Commit | Line | Data |
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44e1e9f8 SK |
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. |