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

["David S. Miller" <davem@redhat.com>]

   From: Roland Dreier <roland@topspin.com>
   Date: 08 Jun 2002 18:26:12 -0700

   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.
   
Yes.

       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?

For non-cacheline aligned chunks in the range "start" to "end" you
must perform a cache writeback and invalidate. To preserve the data
outside of the DMA range.