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Date:   Thu, 13 Apr 2017 18:26:31 -0500
From:   Bjorn Helgaas <>
To:     Logan Gunthorpe <>
Cc:     Benjamin Herrenschmidt <>,
        Christoph Hellwig <>,
        Sagi Grimberg <>,
        "James E.J. Bottomley" <>,
        "Martin K. Petersen" <>,
        Jens Axboe <>,
        Steve Wise <>,
        Stephen Bates <>,
        Max Gurtovoy <>,
        Dan Williams <>,
        Keith Busch <>,
        Jason Gunthorpe <>,,,,,,,
        Jerome Glisse <>
Subject: Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory

On Thu, Apr 13, 2017 at 03:22:06PM -0600, Logan Gunthorpe wrote:
> On 12/04/17 03:55 PM, Benjamin Herrenschmidt wrote:
> > Look at pcibios_resource_to_bus() and pcibios_bus_to_resource(). They
> > will perform the conversion between the struct resource content (CPU
> > physical address) and the actual PCI bus side address.
> Ah, thanks for the tip! On my system, this translation returns the same
> address so it was not necessary. And, yes, that means this would have to
> find its way into the dma mapping routine somehow. This means we'll
> eventually need a way to look-up the p2pmem device from the struct page.
> Which means we will likely need a new flag bit in the struct page or
> something. The big difficulty I see is testing. Do you know what
> architectures or in what circumstances are these translations used?

Any caller of pci_add_resource_offset() uses CPU addresses different from
the PCI bus addresses (unless the offset is zero, of course).  All ACPI
platforms also support this translation (see "translation_offset"), though
in most x86 systems the offset is zero.  I'm aware of one x86 system that
was tested with a non-zero offset but I don't think it was shipped that


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