On Thu, Jan 12, 2017 at 3:14 PM, Jerome Glisse <jglisse@xxxxxxxxxx> wrote: > On Thu, Jan 12, 2017 at 02:43:03PM -0800, Dan Williams wrote: >> Back when we were first attempting to support DMA for DAX mappings of >> persistent memory the plan was to forgo 'struct page' completely and >> develop a pfn-to-scatterlist capability for the dma-mapping-api. That >> effort died in this thread: >> >> https://lkml.org/lkml/2015/8/14/3 >> >> ...where we learned that the dependencies on struct page for dma >> mapping are deeper than a PFN_PHYS() conversion for some >> architectures. That was the moment we pivoted to ZONE_DEVICE and >> arranged for a 'struct page' to be available for any persistent memory >> range that needs to be the target of DMA. ZONE_DEVICE enables any >> device-driver that can target "System RAM" to also be able to target >> persistent memory through a DAX mapping. >> >> Since that time the "page-less" DAX path has continued to mature [1] >> without growing new dependencies on struct page, but at the same time >> continuing to rely on ZONE_DEVICE to satisfy get_user_pages(). >> >> Peer-to-peer DMA appears to be evolving from a niche embedded use case >> to something general purpose platforms will need to comprehend. The >> "map_peer_resource" [2] approach looks to be headed to the same >> destination as the pfn-to-scatterlist effort. It's difficult to avoid >> 'struct page' for describing DMA operations without custom driver >> code. >> >> With that background, a statement and a question to discuss at LSF/MM: >> >> General purpose DMA, i.e. any DMA setup through the dma-mapping-api, >> requires pfn_to_page() support across the entire physical address >> range mapped. > > Note that in my case it is even worse. The pfn of the page does not > correspond to anything so it need to go through a special function > to find if a page can be mapped for another device and to provide a > valid pfn at which the page can be access by other device. I still haven't quite wrapped my head about how these pfn ranges are created. Would this be a use case for a new pfn_t flag? It doesn't sound like something we'd want to risk describing with raw 'unsigned long' pfns. -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
