HW: i.MX7 SabreSD
SW: Android N7.1.1_1.0.0
There is KPP module on i.MX7, but NXP reference board didn't have it. We reworked a "keypad" and has the demo.
For testing, launch an app in Android that can accept text input. KPP also supports multiple input, the "A" is showed by pressing "shift" + "a".
As a side note,
1. The input device driver is drivers/input/keyboard/imx_keypad.c
2. The input event driver is drivers/tty/vt/keyboard.c