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

[Alex Deucher]

Sorry, didn't mean to drop the lists here. re-adding.

On Wed, Mar 28, 2018 at 4:05 PM, Alex Deucher <alexdeucher@xxxxxxxxx> wrote:
> On Wed, Mar 28, 2018 at 3:53 PM, Logan Gunthorpe <logang@xxxxxxxxxxxx> wrote:
>>
>>
>> On 28/03/18 01:44 PM, Christian KÃnig wrote:
>>> 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.
>>
>>> I'm using that with the AMD IOMMU driver and at least there it works
>>> perfectly fine.
>>
>> Yes, it would be nice, but no arch has implemented this yet. We are just
>> lucky in the x86 case because that arch is simple and doesn't need to do
>> anything for P2P (partially due to the Bus and CPU addresses being the
>> same). But in the general case, you can't rely on it.
>
> Could we do something for the arches where it works?  I feel like peer
> to peer has dragged out for years because everyone is trying to boil
> the ocean for all arches.  There are a huge number of use cases for
> peer to peer on these "simple" architectures which actually represent
> a good deal of the users that want this.
>
> Alex
>
>>
>>>>> 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.
>>
>> Can you elaborate? As far as the device is concerned it shouldn't know
>> whether a request comes from a peer or from the host. If it does do
>> crazy stuff like that it's well out of spec. It's up to the switch (or
>> root complex if good support exists) to route the request to the device
>> and it's the root complex that tends to be what drops the load requests
>> which causes the asymmetries.
>>
>> Logan
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