Re: [PATCH v2 16/18] x86: io: implement dummy relaxed accessor macros for writes

[Will Deacon]

On Fri, May 23, 2014 at 03:53:20PM +0100, H. Peter Anvin wrote:
> On 05/23/2014 07:46 AM, Will Deacon wrote:
> > 
> > I would like the relaxed accessors to be ordered with respect to each other...
> > 
> > What do you think?
> > 
> 
> I think "I would like" isn't a very good motivation.  What are the
> semantics of these things supposed to be?  It seems more than a bit odd
> to require them to be ordered with respect to each other and everything
> else (which is what a memory clobber does) and then call them "relaxed".

I suggested some informal semantics in the cover letter:

  https://lkml.org/lkml/2014/4/17/269

Basically, if we define relaxed accesses not to be ordered against anything
apart from other accesses (relaxed or otherwise) to the same device, then
they become a tonne cheaper on arm/arm64/powerpc. Currently we have to
include expensive memory barriers in order to synchronise with accesses to
DMA buffers which is rarely needed.

For those requirements, I don't think we need the "memory" clobber for x86,
but would appreciate your views on this.

Will
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/