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 14:33:16 -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 14:31, Florian Fainelli <f.fainelli@...il.com> wrote:
> On 09/20/2018 02:04 PM, Ard Biesheuvel wrote:
>> 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.
>> """
>>
>
> Then I am confused by your comment, that's what this patch does, it adds
> support for reading "dma-ranges" from Device Tree and setting up inbound
> windows using that. The only caveat is that because the PCIe root
> complex has some ties with the memory bus architecture it is connected
> to (SCB in our case) there is still a requirement to know the
> translation between a given physical address and its backing memory
> controller/aperture.
>

Ah ok, apologies for the noise then.

I was hoping that having working support for dma-ranges would remove
the need for the special phys<->dma conversion routines.