>> I'd like to change NS16550 serial driver, 'com' to add 16bit
>> register access support.  [...]

> The correct way to do this is to add a bus_space for the bus the
> device is connected to.  I'm currently using an embedded chip with a
> '550-alike with registers on 32-bit boundaries without any
> modifications to the bus_space_{read,write}_1's in com.c.

I once had to do something similar; an embedded processor I was working
with had a com.c-compatible chip, but it was connected oddly - it
wasn't actually behind any bus; instead, part of what would normally be
memory space was decoded oddly, such that the chip appeared near the
top of what the memory controller decodes as RAM, with registers 8
bytes apart (as if the address lines were permuted before decoding).

I solved this without touching com.c by creating a new bus_space.  It
wasn't very hard and once any of the accesses worked, everything worked.

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