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f71fea23a2
Add some blurb about /dev/rfkill to the documentation and fix the "transmiter" spelling error. Signed-off-by: Johannes Berg <johannes@sipsolutions.net> Signed-off-by: John W. Linville <linville@tuxdriver.com>
137 lines
5.0 KiB
Plaintext
137 lines
5.0 KiB
Plaintext
rfkill - RF kill switch 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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4. Kernel API
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5. Userspace support
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1. Introduction
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The rfkill subsystem provides a generic interface to disabling any radio
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transmitter in the system. When a transmitter is blocked, it shall not
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radiate any power.
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The subsystem also provides the ability to react on button presses and
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disable all transmitters of a certain type (or all). This is intended for
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situations where transmitters need to be turned off, for example on
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aircraft.
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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 is provided for kernel drivers to register their radio
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transmitter with the kernel, provide methods for turning it on and off and,
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optionally, letting the system know about hardware-disabled states that may
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be implemented on the device. This code is enabled with the CONFIG_RFKILL
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Kconfig option, which drivers can "select".
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The rfkill class code also notifies userspace of state changes, this is
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achieved via uevents. It also provides some sysfs files for userspace to
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check the status of radio transmitters. See the "Userspace support" section
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below.
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The rfkill-input code implements a basic response to rfkill buttons -- it
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implements turning on/off all devices of a certain class (or all).
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When the device is hard-blocked (either by a call to rfkill_set_hw_state()
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or from query_hw_block) set_block() will be invoked but drivers can well
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ignore the method call since they can use the return value of the function
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rfkill_set_hw_state() to sync the software state instead of keeping track
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of calls to set_block().
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The entire functionality is spread over more than one subsystem:
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* The kernel input layer generates KEY_WWAN, KEY_WLAN etc. and
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SW_RFKILL_ALL -- when the user presses a button. Drivers for radio
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transmitters generally do not register to the input layer, unless the
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device really provides an input device (i.e. a button that has no
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effect other than generating a button press event)
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* The rfkill-input code hooks up to these events and switches the soft-block
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of the various radio transmitters, depending on the button type.
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* The rfkill drivers turn off/on their transmitters as requested.
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* The rfkill class will generate userspace notifications (uevents) to tell
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userspace what the current state is.
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3. Kernel driver guidelines
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Drivers for radio transmitters normally implement only the rfkill class.
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These drivers may not unblock the transmitter based on own decisions, they
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should act on information provided by the rfkill class only.
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Platform drivers might implement input devices if the rfkill button is just
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that, a button. If that button influences the hardware then you need to
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implement an rfkill class instead. This also applies if the platform provides
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a way to turn on/off the transmitter(s).
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During suspend/hibernation, transmitters should only be left enabled when
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wake-on wlan or similar functionality requires it and the device wasn't
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blocked before suspend/hibernate. Note that it may be necessary to update
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the rfkill subsystem's idea of what the current state is at resume time if
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the state may have changed over suspend.
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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.
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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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Calling rfkill_set_hw_state() when a state change happens is required from
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rfkill drivers that control devices that can be hard-blocked unless they also
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assign the poll_hw_block() callback (then the rfkill core will poll the
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device). Don't do this unless you cannot get the event in any other way.
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5. Userspace support
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The following sysfs entries exist for every rfkill device:
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name: Name assigned by driver to this key (interface or driver name).
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type: Name of the key type ("wlan", "bluetooth", etc).
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state: Current state of the transmitter
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0: RFKILL_STATE_SOFT_BLOCKED
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transmitter is turned off by software
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1: RFKILL_STATE_UNBLOCKED
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transmitter is (potentially) active
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2: RFKILL_STATE_HARD_BLOCKED
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transmitter is forced off by something outside of
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the driver's control.
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claim: 0: Kernel handles events (currently always reads that value)
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rfkill devices also issue uevents (with an action of "change"), with the
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following 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 contents of these variables corresponds to the "name", "state" and
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"type" sysfs files explained above.
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An alternative userspace interface exists as a misc device /dev/rfkill,
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which allows userspace to obtain and set the state of rfkill devices and
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sets of devices. It also notifies userspace about device addition and
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removal. The API is a simple read/write API that is defined in
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linux/rfkill.h.
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