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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: Dan Williams <dan.j.williams@intel.com>,
        Jason Gunthorpe
	 <jgunthorpe@obsidianresearch.com>
Cc: Logan Gunthorpe <logang@deltatee.com>, Bjorn Helgaas
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        Christoph Hellwig <hch@lst.de>, Sagi Grimberg
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        Jens Axboe
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        Steve Wise <swise@opengridcomputing.com>,
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        Jerome
 Glisse <jglisse@redhat.com>
Date: Wed, 19 Apr 2017 08:46:09 +1000
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On Tue, 2017-04-18 at 10:27 -0700, Dan Williams wrote:
> > FWIW, RDMA probably wouldn't want to use a p2mem device either, we
> > already have APIs that map BAR memory to user space, and would like to
> > keep using them. A 'enable P2P for bar' helper function sounds better
> > to me.
> 
> ...and I think it's not a helper function as much as asking the bus
> provider "can these two device dma to each other". The "helper" is the
> dma api redirecting through a software-iommu that handles bus address
> translation differently than it would handle host memory dma mapping.

Do we even need tat function ? The dma_ops have a dma_supported()
call...

If we have those override ops built into the "dma_target" object,
then these things can make that decision knowing both the source
and target device.

Cheers,
Ben.