[[PageOutline]]

= OpenWrt hotplug =
Our modern OpenWrt based BSP's use OpenWrt procd as PID1. One of procd's features is to catch messages from the kernel and act upon them, much in the way a conventional linux system uses a hotplug helper and udev. The /etc/hotplug.json file configures how various
events are handled:
 * using makedev to create device nodes in /dev on 'add' events and removing the nodes on 'remove' events
 * facilitate firmware loading
 * calling /sbin/hotplug-call on platform subsystem events
 * calling /etc/rc.button/$BUTTON on button events
 * calling /sbin/hotplug-call on various subsystem events (such as net, input, usb, block, atm, tty, button)

Button events:
 * /sbin/hotplug-call will have the following defined:
  - BUTTON - button name
  - ACTION - pressed|released
  - SEQNUM - a numeric value that increments with the event message
  - SEEN - number of seconds since last button event from driver (can be used to determine button held time)
 * see [https://github.com/Gateworks/gw-openwrt-packages/blob/20.06/gateworks/button-hotplug-gw/files/hotplug button-hotplug-gw script] and [wiki:OpenWrt/gpio#button-hotplug-gw button-hotplug-gw config]