On 2018-04-09 11:03 AM, Peter Zijlstra wrote:
On Mon, Apr 09, 2018 at 07:45:22AM -0700, James Bottomley wrote:
https://parisc.wiki.kernel.org/images-parisc/7/73/Parisc2.0.pdf
I don't see the U-bit mentioned in the below blurb, and also, I see
no mention on why SYNCDMA is not needed.
Um, well U is uncached and is used mostly for memory based IO regions.
We also have the odd system that doesn't have the U bit, which is why
there's a network and a scsi driver with dma_cache_sync() in them.
The point here is that U being caching is nothing to do with ordering.
So I clearly had my reading skills disabled this morning (this wasn't
the only such occurence). In appending G, Memory Ordering Model, it
states that U=1 results in strongly ordered stuff, and for some reason I
read it as being required. But you're right, it's just one of many
possible cases.
The O-bit is indeed what we should be looking at.
A paranoid me would do mb() as SYNCDMA and __smp_mb() as SYNC, given
that the manual also states it NO-OPs these instructions when not
needed.
I think you misunderstand how ordering and caching work. Ordering is
the province of barriers and we insert the ordering into the code
assuming we're in the CPU domain (usually everything as seen by the
cpus is fully cache coherent on parisc). Caching is the province of
the coherence domains (usually CPU to bus) and we handle that in the
dma_ IO operations.
So we can quibble about whether we need a syncdma in our dma_
operations, we'd never need one in the execution barriers.
I was under the impression that mb() should (also) order against DMA and
MMIO things, while smp_mb() is just for the SMP/CPU domain.
Also note how with asm-generic/barrier.h the dma_*mb() things default to
being mb().
I have long wondered about this. The only memory instructions that are
weakly ordered
are the cache flush instructions and all the cache routines in pacache.S
have a SYNC instruction
at their end. The routines that are used for DMA have a SYNCDMA
instruction at
their end. I think this is why we haven't needed to define mb() so it
orders DMA and
MMIO things.
--
John David Anglin dave.anglin@xxxxxxxx
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