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

[Paul Mackerras]

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).

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

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

Paul.