FreeBSD/Linux Kernel Cross Reference
sys/Documentation/rfkill.txt

     rfkill - RF kill switch support
     ===============================
     
     1. Introduction
     2. Implementation details
     3. Kernel API
     4. Userspace support
     
     
    1. Introduction
    
    The rfkill subsystem provides a generic interface to disabling any radio
    transmitter in the system. When a transmitter is blocked, it shall not
    radiate any power.
    
    The subsystem also provides the ability to react on button presses and
    disable all transmitters of a certain type (or all). This is intended for
    situations where transmitters need to be turned off, for example on
    aircraft.
    
    The rfkill subsystem has a concept of "hard" and "soft" block, which
    differ little in their meaning (block == transmitters off) but rather in
    whether they can be changed or not:
     - hard block: read-only radio block that cannot be overriden by software
     - soft block: writable radio block (need not be readable) that is set by
                   the system software.
    
    
    2. Implementation details
    
    The rfkill subsystem is composed of three main components:
     * the rfkill core,
     * the deprecated rfkill-input module (an input layer handler, being
       replaced by userspace policy code) and
     * the rfkill drivers.
    
    The rfkill core provides API for kernel drivers to register their radio
    transmitter with the kernel, methods for turning it on and off and, letting
    the system know about hardware-disabled states that may be implemented on
    the device.
    
    The rfkill core code also notifies userspace of state changes, and provides
    ways for userspace to query the current states. See the "Userspace support"
    section below.
    
    When the device is hard-blocked (either by a call to rfkill_set_hw_state()
    or from query_hw_block) set_block() will be invoked for additional software
    block, but drivers can ignore the method call since they can use the return
    value of the function rfkill_set_hw_state() to sync the software state
    instead of keeping track of calls to set_block(). In fact, drivers should
    use the return value of rfkill_set_hw_state() unless the hardware actually
    keeps track of soft and hard block separately.
    
    
    3. Kernel API
    
    
    Drivers for radio transmitters normally implement an rfkill driver.
    
    Platform drivers might implement input devices if the rfkill button is just
    that, a button. If that button influences the hardware then you need to
    implement an rfkill driver instead. This also applies if the platform provides
    a way to turn on/off the transmitter(s).
    
    For some platforms, it is possible that the hardware state changes during
    suspend/hibernation, in which case it will be necessary to update the rfkill
    core with the current state is at resume time.
    
    To create an rfkill driver, driver's Kconfig needs to have
    
            depends on RFKILL || !RFKILL
    
    to ensure the driver cannot be built-in when rfkill is modular. The !RFKILL
    case allows the driver to be built when rfkill is not configured, which which
    case all rfkill API can still be used but will be provided by static inlines
    which compile to almost nothing.
    
    Calling rfkill_set_hw_state() when a state change happens is required from
    rfkill drivers that control devices that can be hard-blocked unless they also
    assign the poll_hw_block() callback (then the rfkill core will poll the
    device). Don't do this unless you cannot get the event in any other way.
    
    
    
    5. Userspace support
    
    The recommended userspace interface to use is /dev/rfkill, which is a misc
    character device that allows userspace to obtain and set the state of rfkill
    devices and sets of devices. It also notifies userspace about device addition
    and removal. The API is a simple read/write API that is defined in
    linux/rfkill.h, with one ioctl that allows turning off the deprecated input
    handler in the kernel for the transition period.
    
    Except for the one ioctl, communication with the kernel is done via read()
    and write() of instances of 'struct rfkill_event'. In this structure, the
    soft and hard block are properly separated (unlike sysfs, see below) and
    userspace is able to get a consistent snapshot of all rfkill devices in the
    system. Also, it is possible to switch all rfkill drivers (or all drivers of
    a specified type) into a state which also updates the default state for
   hotplugged devices.
   
   After an application opens /dev/rfkill, it can read the current state of all
   devices. Changes can be either obtained by either polling the descriptor for
   hotplug or state change events or by listening for uevents emitted by the
   rfkill core framework.
   
   Additionally, each rfkill device is registered in sysfs and emits uevents.
   
   rfkill devices issue uevents (with an action of "change"), with the following
   environment variables set:
   
   RFKILL_NAME
   RFKILL_STATE
   RFKILL_TYPE
   
   The contents of these variables corresponds to the "name", "state" and
   "type" sysfs files explained above.
   
   
   For further details consult Documentation/ABI/stable/sysfs-class-rfkill.

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