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

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Message-ID: <20161205180231.GA28133@obsidianresearch.com>
Date:   Mon, 5 Dec 2016 11:02:31 -0700
From:   Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Stephen Bates <sbates@...thlin.com>,
        Haggai Eran <haggaie@...lanox.com>,
        Logan Gunthorpe <logang@...tatee.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 Mon, Dec 05, 2016 at 09:40:38AM -0800, Dan Williams wrote:

> > If it is kernel only with physical addresess we don't need a uAPI for
> > it, so I'm not sure #1 is at all related to iopmem.
> >
> > Most people who want #1 probably can just mmap
> > /sys/../pci/../resourceX to get a user handle to it, or pass around
> > __iomem pointers in the kernel. This has been asked for before with
> > RDMA.
> >
> > I'm still not really clear what iopmem is for, or why DAX should ever
> > be involved in this..
> 
> Right, by default remap_pfn_range() does not establish DMA capable
> mappings. You can think of iopmem as remap_pfn_range() converted to
> use devm_memremap_pages(). Given the extra constraints of
> devm_memremap_pages() it seems reasonable to have those DMA capable
> mappings be optionally established via a separate driver.

Except the iopmem driver claims the PCI ID, and presents a block
interface which is really *NOT* what people who have asked for this in
the past have wanted. IIRC it was embedded stuff eg RDMA video
directly out of a capture card or a similar kind of thinking.

It is a good point about devm_memremap_pages limitations, but maybe
that just says to create a /sys/.../resource_dmableX ?

Or is there some reason why people want a filesystem on top of BAR
memory? That does not seem to have been covered yet..

Jason