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

Roland Dreier
2002-06-10T15:59:54Z

>>>>> "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.

Thanks,
Roland