Re: [PATCH 2/8] PCI: Add pci_find_common_upstream_dev()

Christian König
2018-03-28T19:44:29Z

Am 28.03.2018 um 20:57 schrieb Logan Gunthorpe:
>
> On 28/03/18 12:28 PM, Christian König wrote:
>> I'm just using amdgpu as blueprint because I'm the co-maintainer of it
>> and know it mostly inside out.
> Ah, I see.
>
>> The resource addresses are translated using dma_map_resource(). As far
>> as I know that should be sufficient to offload all the architecture
>> specific stuff to the DMA subsystem.
> It's not. The dma_map infrastructure currently has no concept of
> peer-to-peer mappings and is designed for system memory only. No
> architecture I'm aware of will translate PCI CPU addresses into PCI Bus
> addresses which is necessary for any transfer that doesn't go through
> the root complex (though on arches like x86 the CPU and Bus address
> happen to be the same). There's a lot of people that would like to see
> this change but it's likely going to be a long road before it does.

Well, isn't that exactly what dma_map_resource() is good for? As far as
I can see it makes sure IOMMU is aware of the access route and
translates a CPU address into a PCI Bus address.

> Furthermore, one of the reasons our patch-set avoids going through the
> root complex at all is that IOMMU drivers will need to be made aware
> that it is operating on P2P memory and do arch-specific things
> accordingly. There will also need to be flags that indicate whether a
> given IOMMU driver supports this. None of this work is done or easy.

I'm using that with the AMD IOMMU driver and at least there it works
perfectly fine.

>> Yeah, but not for ours. See if you want to do real peer 2 peer you need
>> to keep both the operation as well as the direction into account.
> Not sure what you are saying here... I'm pretty sure we are doing "real"
> peer 2 peer...
>
>> For example when you can do writes between A and B that doesn't mean
>> that writes between B and A work. And reads are generally less likely to
>> work than writes. etc...
> If both devices are behind a switch then the PCI spec guarantees that A
> can both read and write B and vice versa.

Sorry to say that, but I know a whole bunch of PCI devices which
horrible ignores that.

For example all AMD APUs fall under that category...

> Only once you involve root
> complexes do you have this problem. Ie. you have unknown support which
> may be no support, or partial support (stores but not loads); or
> sometimes bad performance; or a combination of both... and you need some
> way to figure out all this mess and that is hard. Whoever tries to
> implement a white list will have to sort all this out.

Yes, exactly and unfortunately it looks like I'm the poor guy who needs
to do this :)

Regards,
Christian.

>
> Logan
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