From mboxrd@z Thu Jan 1 00:00:00 1970 From: saeed bishara Subject: RAID/dmaengine violates the dma-streaming API Date: Sun, 17 Apr 2011 19:00:52 +0300 Message-ID: Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Return-path: Sender: linux-raid-owner@vger.kernel.org To: Dan Williams , linux-raid@vger.kernel.org, linux-arm-kernel List-Id: linux-raid.ids 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