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Diffstat (limited to 'kernel/Documentation/leds/ledtrig-transient.txt')
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diff --git a/kernel/Documentation/leds/ledtrig-transient.txt b/kernel/Documentation/leds/ledtrig-transient.txt new file mode 100644 index 000000000..3bd38b487 --- /dev/null +++ b/kernel/Documentation/leds/ledtrig-transient.txt @@ -0,0 +1,152 @@ +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. |