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Documentation / leds / ledtrig-transient.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:40 EST.

1	LED Transient Trigger
2	=====================
3	
4	The leds timer trigger does not currently have an interface to activate
5	a one shot timer. The current support allows for setting two timers, one for
6	specifying how long a state to be on, and the second for how long the state
7	to be off. The delay_on value specifies the time period an LED should stay
8	in on state, followed by a delay_off value that specifies how long the LED
9	should stay in off state. The on and off cycle repeats until the trigger
10	gets deactivated. There is no provision for one time activation to implement
11	features that require an on or off state to be held just once and then stay in
12	the original state forever.
13	
14	Without one shot timer interface, user space can still use timer trigger to
15	set a timer to hold a state, however when user space application crashes or
16	goes away without deactivating the timer, the hardware will be left in that
17	state permanently.
18	
19	As a specific example of this use-case, let's look at vibrate feature on
20	phones. Vibrate function on phones is implemented using PWM pins on SoC or
21	PMIC. There is a need to activate one shot timer to control the vibrate
22	feature, to prevent user space crashes leaving the phone in vibrate mode
23	permanently causing the battery to drain.
24	
25	Transient trigger addresses the need for one shot timer activation. The
26	transient trigger can be enabled and disabled just like the other leds
27	triggers.
28	
29	When an led class device driver registers itself, it can specify all leds
30	triggers it supports and a default trigger. During registration, activation
31	routine for the default trigger gets called. During registration of an led
32	class device, the LED state does not change.
33	
34	When the driver unregisters, deactivation routine for the currently active
35	trigger will be called, and LED state is changed to LED_OFF.
36	
37	Driver suspend changes the LED state to LED_OFF and resume doesn't change
38	the state. Please note that there is no explicit interaction between the
39	suspend and resume actions and the currently enabled trigger. LED state
40	changes are suspended while the driver is in suspend state. Any timers
41	that are active at the time driver gets suspended, continue to run, without
42	being able to actually change the LED state. Once driver is resumed, triggers
43	start functioning again.
44	
45	LED state changes are controlled using brightness which is a common led
46	class device property. When brightness is set to 0 from user space via
47	echo 0 > brightness, it will result in deactivating the current trigger.
48	
49	Transient trigger uses standard register and unregister interfaces. During
50	trigger registration, for each led class device that specifies this trigger
51	as its default trigger, trigger activation routine will get called. During
52	registration, the LED state does not change, unless there is another trigger
53	active, in which case LED state changes to LED_OFF.
54	
55	During trigger unregistration, LED state gets changed to LED_OFF.
56	
57	Transient trigger activation routine doesn't change the LED state. It
58	creates its properties and does its initialization. Transient trigger
59	deactivation routine, will cancel any timer that is active before it cleans
60	up and removes the properties it created. It will restore the LED state to
61	non-transient state. When driver gets suspended, irrespective of the transient
62	state, the LED state changes to LED_OFF.
63	
64	Transient trigger can be enabled and disabled from user space on led class
65	devices, that support this trigger as shown below:
66	
67	echo transient > trigger
68	echo none > trigger
69	
70	NOTE: Add a new property trigger state to control the state.
71	
72	This trigger exports three properties, activate, state, and duration. When
73	transient 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	
113	When timer expires activate goes back to deactivated state, duration is left
114	at the set value to be used when activate is set at a future time. This will
115	allow user app to set the time once and activate it to run it once for the
116	specified value as needed. When timer expires, state is restored to the
117	non-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	
132	What is not supported:
133	======================
134	- Timer activation is one shot and extending and/or shortening the timer
135	  is not supported.
136	
137	Example use-case 1:
138		echo transient > trigger
139		echo n > duration
140		echo 1 > state
141	repeat 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	
146	This 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.
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