Subject: Re: [PATCH 2/4] arch: Add lightweight memory barriers fast_rmb() and fast_wmb()

From: Will Deacon <[email-masked]>
Date: Tue, 18 Nov 2014 11:58:36 +0000

On Tue, Nov 18, 2014 at 03:13:29AM +0000, Alexander Duyck wrote:
> On 11/17/2014 04:39 PM, Benjamin Herrenschmidt wrote:
> > On Mon, 2014-11-17 at 12:24 -0800, Alexander Duyck wrote:
> >> Yes and no.  So for example on ARM I used the dmb() operation, however
> >> I
> >> have to use the barrier at the system level instead of just the inner
> >> shared domain.  However on many other architectures they are just the
> >> same as the smp_* variants.
> >>
> >> Basically the resultant code is somewhere between the smp and non-smp
> >> barriers in terms of what they cover.
> > There I don't quite follow you. You need to explain better especially in
> > the documentation because otherwise people will get it wrong...
> >
> > If it's ordering in the coherent domain, I fail to see how a DMA agent
> > is different than another processor when it comes to barriers, so I fail
> > to see the difference with smp_*
> >
> > I understand the MMIO vs. memory issue, we do have the same on powerpc,
> > but that other aspect eludes me.
> >
> 
> ARM adds some funky things.  They have two different types of 
> primitives, a dmb() which is a data memory barrier, and a dsb() which is 
> a data synchronization barrier.  Then with each of those they have the 
> "domains" the barriers are effective within.
> 
> So for example on ARM a rmb() is dsb(sy) which means it is a system wide 
> synchronization barrier which stops execution on the CPU core until the 
> read completes.  However the smp_rmb() is a dmb(ish) which means it is 
> only a barrier as far as the inner shareable domain which I believe only 
> goes as far as the local shared cache hierarchy and only guarantees read 
> ordering without necessarily halting the CPU or stopping in-order 
> speculative reads.  So what a coherent_rmb() would be in my setup is 
> dmb(sy) which means the barrier runs all the way out to memory, and it 
> is allowed to speculative read as long as it does it in order.
> 
> If it is still unclear you might check out Will Deacon's talk on the 
> topic at https://www.youtube.com/watch?v=6ORn6_35kKo, at about 7:00 in 
> he explains the whole domains thing, and at 13:30 he explains dmb()/dsb().

So actually, this is an interesting case where the barrier would like to
know whether the memory returned by dma_alloc_coherent is h/w coherent
(normal, cacheable) or s/w coherent (normal, non-cacheable). I think Ben
is thinking of the h/w coherent case (i.e. actual snooping into the CPU
caches by the DMA master).

For the former, we could use inner-shareable barriers. For the latter, we'd
need to use outer-shareable barriers.

If we can't tell, then these should be dmb(osh), which will work for both.

Will