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LED handling under Linux

Author:Date:2020-07-07 13:42

Summary:In its simplest form, the LED class just allows control of LEDs from userspace. LEDs appear in /sys/class/leds/. The maximum brightness of the LED is defined in max_brightness file. The brightness file will set the brightness of the LED (taking a value 0-max_brightness). Most LEDs don’t have hardware brightness support so will just be turned on for non-zero brightness settings.

In its simplest form, the LED class just allows control of LEDs from userspace. LEDs appear in /sys/class/leds/. The maximum brightness of the LED is defined in max_brightness file. The brightness file will set the brightness of the LED (taking a value 0-max_brightness). Most LEDs don’t have hardware brightness support so will just be turned on for non-zero brightness settings.

 

The class also introduces the optional concept of an LED trigger. A trigger is a kernel based source of led events. Triggers can either be simple or complex. A simple trigger isn’t configurable and is designed to slot into existing subsystems with minimal additional code. Examples are the disk-activity, nand-disk and sharpsl-charge triggers. With led triggers disabled, the code optimises away.

 

led

 

Complex triggers while available to all LEDs have LED specific parameters and work on a per LED basis. The timer trigger is an example. The timer trigger will periodically change the LED brightness between LED_OFF and the current brightness setting. The “on” and “off” time can be specified via /sys/class/leds/<device>/delay_{on,off} in milliseconds. You can change the brightness value of a LED independently of the timer trigger. However, if you set the brightness value to LED_OFF it will also disable the timer trigger.

 

You can change triggers in a similar manner to the way an IO scheduler is chosen (via /sys/class/leds/<device>/trigger). Trigger specific parameters can appear in /sys/class/leds/<device> once a given trigger is selected.

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