/boot/overlays/README has this text:
act_led_trigger Choose which activity the LED tracks.
Use "heartbeat" for a nice load indicator.
(default "mmc")
But are there other options? If I wanted to create my own (e.g. led constantly on until shutdown), where would I start? Is it recompiling a kernel module, or recompiling the entire kernel?
I have tried describing the values mentioned.
none No trigger
kbd-scrolllock Keyboard scroll lock
kbd-numlock Keyboard num lock
kbd-capslock Keyboard caps lock
kbd-kanalock Keyboard kana lock
kbd-shiftlock Keyboard shift
kbd-altgrlock Keyboard altgr
kbd-ctrllock Keyboard ctrl
kbd-altlock Keyboard alt
kbd-shiftllock Keyboard left shift
kbd-shiftrlock Keyboard right shift
kbd-ctrlllock Keyboard left ctrl
kbd-ctrlrlock Keyboard right ctrl
timer Flash at 1 second intervals
oneshot Flash only once
heartbeat Flash like a heartbeat (1-0-1-00000)
backlight Always on
gpio Flash when a certain GPIO is high???
cpu0 Flash on cpu0 usage
cpu1 Flash on cpu1 usage
cpu2 Flash on cpu2 usage
cpu3 Flash on cpu3 usage
default-on Always on
[input] Default state
panic Flash on kernel panic
mmc0 Flash on mmc0 (primary SD Card interface) activity
mmc1 Flash on mmc1 (secondary SD Card interface) activity
rfkill0 Flash on wifi activity
rfkill1 Flash on bluetooth activity
Try any of the values in cat /sys/devices/platform/leds/leds/led0/trigger
In this Makefile, you can find all the triggers chosen for a 5.4 kernel.
But I noticed that by cat /sys/devices/platform/leds/leds/led0/trigger, some triggers are not listed (e.g. "pattern" and "activity").
It turned out that they are compiled as kernel modules but not loaded by default.
They are located under /lib/modules/[your kernel release]/kernel/drivers/leds/trigger, and you can find out your kernel release by uname -r.
Then you can either manually load the triggers by sudo modprobe ledtrig_xxx or add them to /etc/modules-load.d/xxx.conf to load automatically during boot time.
Finally, try cat /sys/devices/platform/leds/leds/led0/trigger again and play with the triggers :)
I've tried and it works:
In /boot/config.txt:
dtparam=pwr_led_trigger=heartbeat
Enjoy.
Lucian's answer suggesting the Makefile prompted me to look into the related kernel source code.
When /sys/devices/platform/leds/leds/led1/trigger (same for led0) is set to gpio, then the virtual filesystem exposes additional 'files'; I will use gpio 35 to drive the red on-board led ("power led").
$ sudo sh -c "echo 'gpio' > /sys/class/leds/led1/trigger"
$ ls -lA /sys/class/leds/led1/
total 0
-rw-r--r-- 1 root root 4096 Aug 6 10:33 brightness
-rw-r--r-- 1 root root 4096 Aug 6 10:34 desired_brightness
lrwxrwxrwx 1 root root 0 Aug 6 10:33 device -> ../../../leds
-rw-r--r-- 1 root root 4096 Aug 6 10:34 gpio
-rw-r--r-- 1 root root 4096 Aug 6 10:34 inverted
-r--r--r-- 1 root root 4096 Aug 6 10:33 max_brightness
drwxr-xr-x 2 root root 0 Aug 6 10:33 power
lrwxrwxrwx 1 root root 0 Aug 6 10:33 subsystem -> ../../../../../class/leds
-rw-r--r-- 1 root root 0 Aug 6 10:34 trigger
-rw-r--r-- 1 root root 4096 Aug 6 10:33 uevent
$ sudo sh -c "echo '255' > /sys/class/leds/led1/desired_brightness"
$ sudo sh -c "echo '35' > /sys/class/leds/led1/gpio"
$ echo 35 > /sys/class/gpio/export
$ echo 'out' > /sys/class/gpio/gpio35/direction
$ echo '1' > /sys/class/gpio/gpio35/value
