rfkill: document rw rfkill switches and clarify input subsystem interactions

[safe/jmp/linux-2.6] / Documentation / rfkill.txt

rfkill - RF switch subsystem support
====================================

1 Introduction
2 Implementation details
3 Kernel driver guidelines
4 Kernel API
5 Userspace support


1. Introduction:

The rfkill switch subsystem exists to add a generic interface to circuitry that
can enable or disable the signal output of a wireless *transmitter* of any
type.  By far, the most common use is to disable radio-frequency transmitters.

The rfkill switch subsystem offers support for keys and switches often found on
laptops to enable wireless devices like WiFi and Bluetooth to actually perform
an action.

The buttons to enable and disable the wireless transmitters are important in
situations where the user is for example using his laptop on a location where
radio-frequency transmitters _must_ be disabled (e.g. airplanes).

Because of this requirement, userspace support for the keys should not be made
mandatory.  Because userspace might want to perform some additional smarter
tasks when the key is pressed, rfkill provides userspace the possibility to
take over the task to handle the key events.

===============================================================================
2: Implementation details

The rfkill class provides kernel drivers with an interface that allows them to
know when they should enable or disable a wireless network device transmitter.

The rfkill-input module provides the kernel with the ability to implement a
basic response when the user presses a key or button (or toggles a switch)
related to rfkill functionality.  It is an in-kernel implementation of default
policy of reacting to rfkill-related input events and neither mandatory nor
required for wireless drivers to operate.

The rfkill-input module also provides EPO (emergency power-off) functionality
for all wireless transmitters.  This function cannot be overriden, and it is
always active.  rfkill EPO is related to *_RFKILL_ALL input events.

All state changes on rfkill devices are propagated by the rfkill class to a
notification chain and also to userspace through uevents.

The system inside the kernel has been split into 2 separate sections:
        1 - RFKILL
        2 - RFKILL_INPUT

The first option enables rfkill support and will make sure userspace will be
notified of any events through uevents.  It provides a notification chain for
interested parties in the kernel to also get notified of rfkill state changes
in other drivers.  It creates several sysfs entries which can be used by
userspace.  See section "Userspace support".

The second option provides an rfkill input handler. This handler will listen to
all rfkill key events and will toggle the radio accordingly.  With this option
enabled userspace could either do nothing or simply perform monitoring tasks.

When a rfkill switch is in the RFKILL_STATE_ON, the wireless transmitter (radio
TX circuit for example) is *enabled*.  When the rfkill switch is in the
RFKILL_STATE_OFF, the wireless transmitter is to be *blocked* from operating.

Full rfkill functionality requires two different subsystems to cooperate: the
input layer and the rfkill class.  The input layer issues *commands* to the
entire system requesting that devices registered to the rfkill class change
state.  The way this interaction happens is not complex, but it is not obvious
either:

Kernel Input layer:

        * Generates KEY_WWAN, KEY_WLAN, KEY_BLUETOOTH, SW_RFKILL_ALL, and
          other such events when the user presses certain keys, buttons, or
          toggles certain physical switches.

        THE INPUT LAYER IS NEVER USED TO PROPAGATE STATUS, NOTIFICATIONS OR THE
        KIND OF STUFF AN ON-SCREEN-DISPLAY APPLICATION WOULD REPORT.  It is
        used to issue *commands* for the system to change behaviour, and these
        commands may or may not be carried out by some kernel driver or
        userspace application.  It follows that doing user feedback based only
        on input events is broken, there is no guarantee that an input event
        will be acted upon.

        Most wireless communication device drivers implementing rfkill
        functionality MUST NOT generate these events, and have no reason to
        register themselves with the input layer.  This is a common
        misconception.  There is an API to propagate rfkill status change
        information, and it is NOT the input layer.

rfkill class:

        * Calls a hook in a driver to effectively change the wireless
          transmitter state;
        * Keeps track of the wireless transmitter state (with help from
          the driver);
        * Generates userspace notifications (uevents) and a call to a
          notification chain (kernel) when there is a wireless transmitter
          state change;
        * Connects a wireless communications driver with the common rfkill
          control system, which, for example, allows actions such as
          "switch all bluetooth devices offline" to be carried out by
          userspace or by rfkill-input.

        THE RFKILL CLASS NEVER ISSUES INPUT EVENTS.  THE RFKILL CLASS DOES
        NOT LISTEN TO INPUT EVENTS.  NO DRIVER USING THE RFKILL CLASS SHALL
        EVER LISTEN TO, OR ACT ON RFKILL INPUT EVENTS.

        Most wireless data communication drivers in the kernel have just to
        implement the rfkill class API to work properly.  Interfacing to the
        input layer is not often required (and is very often a *bug*).

Userspace input handlers (uevents) or kernel input handlers (rfkill-input):

        * Implements the policy of what should happen when one of the input
          layer events related to rfkill operation is received.
        * Uses the sysfs interface (userspace) or private rfkill API calls
          to tell the devices registered with the rfkill class to change
          their state (i.e. translates the input layer event into real
          action).
        * rfkill-input implements EPO by handling EV_SW SW_RFKILL_ALL 0
          (power off all transmitters) in a special way: it ignores any
          overrides and local state cache and forces all transmitters to
          the OFF state (including those which are already supposed to be
          OFF).  Note that the opposite event (power on all transmitters)
          is handled normally.

Userspace uevent handler or kernel platform-specific drivers hooked to the
rfkill notifier chain:

        * Taps into the rfkill notifier chain or to KOBJ_CHANGE uevents,
          in order to know when a device that is registered with the rfkill
          class changes state;
        * Issues feedback notifications to the user;
        * In the rare platforms where this is required, synthesizes an input
          event to command all *OTHER* rfkill devices to also change their
          statues when a specific rfkill device changes state.


===============================================================================
3: Kernel driver guidelines

The first thing one needs to know is whether his driver should be talking to
the rfkill class or to the input layer.

Do not mistake input devices for rfkill devices.  The only type of "rfkill
switch" device that is to be registered with the rfkill class are those
directly controlling the circuits that cause a wireless transmitter to stop
working (or the software equivalent of them).  Every other kind of "rfkill
switch" is just an input device and MUST NOT be registered with the rfkill
class.

A driver should register a device with the rfkill class when ALL of the
following conditions are met:

1. The device is/controls a data communications wireless transmitter;

2. The kernel can interact with the hardware/firmware to CHANGE the wireless
   transmitter state (block/unblock TX operation);

A driver should register a device with the input subsystem to issue
rfkill-related events (KEY_WLAN, KEY_BLUETOOTH, KEY_WWAN, KEY_WIMAX,
SW_RFKILL_ALL, etc) when ALL of the folowing conditions are met:

1. It is directly related to some physical device the user interacts with, to
   command the O.S./firmware/hardware to enable/disable a data communications
   wireless transmitter.

   Examples of the physical device are: buttons, keys and switches the user
   will press/touch/slide/switch to enable or disable the wireless
   communication device.

2. It is NOT slaved to another device, i.e. there is no other device that
   issues rfkill-related input events in preference to this one.

   Typically, the ACPI "radio kill" switch of a laptop is the master input
   device to issue rfkill events, and, e.g., the WLAN card is just a slave
   device that gets disabled by its hardware radio-kill input pin.

When in doubt, do not issue input events.  For drivers that should generate
input events in some platforms, but not in others (e.g. b43), the best solution
is to NEVER generate input events in the first place.  That work should be
deferred to a platform-specific kernel module (which will know when to generate
events through the rfkill notifier chain) or to userspace.  This avoids the
usual maintenance problems with DMI whitelisting.


Corner cases and examples:
====================================

1. If the device is an input device that, because of hardware or firmware,
causes wireless transmitters to be blocked regardless of the kernel's will, it
is still just an input device, and NOT to be registered with the rfkill class.

2. If the wireless transmitter switch control is read-only, it is an input
device and not to be registered with the rfkill class (and maybe not to be made
an input layer event source either, see below).

3. If there is some other device driver *closer* to the actual hardware the
user interacted with (the button/switch/key) to issue an input event, THAT is
the device driver that should be issuing input events.

E.g:
  [RFKILL slider switch] -- [GPIO hardware] -- [WLAN card rf-kill input]
                           (platform driver)    (wireless card driver)

The user is closer to the RFKILL slide switch plaform driver, so the driver
which must issue input events is the platform driver looking at the GPIO
hardware, and NEVER the wireless card driver (which is just a slave).  It is
very likely that there are other leaves than just the WLAN card rf-kill input
(e.g. a bluetooth card, etc)...

On the other hand, some embedded devices do this:

  [RFKILL slider switch] -- [WLAN card rf-kill input]
                             (wireless card driver)

In this situation, the wireless card driver *could* register itself as an input
device and issue rf-kill related input events... but in order to AVOID the need
for DMI whitelisting, the wireless card driver does NOT do it.  Userspace (HAL)
or a platform driver (that exists only on these embedded devices) will do the
dirty job of issuing the input events.


COMMON MISTAKES in kernel drivers, related to rfkill:
====================================

1. NEVER confuse input device keys and buttons with input device switches.

  1a. Switches are always set or reset.  They report the current state
      (on position or off position).

  1b. Keys and buttons are either in the pressed or not-pressed state, and
      that's it.  A "button" that latches down when you press it, and
      unlatches when you press it again is in fact a switch as far as input
      devices go.

Add the SW_* events you need for switches, do NOT try to emulate a button using
KEY_* events just because there is no such SW_* event yet.  Do NOT try to use,
for example, KEY_BLUETOOTH when you should be using SW_BLUETOOTH instead.

2. Input device switches (sources of EV_SW events) DO store their current
state, and that state CAN be queried from userspace through IOCTLs.  There is
no sysfs interface for this, but that doesn't mean you should break things
trying to hook it to the rfkill class to get a sysfs interface :-)

3. Do not issue *_RFKILL_ALL events, unless you are sure it is the correct
event for your switch/button.  These events are emergency power-off events when
they are trying to turn the transmitters off.  An example of an input device
which SHOULD generate *_RFKILL_ALL events is the wireless-kill switch in a
laptop which is NOT a hotkey, but a real switch that kills radios in hardware,
even if the O.S. has gone to lunch.  An example of an input device which SHOULD
NOT generate *_RFKILL_ALL events is any sort of hot key that does nothing by
itself, as well as any hot key that is type-specific (e.g. the one for WLAN).


===============================================================================
4: Kernel API

To build a driver with rfkill subsystem support, the driver should depend on
the Kconfig symbol RFKILL; it should _not_ depend on RKFILL_INPUT.

The hardware the driver talks to may be write-only (where the current state
of the hardware is unknown), or read-write (where the hardware can be queried
about its current state).

The rfkill class will call the get_state hook of a device every time it needs
to know the *real* current state of the hardware.  This can happen often.

Some hardware provides events when its status changes.  In these cases, it is
best for the driver to not provide a get_state hook, and instead register the
rfkill class *already* with the correct status, and keep it updated using
rfkill_force_state() when it gets an event from the hardware.

There is no provision for a statically-allocated rfkill struct.  You must
use rfkill_allocate() to allocate one.

You should:
        - rfkill_allocate()
        - modify rfkill fields (flags, name)
        - modify state to the current hardware state (THIS IS THE ONLY TIME
          YOU CAN ACCESS state DIRECTLY)
        - rfkill_register()

Please refer to the source for more documentation.

===============================================================================
5: Userspace support

rfkill devices issue uevents (with an action of "change"), with the following
environment variables set:

RFKILL_NAME
RFKILL_STATE
RFKILL_TYPE

The ABI for these variables is defined by the sysfs attributes.  It is best
to take a quick look at the source to make sure of the possible values.

It is expected that HAL will trap those, and bridge them to DBUS, etc.  These
events CAN and SHOULD be used to give feedback to the user about the rfkill
status of the system.

Input devices may issue events that are related to rfkill.  These are the
various KEY_* events and SW_* events supported by rfkill-input.c.

******IMPORTANT******
When rfkill-input is ACTIVE, userspace is NOT TO CHANGE THE STATE OF AN RFKILL
SWITCH IN RESPONSE TO AN INPUT EVENT also handled by rfkill-input, unless it
has set to true the user_claim attribute for that particular switch.  This rule
is *absolute*; do NOT violate it.
******IMPORTANT******

Userspace must not assume it is the only source of control for rfkill switches.
Their state CAN and WILL change on its own, due to firmware actions, direct
user actions, and the rfkill-input EPO override for *_RFKILL_ALL.

When rfkill-input is not active, userspace must initiate an rfkill status
change by writing to the "state" attribute in order for anything to happen.

Take particular care to implement EV_SW SW_RFKILL_ALL properly.  When that
switch is set to OFF, *every* rfkill device *MUST* be immediately put into the
OFF state, no questions asked.

The following sysfs entries will be created:

        name: Name assigned by driver to this key (interface or driver name).
        type: Name of the key type ("wlan", "bluetooth", etc).
        state: Current state of the key. 1: On, 0: Off.
        claim: 1: Userspace handles events, 0: Kernel handles events

Both the "state" and "claim" entries are also writable. For the "state" entry
this means that when 1 or 0 is written, the device rfkill state (if not yet in
the requested state), will be will be toggled accordingly.

For the "claim" entry writing 1 to it means that the kernel no longer handles
key events even though RFKILL_INPUT input was enabled. When "claim" has been
set to 0, userspace should make sure that it listens for the input events or
check the sysfs "state" entry regularly to correctly perform the required tasks
when the rkfill key is pressed.

A note about input devices and EV_SW events:

In order to know the current state of an input device switch (like
SW_RFKILL_ALL), you will need to use an IOCTL.  That information is not
available through sysfs in a generic way at this time, and it is not available
through the rfkill class AT ALL.