linux-kernel - Re: [PATCH 06/12] IB/core: Add optional PCI P2P flag to rdma_rw_ctx

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Message-ID: <3daea7fe-f64a-36a9-ca80-0cb4d9acf171@deltatee.com>
Date:   Mon, 8 Jan 2018 11:17:38 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jason Gunthorpe <jgg@...pe.ca>, Christoph Hellwig <hch@....de>
Cc:     linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-rdma@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-block@...r.kernel.org,
        Stephen Bates <sbates@...thlin.com>,
        Jens Axboe <axboe@...nel.dk>,
        Keith Busch <keith.busch@...el.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH 06/12] IB/core: Add optional PCI P2P flag to
 rdma_rw_ctx_[init|destroy]()



On 08/01/18 11:09 AM, Jason Gunthorpe wrote:
> It could, if we had a DMA op for p2p then the drivers that provide
> their own ops can implement it appropriately or not at all.

I was thinking of doing something like this. I'll probably rough out a 
patch and send it along today or tomorrow.

>> If at all it should be in the dma_map* wrappers, but for that we'd need
>> a good identifier.  And it still would not solve the whole fake dma
>> ops issue.
> 
> Very long term the IOMMUs under the ops will need to care about this,
> so the wrapper is not an optimal place to put it - but I wouldn't
> object if it gets it out of RDMA :)

Well, creating the extra op doesn't really change anything to the RDMA 
patch in this series. The point is, for the time being, we need to track 
whether we are doing a P2P or normal mapping using a side channel as we 
don't have a good way of tracking it in the SGL at this time.

Adding an extra DMA op is just one way to allow the existing DMA 
providers to opt-in/out.

Logan