Re: [PATCH 15/15] rfkill: document rw rfkill switches and clarifyinput subsystem interactions

From: Randy Dunlap
Date: Mon May 19 2008 - 13:52:46 EST

On Sun, 18 May 2008 15:48:05 -0300 Henrique de Moraes Holschuh wrote:

> Rework the documentation so as to make sure driver writers understand
> exactly where the boundaries are for input drivers related to rfkill
> switches, buttons and keys, and rfkill class drivers.
> Signed-off-by: Henrique de Moraes Holschuh <hmh@xxxxxxxxxx>
> Cc: Ivo van Doorn <IvDoorn@xxxxxxxxx>
> Cc: Dmitry Torokhov <dtor@xxxxxxx>
> ---
> Documentation/rfkill.txt | 329 ++++++++++++++++++++++++++++++++++++++-------
> 1 files changed, 277 insertions(+), 52 deletions(-)
> diff --git a/Documentation/rfkill.txt b/Documentation/rfkill.txt
> index ec75d6d..bf8709c 100644
> --- a/Documentation/rfkill.txt
> +++ b/Documentation/rfkill.txt
> @@ -2,82 +2,299 @@ rfkill - RF switch subsystem support
> ====================================

Add INTRODUCTION item here?

> 1 Implementation details
> -2 Driver support
> -3 Userspace support
> +2 Kernel driver guidelines
> +3 Kernel API
> +4 Userspace support
> -===============================================================================
> -1: Implementation details
> +


> +===============================================================================
> +1: Implementation details
> +
> +The rfkill class provides kernel drivers with an interface that allows them to
> +know when they should enable or disable a wireless network device transmitter.
> +
> +The rfkill-input module provides the kernel with the hability to implement


> +a basic response to when the user presses a key or button (or toggles a switch)

s/to when/when/

> +related to rfkill functionality. This is optional, and can also be done in
> +userspace.
> +
> +All state changes on rfkill devices is propagated by the rfkill class to a


> +notification chain and also to userspace through uevents.
> The system inside the kernel has been split into 2 separate sections:
> 1 - RFKILL
> -The first option enables rfkill support and will make sure userspace will
> -be notified of any events through the input device. It also creates several
> -sysfs entries which can be used by userspace. See section "Userspace support".
> +The first option enables rfkill support and will make sure userspace will be
> +notified of any events through uevents. It provides a notification chain for
> +interested parties in the kernel to also get notified of rfkill state changes
> +in other drivers. It creates several sysfs entries which can be used by
> +userspace. See section "Userspace support".
> +
> +The second option provides an rfkill input handler. This handler will listen to
> +all rfkill key events and will toggle the radio accordingly. With this option
> +enabled userspace could either do nothing or simply perform monitoring tasks.
> +
> +When a rfkill switch is in the RFKILL_STATE_ON, the wireless transmitter (radio
> +TX circuit for example) is *enabled*. When the rfkill switch is in the
> +RFKILL_STATE_OFF, the wireless transmitter is to be *blocked* from operating.
> +
> +Full rfkill functionality requires two different subsystems to cooperate: the
> +input layer, and the rfkill class. The input layer issues *commands* to the

no comma.

> +entire system requesting that devices registered to the rfkill class change
> +state. The way this interation happens is not complex, but it is not


> +either:
> +
> +Kernel Input layer:
> + other such events when the user presses certain keys, buttons, or
> + toggles certain physical switches.


> +===============================================================================
> +2: Kernel driver guidelines
> +
> +The first thing one needs to know is whether his driver should be talking to
> +the rfkill class, or to the input layer.

no comma.

> +
> +Do not mistake input devices for rfkill devices. The only type of "rfkill
> +switch" device that is to be registered with the rfkill class are those
> +directly controlling the circuits that cause a wireless transmitter to stop
> +working (or the software equivalent of them). Every other kind of "rfkill
> +switch" is just an input device and MUST NOT be registered with the rfkill
> +class.


> + Typically, the ACPI "radio kill" switch of a laptop is the master input
> + device to issue rfkill events, and, e.g., the WLAN card is just a slave
> + device that gets disabled by its hardware radio-kill input pin.
> -The second option provides an rfkill input handler. This handler will
> -listen to all rfkill key events and will toggle the radio accordingly.
> -With this option enabled userspace could either do nothing or simply
> -perform monitoring tasks.
> +When in doubt, do not issue input events. For drivers that should generate
> +input events in some platforms, but not in others (e.g. b43), the best solution
> +is to NEVER generate input events in the first place. That work should be
> +deferred to a platform-specific kernel module (which will known when to


> +generate events through the rfkill notifier chain), or to userspace. This

no comma.

> +avoids the usual maintenance problems with DMI whitelisting.
> +
> +Corner cases and examples:
> ====================================
> -2: Driver support
> -To build a driver with rfkill subsystem support, the driver should
> -depend on the Kconfig symbol RFKILL; it should _not_ depend on
> +1. If the device is an input device that, because of hardware or firmware,
> +causes wireless transmitters to be blocked regardless of the kernel's will, it
> +is still just an input device, and NOT to be registered with the rfkill class.
> -Unless key events trigger an interrupt to which the driver listens, polling
> -will be required to determine the key state changes. For this the input
> -layer providers the input-polldev handler.
> +2. If the wireless transmitter switch control is read-only, it is an input
> +device and not to be registered with the rfkill class (and maybe not to be made
> +an input layer event source either, see below).
> -A driver should implement a few steps to correctly make use of the
> -rfkill subsystem. First for non-polling drivers:
> +3. If there is some other device driver *closer* to the actuall hardware the


> +user interacted with (the button/switch/key) to issue an input event, THAT is


> +the device driver that should be issuing input events.
> +COMMON MISTAKES in kernel drivers, related to rfkill:
> +====================================
> +
> +1. NEVER confuse input device keys and buttons with input device switches.
> +
> + 1a. Switches are always set or reset. They report the current state
> + (on position or off position).
> +
> + 1b. Keys and buttons are either in the pressed or not-pressed state, and
> + that's it. A "button" that latches down when you press it, and
> + unlatches when you press it again is in fact a switch as far as input
> + devices go.
> +
> +Add the SW_* events you need them for switches, do NOT try to emulate a button

Delete "them"? A little misworded/confusing.

> +using KEY_* events just because there is no such SW_* event yet. Do NOT try to
> +use, for example, KEY_BLUETOOTH when you should be using SW_BLUETOOTH instead.
> +
> +2. Input device switches (sources of EV_SW events) DO store their current
> +state, and that state CAN be queried from userspace through IOCTLs. There is
> +no sysfs interface for this, but that doesn't mean you should break things
> +trying to hook it to the rfkill class to get a sysfs interface :-)
> +
> +
> +===============================================================================
> +3: Kernel API
> +
> +To build a driver with rfkill subsystem support, the driver should depend on
> +the Kconfig symbol RFKILL; it should _not_ depend on RKFILL_INPUT.
> +
> +The hardware the driver talks to may be write-only (where the current state
> +of the hardware is unknown), or read-write (where the hardware can be queried
> +about its current state).
> +
> +The rfkill class will call the get_state hook of a device every time it needs
> +to know the *real* current state of the hardware. This can happen often.
> +
> +Some hardware provide events when its status change. In these cases, it is


> +best for the driver to not provide a get_state hook, and instead register the
> +rfkill class *already* with the correct status, and keep it updated using
> +rfkill_force_state() when it gets an event from the hardware.


> +4: Userspace support
> +
> +rfkill devices issue uevents (with an action of "change"), with the following
> +environment variables set:
> +
> -For each key an input device will be created which will send out the correct
> -key event when the rfkill key has been pressed.
> +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.
> +
> +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.
> +
> +Input devices may issue events that are related to rfkill. These are the
> +various KEY_* events and SW_* events supported by rfkill-input.c.
> +
> +******IMPORTANT******
> +When rfkill-input is ACTIVE, userspace is NOT TO CHANGE THE STATE OF A RFKILL

i.e., does one say "R F KILL" or does one say "rifkill" or "rufkill" or
something else?

> +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******
> +
> +When rfkill-input is not active, userspace must initiate an 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
> +OFF state, no questions asked.
> The following sysfs entries will be created:


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