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Subject: Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>,
        Dan Williams
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Cc: Logan Gunthorpe <logang@deltatee.com>, Bjorn Helgaas
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Date: Wed, 19 Apr 2017 11:20:06 +1000
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On Tue, 2017-04-18 at 15:03 -0600, Jason Gunthorpe wrote:
> I don't follow, when does get_dma_ops() return a p2p aware provider?
> It has no way to know if the DMA is going to involve p2p, get_dma_ops
> is called with the device initiating the DMA.
> 
> So you'd always return the P2P shim on a system that has registered
> P2P memory?
> 
> Even so, how does this shim work? dma_ops are not really intended to
> be stacked. How would we make unmap work, for instance? What happens
> when the underlying iommu dma ops actually natively understands p2p
> and doesn't want the shim?

Good point. We only know on a per-page basis ... ugh.

So we really need to change the arch main dma_ops. I'm not opposed to
that. What we then need to do is have that main arch dma_map_sg,
when it encounters a "device" page, call into a helper attached to
the devmap to handle *that page*, providing sufficient context.

That helper wouldn't perform the actual iommu mapping. It would simply
return something along the lines of:

 - "use that alternate bus address and don't map in the iommu"
 - "use that alternate bus address and do map in the iommu"
 - "proceed as normal"
 - "fail"

What do you think ?

Cheers,
Ben.