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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 overriden by software | |
25 | - soft block: writable radio block (need not be readable) that is set by | |
26 | the system software. | |
27 | ||
28 | ||
29 | 2. Implementation details | |
30 | ||
31 | The 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 | ||
37 | The rfkill core provides API for kernel drivers to register their radio | |
38 | transmitter with the kernel, methods for turning it on and off and, letting | |
39 | the system know about hardware-disabled states that may be implemented on | |
40 | the device. | |
41 | ||
42 | The rfkill core code also notifies userspace of state changes, and provides | |
43 | ways for userspace to query the current states. See the "Userspace support" | |
44 | section below. | |
45 | ||
46 | When the device is hard-blocked (either by a call to rfkill_set_hw_state() | |
47 | or from query_hw_block) set_block() will be invoked for additional software | |
48 | block, but drivers can ignore the method call since they can use the return | |
49 | value of the function rfkill_set_hw_state() to sync the software state | |
50 | instead of keeping track of calls to set_block(). In fact, drivers should | |
51 | use the return value of rfkill_set_hw_state() unless the hardware actually | |
52 | keeps track of soft and hard block separately. | |
53 | ||
54 | ||
55 | 3. Kernel API | |
56 | ||
57 | ||
58 | Drivers for radio transmitters normally implement an rfkill driver. | |
59 | ||
60 | Platform drivers might implement input devices if the rfkill button is just | |
61 | that, a button. If that button influences the hardware then you need to | |
62 | implement an rfkill driver instead. This also applies if the platform provides | |
63 | a way to turn on/off the transmitter(s). | |
64 | ||
65 | For some platforms, it is possible that the hardware state changes during | |
66 | suspend/hibernation, in which case it will be necessary to update the rfkill | |
67 | core with the current state is at resume time. | |
68 | ||
69 | To create an rfkill driver, driver's Kconfig needs to have | |
70 | ||
71 | depends on RFKILL || !RFKILL | |
72 | ||
73 | to ensure the driver cannot be built-in when rfkill is modular. The !RFKILL | |
74 | case allows the driver to be built when rfkill is not configured, which which | |
75 | case all rfkill API can still be used but will be provided by static inlines | |
76 | which compile to almost nothing. | |
77 | ||
78 | Calling rfkill_set_hw_state() when a state change happens is required from | |
79 | rfkill drivers that control devices that can be hard-blocked unless they also | |
80 | assign the poll_hw_block() callback (then the rfkill core will poll the | |
81 | device). Don't do this unless you cannot get the event in any other way. | |
82 | ||
83 | ||
84 | ||
85 | 5. Userspace support | |
86 | ||
87 | The recommended userspace interface to use is /dev/rfkill, which is a misc | |
88 | character device that allows userspace to obtain and set the state of rfkill | |
89 | devices and sets of devices. It also notifies userspace about device addition | |
90 | and removal. The API is a simple read/write API that is defined in | |
91 | linux/rfkill.h, with one ioctl that allows turning off the deprecated input | |
92 | handler in the kernel for the transition period. | |
93 | ||
94 | Except for the one ioctl, communication with the kernel is done via read() | |
95 | and write() of instances of 'struct rfkill_event'. In this structure, the | |
96 | soft and hard block are properly separated (unlike sysfs, see below) and | |
97 | userspace is able to get a consistent snapshot of all rfkill devices in the | |
98 | system. Also, it is possible to switch all rfkill drivers (or all drivers of | |
99 | a specified type) into a state which also updates the default state for | |
100 | hotplugged devices. | |
101 | ||
102 | After an application opens /dev/rfkill, it can read the current state of all | |
103 | devices. Changes can be either obtained by either polling the descriptor for | |
104 | hotplug or state change events or by listening for uevents emitted by the | |
105 | rfkill core framework. | |
106 | ||
107 | Additionally, each rfkill device is registered in sysfs and emits uevents. | |
108 | ||
109 | rfkill devices issue uevents (with an action of "change"), with the following | |
110 | environment variables set: | |
111 | ||
112 | RFKILL_NAME | |
113 | RFKILL_STATE | |
114 | RFKILL_TYPE | |
115 | ||
116 | The contents of these variables corresponds to the "name", "state" and | |
117 | "type" sysfs files explained above. | |
118 | ||
119 | ||
120 | For further details consult Documentation/ABI/stable/dev-rfkill and | |
121 | Documentation/ABI/stable/sysfs-class-rfkill. |