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-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
