linux-kernel - Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory

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Message-ID: <1492555569.25766.99.camel@kernel.crashing.org>
Date:   Wed, 19 Apr 2017 08:46:09 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Dan Williams <dan.j.williams@...el.com>,
        Jason Gunthorpe <jgunthorpe@...idianresearch.com>
Cc:     Logan Gunthorpe <logang@...tatee.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Jens Axboe <axboe@...nel.dk>,
        Steve Wise <swise@...ngridcomputing.com>,
        Stephen Bates <sbates@...thlin.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Keith Busch <keith.busch@...el.com>, linux-pci@...r.kernel.org,
        linux-scsi <linux-scsi@...r.kernel.org>,
        linux-nvme@...ts.infradead.org, linux-rdma@...r.kernel.org,
        linux-nvdimm <linux-nvdimm@...1.01.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jerome Glisse <jglisse@...hat.com>
Subject: Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory

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.