On 17/11/14 14:11, Stefano Stabellini wrote: > Hi all, > I am writing this email to ask for your advice. > > On architectures where dma addresses are different from physical > addresses, it can be difficult to retrieve the physical address of a > page from its dma address. > > Specifically this is the case for Xen on arm and arm64 but I think that > other architectures might have the same issue. > > Knowing the physical address is necessary to be able to issue any > required cache maintenance operations when unmap_page, > sync_single_for_cpu and sync_single_for_device are called. > > Adding a struct page* parameter to unmap_page, sync_single_for_cpu and > sync_single_for_device would make Linux dma handling on Xen on arm and > arm64 much easier and quicker.
Using an opaque handle instead of struct page * would be more beneficial for the Intel IOMMU driver. e.g., typedef dma_addr_t dma_handle_t; dma_handle_t dma_map_single(struct device *dev, void *va, size_t size, enum dma_data_direction dir); void dma_unmap_single(struct device *dev, dma_handle_t handle, size_t size, enum dma_data_direction dir); etc. Drivers would then use: dma_addr_t dma_addr(dma_handle_t handle); To obtain the bus address from the handle. > I think that other drivers have similar problems, such as the Intel > IOMMU driver having to call find_iova and walking down an rbtree to get > the physical address in its implementation of unmap_page. > > Callers have the struct page* in their hands already from the previous > map_page call so it shouldn't be an issue for them. A problem does > exist however: there are about 280 callers of dma_unmap_page and > pci_unmap_page. We have even more callers of the dma_sync_single_for_* > functions. You will also need to fix dma_unmap_single() and pci_unmap_single() (another 1000+ callers). You may need to consider a parallel set of map/unmap API calls that return/accept a handle, and then converting drivers one-by-one as required, instead of trying to convert every single driver at once. David