According to the DragonBoard's hardware manual, two user LEDs are connected to APG GPIO 21 and APG GPIO 120, the other two are connected to PM PGIO 1 and PM GPIO 2.
How to I control them from user space, preferably through the same sysfs as documented in "GPIO Usage on Low Speed Connector Application Note"?
I tried to do:
export PIN=$((21 + 902))
echo $PIN > export # this failed
[69040.431307] _gpio_request: gpio-923 (sysfs) status -16
[69040.431314] export_store: status -16
Apparently, these GPIO pins are not registered to the sysfs.