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

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Date:   Fri, 20 Oct 2017 18:17:19 +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 <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 Fri, Oct 20, 2017 at 11:27:41AM -0400, Jim Quinlan wrote:
>       memc0-a@[        0....3fffefff] <=> pci@[        0....3fffefff]
>       memc0-b@[100000000...13fffefff] <=> pci@[ 40000000....7fffefff]
>       memc1-a@[ 40000000....7fffefff] <=> pci@[ 80000000....bfffefff]
>       memc1-b@[300000000...33fffefff] <=> pci@[ c0000000....ffffefff]
>       memc2-a@[ 80000000....bfffefff] <=> pci@[100000000...13fffefff]
>       memc2-b@[c00000000...c3fffffff] <=> pci@[140000000...17fffffff]
> 
> This leaves a one-page gap between phsyical memory regions which would
> normally be contiguous. One cannot have a dma alloc that spans any two
> regions.  This is a drastic step, but I don't see an alternative.
> Perhaps  I may be missing what you are saying...

Ok, IFF we are guranteed to always have a gap between physical memory
locations we are fine indeed.