rfkill - RF switch subsystem support
====================================
-1 Implementation details
-2 Driver support
-3 Userspace support
+1 Introduction
+2 Implementation details
+3 Kernel driver guidelines
+4 Kernel API
+5 Userspace support
-===============================================================================
-1: Implementation details
+
+1. Introduction:
The rfkill switch subsystem exists to add a generic interface to circuitry that
-can enable or disable the RF output of a radio *transmitter* of any type.
+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.
-When a rfkill switch is in the RFKILL_STATE_ON, the radio transmitter is
-*enabled*. When the rfkill switch is in the RFKILL_STATE_OFF, the radio
-transmitter is *disabled*.
+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 rfkill switch subsystem offers support for keys often found on laptops
-to enable wireless devices like WiFi and Bluetooth.
+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).
-This is done by providing the user 3 possibilities:
- 1 - The rfkill system handles all events; userspace is not aware of events.
- 2 - The rfkill system handles all events; userspace is informed about the events.
- 3 - The rfkill system does not handle events; userspace handles all events.
+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.
-The buttons to enable and disable the wireless radios are important in
-situations where the user is for example using his laptop on a location where
-wireless radios _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 still 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 the input device. It also creates several
-sysfs entries which can be used by userspace. See section "Userspace support".
+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.
-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 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:
====================================
-2: Driver support
-To build a driver with rfkill subsystem support, the driver should
-depend on the Kconfig symbol RFKILL; it should _not_ depend on
-RKFILL_INPUT.
+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.
-Unless key events trigger an interrupt to which the driver listens, polling
-will be required to determine the key state changes. For this the input
-layer providers the input-polldev handler.
+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).
-A driver should implement a few steps to correctly make use of the
-rfkill subsystem. First for non-polling drivers:
+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.
- - rfkill_allocate()
- - input_allocate_device()
- - rfkill_register()
- - input_register_device()
+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.
-For polling drivers:
+There is no provision for a statically-allocated rfkill struct. You must
+use rfkill_allocate() to allocate one.
+You should:
- rfkill_allocate()
- - input_allocate_polled_device()
+ - modify rfkill fields (flags, name)
+ - modify state to the current hardware state (THIS IS THE ONLY TIME
+ YOU CAN ACCESS state DIRECTLY)
- rfkill_register()
- - input_register_polled_device()
-When a key event has been detected, the correct event should be
-sent over the input device which has been registered by the driver.
+Please refer to the source for more documentation.
-====================================
-3: Userspace support
+===============================================================================
+5: Userspace support
+
+rfkill devices issue uevents (with an action of "change"), with the following
+environment variables set:
+
+RFKILL_NAME
+RFKILL_STATE
+RFKILL_TYPE
-For each key an input device will be created which will send out the correct
-key event when the rfkill key has been pressed.
+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:
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 all radios, not yet in the requested
-state, will be will be toggled accordingly.
+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.
+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.
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