While your points are well taken, and in fact many of us have already
thought about them, most of them are irrelevant to the particular
topic at hand.

One specific item, though:

   > * In some weirder cases, you might even have one device mapped in I/O
   > space, and another one of the same type mapped in memory space, on the
   > same machine.

   I can't see this, unless you are defining a device by the driver it
   uses rather than the actual hardware itself.

No; I mean the hardware itself.  One could imagine a scenario like
this:

* 1 DEPCA card in an ISA slot

* 1 DEPCA card in an ISA slot, which is connected via a PCI-to-ISA
bridge that maps I/O space to memory space.

In both cases, you have the same device, but the method of accessing
the registers is necessarily different.

Right now, we don't have any cases of this happening in a single
machine, but we can have both cases in separate machines.  Do we have
to use separate drivers for the two, or do we make the DEPCA driver
know how to talk to both kinds of busses?  That's part of the problem
I'm addressing.