oops, missed this:

>I will assert (again) that mapping the device registers is an inherently
                                               ^^^^^^^^^
>bus-specific operation and should be done by the driver through the use of
>bus_space*() routines rather than the VM system.

I'm talking about mapping device *memory*, not registers.  if an app
is going to map the memory and treat it like memory-- e.g., to blit
into it -- then it better behave like memory once it's mapped,
(at both  the  bus_space and d_mmap level!)

Device registers are (or can be) another kettle of fish:
memory barriers, &c.