Re: [PATCH v2 00/18] Cross-architecture definitions of relaxed MMIO accessors

[Will Deacon <will.deacon@arm.com>]

On Tue, May 27, 2014 at 09:21:38PM +0100, Benjamin Herrenschmidt wrote:
> On Tue, 2014-05-27 at 20:32 +0100, Will Deacon wrote:
> 
> > Why would you need two barriers? I would have though an mmiowb() inlined
> > into writel after the store operation would be sufficient. Or is this to
> > ensure a non-relaxed write is ordered with respect to a relaxed write?
> 
> Well, so the non-relaxed writel would have to do:
> 
> 	sync
> 	store
> 	sync
> 
> The first sync is to synchronize with DMAs, so that a sequence of
> 
> 	store to mem
> 	writel
> 
> Remains ordered vs. the device (ie, when the writel causes the device
> to do a DMA, it will see the previous store to mem).
> 
> The second sync is needed as mmiowb, to order with unlocks.

Ah yeah, thanks. I was so hung up on the ordering against locks that I
completely forgot about DMA!

> At this point, I'm keen on keeping my per-cpu trick to avoid that
> second one in most cases.

Makes sense. The alternative is dropping that requirement and instead
relying on drivers to use mmiowb() even with the non-relaxed accessors,
but I think that's going to be fairly painful (and hence why you have the
trick to start with).

Will