1LED Transient Trigger
  2=====================
  3
  4The leds timer trigger does not currently have an interface to activate
  5a one shot timer. The current support allows for setting two timers, one for
  6specifying how long a state to be on, and the second for how long the state
  7to be off. The delay_on value specifies the time period an LED should stay
  8in on state, followed by a delay_off value that specifies how long the LED
  9should stay in off state. The on and off cycle repeats until the trigger
 10gets deactivated. There is no provision for one time activation to implement
 11features that require an on or off state to be held just once and then stay in
 12the original state forever.
 13
 14Without one shot timer interface, user space can still use timer trigger to
 15set a timer to hold a state, however when user space application crashes or
 16goes away without deactivating the timer, the hardware will be left in that
 17state permanently.
 18
 19As a specific example of this use-case, let's look at vibrate feature on
 20phones. Vibrate function on phones is implemented using PWM pins on SoC or
 21PMIC. There is a need to activate one shot timer to control the vibrate
 22feature, to prevent user space crashes leaving the phone in vibrate mode
 23permanently causing the battery to drain.
 24
 25Transient trigger addresses the need for one shot timer activation. The
 26transient trigger can be enabled and disabled just like the other leds
 27triggers.
 28
 29When an led class device driver registers itself, it can specify all leds
 30triggers it supports and a default trigger. During registration, activation
 31routine for the default trigger gets called. During registration of an led
 32class device, the LED state does not change.
 33
 34When the driver unregisters, deactivation routine for the currently active
 35trigger will be called, and LED state is changed to LED_OFF.
 36
 37Driver suspend changes the LED state to LED_OFF and resume doesn't change
 38the state. Please note that there is no explicit interaction between the
 39suspend and resume actions and the currently enabled trigger. LED state
 40changes are suspended while the driver is in suspend state. Any timers
 41that are active at the time driver gets suspended, continue to run, without
 42being able to actually change the LED state. Once driver is resumed, triggers
 43start functioning again.
 44
 45LED state changes are controlled using brightness which is a common led
 46class device property. When brightness is set to 0 from user space via
 47echo 0 > brightness, it will result in deactivating the current trigger.
 48
 49Transient trigger uses standard register and unregister interfaces. During
 50trigger registration, for each led class device that specifies this trigger
 51as its default trigger, trigger activation routine will get called. During
 52registration, the LED state does not change, unless there is another trigger
 53active, in which case LED state changes to LED_OFF.
 54
 55During trigger unregistration, LED state gets changed to LED_OFF.
 56
 57Transient trigger activation routine doesn't change the LED state. It
 58creates its properties and does its initialization. Transient trigger
 59deactivation routine, will cancel any timer that is active before it cleans
 60up and removes the properties it created. It will restore the LED state to
 61non-transient state. When driver gets suspended, irrespective of the transient
 62state, the LED state changes to LED_OFF.
 63
 64Transient trigger can be enabled and disabled from user space on led class
 65devices, that support this trigger as shown below:
 66
 67echo transient > trigger
 68echo none > trigger
 69
 70NOTE: Add a new property trigger state to control the state.
 71
 72This trigger exports three properties, activate, state, and duration. When
 73transient trigger is activated these properties are set to default values.
 74
 75- duration allows setting timer value in msecs. The initial value is 0.
 76- activate allows activating and deactivating the timer specified by
 77  duration as needed. The initial and default value is 0.  This will allow
 78  duration to be set after trigger activation.
 79- state allows user to specify a transient state to be held for the specified
 80  duration.
 81
 82	activate - one shot timer activate mechanism.
 83		1 when activated, 0 when deactivated.
 84		default value is zero when transient trigger is enabled,
 85		to allow duration to be set.
 86
 87		activate state indicates a timer with a value of specified
 88		duration running.
 89		deactivated state indicates that there is no active timer
 90		running.
 91
 92	duration - one shot timer value. When activate is set, duration value
 93		is used to start a timer that runs once. This value doesn't
 94		get changed by the trigger unless user does a set via
 95		echo new_value > duration
 96
 97	state - transient state to be held. It has two values 0 or 1. 0 maps
 98		to LED_OFF and 1 maps to LED_FULL. The specified state is
 99		held for the duration of the one shot timer and then the
100		state gets changed to the non-transient state which is the
101		inverse of transient state.
102		If state = LED_FULL, when the timer runs out the state will
103		go back to LED_OFF.
104		If state = LED_OFF, when the timer runs out the state will
105		go back to LED_FULL.
106		Please note that current LED state is not checked prior to
107		changing the state to the specified state.
108		Driver could map these values to inverted depending on the
109		default states it defines for the LED in its brightness_set()
110		interface which is called from the led brightness_set()
111		interfaces to control the LED state.
112
113When timer expires activate goes back to deactivated state, duration is left
114at the set value to be used when activate is set at a future time. This will
115allow user app to set the time once and activate it to run it once for the
116specified value as needed. When timer expires, state is restored to the
117non-transient state which is the inverse of the transient state.
118
119	echo 1 > activate - starts timer = duration when duration is not 0.
120	echo 0 > activate - cancels currently running timer.
121	echo n > duration - stores timer value to be used upon next
122                            activate. Currently active timer if
123                            any, continues to run for the specified time.
124	echo 0 > duration - stores timer value to be used upon next
125                            activate. Currently active timer if any,
126                            continues to run for the specified time.
127	echo 1 > state    - stores desired transient state LED_FULL to be
128			    held for the specified duration.
129	echo 0 > state    - stores desired transient state LED_OFF to be
130			    held for the specified duration.
131
132What is not supported:
133======================
134- Timer activation is one shot and extending and/or shortening the timer
135  is not supported.
136
137Example use-case 1:
138	echo transient > trigger
139	echo n > duration
140	echo 1 > state
141repeat the following step as needed:
142	echo 1 > activate - start timer = duration to run once
143	echo 1 > activate - start timer = duration to run once
144	echo none > trigger
145
146This trigger is intended to be used for for the following example use cases:
147 - Control of vibrate (phones, tablets etc.) hardware by user space app.
148 - Use of LED by user space app as activity indicator.
149 - Use of LED by user space app as a kind of watchdog indicator -- as
150       long as the app is alive, it can keep the LED illuminated, if it dies
151       the LED will be extinguished automatically.
152 - Use by any user space app that needs a transient GPIO output.