On Mon, Dec 5, 2016 at 9:18 AM, Jason Gunthorpe <jgunthorpe@xxxxxxxxxxxxxxxxxxxx> wrote: > On Sun, Dec 04, 2016 at 07:23:00AM -0600, Stephen Bates wrote: >> Hi All >> >> This has been a great thread (thanks to Alex for kicking it off) and I >> wanted to jump in and maybe try and put some summary around the >> discussion. I also wanted to propose we include this as a topic for LFS/MM >> because I think we need more discussion on the best way to add this >> functionality to the kernel. >> >> As far as I can tell the people looking for P2P support in the kernel fall >> into two main camps: >> >> 1. Those who simply want to expose static BARs on PCIe devices that can be >> used as the source/destination for DMAs from another PCIe device. This >> group has no need for memory invalidation and are happy to use >> physical/bus addresses and not virtual addresses. > > I didn't think there was much on this topic except for the CMB > thing.. Even that is really a mapped kernel address.. > >> I think something like the iopmem patches Logan and I submitted recently >> come close to addressing use case 1. There are some issues around >> routability but based on feedback to date that does not seem to be a >> show-stopper for an initial inclusion. > > If it is kernel only with physical addresess we don't need a uAPI for > it, so I'm not sure #1 is at all related to iopmem. > > Most people who want #1 probably can just mmap > /sys/../pci/../resourceX to get a user handle to it, or pass around > __iomem pointers in the kernel. This has been asked for before with > RDMA. > > I'm still not really clear what iopmem is for, or why DAX should ever > be involved in this.. Right, by default remap_pfn_range() does not establish DMA capable mappings. You can think of iopmem as remap_pfn_range() converted to use devm_memremap_pages(). Given the extra constraints of devm_memremap_pages() it seems reasonable to have those DMA capable mappings be optionally established via a separate driver. -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
