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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.