Re: PCI DMA to small buffers on cache-incoherent arch

Tom Rini
2002-06-10T17:03:18Z

On Mon, Jun 10, 2002 at 08:59:48AM -0700, Roland Dreier wrote:
> >>>>> "David" == David S Miller <davem@redhat.com> writes:
>
> Paul> This is the problem scenario. Suppose we are doing DMA to a
> Paul> buffer B and also independently accessing a variable X which
> Paul> is not part of B. Suppose that X and the beginning of B are
> Paul> both in cache line C.
>
> David> I see what the problem is. Hmmm...
>
> David> I'm trying to specify this such that knowledge of
> David> cachelines and whatnot don't escape the arch specific code,
> David> ho hum... Looks like that isn't possible.
>
> So is the consensus now that in general drivers should make sure any
> buffers passed to pci_map/unmap are aligned to SMP_CACHE_BYTES (and a
> multiple of SMP_CACHE_BYTES in size)? In other words if a driver uses
> an unaligned buffer it should be fixed unless we can prove (and
> document in a comment :) that it won't cause problems on an arch
> without cache coherency and with a writeback cache.

And how about we don't call it SMP_CACHE_BYTES too? The processors
where this matters certainly aren't doing SMP...

--
Tom Rini (TR1265)