On Wed, Apr 27, 2011 at 1:20 PM, Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx> wrote: > 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@xxxxxxxxx> 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.) > This is still on my plate and is waiting for me to get out from underneath the isci driver effort. I was thinking to push it all to md and kill the api, but then had an idea of an async_session data structure that could automatically marshal a chain of transfers between mapping domains. The fast path would be a chain that stays within one mapping domain, but the infrastructure would be able to devolve to support pathological cases like dma_domain1->cpu->dma_domain2 chains. So md would need to manipulate async_sessions, but it could rely on the async_tx api to handle dma mapping details. That's my 10,000 foot view at least. -- Dan -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
