linux-kernel - Re: [PATCH v5 04/12] PCI: brcmstb: add dma-range mapping for inbound traffic

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Date:   Thu, 20 Sep 2018 14:04:36 -0700
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Jim Quinlan <jim2101024@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        linux-pci <linux-pci@...r.kernel.org>,
        BCM Kernel Feedback <bcm-kernel-feedback-list@...adcom.com>,
        Gregory Fong <gregory.0xf0@...il.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Brian Norris <computersforpeace@...il.com>,
        Christoph Hellwig <hch@....de>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v5 04/12] PCI: brcmstb: add dma-range mapping for inbound traffic

On 20 September 2018 at 13:55, Florian Fainelli <f.fainelli@...il.com> wrote:
> On 09/19/2018 07:19 PM, Ard Biesheuvel wrote:
>> On 19 September 2018 at 07:31, Jim Quinlan <jim2101024@...il.com> wrote:
>>> 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]
>>>
>>
>> So is describing this as
>>
>> dma-ranges = <0x0 0x0 0x0 0x0 0x0 0x40000000>,
>>              <0x0 0x40000000 0x1 0x0 0x0 0x40000000>,
>>              <0x0 0x80000000 0x0 0x40000000 0x0 0x40000000>,
>>              <0x0 0xc0000000 0x3 0x0 0x0 0x40000000>,
>>              <0x1 0x0 0x0 0x80000000 0x0 0x40000000>,
>>              <0x1 0x40000000 0x0 0xc0000000 0x0 0x40000000>;
>>
>> not working for you? I haven't tried this myself, but since DT permits
>> describing the inbound mappings this way, we should fix the code if it
>> doesn't work at the moment.
>
> You mean encoding the memory controller index in the first cell? If that
> works, that's indeed a much cleaner solution, though is it standard
> compliant in any form?

No those are just memory addresses (although I may have screwed up the
order). From Documentation/devicetree/booting-without-of.txt:

"""
Optional property:
- dma-ranges: <prop-encoded-array> encoded as arbitrary number of triplets of
        (child-bus-address, parent-bus-address, length). Each triplet specified
        describes a contiguous DMA address range.
"""