On Fri, Mar 27, 2020 at 6:31 PM Ruhl, Michael J <michael.j.ruhl@xxxxxxxxx> wrote: > Is there an example of what the scatterlist would look like in this case? > > Does each SG entry always have the page and dma info? or could you have > entries that have page information only, and entries that have dma info only? > > If the same entry has different size info (page_len = PAGE_SIZE, > dma_len = 4 * PAGE_SIZE?), are we guaranteed that the arrays (page and addrs) have > been sized correctly? > > Just trying to get my head wrapped around this. > > Thanks, > > Mike > My understanding is that page_len and dma_len in this case could have different values (looking at iommu_dma_map_sg within dma-iommu.c), this seems to add some padding calculated by using the device iova domain to s_length but sg_dma_len is set to the original length The scatterlists table can also get reduced down within "__finalise_sg" possibly causing (if reduced) the dma_len of the last table elements to be 0 (page_len would not be 0 in this case). Documentation around looping & accessing scatterlists in DMA-API.txt states that sg_dma_address() and sg_dma_len() should be used when accessing addr and len rather than sg->address and sg->length. Maybe it would be worth splitting this out into 2 functions to avoid potential issues with the above use case ? Regards, Shane Francis
