LED Transient Trigger¶

The leds timer trigger does not currently have an interface to activatea one shot timer. The current support allows for setting two timers, one forspecifying how long a state to be on, and the second for how long the stateto be off. The delay_on value specifies the time period an LED should stayin on state, followed by a delay_off value that specifies how long the LEDshould stay in off state. The on and off cycle repeats until the triggergets deactivated. There is no provision for one time activation to implementfeatures that require an on or off state to be held just once and then stay inthe original state forever.

Without one shot timer interface, user space can still use timer trigger toset a timer to hold a state, however when user space application crashes orgoes away without deactivating the timer, the hardware will be left in thatstate permanently.

As a specific example of this use-case, let’s look at vibrate feature onphones. Vibrate function on phones is implemented using PWM pins on SoC orPMIC. There is a need to activate one shot timer to control the vibratefeature, to prevent user space crashes leaving the phone in vibrate modepermanently causing the battery to drain.

Transient trigger addresses the need for one shot timer activation. Thetransient trigger can be enabled and disabled just like the other ledstriggers.

When an led class device driver registers itself, it can specify all ledstriggers it supports and a default trigger. During registration, activationroutine for the default trigger gets called. During registration of an ledclass device, the LED state does not change.

When the driver unregisters, deactivation routine for the currently activetrigger will be called, and LED state is changed to LED_OFF.

Driver suspend changes the LED state to LED_OFF and resume doesn’t changethe state. Please note that there is no explicit interaction between thesuspend and resume actions and the currently enabled trigger. LED statechanges are suspended while the driver is in suspend state. Any timersthat are active at the time driver gets suspended, continue to run, withoutbeing able to actually change the LED state. Once driver is resumed, triggersstart functioning again.

LED state changes are controlled using brightness which is a common ledclass device property. When brightness is set to 0 from user space viaecho 0 > brightness, it will result in deactivating the current trigger.

Transient trigger uses standard register and unregister interfaces. Duringtrigger registration, for each led class device that specifies this triggeras its default trigger, trigger activation routine will get called. Duringregistration, the LED state does not change, unless there is another triggeractive, in which case LED state changes to LED_OFF.

During trigger unregistration, LED state gets changed to LED_OFF.

Transient trigger activation routine doesn’t change the LED state. Itcreates its properties and does its initialization. Transient triggerdeactivation routine, will cancel any timer that is active before it cleansup and removes the properties it created. It will restore the LED state tonon-transient state. When driver gets suspended, irrespective of the transientstate, the LED state changes to LED_OFF.

Transient trigger can be enabled and disabled from user space on led classdevices, that support this trigger as shown below:

echo transient > triggerecho none > trigger

NOTE:

Add a new property trigger state to control the state.

This trigger exports three properties, activate, state, and duration. Whentransient trigger is activated these properties are set to default values.

• duration allows setting timer value in msecs. The initial value is 0.

• activate allows activating and deactivating the timer specified byduration as needed. The initial and default value is 0. This will allowduration to be set after trigger activation.

• state allows user to specify a transient state to be held for the specifiedduration.

  activate

  • one shot timer activate mechanism.1 when activated, 0 when deactivated.default value is zero when transient trigger is enabled,to allow duration to be set.

    activate state indicates a timer with a value of specifiedduration running.deactivated state indicates that there is no active timerrunning.

  duration

  • one shot timer value. When activate is set, duration valueis used to start a timer that runs once. This value doesn’tget changed by the trigger unless user does a set viaecho new_value > duration

  state

  • transient state to be held. It has two values 0 or 1. 0 mapsto LED_OFF and 1 maps to LED_FULL. The specified state isheld for the duration of the one shot timer and then thestate gets changed to the non-transient state which is theinverse of transient state.If state = LED_FULL, when the timer runs out the state willgo back to LED_OFF.If state = LED_OFF, when the timer runs out the state willgo back to LED_FULL.Please note that current LED state is not checked prior tochanging the state to the specified state.Driver could map these values to inverted depending on thedefault states it defines for the LED in its brightness_set()interface which is called from the led brightness_set()interfaces to control the LED state.

When timer expires activate goes back to deactivated state, duration is leftat the set value to be used when activate is set at a future time. This willallow user app to set the time once and activate it to run it once for thespecified value as needed. When timer expires, state is restored to thenon-transient state which is the inverse of the transient state:

echo 1 > activate	starts timer = duration when duration is not 0.
echo 0 > activate	cancels currently running timer.
echo n > duration	stores timer value to be used upon nextactivate. Currently active timer ifany, continues to run for the specified time.
echo 0 > duration	stores timer value to be used upon nextactivate. Currently active timer if any,continues to run for the specified time.
echo 1 > state	stores desired transient state LED_FULL to beheld for the specified duration.
echo 0 > state	stores desired transient state LED_OFF to beheld for the specified duration.

echo transient > triggerecho n > durationecho 1 > state

echo 1 > activate - start timer = duration to run onceecho 1 > activate - start timer = duration to run onceecho none > trigger