rfkill - RF kill switch support

Introduction

The rfkill subsystem provides a generic interface for disabling any radiotransmitter in the system. When a transmitter is blocked, it shall notradiate any power.

The subsystem also provides the ability to react on button presses anddisable all transmitters of a certain type (or all). This is intended forsituations where transmitters need to be turned off, for example onaircraft.

The rfkill subsystem has a concept of “hard” and “soft” block, whichdiffer little in their meaning (block == transmitters off) but rather inwhether they can be changed or not:

  • hard block
    read-only radio block that cannot be overridden by software
  • soft block
    writable radio block (need not be readable) that is set bythe system software.

The rfkill subsystem has two parameters, rfkill.default_state andrfkill.master_switch_mode, which are documented inadmin-guide/kernel-parameters.rst.

Implementation details

The rfkill subsystem is composed of three main components:

  • the rfkill core,
  • the deprecated rfkill-input module (an input layer handler, beingreplaced by userspace policy code) and
  • the rfkill drivers.

The rfkill core provides API for kernel drivers to register their radiotransmitter with the kernel, methods for turning it on and off, and lettingthe system know about hardware-disabled states that may be implemented onthe device.

The rfkill core code also notifies userspace of state changes, and providesways for userspace to query the current states. See the “Userspace support”section below.

When the device is hard-blocked (either by a call to rfkill_set_hw_state()or from query_hw_block), set_block() will be invoked for additional softwareblock, but drivers can ignore the method call since they can use the returnvalue of the function rfkill_set_hw_state() to sync the software stateinstead of keeping track of calls to set_block(). In fact, drivers shoulduse the return value of rfkill_set_hw_state() unless the hardware actuallykeeps track of soft and hard block separately.

Kernel API

Drivers for radio transmitters normally implement an rfkill driver.

Platform drivers might implement input devices if the rfkill button is justthat, a button. If that button influences the hardware then you need toimplement an rfkill driver instead. This also applies if the platform providesa way to turn on/off the transmitter(s).

For some platforms, it is possible that the hardware state changes duringsuspend/hibernation, in which case it will be necessary to update the rfkillcore with the current state at resume time.

To create an rfkill driver, driver’s Kconfig needs to have:

depends on RFKILL || !RFKILL

to ensure the driver cannot be built-in when rfkill is modular. The !RFKILLcase allows the driver to be built when rfkill is not configured, in whichcase all rfkill API can still be used but will be provided by static inlineswhich compile to almost nothing.

Calling rfkill_set_hw_state() when a state change happens is required fromrfkill drivers that control devices that can be hard-blocked unless they alsoassign the poll_hw_block() callback (then the rfkill core will poll thedevice). Don’t do this unless you cannot get the event in any other way.

rfkill provides per-switch LED triggers, which can be used to drive LEDsaccording to the switch state (LED_FULL when blocked, LED_OFF otherwise).

Userspace support

The recommended userspace interface to use is /dev/rfkill, which is a misccharacter device that allows userspace to obtain and set the state of rfkilldevices and sets of devices. It also notifies userspace about device additionand removal. The API is a simple read/write API that is defined inlinux/rfkill.h, with one ioctl that allows turning off the deprecated inputhandler 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, thesoft and hard block are properly separated (unlike sysfs, see below) anduserspace is able to get a consistent snapshot of all rfkill devices in thesystem. Also, it is possible to switch all rfkill drivers (or all drivers ofa specified type) into a state which also updates the default state forhotplugged devices.

After an application opens /dev/rfkill, it can read the current state of alldevices. Changes can be obtained by either polling the descriptor forhotplug or state change events or by listening for uevents emitted by therfkill core framework.

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

rfkill devices issue uevents (with an action of “change”), with the followingenvironment variables set:

RFKILL_NAMERFKILL_STATERFKILL_TYPE

The content of these variables corresponds to the “name”, “state” and“type” sysfs files explained above.

For further details consult Documentation/ABI/stable/sysfs-class-rfkill.