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

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Message-ID: <CANCKTBsCB+x2XgrND9AhRtxPkCXfps1nA+YymkZjKHOUZfjSHQ@mail.gmail.com>
Date:   Tue, 17 Oct 2017 12:11:55 -0400
From:   Jim Quinlan <jim2101024@...il.com>
To:     Christoph Hellwig <hch@....de>
Cc:     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 4:14 AM, Christoph Hellwig <hch@....de> wrote:
> Just took a quick look over this and I basically agree with the comments
> from Robin.
>
> What I don't understand is why you're even trying to do all these
> hacky things.
>
> It seems like the controller should simply set dma_pfn_offset for
> each device hanging off it, and all the supported architectures
> should be updated to obey that if they don't do so yet, and
> you're done without needing this giant mess.

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.  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?