rfkill.rst 5.0 KB

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  1. ===============================
  2. rfkill - RF kill switch support
  3. ===============================
  4. .. contents::
  5. :depth: 2
  6. Introduction
  7. ============
  8. The rfkill subsystem provides a generic interface for disabling any radio
  9. transmitter in the system. When a transmitter is blocked, it shall not
  10. radiate any power.
  11. The subsystem also provides the ability to react on button presses and
  12. disable all transmitters of a certain type (or all). This is intended for
  13. situations where transmitters need to be turned off, for example on
  14. aircraft.
  15. The rfkill subsystem has a concept of "hard" and "soft" block, which
  16. differ little in their meaning (block == transmitters off) but rather in
  17. whether they can be changed or not:
  18. - hard block
  19. read-only radio block that cannot be overridden by software
  20. - soft block
  21. writable radio block (need not be readable) that is set by
  22. the system software.
  23. The rfkill subsystem has two parameters, rfkill.default_state and
  24. rfkill.master_switch_mode, which are documented in
  25. admin-guide/kernel-parameters.rst.
  26. Implementation details
  27. ======================
  28. The rfkill subsystem is composed of three main components:
  29. * the rfkill core,
  30. * the deprecated rfkill-input module (an input layer handler, being
  31. replaced by userspace policy code) and
  32. * the rfkill drivers.
  33. The rfkill core provides API for kernel drivers to register their radio
  34. transmitter with the kernel, methods for turning it on and off, and letting
  35. the system know about hardware-disabled states that may be implemented on
  36. the device.
  37. The rfkill core code also notifies userspace of state changes, and provides
  38. ways for userspace to query the current states. See the "Userspace support"
  39. section below.
  40. When the device is hard-blocked (either by a call to rfkill_set_hw_state()
  41. or from query_hw_block), set_block() will be invoked for additional software
  42. block, but drivers can ignore the method call since they can use the return
  43. value of the function rfkill_set_hw_state() to sync the software state
  44. instead of keeping track of calls to set_block(). In fact, drivers should
  45. use the return value of rfkill_set_hw_state() unless the hardware actually
  46. keeps track of soft and hard block separately.
  47. Kernel API
  48. ==========
  49. Drivers for radio transmitters normally implement an rfkill driver.
  50. Platform drivers might implement input devices if the rfkill button is just
  51. that, a button. If that button influences the hardware then you need to
  52. implement an rfkill driver instead. This also applies if the platform provides
  53. a way to turn on/off the transmitter(s).
  54. For some platforms, it is possible that the hardware state changes during
  55. suspend/hibernation, in which case it will be necessary to update the rfkill
  56. core with the current state at resume time.
  57. To create an rfkill driver, driver's Kconfig needs to have::
  58. depends on RFKILL || !RFKILL
  59. to ensure the driver cannot be built-in when rfkill is modular. The !RFKILL
  60. case allows the driver to be built when rfkill is not configured, in which
  61. case all rfkill API can still be used but will be provided by static inlines
  62. which compile to almost nothing.
  63. Calling rfkill_set_hw_state() when a state change happens is required from
  64. rfkill drivers that control devices that can be hard-blocked unless they also
  65. assign the poll_hw_block() callback (then the rfkill core will poll the
  66. device). Don't do this unless you cannot get the event in any other way.
  67. rfkill provides per-switch LED triggers, which can be used to drive LEDs
  68. according to the switch state (LED_FULL when blocked, LED_OFF otherwise).
  69. Userspace support
  70. =================
  71. The recommended userspace interface to use is /dev/rfkill, which is a misc
  72. character device that allows userspace to obtain and set the state of rfkill
  73. devices and sets of devices. It also notifies userspace about device addition
  74. and removal. The API is a simple read/write API that is defined in
  75. linux/rfkill.h, with one ioctl that allows turning off the deprecated input
  76. handler in the kernel for the transition period.
  77. Except for the one ioctl, communication with the kernel is done via read()
  78. and write() of instances of 'struct rfkill_event'. In this structure, the
  79. soft and hard block are properly separated (unlike sysfs, see below) and
  80. userspace is able to get a consistent snapshot of all rfkill devices in the
  81. system. Also, it is possible to switch all rfkill drivers (or all drivers of
  82. a specified type) into a state which also updates the default state for
  83. hotplugged devices.
  84. After an application opens /dev/rfkill, it can read the current state of all
  85. devices. Changes can be obtained by either polling the descriptor for
  86. hotplug or state change events or by listening for uevents emitted by the
  87. rfkill core framework.
  88. Additionally, each rfkill device is registered in sysfs and emits uevents.
  89. rfkill devices issue uevents (with an action of "change"), with the following
  90. environment variables set::
  91. RFKILL_NAME
  92. RFKILL_STATE
  93. RFKILL_TYPE
  94. The content of these variables corresponds to the "name", "state" and
  95. "type" sysfs files explained above.
  96. For further details consult Documentation/ABI/stable/sysfs-class-rfkill.