On Wednesday 22 June 2011, Jordan Crouse wrote: > >> I have a query in similar lines, but related to user virtual address > >> space. Is it feasible to extend these DMA interfaces(and IOMMU), to map > >> a user allocated buffer into the hardware? > > > > This can be done with the current API, although it may not look so > > straightforward. You just need to create a scatter list of user pages > > (these can be gathered with get_user_pages function) and use dma_map_sg() > > function. If the dma-mapping support iommu, it can map all these pages > > into a single contiguous buffer on device (DMA) address space. > > > > Some additional 'magic' might be required to get access to pages that are > > mapped with pure PFN (VM_PFNMAP flag), but imho it still can be done. > > > > I will try to implement this feature in videobuf2-dma-config allocator > > together with the next version of my patches for dma-mapping&iommu. > > With luck DMA_ATTRIB_NO_KERNEL_MAPPING should remove any lingering arguments > for trying to map user pages. Given that our ultimate goal here is buffer > sharing, user allocated pages have limited value and appeal. If anything, I > vote that this be a far lower priority compared to the rest of the win you > have here. I agree. Mapping user-allocated buffers is extremely hard to get right when there are extra constraints. If it doesn't work already for some driver, I wouldn't put too much effort into making it work for more special cases. Arnd -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>