Berg committed
66
a way to turn on/off the transmitter(s).
67

Johannes Berg's avatar
Johannes Berg committed
68 69 70
For some platforms, it is possible that the hardware state changes during
suspend/hibernation, in which case it will be necessary to update the rfkill
core with the current state is at resume time.
71

Johannes Berg's avatar
Johannes Berg committed
72
To create an rfkill driver, driver's Kconfig needs to have
73

Johannes Berg's avatar
Johannes Berg committed
74
	depends on RFKILL || !RFKILL
75

Johannes Berg's avatar
Johannes Berg committed
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
Johannes Berg's avatar
Johannes Berg committed
78 79
case all rfkill API can still be used but will be provided by static inlines
which compile to almost nothing.
80

Johannes Berg's avatar
Johannes Berg committed
81 82 83 84
Calling rfkill_set_hw_state() when a state change happens is required from
rfkill drivers that control devices that can be hard-blocked unless they also
assign the poll_hw_block() callback (then the rfkill core will poll the
device). Don't do this unless you cannot get the event in any other way.
85

86 87
RFKill provides per-switch LED triggers, which can be used to drive LEDs
according to the switch state (LED_FULL when blocked, LED_OFF otherwise).
88 89


Johannes Berg's avatar
Johannes Berg committed
90
5. Userspace support
91

Johannes Berg's avatar
Johannes Berg committed
92 93 94 95 96 97 98 99 100 101 102 103 104 105 106
The recommended userspace interface to use is /dev/rfkill, which is a misc
character device that allows userspace to obtain and set the state of rfkill
devices and sets of devices. It also notifies userspace about device addition
and removal. The API is a simple read/write API that is defined in
linux/rfkill.h, with one ioctl that allows turning off the deprecated input
handler in the kernel for the transition period.

Except for the one ioctl, communication with the kernel is done via read()
and write() of instances of 'struct rfkill_event'. In this structure, the
soft and hard block are properly separated (unlike sysfs, see below) and
userspace is able to get a consistent snapshot of all rfkill devices in the
system. Also, it is possible to switch all rfkill drivers (or all drivers of
a specified type) into a state which also updates the default state for
hotplugged devices.

107 108 109 110 111 112 113 114 115
After an application opens /dev/rfkill, it can read the current state of all
devices. Changes can be either obtained by either polling the descriptor for
hotplug or state change events or by listening for uevents emitted by the
rfkill core framework.

Additionally, each rfkill device is registered in sysfs and emits uevents.

rfkill devices issue uevents (with an action of "change"), with the following
environment variables set:
Johannes Berg's avatar
Johannes Berg committed
116 117 118 119 120 121 122

RFKILL_NAME
RFKILL_STATE
RFKILL_TYPE

The contents of these variables corresponds to the "name", "state" and
"type" sysfs files explained above.
123 124


Paul Bolle's avatar
Paul Bolle committed
125
For further details consult Documentation/ABI/stable/sysfs-class-rfkill.