On Thu, Jun 18, 2015 at 11:33:24AM +0200, Boris Brezillon wrote: > Hi Russel, > > On Thu, 18 Jun 2015 10:04:00 +0100 > Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx> wrote: > > > On Wed, Jun 17, 2015 at 05:50:01PM +0800, Herbert Xu wrote: > > > On Wed, Jun 17, 2015 at 09:45:33AM +0200, Boris Brezillon wrote: > > > > > > > > + ret = dma_map_sg(cesa_dev->dev, req->src, creq->src_nents, > > > > + DMA_TO_DEVICE); > > > > + if (!ret) > > > > + return -ENOMEM; > > > > + > > > > + creq->src_nents = ret; > > > > > > DMA-API-HOWTO says that you must retain the original nents and > > > use it when you call dma_unmap_sg. So I'm afraid one more repost > > > is needed :) > > > > It's worse than that... You're right on that point, but there's an > > additional point. > > > > If dma_map_sg() coalesces scatterlist entries, then ret will be smaller > > than src_nents, and ret indicates how many scatterlist entries to be > > walked during DMA - you should not use src_nents for that. I couldn't > > see where the driver used that information. In fact, the driver seems > > to be capable of walking more than src_nents/ret numbers of scatterlist > > entries: it just keeps going with sg_next() until it hits the end of > > the allocated scatterlist. > > Yes, I realized that, and I never used the value returned by > dma_map_sg() to walk the scatterlist anyway: I was using the sg_next() > and sg->length value (which I replaced by sg_dma_len() in v7 as > suggested by Herbert). > So the ->src_nents assignment to dma_map_sg() return value was just a > silly mistake caused by an uncareful read of the DMA-API-HOWTO. > > Am I missing something else ? Yes. 'ret' should be used to indicate the number of scatterlist entries to walk for DMA purposes after the scatterlist has been mapped. For PIO purposes, using src_nents is still acceptable. As Herbert points out, you're stopping after the sum of transferred bytes matches, so I suppose that's fine. One other point though: you should use sg_dma_address() rather than dereferencing sg->dma_address directly. -- FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up according to speedtest.net. -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
