Hi Anurag, On Fri, Jun 07, 2019 at 09:49:59AM +0300, Felipe Balbi wrote: > Anurag Kumar Vulisha <anuragku@xxxxxxxxxx> writes: > >>> The dma_map_sg() merges sg1 & sg2 memory regions into sg1- > >>>dma_address. > >>> Similarly sg3 & sg4 into sg2->dma_address, sg5 & sg6 into the > >>> sg3->dma_address and sg6 & sg8 into sg4->dma_address. Here the > >>memory > >>> regions are merged but the page_link properties like SG_END are not > >>> retained into the merged sgs. > >> > >>isn't this a bug in the scatterlist mapping code? Why doesn't it keep > >>SG_END? > >> > > > > Thanks for providing your comment. > > > > I don't think it is a bug, instead I feel some enhancement needs to be done in > > dma-mapping code. > > > > SG_END represents the last sg entry in the sglist and it is correctly getting > > set to the last sg entry. > > > > The issue happens only when 2 or more sg entry pages are merged into > > contiguous dma-able address and sg_is_last() is used to find the last sg entry > > with valid dma address. > > Right, and that's something that's bound to happen. I'm arguing that, perhaps, > dma API should move SG_END in case entries are merged. > > > I think that along with sg_is_last() a new flag (SG_DMA_END) and function > > (something like sg_dma_is_last() ) needs to be added into dma-mapping code for > > identifying the last valid sg entry with valid dma address. So that we can > > make use of that function instead of sg_is_last(). > > Sure, propose a patch to DMA API. I'm curious if this was ever resolved. I just ran into this exact issue with Android ADB which uses 16KB buffers, along with f_fs supporting S/G since 5.0, combined with our IOMMU which performs this merging behavior, so it resulted in a single TRB getting queued with CHN=1 and LST=0 and thus the transfer never completes. Your initial patch resolves the issue for me, but upon revisiting this discussion I couldn't tell if you had attempted to patch DMA API instead as per Felipe's suggestion. Thanks, Jack -- The Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
