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Commit ce0879e3 authored by Johannes Berg's avatar Johannes Berg Committed by John W. Linville
Browse files

rfkill: improve docs 



Now that the dust has settled a bit, improve the docs on rfkill
and include more information about /dev/rfkill.

Signed-off-by: default avatarJohannes Berg <johannes@sipsolutions.net>
Signed-off-by: default avatarJohn W. Linville <linville@tuxdriver.com>
parent 7e9debe9

Documentation/rfkill.txt

+69 −68
Original line number Original line Diff line number Diff line
@@ -3,9 +3,8 @@ rfkill - RF kill switch support 




1. Introduction

1. Introduction

2. Implementation details

2. Implementation details

3. Kernel driver guidelines

3. Kernel API

4. Kernel API

4. Userspace support

5. Userspace support









1. Introduction

1. Introduction
@@ -19,82 +18,62 @@ disable all transmitters of a certain type (or all). This is intended for 
situations where transmitters need to be turned off, for example on

situations where transmitters need to be turned off, for example on

aircraft.

aircraft.





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:

 - hard block: read-only radio block that cannot be overriden by software

 - soft block: writable radio block (need not be readable) that is set by

               the system software.









2. Implementation details

2. Implementation details





The rfkill subsystem is composed of various components: the rfkill class, the

The rfkill subsystem is composed of three main components:

rfkill-input module (an input layer handler), and some specific input layer

 * the rfkill core,

events.

 * the deprecated rfkill-input module (an input layer handler, being



   replaced by userspace policy code) and

The rfkill class is provided for kernel drivers to register their radio

 * the rfkill drivers.

transmitter with the kernel, provide methods for turning it on and off and,

optionally, letting the system know about hardware-disabled states that may

be implemented on the device. This code is enabled with the CONFIG_RFKILL

Kconfig option, which drivers can "select".



The rfkill class code also notifies userspace of state changes, this is

achieved via uevents. It also provides some sysfs files for userspace to

check the status of radio transmitters. See the "Userspace support" section

below.





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.





The rfkill-input code implements a basic response to rfkill buttons -- it

The rfkill core code also notifies userspace of state changes, and provides

implements turning on/off all devices of a certain class (or all).

ways 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()

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 but drivers can well

or from query_hw_block) set_block() will be invoked for additional software

ignore the method call since they can use the return value of the function

block, but drivers can ignore the method call since they can use the return

rfkill_set_hw_state() to sync the software state instead of keeping track

value of the function rfkill_set_hw_state() to sync the software state

of calls to set_block().

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.

The entire functionality is spread over more than one subsystem:



 * The kernel input layer generates KEY_WWAN, KEY_WLAN etc. and

   SW_RFKILL_ALL -- when the user presses a button. Drivers for radio

   transmitters generally do not register to the input layer, unless the

   device really provides an input device (i.e. a button that has no

   effect other than generating a button press event)



 * The rfkill-input code hooks up to these events and switches the soft-block

   of the various radio transmitters, depending on the button type.



 * The rfkill drivers turn off/on their transmitters as requested.



 * The rfkill class will generate userspace notifications (uevents) to tell

   userspace what the current state is.









3. Kernel API





3. Kernel driver guidelines







Drivers for radio transmitters normally implement an rfkill driver.

Drivers for radio transmitters normally implement only the rfkill class.

These drivers may not unblock the transmitter based on own decisions, they

should act on information provided by the rfkill class only.





Platform drivers might implement input devices if the rfkill button is just

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

that, a button. If that button influences the hardware then you need to

implement an rfkill class instead. This also applies if the platform provides

implement an rfkill driver instead. This also applies if the platform provides

a way to turn on/off the transmitter(s).

a way to turn on/off the transmitter(s).





During suspend/hibernation, transmitters should only be left enabled when

For some platforms, it is possible that the hardware state changes during

wake-on wlan or similar functionality requires it and the device wasn't

suspend/hibernation, in which case it will be necessary to update the rfkill

blocked before suspend/hibernate. Note that it may be necessary to update

core with the current state is at resume time.

the rfkill subsystem's idea of what the current state is at resume time if

the state may have changed over suspend.







To create an rfkill driver, driver's Kconfig needs to have





4. Kernel API

	depends on RFKILL || !RFKILL





To build a driver with rfkill subsystem support, the driver should depend on

to ensure the driver cannot be built-in when rfkill is modular. The !RFKILL

(or select) the Kconfig symbol RFKILL.

case allows the driver to be built when rfkill is not configured, which which



case all rfkill API can still be used but will be provided by static inlines

The hardware the driver talks to may be write-only (where the current state

which compile to almost nothing.

of the hardware is unknown), or read-write (where the hardware can be queried

about its current state).





Calling rfkill_set_hw_state() when a state change happens is required from

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

rfkill drivers that control devices that can be hard-blocked unless they also
@@ -105,10 +84,33 @@ device). Don't do this unless you cannot get the event in any other way. 




5. Userspace support

5. Userspace support





The following sysfs entries exist for every rfkill device:

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.



After an application opens /dev/rfkill, it can read the current state of

all devices, and afterwards can poll the descriptor for hotplug or state

change events.



Applications must ignore operations (the "op" field) they do not handle,

this allows the API to be extended in the future.



Additionally, each rfkill device is registered in sysfs and there has the

following attributes:





	name: Name assigned by driver to this key (interface or driver name).

	name: Name assigned by driver to this key (interface or driver name).

	type: Name of the key type ("wlan", "bluetooth", etc).

	type: Driver type string ("wlan", "bluetooth", etc).

	state: Current state of the transmitter

	state: Current state of the transmitter

		0: RFKILL_STATE_SOFT_BLOCKED

		0: RFKILL_STATE_SOFT_BLOCKED

			transmitter is turned off by software

			transmitter is turned off by software
@@ -117,7 +119,12 @@ The following sysfs entries exist for every rfkill device: 
		2: RFKILL_STATE_HARD_BLOCKED

		2: RFKILL_STATE_HARD_BLOCKED

			transmitter is forced off by something outside of

			transmitter is forced off by something outside of

			the driver's control.

			the driver's control.

	claim: 0: Kernel handles events (currently always reads that value)

	       This file is deprecated because it can only properly show

	       three of the four possible states, soft-and-hard-blocked is

	       missing.

	claim: 0: Kernel handles events

	       This file is deprecated because there no longer is a way to

	       claim just control over a single rfkill instance.





rfkill devices also issue uevents (with an action of "change"), with the

rfkill devices also issue uevents (with an action of "change"), with the

following environment variables set:

following environment variables set:
@@ -128,9 +135,3 @@ RFKILL_TYPE 




The contents of these variables corresponds to the "name", "state" and

The contents of these variables corresponds to the "name", "state" and

"type" sysfs files explained above.
 
"type" sysfs files explained above.
 


An alternative userspace interface exists as a misc device /dev/rfkill,

which 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.