linux-kernel - Re: [PATCH 5/9] PCI: host: brcmstb: add dma-ranges for inbound traffic

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Message-ID: <20171018065316.GA11183@lst.de>
Date:   Wed, 18 Oct 2017 08:53:16 +0200
From:   Christoph Hellwig <hch@....de>
To:     Jim Quinlan <jim2101024@...il.com>
Cc:     Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>,
        linux-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        linux-mips@...ux-mips.org, Florian Fainelli <f.fainelli@...il.com>,
        devicetree@...r.kernel.org, linux-pci <linux-pci@...r.kernel.org>,
        Kevin Cernekee <cernekee@...il.com>,
        Will Deacon <will.deacon@....com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Rob Herring <robh+dt@...nel.org>,
        bcm-kernel-feedback-list <bcm-kernel-feedback-list@...adcom.com>,
        Gregory Fong <gregory.0xf0@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Brian Norris <computersforpeace@...il.com>,
        linux-arm-kernel@...ts.infradead.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        iommu@...ts.linux-foundation.org
Subject: Re: [PATCH 5/9] PCI: host: brcmstb: add dma-ranges for inbound
        traffic

On Tue, Oct 17, 2017 at 12:11:55PM -0400, Jim Quinlan wrote:
> My understanding is that dma_pfn_offset is that it is a single
> constant offset from RAM, in our case, to map to PCIe space.

Yes.

> But in
> my commit message I detail how our PCIe controller presents memory
> with multiple regions with multiple different offsets. If an EP device
> maps to a region on the host memory, yes we can set the dma_pfn_offset
> for that device for that location within that range,.  But if the
> device then unmaps and allocates from another region with a different
> offset, it won't work.  If  I set dma_pfn_offset I have to assume that
> the device is using only one region of memory only, not more than one,
> and that it is not unmapping that region and mapping another (with a
> different offset).  Can I make those assumptions?

No, we can't make that assumption unfortunately.  But how is your
code going to work if the mapping spans multiple of your translation
regions?

Also I really don't think the stacking of dma_ops as in this patch
is a good idea.  For MIPS you should do the variant suggested in
the patch description and hook into dma_to_phys/phys_to_dma helpers,
and for ARM/ARM64 you should talk to the maintainers on how they
want the translation integrated.