Re: consistent_sync_for_cpu() and friends on ppc32

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

> See, the direction really doesn't matter for the sync ops.

Well, the direction makes the difference between a flush and an
invalidation ;)

> If you flush the cpu caches at MAP time, and your PCI controller doesn't
> have DMA caching or something like that, then sync for CPU can always be
> a nop.  You will have always previously flushed the cpu caches before
> giving the buffer back to the device, either via MAP or sync for device
> calls.
> 
> So basically, make MAP and sync for device writeback flush the cpu caches.

No, flush on TO_DEVICE and BIDIRECTIONAL, invalidate on FROM_DEVICE,
it's less expensive to invalidate than flush in that case, since we
don't care about writing to real memory whatever junk the cache
contained for this area.

Ben.