I'm not a kernel developer so I'm a bit guessing here.

To my understanding many peripherals need a kernel driver and a configuration for the hardware connection. ADP5589 was mentioned in this thread earlier and there is a kernel driver for it. But in addition to that the kernel needs to know how the ADP5589 is connected to the main CPU on the device (e.g. on the RPi).

E.g. I2C devices are usually static so everything is hardcoded to kernel's board files or nowadays to Device Tree files.

To use ADP5589 with RPi you need to probably tell in which I2C bus it is, what's the I2C slave address (7 or 10bit) and what are the possible interrupt lines. I tried quickly googling for some devices using the ADP5589 for an example but couldn't find any (but I'm sure there are such).

So, the actual I2C communication between CPU and ADP5589 should be covered by the driver and as such is plug'n'play but a hardware connection configuration is still needed. Some sort of user-space keymapping is probably needed as well.


EDIT: related to that particular driver I found an interesting PDF, google for "ADP5589 Input Keyboard and GPIO Linux Driver".