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

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Date:   Thu, 19 Oct 2017 18:47:45 -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 Thu, Oct 19, 2017 at 5:16 AM, Christoph Hellwig <hch@....de> wrote:
> On Wed, Oct 18, 2017 at 10:41:17AM -0400, Jim Quinlan wrote:
>> That's what brcm_to_{pci,cpu} are for -- they keep a list of the
>> dma-ranges given in the PCIe DT node, and translate from system memory
>> addresses to pci-space addresses, and vice versa.  As long as people
>> are using the DMA API it should work.  It works for all of the ARM,
>> ARM64, and MIPS Broadcom systems I've tested, using eight different EP
>> devices.  Note that I am not thrilled to be advocating this mechanism
>> but it seemed the best alternative.
>
> Say we are using your original example ranges:
>
>  memc0-a@[        0....3fffffff] <=> pci@[        0....3fffffff]
>  memc0-b@[100000000...13fffffff] <=> pci@[ 40000000....7fffffff]
>  memc1-a@[ 40000000....7fffffff] <=> pci@[ 80000000....bfffffff]
>  memc1-b@[300000000...33fffffff] <=> pci@[ c0000000....ffffffff]
>  memc2-a@[ 80000000....bfffffff] <=> pci@[100000000...13fffffff]
>  memc2-b@[c00000000...c3fffffff] <=> pci@[140000000...17fffffff]
>
> and now you get a dma mapping request for physical addresses
> 3fffff00 to 4000000f, which would span two of your ranges.  How
> is this going to work?

The only way to prevent this is to reserve a single page at the end of
the first memory region of any pair that are adjacent in physical
memory.  A hack, yes, but I don't see an easier way out of this.  Many
if not most of our boards do not have adjacent regions and would not
need this.

Overriding phys_to_dma/dma_to_phys comes with the same overlap problem
(MIPS solution and possible ARM/ARM64 solution).

>
>> I would prefer that the same code work for all three architectures.
>> What I would like from ARM/ARM64 is the ability to override
>> phys_to_dma() and dma_to_phys(); I thought the chances of that being
>> accepted would be slim.  But you are right, I should ask the
>> maintainers.
>
> It is still better than trying to stack dma ops, which is a receipe
> for problems down the road.

Let me send out V2 of my patchset and also send it to the ARM/ARM64
maintainers as you suggested; perhaps there is an alternative.