On Sat, Jul 04, 2009 at 11:41:46PM +0800, Ming Lei wrote:
quoted text
> dma_map_*() can invalidate cache lines, so dma_unmap_*() may do nothing.
> But if someone accesses the dma buffer during dma transfer, you may
> get the  inconsistent result.

You missed an important statement: accessing the dma buffer during a dma
transfer is not permitted without an intervening call to the dma_sync_*
functions.

DMA buffers have the idea of "ownership" - they initially start off being
owned by the CPU.  Upon dma_map_*(), ownership transfers to the DMA agent,
and remains there until either a dma_unmap_*() or dma_sync_single_for_cpu().
In the case of dma_sync_single_for_cpu(), it can only be given back to the
DMA agent using dma_sync_single_for_device().

The important point from the above is that _only_ the current owner may
access the buffer.  So, if you've mapped a buffer, accessing the buffer
subsequently without a call to dma_sync_single_for_cpu() is illegal.
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