[PATCH v3] leds: add new transient trigger for one shot timeractivation

From: Shuah Khan
Date: Mon Apr 30 2012 - 16:33:48 EST


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.

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.