f7983f7301
Improve the documentation of how to use the rfkill class in kernel drivers, based on the doubts that came up in a thread in linux-wireless. Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br> Acked-by: Ivo van Doorn <IvDoorn@gmail.com> Signed-off-by: John W. Linville <linville@tuxdriver.com>
529 lines
24 KiB
Plaintext
529 lines
24 KiB
Plaintext
rfkill - RF switch subsystem support
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====================================
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1 Introduction
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2 Implementation details
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3 Kernel driver guidelines
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3.1 wireless device drivers
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3.2 platform/switch drivers
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3.3 input device drivers
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4 Kernel API
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5 Userspace support
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1. Introduction:
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The rfkill switch subsystem exists to add a generic interface to circuitry that
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can enable or disable the signal output of a wireless *transmitter* of any
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type. By far, the most common use is to disable radio-frequency transmitters.
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Note that disabling the signal output means that the the transmitter is to be
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made to not emit any energy when "blocked". rfkill is not about blocking data
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transmissions, it is about blocking energy emission.
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The rfkill subsystem offers support for keys and switches often found on
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laptops to enable wireless devices like WiFi and Bluetooth, so that these keys
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and switches actually perform an action in all wireless devices of a given type
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attached to the system.
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The buttons to enable and disable the wireless transmitters are important in
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situations where the user is for example using his laptop on a location where
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radio-frequency transmitters _must_ be disabled (e.g. airplanes).
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Because of this requirement, userspace support for the keys should not be made
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mandatory. Because userspace might want to perform some additional smarter
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tasks when the key is pressed, rfkill provides userspace the possibility to
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take over the task to handle the key events.
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===============================================================================
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2: Implementation details
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The rfkill subsystem is composed of various components: the rfkill class, the
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rfkill-input module (an input layer handler), and some specific input layer
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events.
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The rfkill class provides kernel drivers with an interface that allows them to
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know when they should enable or disable a wireless network device transmitter.
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This is enabled by the CONFIG_RFKILL Kconfig option.
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The rfkill class support makes sure userspace will be notified of all state
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changes on rfkill devices through uevents. It provides a notification chain
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for interested parties in the kernel to also get notified of rfkill state
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changes in other drivers. It creates several sysfs entries which can be used
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by userspace. See section "Userspace support".
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The rfkill-input module provides the kernel with the ability to implement a
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basic response when the user presses a key or button (or toggles a switch)
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related to rfkill functionality. It is an in-kernel implementation of default
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policy of reacting to rfkill-related input events and neither mandatory nor
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required for wireless drivers to operate. It is enabled by the
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CONFIG_RFKILL_INPUT Kconfig option.
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rfkill-input is a rfkill-related events input layer handler. This handler will
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listen to all rfkill key events and will change the rfkill state of the
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wireless devices accordingly. With this option enabled userspace could either
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do nothing or simply perform monitoring tasks.
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The rfkill-input module also provides EPO (emergency power-off) functionality
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for all wireless transmitters. This function cannot be overridden, and it is
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always active. rfkill EPO is related to *_RFKILL_ALL input layer events.
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Important terms for the rfkill subsystem:
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In order to avoid confusion, we avoid the term "switch" in rfkill when it is
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referring to an electronic control circuit that enables or disables a
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transmitter. We reserve it for the physical device a human manipulates
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(which is an input device, by the way):
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rfkill switch:
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A physical device a human manipulates. Its state can be perceived by
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the kernel either directly (through a GPIO pin, ACPI GPE) or by its
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effect on a rfkill line of a wireless device.
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rfkill controller:
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A hardware circuit that controls the state of a rfkill line, which a
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kernel driver can interact with *to modify* that state (i.e. it has
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either write-only or read/write access).
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rfkill line:
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An input channel (hardware or software) of a wireless device, which
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causes a wireless transmitter to stop emitting energy (BLOCK) when it
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is active. Point of view is extremely important here: rfkill lines are
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always seen from the PoV of a wireless device (and its driver).
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soft rfkill line/software rfkill line:
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A rfkill line the wireless device driver can directly change the state
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of. Related to rfkill_state RFKILL_STATE_SOFT_BLOCKED.
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hard rfkill line/hardware rfkill line:
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A rfkill line that works fully in hardware or firmware, and that cannot
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be overridden by the kernel driver. The hardware device or the
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firmware just exports its status to the driver, but it is read-only.
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Related to rfkill_state RFKILL_STATE_HARD_BLOCKED.
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The enum rfkill_state describes the rfkill state of a transmitter:
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When a rfkill line or rfkill controller is in the RFKILL_STATE_UNBLOCKED state,
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the wireless transmitter (radio TX circuit for example) is *enabled*. When the
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it is in the RFKILL_STATE_SOFT_BLOCKED or RFKILL_STATE_HARD_BLOCKED, the
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wireless transmitter is to be *blocked* from operating.
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RFKILL_STATE_SOFT_BLOCKED indicates that a call to toggle_radio() can change
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that state. RFKILL_STATE_HARD_BLOCKED indicates that a call to toggle_radio()
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will not be able to change the state and will return with a suitable error if
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attempts are made to set the state to RFKILL_STATE_UNBLOCKED.
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RFKILL_STATE_HARD_BLOCKED is used by drivers to signal that the device is
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locked in the BLOCKED state by a hardwire rfkill line (typically an input pin
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that, when active, forces the transmitter to be disabled) which the driver
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CANNOT override.
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Full rfkill functionality requires two different subsystems to cooperate: the
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input layer and the rfkill class. The input layer issues *commands* to the
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entire system requesting that devices registered to the rfkill class change
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state. The way this interaction happens is not complex, but it is not obvious
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either:
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Kernel Input layer:
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* Generates KEY_WWAN, KEY_WLAN, KEY_BLUETOOTH, SW_RFKILL_ALL, and
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other such events when the user presses certain keys, buttons, or
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toggles certain physical switches.
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THE INPUT LAYER IS NEVER USED TO PROPAGATE STATUS, NOTIFICATIONS OR THE
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KIND OF STUFF AN ON-SCREEN-DISPLAY APPLICATION WOULD REPORT. It is
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used to issue *commands* for the system to change behaviour, and these
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commands may or may not be carried out by some kernel driver or
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userspace application. It follows that doing user feedback based only
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on input events is broken, as there is no guarantee that an input event
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will be acted upon.
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Most wireless communication device drivers implementing rfkill
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functionality MUST NOT generate these events, and have no reason to
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register themselves with the input layer. Doing otherwise is a common
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misconception. There is an API to propagate rfkill status change
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information, and it is NOT the input layer.
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rfkill class:
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* Calls a hook in a driver to effectively change the wireless
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transmitter state;
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* Keeps track of the wireless transmitter state (with help from
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the driver);
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* Generates userspace notifications (uevents) and a call to a
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notification chain (kernel) when there is a wireless transmitter
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state change;
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* Connects a wireless communications driver with the common rfkill
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control system, which, for example, allows actions such as
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"switch all bluetooth devices offline" to be carried out by
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userspace or by rfkill-input.
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THE RFKILL CLASS NEVER ISSUES INPUT EVENTS. THE RFKILL CLASS DOES
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NOT LISTEN TO INPUT EVENTS. NO DRIVER USING THE RFKILL CLASS SHALL
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EVER LISTEN TO, OR ACT ON RFKILL INPUT EVENTS. Doing otherwise is
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a layering violation.
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Most wireless data communication drivers in the kernel have just to
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implement the rfkill class API to work properly. Interfacing to the
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input layer is not often required (and is very often a *bug*) on
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wireless drivers.
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Platform drivers often have to attach to the input layer to *issue*
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(but never to listen to) rfkill events for rfkill switches, and also to
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the rfkill class to export a control interface for the platform rfkill
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controllers to the rfkill subsystem. This does NOT mean the rfkill
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switch is attached to a rfkill class (doing so is almost always wrong).
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It just means the same kernel module is the driver for different
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devices (rfkill switches and rfkill controllers).
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Userspace input handlers (uevents) or kernel input handlers (rfkill-input):
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* Implements the policy of what should happen when one of the input
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layer events related to rfkill operation is received.
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* Uses the sysfs interface (userspace) or private rfkill API calls
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to tell the devices registered with the rfkill class to change
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their state (i.e. translates the input layer event into real
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action).
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* rfkill-input implements EPO by handling EV_SW SW_RFKILL_ALL 0
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(power off all transmitters) in a special way: it ignores any
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overrides and local state cache and forces all transmitters to the
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RFKILL_STATE_SOFT_BLOCKED state (including those which are already
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supposed to be BLOCKED). Note that the opposite event (power on all
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transmitters) is handled normally.
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Userspace uevent handler or kernel platform-specific drivers hooked to the
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rfkill notifier chain:
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* Taps into the rfkill notifier chain or to KOBJ_CHANGE uevents,
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in order to know when a device that is registered with the rfkill
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class changes state;
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* Issues feedback notifications to the user;
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* In the rare platforms where this is required, synthesizes an input
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event to command all *OTHER* rfkill devices to also change their
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statues when a specific rfkill device changes state.
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===============================================================================
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3: Kernel driver guidelines
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Remember: point-of-view is everything for a driver that connects to the rfkill
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subsystem. All the details below must be measured/perceived from the point of
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view of the specific driver being modified.
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The first thing one needs to know is whether his driver should be talking to
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the rfkill class or to the input layer. In rare cases (platform drivers), it
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could happen that you need to do both, as platform drivers often handle a
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variety of devices in the same driver.
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Do not mistake input devices for rfkill controllers. The only type of "rfkill
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switch" device that is to be registered with the rfkill class are those
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directly controlling the circuits that cause a wireless transmitter to stop
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working (or the software equivalent of them), i.e. what we call a rfkill
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controller. Every other kind of "rfkill switch" is just an input device and
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MUST NOT be registered with the rfkill class.
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A driver should register a device with the rfkill class when ALL of the
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following conditions are met (they define a rfkill controller):
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1. The device is/controls a data communications wireless transmitter;
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2. The kernel can interact with the hardware/firmware to CHANGE the wireless
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transmitter state (block/unblock TX operation);
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3. The transmitter can be made to not emit any energy when "blocked":
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rfkill is not about blocking data transmissions, it is about blocking
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energy emission;
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A driver should register a device with the input subsystem to issue
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rfkill-related events (KEY_WLAN, KEY_BLUETOOTH, KEY_WWAN, KEY_WIMAX,
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SW_RFKILL_ALL, etc) when ALL of the folowing conditions are met:
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1. It is directly related to some physical device the user interacts with, to
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command the O.S./firmware/hardware to enable/disable a data communications
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wireless transmitter.
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Examples of the physical device are: buttons, keys and switches the user
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will press/touch/slide/switch to enable or disable the wireless
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communication device.
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2. It is NOT slaved to another device, i.e. there is no other device that
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issues rfkill-related input events in preference to this one.
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Please refer to the corner cases and examples section for more details.
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When in doubt, do not issue input events. For drivers that should generate
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input events in some platforms, but not in others (e.g. b43), the best solution
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is to NEVER generate input events in the first place. That work should be
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deferred to a platform-specific kernel module (which will know when to generate
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events through the rfkill notifier chain) or to userspace. This avoids the
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usual maintenance problems with DMI whitelisting.
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Corner cases and examples:
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====================================
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1. If the device is an input device that, because of hardware or firmware,
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causes wireless transmitters to be blocked regardless of the kernel's will, it
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is still just an input device, and NOT to be registered with the rfkill class.
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2. If the wireless transmitter switch control is read-only, it is an input
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device and not to be registered with the rfkill class (and maybe not to be made
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an input layer event source either, see below).
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3. If there is some other device driver *closer* to the actual hardware the
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user interacted with (the button/switch/key) to issue an input event, THAT is
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the device driver that should be issuing input events.
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E.g:
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[RFKILL slider switch] -- [GPIO hardware] -- [WLAN card rf-kill input]
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(platform driver) (wireless card driver)
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The user is closer to the RFKILL slide switch plaform driver, so the driver
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which must issue input events is the platform driver looking at the GPIO
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hardware, and NEVER the wireless card driver (which is just a slave). It is
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very likely that there are other leaves than just the WLAN card rf-kill input
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(e.g. a bluetooth card, etc)...
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On the other hand, some embedded devices do this:
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[RFKILL slider switch] -- [WLAN card rf-kill input]
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(wireless card driver)
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In this situation, the wireless card driver *could* register itself as an input
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device and issue rf-kill related input events... but in order to AVOID the need
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for DMI whitelisting, the wireless card driver does NOT do it. Userspace (HAL)
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or a platform driver (that exists only on these embedded devices) will do the
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dirty job of issuing the input events.
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COMMON MISTAKES in kernel drivers, related to rfkill:
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====================================
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1. NEVER confuse input device keys and buttons with input device switches.
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1a. Switches are always set or reset. They report the current state
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(on position or off position).
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1b. Keys and buttons are either in the pressed or not-pressed state, and
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that's it. A "button" that latches down when you press it, and
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unlatches when you press it again is in fact a switch as far as input
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devices go.
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Add the SW_* events you need for switches, do NOT try to emulate a button using
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KEY_* events just because there is no such SW_* event yet. Do NOT try to use,
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for example, KEY_BLUETOOTH when you should be using SW_BLUETOOTH instead.
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2. Input device switches (sources of EV_SW events) DO store their current state
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(so you *must* initialize it by issuing a gratuitous input layer event on
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driver start-up and also when resuming from sleep), and that state CAN be
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queried from userspace through IOCTLs. There is no sysfs interface for this,
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but that doesn't mean you should break things trying to hook it to the rfkill
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class to get a sysfs interface :-)
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3. Do not issue *_RFKILL_ALL events by default, unless you are sure it is the
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correct event for your switch/button. These events are emergency power-off
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events when they are trying to turn the transmitters off. An example of an
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input device which SHOULD generate *_RFKILL_ALL events is the wireless-kill
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switch in a laptop which is NOT a hotkey, but a real switch that kills radios
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in hardware, even if the O.S. has gone to lunch. An example of an input device
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which SHOULD NOT generate *_RFKILL_ALL events by default, is any sort of hot
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key that does nothing by itself, as well as any hot key that is type-specific
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(e.g. the one for WLAN).
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3.1 Guidelines for wireless device drivers
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------------------------------------------
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1. Each independent transmitter in a wireless device (usually there is only one
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transmitter per device) should have a SINGLE rfkill class attached to it.
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2. If the device does not have any sort of hardware assistance to allow the
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driver to rfkill the device, the driver should emulate it by taking all actions
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required to silence the transmitter.
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3. If it is impossible to silence the transmitter (i.e. it still emits energy,
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even if it is just in brief pulses, when there is no data to transmit and there
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is no hardware support to turn it off) do NOT lie to the users. Do not attach
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it to a rfkill class. The rfkill subsystem does not deal with data
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transmission, it deals with energy emission. If the transmitter is emitting
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energy, it is not blocked in rfkill terms.
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4. It doesn't matter if the device has multiple rfkill input lines affecting
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the same transmitter, their combined state is to be exported as a single state
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per transmitter (see rule 1).
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This rule exists because users of the rfkill subsystem expect to get (and set,
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when possible) the overall transmitter rfkill state, not of a particular rfkill
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line.
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Example of a WLAN wireless driver connected to the rfkill subsystem:
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--------------------------------------------------------------------
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A certain WLAN card has one input pin that causes it to block the transmitter
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and makes the status of that input pin available (only for reading!) to the
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kernel driver. This is a hard rfkill input line (it cannot be overridden by
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the kernel driver).
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The card also has one PCI register that, if manipulated by the driver, causes
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it to block the transmitter. This is a soft rfkill input line.
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It has also a thermal protection circuitry that shuts down its transmitter if
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the card overheats, and makes the status of that protection available (only for
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reading!) to the kernel driver. This is also a hard rfkill input line.
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If either one of these rfkill lines are active, the transmitter is blocked by
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the hardware and forced offline.
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The driver should allocate and attach to its struct device *ONE* instance of
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the rfkill class (there is only one transmitter).
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It can implement the get_state() hook, and return RFKILL_STATE_HARD_BLOCKED if
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either one of its two hard rfkill input lines are active. If the two hard
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rfkill lines are inactive, it must return RFKILL_STATE_SOFT_BLOCKED if its soft
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rfkill input line is active. Only if none of the rfkill input lines are
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active, will it return RFKILL_STATE_UNBLOCKED.
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If it doesn't implement the get_state() hook, it must make sure that its calls
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to rfkill_force_state() are enough to keep the status always up-to-date, and it
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must do a rfkill_force_state() on resume from sleep.
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Every time the driver gets a notification from the card that one of its rfkill
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lines changed state (polling might be needed on badly designed cards that don't
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generate interrupts for such events), it recomputes the rfkill state as per
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above, and calls rfkill_force_state() to update it.
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The driver should implement the toggle_radio() hook, that:
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1. Returns an error if one of the hardware rfkill lines are active, and the
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caller asked for RFKILL_STATE_UNBLOCKED.
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2. Activates the soft rfkill line if the caller asked for state
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RFKILL_STATE_SOFT_BLOCKED. It should do this even if one of the hard rfkill
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lines are active, effectively double-blocking the transmitter.
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3. Deactivates the soft rfkill line if none of the hardware rfkill lines are
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active and the caller asked for RFKILL_STATE_UNBLOCKED.
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===============================================================================
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4: Kernel API
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To build a driver with rfkill subsystem support, the driver should depend on
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(or select) the Kconfig symbol RFKILL; it should _not_ depend on RKFILL_INPUT.
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The hardware the driver talks to may be write-only (where the current state
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of the hardware is unknown), or read-write (where the hardware can be queried
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about its current state).
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The rfkill class will call the get_state hook of a device every time it needs
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to know the *real* current state of the hardware. This can happen often.
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Some hardware provides events when its status changes. In these cases, it is
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best for the driver to not provide a get_state hook, and instead register the
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rfkill class *already* with the correct status, and keep it updated using
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rfkill_force_state() when it gets an event from the hardware.
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There is no provision for a statically-allocated rfkill struct. You must
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use rfkill_allocate() to allocate one.
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You should:
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- rfkill_allocate()
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- modify rfkill fields (flags, name)
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- modify state to the current hardware state (THIS IS THE ONLY TIME
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YOU CAN ACCESS state DIRECTLY)
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- rfkill_register()
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The only way to set a device to the RFKILL_STATE_HARD_BLOCKED state is through
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a suitable return of get_state() or through rfkill_force_state().
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When a device is in the RFKILL_STATE_HARD_BLOCKED state, the only way to switch
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it to a different state is through a suitable return of get_state() or through
|
|
rfkill_force_state().
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|
|
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If toggle_radio() is called to set a device to state RFKILL_STATE_SOFT_BLOCKED
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|
when that device is already at the RFKILL_STATE_HARD_BLOCKED state, it should
|
|
not return an error. Instead, it should try to double-block the transmitter,
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|
so that its state will change from RFKILL_STATE_HARD_BLOCKED to
|
|
RFKILL_STATE_SOFT_BLOCKED should the hardware blocking cease.
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|
|
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Please refer to the source for more documentation.
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|
|
|
===============================================================================
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5: Userspace support
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|
|
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rfkill devices issue uevents (with an action of "change"), with the following
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|
environment variables set:
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|
|
|
RFKILL_NAME
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|
RFKILL_STATE
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|
RFKILL_TYPE
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|
|
|
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.
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|
|
|
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.
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|
|
|
Input devices may issue events that are related to rfkill. These are the
|
|
various KEY_* events and SW_* events supported by rfkill-input.c.
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|
|
|
******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 due to firmware actions, direct user actions,
|
|
and the rfkill-input EPO override for *_RFKILL_ALL.
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|
|
|
When rfkill-input is not active, userspace must initiate a 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
|
|
RFKILL_STATE_SOFT_BLOCKED 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 transmitter
|
|
0: RFKILL_STATE_SOFT_BLOCKED
|
|
transmitter is forced off, but one can override it
|
|
by a write to the state attribute;
|
|
1: RFKILL_STATE_UNBLOCKED
|
|
transmiter is NOT forced off, and may operate if
|
|
all other conditions for such operation are met
|
|
(such as interface is up and configured, etc);
|
|
2: RFKILL_STATE_HARD_BLOCKED
|
|
transmitter is forced off by something outside of
|
|
the driver's control. One cannot set a device to
|
|
this state through writes to the state attribute;
|
|
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.
|