Documentation/leds/ledtrig-transient.txt - linux - Git at Google

 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.
	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.