Re: [PATCH 3/3] tile pci: enable IOMMU to support DMA for legacy devices

[Bjorn Helgaas]

On Wed, Jul 18, 2012 at 2:10 PM, Chris Metcalf <cmetcalf@xxxxxxxxxx> wrote:
> On 7/18/2012 12:50 PM, Bjorn Helgaas wrote:
>> On Wed, Jul 18, 2012 at 10:15 AM, Chris Metcalf <cmetcalf@xxxxxxxxxx> wrote:
>>> On 7/13/2012 1:25 PM, Bjorn Helgaas wrote:
>>>> On Fri, Jul 13, 2012 at 11:52:11AM -0400, Chris Metcalf wrote:
>>>>> We use the same pci_iomem_resource for different domains or host
>>>>> bridges, but the MMIO apertures for each bridge do not overlap because
>>>>> non-overlapping resource ranges are allocated for each domains.
>>>> You should not use the same pci_iomem_resource for different host bridges
>>>> because that tells the PCI core that everything in pci_iomem_resource is
>>>> available for devices under every host bridge, which I doubt is the case.
>>>>
>>>> The fact that your firmware assigns non-overlapping resources is good and
>>>> works now, but if the kernel ever needs to allocate resources itself,
>>> Actually, we were not using any firmware. It was indeed the kernel which
>>> allocates resources from the shared pci_iomem_resource.
>> Wow.  I wonder how that managed to work.  Is there some information
>> that would have helped the PCI core do the right allocations?  Or
>> maybe the host bridges forward everything they receive to PCI,
>> regardless of address, and any given MMIO address is only routed to
>> one of the host bridges because of the routing info in the page
>> tables?
>
> Since each host bridge contains non-overlapping ranges in its bridge config
> header, ioremap() locates the right host bridge for the target PCI resource
> address and programs the host bridge info into the MMIO mapping. The end
> result is the MMIO address is routed to the right host bridge. On Tile
> processors, different host bridges are like separate IO devices, in
> completely separate domains.
>
>> I guess in that case, the "apertures" would basically be
>> defined by the page tables, not by the host bridges.  But that still
>> doesn't explain how we would assign non-overlapping ranges to each
>> domain.
>
> Since all domains share the single resource, allocate_resource() "allocate
> empty slot in the resource tree", giving non-overlapping ranges to each
> devices.
>
> Just to confirm, I'm assuming I'll ask Linus to pull this code out of my
> tile tree when the merge window opens, right?  Would you like me to add
> your name to the commit as acked or reviewed?  Thanks!

Yep, just ask Linus to pull it; I don't think there's any need to
coordinate with my PCI tree since you're not using any interfaces we
changed in this cycle.

Reviewed-by: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
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