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

[Roland Dreier <roland@topspin.com>]

>>>>> "David" == David S Miller <davem@redhat.com> writes:

    Roland> Or should we leave that usage unless it is observed
    Roland> causing problems (since we almost always get lucky and
    Roland> don't touch the rest of the cache line near the DMA)?
   
    David> I think passing in a 4 byte chunk and assuming the rest of
    David> the cacheline is unmodified is a valid expectation the more
    David> I think about it.

Just to make sure I'm reading this correctly, you're saying that as
long as a buffer is OK for DMA, it should be OK to use a
sub-cache-line chunk as a DMA buffer via pci_map_single(), and
accessing the rest of the cache line should be OK at any time before,
during and after the DMA.

    David> This means what MIPS is doing is wrong.  For partial
    David> cacheline bits it can't do the invalidate thing.

If I understand you, this means non-cache-coherent PPC is wrong as
well -- pci_map_single() goes through consistent_sync() and turns
into:

	case PCI_DMA_FROMDEVICE:	/* invalidate only */
		invalidate_dcache_range(start, end);
		break;

What alternate implementation are you proposing?

Thanks,
  Roland