On Mon, Apr 3, 2017 at 3:10 PM, Logan Gunthorpe <logang@xxxxxxxxxxxx> wrote: > > > On 03/04/17 03:44 PM, Dan Williams wrote: >> On Mon, Apr 3, 2017 at 2:20 PM, Logan Gunthorpe <logang@xxxxxxxxxxxx> wrote: >>> Hi Christoph, >>> >>> What are your thoughts on an approach like the following untested >>> draft patch. >>> >>> The patch (if fleshed out) makes it so iomem can be used in an sgl >>> and WARN_ONs will occur in places where drivers attempt to access >>> iomem directly through the sgl. >>> >>> I'd also probably create a p2pmem_alloc_sgl helper function so driver >>> writers wouldn't have to mess with sg_set_iomem_page. >>> >>> With all that in place, it should be relatively safe for drivers to >>> implement p2pmem even though we'd still technically be violating the >>> __iomem boundary in some places. >> >> Just reacting to this mail, I still haven't had a chance to take a >> look at the rest of the series. >> >> The pfn_t type was invented to carry extra type and page lookup >> information about the memory behind a given pfn. At first glance that >> seems a more natural place to carry an indication that this is an >> "I/O" pfn. > > I agree... But what are the plans for pfn_t? Is anyone working on using > it in the scatterlist code? Currently it's not there yet and given the > assertion that we will continue to be using struct page for DMA is that > a direction we'd want to go? > I wouldn't necessarily conflate supporting pfn_t in the scatterlist with the stalled stuct-page-less DMA effor. A pfn_t_to_page() conversion will still work and be required. However you're right, the minute we use pfn_t for this we're into the realm of special case drivers that understand scatterlists with special "I/O-pfn_t" entries. However, maybe that's what we want? I think peer-to-peer DMA is not a general purpose feature unless/until we get it standardized in PCI. So maybe drivers with special case scatterlist support is exactly what we want for now. Thoughts?
