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

[Jim Quinlan]

On Wed, Oct 25, 2017 at 5:46 AM, David Laight <David.Laight@xxxxxxxxxx> 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
>