Re: RAID/dmaengine violates the dma-streaming API

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Wed, Apr 27, 2011 at 02:42:57PM +0300, saeed bishara wrote:
> On Sun, Apr 17, 2011 at 7:00 PM, saeed bishara <saeed.bishara@gmail.com> wrote:
> > 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!!.
> >
> Russell/Dan,
>      can you have a look into this issue? what I see here is that the
> raid stack issues dma_map_page to a buffer that still owned by DMA.

I already mentioned this issue to Dan, and pointed out that it's
a violation of the buffer ownership rules.  I don't remember clearly
what the outcome of it is, but there's not a lot which can be done at
architecture level about it.

I think the buffer mapping was going to be moved upwards, to prevent
the multiple buffer mapping issue.  I don't know if patches were
produced though (and I don't have hardware to be able to produce and
test such patches against.)
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