On Thu, Mar 28, 2019 at 08:09:23PM +0200, Gal Pressman wrote: > >> I'm not entirely up to date with Shiraz's work, wanna make sure I understand: > >> Currently the umem sgl is built out of PAGE_SIZE elements. The reason we can't > >> assume each element is PAGE_SIZE long is because the DMA mapping (iommu) might > >> combine multiple elements or because Shiraz's work is going to combine the > >> elements manually? > > > > That is the existing reason, but this patch: > > > > https://patchwork.kernel.org/patch/10875419/ > > > > Just directly makes larger SGL. > > Why do we want to combine pages if the iommu does that for us? Isn't > the only thing we need here is Shiraz's iterator that uses the device > supported page sizes? Only some iommu's do it (and sometimes only in some configurations), and it doesn't happen if the iommu isn't used. We want a consistent operation for IB drivers. > >> Reading the documentation, sg_page_iter kinda looks like it covers > >> sg_dma_page_iter functionality? Don't they both retrieve the DMA > >> address page by page? Looks like the only difference is that in > >> sg_page_iter I can get the struct page as well? > > > > No. > > > > A SGL contains two lists of different size, a 'dma mapped' list and a > > 'CPU page list'. The act of DMA mapping creates the first list. > > > > These lists cannot be intermixed, if you run over the CPU page list > > you can't get DMA addresses, and vice versa. > > The sg_page_iter documentation: > > /* > * sg page iterator > * > * Iterates over sg entries page-by-page. On each successful iteration, you > * can call sg_page_iter_page(@piter) to get the current page and its dma > * address. @piter->sg will point to the sg holding this page and > * @piter->sg_pgoffset to the page's page offset within the sg. The iteration > * will stop either when a maximum number of sg entries was reached or a > * terminating sg (sg_last(sg) == true) was reached. > */ > > It says you can call sg_page_iter_page() to get the current page and its dma > address, should the documentation be updated to remove this part? I missed that one 'dma address' when I fixed this :\ Send a patch? Jason
