Documentation/leds/ledtrig-transient.rst

5 The leds timer trigger does not currently have an interface to activate
7 specifying how long a state to be on, and the second for how long the state
8 to be off. The delay_on value specifies the time period an LED should stay
11 gets deactivated. There is no provision for one time activation to implement
12 features that require an on or off state to be held just once and then stay in
15 Without one shot timer interface, user space can still use timer trigger to
16 set a timer to hold a state, however when user space application crashes or
30 trigger will be called, and LED state is changed to LED_OFF.
32 Driver suspend changes the LED state to LED_OFF and resume doesn't change
36 that are active at the time driver gets suspended, continue to run, without
37 being able to actually change the LED state. Once driver is resumed, triggers
41 class device property. When brightness is set to 0 from user space via
48 active, in which case LED state changes to LED_OFF.
50 During trigger unregistration, LED state gets changed to LED_OFF.
55 up and removes the properties it created. It will restore the LED state to
56 non-transient state. When driver gets suspended, irrespective of the transient
57 state, the LED state changes to LED_OFF.
66 	Add a new property trigger state to control the state.
68 This trigger exports three properties, activate, state, and duration. When
69 transient trigger is activated these properties are set to default values.
71 - duration allows setting timer value in msecs. The initial value is 0.
72 - activate allows activating and deactivating the timer specified by
74   duration to be set after trigger activation.
75 - state allows user to specify a transient state to be held for the specified
78 	activate
79 	      - one shot timer activate mechanism.
82 		to allow duration to be set.
84 		activate state indicates a timer with a value of specified
90 	      - one shot timer value. When activate is set, duration value
91 		is used to start a timer that runs once. This value doesn't
96 	      - transient state to be held. It has two values 0 or 1. 0 maps
97 		to LED_OFF and 1 maps to LED_FULL. The specified state is
99 		state gets changed to the non-transient state which is the
102 		go back to LED_OFF.
104 		go back to LED_FULL.
105 		Please note that current LED state is not checked prior to
106 		changing the state to the specified state.
107 		Driver could map these values to inverted depending on the
110 		interfaces to control the LED state.
112 When timer expires activate goes back to deactivated state, duration is left
113 at the set value to be used when activate is set at a future time. This will
114 allow user app to set the time once and activate it to run it once for the
115 specified value as needed. When timer expires, state is restored to the
116 non-transient state which is the inverse of the transient state:
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
129 	echo 0 > state      stores desired transient state LED_OFF to be
136 - Timer activation is one shot and extending and/or shortening the timer
142 use-case 1::
150 	echo 1 > activate - start timer = duration to run once
151 	echo 1 > activate - start timer = duration to run once
154 This trigger is intended to be used for the following example use cases:
156  - Use of LED by user space app as activity indicator.
157  - Use of LED by user space app as a kind of watchdog indicator -- as
160  - Use by any user space app that needs a transient GPIO output.