Adding buttons on the Linkit Smart

Here’s what I found out while trying to add buttons to the OpenWrt based Linkit Smart.

The flow is:

• Keys are defined in target/linux/ramips/dts/LINKIT7688.dts
  • gpio-button-hotplug emulates gpio-keys-polled and generates uevents.
    • The uevent is received onver netlink by procd in plug/hotplug.c

      • The event is dispatched via /etc/hotplug.json

        • The standard dispatch rule calls /etc/rc.buttons/$BUTTON

        The DTS fragment is:

        gpio-keys-polled {
        

      … btn2 { label = “btn2”; gpios = <&gpio0 2 1>; linux,code = <0x102>; };

      where the `<&#038;gpio0 2 1>` picks the second pin from the zeroth port i.e. GPIO 2 and
      
      maps it to BTN_2. See [`uapi/linux/input.h`][6] for the other key codes.
      
      gpio-button-hotplug supports a limited number of keys. See the source for the
      
      current list, which is `BTN_0` to `BTN_9`, `power`, `reset`, `rfkill`, `wps`, and `wwan`.
      
      Also, the LinkIt uses a binary wifi driver which seems to be _very_ sensitive to
      
      the kernel version. I ended up reverting [`363508bcabd8e9205f5fffc8ff282439e61d618f`][7]
      
      to bring OpenWrt back to 3.18.21 to stop it crashing when the interface comes up.