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

[Christoph Hellwig]

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.

> GPU driver
> 
> https://lore.kernel.org/dri-devel/20240117221223.18540-7-oak.zeng@xxxxxxxxx/

Eww, that's horrible.  Converting this to Leon's new hmm helpers
would be really nice (and how that they are useful for more than
mlx5).