RAID/dmaengine violates the dma-streaming API

[saeed bishara <saeed.bishara@gmail.com>]

Hi,
     when md uses the dma for offloading xor and memcpy operations, it
violates the dma-mapping API.  here is the scenario I'm taking about
(under write to degraded raid5):
1. ops_run_prexor sends xor operation from buffers A and B, and the
destination is A.
2. ops_run_biodrain: sends mempcy operation from C to B.
3. ops_run_reconstruct5: sends xor operation from A and B, and the
destination is A again.

in step 1, the async tx maps A using dma_map_page, and in step 3, it
maps again the same buffer. but, if the request from step 1 still
being handled the dma engine, then we end with a case where the buffer
mapped while it still belongs to the dma hw.
when the arch is ARMv6/SMP mode (without io coherency), the cache
maintenance involves read/write access to the buffers, that means, the
second mapping above may access the buffer(with read/write) while the
dma is writing to it!!.

also, from performance sake, it would be great if this multiple
mappings get avoided.

saeed