[ The sample code in my previous message was not quite right. -gwr ]

> If the port wants to support the driver on multiple bus types, then it
> would needs to define things like ESP_READ_REG() as:
> 
> #define ESP_READ_REG(sc, reg) \
> 	BUS_READ_8((sc)->sc_reg[(reg) * 4])

Actually, the MD definition would look like this:

#define ESP_READ_REG(sc, reg) \
	bus_space_read_1((sc)->sc_md.md_bustag, \
	                 (sc)->sc_md.md_bushandle, \
	                 (reg))