On 2019-01-30 12:59 p.m., Jason Gunthorpe wrote: > On Wed, Jan 30, 2019 at 12:45:46PM -0700, Logan Gunthorpe wrote: >> >> >> On 2019-01-30 12:06 p.m., Jason Gunthorpe wrote: >>>> Way less problems than not having struct page for doing anything >>>> non-trivial. If you map the BAR to userspace with remap_pfn_range >>>> and friends the mapping is indeed very simple. But any operation >>>> that expects a page structure, which is at least everything using >>>> get_user_pages won't work. >>> >>> GUP doesn't work anyhow today, and won't work with BAR struct pages in >>> the forseeable future (Logan has sent attempts on this before). >> >> I don't recall ever attempting that... But patching GUP for special >> pages or VMAS; or working around by not calling it in some cases seems >> like the thing that's going to need to be done one way or another. > > Remember, the long discussion we had about how to get the IOMEM > annotation into SGL? That is a necessary pre-condition to doing > anything with GUP in DMA using drivers as GUP -> SGL -> DMA map is > pretty much the standard flow. Yes, but that was unrelated to GUP even if that might have been the eventual direction. And I feel the GUP->SGL->DMA flow should still be what we are aiming for. Even if we need a special GUP for special pages, and a special DMA map; and the SGL still has to be homogenous.... > So, I see Jerome solving the GUP problem by replacing GUP entirely > using an API that is more suited to what these sorts of drivers > actually need. Yes, this is what I'm expecting and what I want. Not bypassing the whole thing by doing special things with VMAs. Logan
