Hi, Anurag Kumar Vulisha <anurag.kumar.vulisha@xxxxxxxxxx> writes: > As a process of preparing TRBs usb_gadget_map_request_by_dev() is > called from dwc3_prepare_trbs() for mapping the request. This will > call dma_map_sg() if req->num_sgs are greater than 0. dma_map_sg() > will map the sg entries in sglist and return the number of mapped SGs. > As a part of mapping, some sg entries having contigous memory may be > merged together into a single sg (when IOMMU used). So, the number of > mapped sg entries may not be equal to the number of orginal sg entries > in the request (req->num_sgs). > > As a part of preparing the TRBs, dwc3_prepare_one_trb_sg() iterates over > the sg entries present in the sglist and calls sg_is_last() to identify > whether the sg entry is last and set IOC bit for the last sg entry. The > sg_is_last() determines last sg if SG_END is set in sg->page_link. When > IOMMU used, dma_map_sg() merges 2 or more sgs into a single sg and it > doesn't retain the page_link properties. Because of this reason the > sg_is_last() may not find SG_END and thus resulting in IOC bit never > getting set. > > For example: > > Consider a request having 8 sg entries with each entry having a length of > 4096 bytes. Assume that sg1 & sg2, sg3 & sg4, sg5 & sg6, sg7 & sg8 are > having contigous memory regions. > > Before calling dma_map_sg(): > sg1-->sg2-->sg3-->sg4-->sg6-->sg7-->sg8 > dma_length: 4K 4K 4K 4K 4K 4K 4K > SG_END: False False False False False False True > num_sgs = 8 > num_mapped_sgs = 0 > > 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? -- balbi
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