On Thu, Nov 07, 2024 at 09:32:56AM +0100, Christoph Hellwig wrote:
> On Tue, Nov 05, 2024 at 03:53:57PM -0400, Jason Gunthorpe wrote:
> > > Yeah, I don't really get the struct page argument. In fact if we look
> > > at the nitty-gritty details of dma_map_page it doesn't really need a
> > > page at all.
> >
> > Today, if you want to map a P2P address you must have a struct page,
> > because page->pgmap is the only source of information on the P2P
> > topology.
> >
> > So the logic is, to get P2P without struct page we need a way to have
> > all the features of dma_map_sg() but without a mandatory scatterlist
> > because we cannot remove struct page from scatterlist.
>
> Well, that is true but also not the point. The hard part is to
> find the P2P routing information without the page. After that
> any physical address based interface will work, including a trivial
> dma_map_phys.
Once we are freed from scatterlist we can explore a design that would
pass the P2P routing information directly. For instance imagine
something like:
dma_map_p2p(dev, phys, p2p_provider);
Then dma_map_page(dev, page) could be something like
if (is_pci_p2pdma_page(page))
dev_map_p2p(dev, page_to_phys(page), page->pgmap->p2p_provider)
