linux-kernel - Re: [PATCH 4/8] PCI: host: brcmstb: add dma-ranges for inbound traffic

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Date:   Wed, 25 Oct 2017 12:00:03 -0400
From:   Jim Quinlan <jim2101024@...il.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Brian Norris <computersforpeace@...il.com>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@....de>,
        Florian Fainelli <f.fainelli@...il.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "bcm-kernel-feedback-list@...adcom.com" 
        <bcm-kernel-feedback-list@...adcom.com>,
        Gregory Fong <gregory.0xf0@...il.com>,
        Kevin Cernekee <cernekee@...il.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-mips@...ux-mips.org" <linux-mips@...ux-mips.org>,
        Ralf Baechle <ralf@...ux-mips.org>
Subject: Re: [PATCH 4/8] PCI: host: brcmstb: add dma-ranges for inbound traffic

On Wed, Oct 25, 2017 at 5:46 AM, David Laight <David.Laight@...lab.com> wrote:
> From: Jim QuinlanPCIE_IPROC_MSI
>> Sent: 24 October 2017 19:16
>> The Broadcom STB PCIe host controller is intimately related to the
>> memory subsystem.  This close relationship adds complexity to how cpu
>> system memory is mapped to PCIe memory.  Ideally, this mapping is an
>> identity mapping, or an identity mapping off by a constant.  Not so in
>> this case.
>>
>> Consider the Broadcom reference board BCM97445LCC_4X8 which has 6 GB
>> of system memory.  Here is how the PCIe controller maps the
>> system memory to PCIe memory:
>>
>>   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]
>
> I presume the first column is the 'cpu physical address'
> and the second the 'pci' address?
>
Yes.  I probably made this more difficult to read because I ordered
the rows by PCI addresses.

> ...
>
> Isn't this just the same as having an iommu that converts 'bus'
> addresses into 'physical' ones?

Pretty much, but for PCIe devices only.  This could be done by somehow
overriding the arch specific phys_to_dma() and dma_to_phys() calls.

>
> A simple table lookup of the high address bits will do the
> conversion.
True, but this table could be passed something like ARM_MAPPING_ERROR,
which may be out the table (the driver is not privy to
ARM_MAPPING_ERROR's definition).

Thanks,
Jim

>
>         David
>