linux-kernel - Re: Enabling peer to peer device transactions for PCIe devices

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Date:   Mon, 5 Dec 2016 12:27:20 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        Dan Williams <dan.j.williams@...el.com>
Cc:     Stephen Bates <sbates@...thlin.com>,
        Haggai Eran <haggaie@...lanox.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-nvdimm@...1.01.org" <linux-nvdimm@...1.01.org>,
        "christian.koenig@....com" <christian.koenig@....com>,
        "Suravee.Suthikulpanit@....com" <suravee.suthikulpanit@....com>,
        "John.Bridgman@....com" <john.bridgman@....com>,
        "Alexander.Deucher@....com" <alexander.deucher@....com>,
        "Linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Max Gurtovoy <maxg@...lanox.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "serguei.sagalovitch@....com" <serguei.sagalovitch@....com>,
        "Paul.Blinzer@....com" <paul.blinzer@....com>,
        "Felix.Kuehling@....com" <felix.kuehling@....com>,
        "ben.sander@....com" <ben.sander@....com>
Subject: Re: Enabling peer to peer device transactions for PCIe devices



On 05/12/16 12:14 PM, Jason Gunthorpe wrote:
> But CMB sounds much more like the GPU case where there is a
> specialized allocator handing out the BAR to consumers, so I'm not
> sure a general purpose chardev makes a lot of sense?

I don't think it will ever need to be as complicated as the GPU case. 
There will probably only ever be a relatively small amount of memory 
behind the CMB and really the only users are those doing P2P work. Thus 
the specialized allocator could be pretty simple and I expect it would 
be fine to just return -ENOMEM if there is not enough memory.

Also, if it was implemented this way, if there was a need to make the 
allocator more complicated it could easily be added later as the 
userspace interface is just mmap to obtain a buffer.

Logan