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

From: David S. Miller (
Date: Sun Jun 09 2002 - 00:29:03 EST

   From: Roland Dreier <>
   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.

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