Re: [PATCH 11/16] iommu/dma: Support PCI P2PDMA pages in dma-iommu map_sg

[Jason Gunthorpe]

On Thu, Apr 08, 2021 at 11:01:18AM -0600, Logan Gunthorpe wrote:
> When a PCI P2PDMA page is seen, set the IOVA length of the segment
> to zero so that it is not mapped into the IOVA. Then, in finalise_sg(),
> apply the appropriate bus address to the segment. The IOVA is not
> created if the scatterlist only consists of P2PDMA pages.

I expect P2P to work with systems that use ATS, so we'd want to see
those systems have the IOMMU programmed with the bus address.

Is it OK like this because the other logic prohibits all PCI cases
that would lean on the IOMMU, like ATS, hairpinning through the root
port, or transiting the root complex?

If yes, the code deserves a big comment explaining this is incomplete,
and I'd want to know we can finish this to include ATS at least based
on this series.

Jason