[PATCH] Document Linux's memory barriers

[Jesse Barnes]

On Tuesday, March 7, 2006 10:30 am, David Howells wrote:
> True, I suppose. I should make it clear that these accessor functions
> imply memory barriers, if indeed they do, and that you should use them
> rather than accessing I/O registers directly (at least, outside the
> arch you should).

But they don't, that's why we have mmiowb().  There are lots of cases to 
handle:
  1) memory vs. memory
  2) memory vs. I/O
  3) I/O vs. I/O
(reads and writes for every case).

AFAIK, we have (1) fairly well handled with a plethora of barrier ops.  
(2) is a bit fuzzy with the current operations I think, and for (3) all 
we have is mmiowb() afaik.  Maybe one of the ppc64 guys can elaborate on 
the barriers their hw needs for the above cases (I think they're the 
pathological case, so covering them should be good enough everybody).

Btw, thanks for putting together this documentation, it's desperately 
needed.

Jesse