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  1 rfkill - RF kill switch support
  2 ===============================
  4 1. Introduction
  5 2. Implementation details
  6 3. Kernel API
  7 4. Userspace support
 10 1. Introduction
 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.
 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.
 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.
 28 The rfkill subsystem has two parameters, rfkill.default_state and
 29 rfkill.master_switch_mode, which are documented in admin-guide/kernel-parameters.rst.
 32 2. Implementation details
 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.
 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.
 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.
 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.
 58 3. Kernel API
 61 Drivers for radio transmitters normally implement an rfkill driver.
 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).
 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.
 72 To create an rfkill driver, driver's Kconfig needs to have
 74         depends on RFKILL || !RFKILL
 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.
 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.
 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).
 90 5. Userspace support
 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.
 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.
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.
112 Additionally, each rfkill device is registered in sysfs and emits uevents.
114 rfkill devices issue uevents (with an action of "change"), with the following
115 environment variables set:
121 The contents of these variables corresponds to the "name", "state" and
122 "type" sysfs files explained above.
125 For further details consult Documentation/ABI/stable/sysfs-class-rfkill.

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