Documentation/leds/ledtrig-transient.rst

Source file repositories/reference/linux-study-clean/Documentation/leds/ledtrig-transient.rst

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Documentation/leds/ledtrig-transient.rst
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=====================
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.

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.

Annotation

Implementation Notes