133 lines
5 KiB
Plaintext
133 lines
5 KiB
Plaintext
===============================
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rfkill - RF kill switch support
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===============================
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.. contents::
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:depth: 2
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Introduction
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============
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The rfkill subsystem provides a generic interface for 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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The rfkill subsystem has a concept of "hard" and "soft" block, which
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differ little in their meaning (block == transmitters off) but rather in
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whether they can be changed or not:
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- hard block
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read-only radio block that cannot be overridden by software
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- soft block
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writable radio block (need not be readable) that is set by
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the system software.
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The rfkill subsystem has two parameters, rfkill.default_state and
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rfkill.master_switch_mode, which are documented in
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admin-guide/kernel-parameters.rst.
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Implementation details
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======================
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The rfkill subsystem is composed of three main components:
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* the rfkill core,
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* the deprecated rfkill-input module (an input layer handler, being
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replaced by userspace policy code) and
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* the rfkill drivers.
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The rfkill core provides API for kernel drivers to register their radio
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transmitter with the kernel, methods for turning it on and off, and letting
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the system know about hardware-disabled states that may be implemented on
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the device.
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The rfkill core code also notifies userspace of state changes, and provides
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ways for userspace to query the current states. See the "Userspace support"
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section below.
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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 for additional software
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block, but drivers can ignore the method call since they can use the return
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value of the function rfkill_set_hw_state() to sync the software state
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instead of keeping track of calls to set_block(). In fact, drivers should
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use the return value of rfkill_set_hw_state() unless the hardware actually
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keeps track of soft and hard block separately.
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Kernel API
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==========
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Drivers for radio transmitters normally implement an rfkill driver.
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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 driver 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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For some platforms, it is possible that the hardware state changes during
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suspend/hibernation, in which case it will be necessary to update the rfkill
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core with the current state at resume time.
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To create an rfkill driver, driver's Kconfig needs to have::
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depends on RFKILL || !RFKILL
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to ensure the driver cannot be built-in when rfkill is modular. The !RFKILL
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case allows the driver to be built when rfkill is not configured, in which
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case all rfkill API can still be used but will be provided by static inlines
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which compile to almost nothing.
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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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rfkill provides per-switch LED triggers, which can be used to drive LEDs
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according to the switch state (LED_FULL when blocked, LED_OFF otherwise).
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Userspace support
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=================
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The recommended userspace interface to use is /dev/rfkill, which is a misc
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character device that allows userspace to obtain and set the state of rfkill
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devices and sets of devices. It also notifies userspace about device addition
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and removal. The API is a simple read/write API that is defined in
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linux/rfkill.h, with one ioctl that allows turning off the deprecated input
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handler in the kernel for the transition period.
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Except for the one ioctl, communication with the kernel is done via read()
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and write() of instances of 'struct rfkill_event'. In this structure, the
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soft and hard block are properly separated (unlike sysfs, see below) and
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userspace is able to get a consistent snapshot of all rfkill devices in the
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system. Also, it is possible to switch all rfkill drivers (or all drivers of
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a specified type) into a state which also updates the default state for
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hotplugged devices.
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After an application opens /dev/rfkill, it can read the current state of all
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devices. Changes can be obtained by either polling the descriptor for
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hotplug or state change events or by listening for uevents emitted by the
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rfkill core framework.
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Additionally, each rfkill device is registered in sysfs and emits uevents.
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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 content of these variables corresponds to the "name", "state" and
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"type" sysfs files explained above.
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For further details consult Documentation/ABI/stable/sysfs-class-rfkill.
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