Controlling LEDs

Table of Contents

1. Preparation
2. Controlling LEDs
   1. Controlling LEDs Brightness
   2. LEDs Triggers
3. Further Resources

Preparation

The leds_mlxcpld kernel module is in charge of providing standard sysfs access to the LEDs on Mellanox switches. Hence, in order to control the LEDs, one must make sure that the module is compiled and loaded.

Module compilation is controlled via the CONFIG_LEDS_MLXCPLD config option.

To verify whether the module is loaded, the following command may be used:

$ lsmod | grep leds_mlxcpld

If the module is not loaded, it can be loaded by running:

$ modprobe leds_mlxcpld

When installing the provided .iso file according to the instructions, the module will be automatically loaded upon boot via /etc/modules-load.d/mlxleds.conf.

Controlling LEDs

To control the LEDs' status from the user space, the sysfs interface can be used. For every LED and every color, there exists a directory named:

/sys/class/leds/<driver>:<LED name>:<LED color>

For example, on a Mellanox Spectrum machine, the following directories are available:

$ ls /sys/class/leds/
mlxcpld:fan1:green
mlxcpld:fan1:red
mlxcpld:fan2:green
mlxcpld:fan2:red
mlxcpld:fan3:green
mlxcpld:fan3:red
mlxcpld:fan4:green
mlxcpld:fan4:red
mlxcpld:psu:green
mlxcpld:psu:red
mlxcpld:status:green
mlxcpld:status:red

Controlling LEDs Brightness

Controlling or querying the brightness of a specific LED and color can be done via the file <LED dir>/brightness. This file contains integer values that vary from 0 to <LED dir>/max_brightness.

For example, to set the status LED to be green, run:

$ cd /sys/class/leds
$ echo 0 > mlxcpld\:status\:red/brightness
$ cat mlxcpld\:status\:green/max_brightness > mlxcpld\:status\:green/brightness

LEDs Triggers

The kernel's LED infrastructure allows connecting certain LEDs to certain triggers. To control a LED trigger, the file <LED dir>/trigger may be used.

To list all available triggers, use the command:

$ cat <LED dir>/trigger

For example, to make a LED blink, the timer trigger may be used:

$ echo timer > <LED dir>/trigger

The files delay_on and delay_off are added under <LED dir>. With those two files, it is possible to set the LED frequency and duty-cycle, as the delay_on sets the number of milliseconds for which the LED would be ON, and the delay_off sets the number of milliseconds for which the LED would be OFF. Hence, the total frequency is 1.0/(delay_on + delay_off) KHz.

Note: To use the timer trigger, make sure that the ledtrig-timer module is loaded. In the provided .iso file, the module is loaded on startup.

Further Resources

1. LEDs class documentation in kernel source tree
2. leds_mlxcpld documentation in kernel source tree