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-LED Transient Trigger
-=====================
-
-The leds timer trigger does not currently have an interface to activate
-a one shot timer. The current support allows for setting two timers, one for
-specifying how long a state to be on, and the second for how long the state
-to be off. The delay_on value specifies the time period an LED should stay
-in on state, followed by a delay_off value that specifies how long the LED
-should stay in off state. The on and off cycle repeats until the trigger
-gets deactivated. There is no provision for one time activation to implement
-features that require an on or off state to be held just once and then stay in
-the original state forever.
-
-Without one shot timer interface, user space can still use timer trigger to
-set a timer to hold a state, however when user space application crashes or
-goes away without deactivating the timer, the hardware will be left in that
-state permanently.
-
-As a specific example of this use-case, let's look at vibrate feature on
-phones. Vibrate function on phones is implemented using PWM pins on SoC or
-PMIC. There is a need to activate one shot timer to control the vibrate
-feature, to prevent user space crashes leaving the phone in vibrate mode
-permanently causing the battery to drain.
-
-Transient trigger addresses the need for one shot timer activation. The
-transient trigger can be enabled and disabled just like the other leds
-triggers.
-
-When an led class device driver registers itself, it can specify all leds
-triggers it supports and a default trigger. During registration, activation
-routine for the default trigger gets called. During registration of an led
-class device, the LED state does not change.
-
-When the driver unregisters, deactivation routine for the currently active
-trigger will be called, and LED state is changed to LED_OFF.
-
-Driver suspend changes the LED state to LED_OFF and resume doesn't change
-the state. Please note that there is no explicit interaction between the
-suspend and resume actions and the currently enabled trigger. LED state
-changes are suspended while the driver is in suspend state. Any timers
-that are active at the time driver gets suspended, continue to run, without
-being able to actually change the LED state. Once driver is resumed, triggers
-start functioning again.
-
-LED state changes are controlled using brightness which is a common led
-class device property. When brightness is set to 0 from user space via
-echo 0 > brightness, it will result in deactivating the current trigger.
-
-Transient trigger uses standard register and unregister interfaces. During
-trigger registration, for each led class device that specifies this trigger
-as its default trigger, trigger activation routine will get called. During
-registration, the LED state does not change, unless there is another trigger
-active, 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. It
-creates its properties and does its initialization. Transient trigger
-deactivation routine, will cancel any timer that is active before it cleans
-up and removes the properties it created. It will restore the LED state to
-non-transient state. When driver gets suspended, irrespective of the transient
-state, the LED state changes to LED_OFF.
-
-Transient trigger can be enabled and disabled from user space on led class
-devices, that support this trigger as shown below:
-
-echo transient > trigger
-echo none > trigger
-
-NOTE: Add a new property trigger state to control the state.
-
-This trigger exports three properties, activate, state, and duration. When
-transient 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 by
- duration as needed. The initial and default value is 0. This will allow
- duration to be set after trigger activation.
-- state allows user to specify a transient state to be held for the specified
- duration.
-
- 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 specified
- duration running.
- deactivated state indicates that there is no active timer
- running.
-
- duration - one shot timer value. When activate is set, duration value
- is used to start a timer that runs once. This value doesn't
- get changed by the trigger unless user does a set via
- echo new_value > duration
-
- state - transient state to be held. It has two values 0 or 1. 0 maps
- to LED_OFF and 1 maps to LED_FULL. The specified state is
- held for the duration of the one shot timer and then the
- state gets changed to the non-transient state which is the
- inverse of transient state.
- If state = LED_FULL, when the timer runs out the state will
- go back to LED_OFF.
- If state = LED_OFF, when the timer runs out the state will
- go back to LED_FULL.
- Please note that current LED state is not checked prior to
- changing the state to the specified state.
- Driver could map these values to inverted depending on the
- default 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 left
-at the set value to be used when activate is set at a future time. This will
-allow user app to set the time once and activate it to run it once for the
-specified value as needed. When timer expires, state is restored to the
-non-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 next
- activate. Currently active timer if
- any, continues to run for the specified time.
- echo 0 > duration - stores timer value to be used upon next
- activate. Currently active timer if any,
- continues to run for the specified time.
- echo 1 > state - stores desired transient state LED_FULL to be
- held for the specified duration.
- echo 0 > state - stores desired transient state LED_OFF to be
- held for the specified duration.
-
-What is not supported:
-======================
-- Timer activation is one shot and extending and/or shortening the timer
- is not supported.
-
-Example use-case 1:
- echo transient > trigger
- echo n > duration
- echo 1 > state
-repeat the following step as needed:
- echo 1 > activate - start timer = duration to run once
- echo 1 > activate - start timer = duration to run once
- echo none > trigger
-
-This trigger is intended to be used for for the following example use cases:
- - Control of vibrate (phones, tablets etc.) hardware by user space app.
- - Use of LED by user space app as activity indicator.
- - Use of LED by user space app as a kind of watchdog indicator -- as
- long as the app is alive, it can keep the LED illuminated, if it dies
- the LED will be extinguished automatically.
- - Use by any user space app that needs a transient GPIO output.