Merge tag 'sound-4.2-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/tiwai...
[linux-2.6-block.git] / Documentation / rfkill.txt
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1rfkill - RF kill switch support
2===============================
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41. Introduction
52. Implementation details
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63. Kernel API
74. Userspace support
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19d337df 101. Introduction
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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.
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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.
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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:
c98be0c9 24 - hard block: read-only radio block that cannot be overridden by software
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25 - soft block: writable radio block (need not be readable) that is set by
26 the system software.
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28The rfkill subsystem has two parameters, rfkill.default_state and
29rfkill.master_switch_mode, which are documented in kernel-parameters.txt.
30
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19d337df 322. Implementation details
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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.
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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.
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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.
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19d337df 49When the device is hard-blocked (either by a call to rfkill_set_hw_state()
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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.
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ce0879e3 583. Kernel API
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ce0879e3 61Drivers for radio transmitters normally implement an rfkill driver.
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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
ce0879e3 65implement an rfkill driver instead. This also applies if the platform provides
19d337df 66a way to turn on/off the transmitter(s).
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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.
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ce0879e3 72To create an rfkill driver, driver's Kconfig needs to have
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ce0879e3 74 depends on RFKILL || !RFKILL
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ce0879e3 76to ensure the driver cannot be built-in when rfkill is modular. The !RFKILL
c79a8d85 77case allows the driver to be built when rfkill is not configured, which
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78case all rfkill API can still be used but will be provided by static inlines
79which compile to almost nothing.
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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.
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19d337df 885. Userspace support
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90The recommended userspace interface to use is /dev/rfkill, which is a misc
91character device that allows userspace to obtain and set the state of rfkill
92devices and sets of devices. It also notifies userspace about device addition
93and removal. The API is a simple read/write API that is defined in
94linux/rfkill.h, with one ioctl that allows turning off the deprecated input
95handler in the kernel for the transition period.
96
97Except for the one ioctl, communication with the kernel is done via read()
98and write() of instances of 'struct rfkill_event'. In this structure, the
99soft and hard block are properly separated (unlike sysfs, see below) and
100userspace is able to get a consistent snapshot of all rfkill devices in the
101system. Also, it is possible to switch all rfkill drivers (or all drivers of
102a specified type) into a state which also updates the default state for
103hotplugged devices.
104
69c86373 105After an application opens /dev/rfkill, it can read the current state of all
106devices. Changes can be either obtained by either polling the descriptor for
107hotplug or state change events or by listening for uevents emitted by the
108rfkill core framework.
109
110Additionally, each rfkill device is registered in sysfs and emits uevents.
111
112rfkill devices issue uevents (with an action of "change"), with the following
113environment variables set:
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114
115RFKILL_NAME
116RFKILL_STATE
117RFKILL_TYPE
118
119The contents of these variables corresponds to the "name", "state" and
120"type" sysfs files explained above.
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122
395cf969 123For further details consult Documentation/ABI/stable/sysfs-class-rfkill.