Trent Piepho wrote:

Is there an issue with anything _besides_ coherent DMA?

Could one have a special version of the accessors for drivers that
want to assume they are strictly ordered vs coherent DMA memory?
That would be much easier to get right, without slowing _everything_
down.

It's better to be safe by default and then optimize the fast paths than 
to be relaxed by default and hang the machine in some piece of code that 
runs once a month.  "Premature optimization is the root of all evil", 
and what not.

One could even go as far as to allow a driver to "#define 
WANT_STRICT_IO" and then it would get the strict versions.  Add that
to any driver that uses DMA and then worry about vetting those
drivers.

See above -- if you must have a #define, then it should be WANT_RELAXED_IO.

-Scott