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| -rw-r--r-- | Documentation/rfkill.txt | 363 |
1 files changed, 308 insertions, 55 deletions
diff --git a/Documentation/rfkill.txt b/Documentation/rfkill.txt index ec75d6d..cf230c1 100644 --- a/Documentation/rfkill.txt +++ b/Documentation/rfkill.txt @@ -1,83 +1,328 @@ 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: @@ -87,10 +332,18 @@ 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. |
