Re: [PATCH] Document Linux's memory barriers

[Stephen Hemminger <shemminger@osdl.org>]

On Tue, 07 Mar 2006 19:24:03 +0000
David Howells <dhowells@redhat.com> wrote:

> Andi Kleen <ak@suse.de> wrote:
> 
> > > > You're not supposed to do it this way anyways. The official way to access
> > > > MMIO space is using read/write[bwlq]
> > > 
> > > True, I suppose. I should make it clear that these accessor functions imply
> > > memory barriers, if indeed they do, 
> > 
> > I don't think they do.
> 
> Hmmm.. Seems Stephen Hemminger disagrees:
> 
> | > > 1) Access to i/o mapped memory does not need memory barriers.
> | > 
> | > There's no guarantee of that. On FRV you have to insert barriers as
> | > appropriate when you're accessing I/O mapped memory if ordering is required
> | > (accessing an ethernet card vs accessing a frame buffer), but support for
> | > inserting the appropriate barriers is built into gcc - which knows the rules
> | > for when to insert them.
> | > 
> | > Or are you referring to the fact that this should be implicit in inX(),
> | > outX(), readX(), writeX() and similar?
> | 

The problem with all this is like physics it is all relative to the observer.
I get confused an lost when talking about the general case because there are so many possible
specific examples where a barrier is or is not needed.