[PATCH] Document Linux's memory barriers [try #2]

[Jes Sorensen]

>>>>> "Paul" == Paul Mackerras <paulus at samba.org> writes:

Paul> Jesse Barnes writes:
>> So ultimately mmiowb() is the only thing drivers really have to
>> care about on Altix (assuming they do DMA mapping correctly), and
>> the rules for that are fairly simple.  Then they can additionally
>> use read_relaxed() to optimize performance a bit (quite a bit on
>> big systems).

Paul> If I can be sure that all the drivers we care about on PPC use
Paul> mmiowb correctly, I can reduce or eliminate the barrier in
Paul> write*, which would be nice.

Paul> Which drivers have been audited to make sure they use mmiowb
Paul> correctly?  In particular, has the USB driver been audited?

I think the primary drivers we've looked at are drivers/net/tg3.c,
drivers/net/s2io.c, drivers/scsi/qla1280.c, and possibly the
qla[234]xxx series - thats probably it!

While we have USB on the systems, I don't think anyone has spend a lot
of time verifying it in this context. At least the keyboard and mouse
I have on this box seems to behave.

Cheers,
Jes