[PATCH v1 00/17] Provide a new two step DMA mapping API

[Jason Gunthorpe]

On Thu, Nov 07, 2024 at 02:50:25PM +0100, Christoph Hellwig wrote:
> On Thu, Nov 07, 2024 at 09:28:08AM -0400, Jason Gunthorpe wrote:
> > 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)
> 
> One thing that this series does is to move the P2P mapping decisions out
> of the low-level dma mapping helpers and into the caller (again) for
> the non-sg callers and moves the special switch based bus mapping into
> a routine that can be called directly.
> 
> Take a look at blk_rq_dma_map_iter_start, which now literally uses
> dma_map_page for the no-iommu, no-switch P2P case.  It also is a good
> use case for the proposed dma_map_phys.

It is fully OK? Can't dma_map_page() trigger swiotlb? It must not do
that for P2P. How does it know the difference if it just gets a phys?

Jason