Improving virtIO <-> sDDF interface

[erichchan999]

Currently each virtIO device class talks to the sDDF interface via the same sddf_handler struct, which has been made generic. Furthermore, we have each device initialised via the same function virtio_mmio_device_init, which takes in an enum that specifies the device class type.

However a better design would be to have:

• different sddf_handler structs specific to each device class.
• different init function for each device class.

[alexandermbrown]

Not too sure about the name "handler". I think "handler" came from "queue_handle" then for some reason we put an r on the end. E.g., the global array sddf_handler_t sddf_serial_handlers[SDDF_SERIAL_NUM_HANDLES] has NUM_HANDLES as the size, doesn't really make sense. All this needs to be is something like void *device_data, it doens't need to be sDDF specific.

I agree we should have a per device class init function. virtio_mmio_device_init doesn't really do much extra apart from register the exception handler and virq. We could either

• remove virtio_mmio_device_init and put exception and virq registration in per-device functions, or
• rename and switch sddf_handlers to be a device-specific struct passed with void *.
  • if we wanted to remove the switch statement we could also pass in the virtio_*_init function as a function pointer.

[alexandermbrown]

FYI: I've made a draft of these changes in #37