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

[Logan Gunthorpe]

On 30/03/18 01:45 PM, Jerome Glisse wrote:
> Looking at upstream code it seems that the x86 bits never made it upstream
> and thus what is now upstream is only for ARM. See [1] for x86 code. Dunno
> what happen, i was convince it got merge. So yes current code is broken on
> x86. ccing Joerg maybe he remembers what happened there.
> 
> [1] https://lwn.net/Articles/646605/

That looks like a significant improvement over what's upstream. But it's
three years old and looks to have been abandoned. I think I agree with
Bjorn's comments in that if it's going to be a general P2P API it will
need the device the resource belongs to in addition to the device that's
initiating the DMA request.

> Given it is currently only used by ARM folks it appear to at least work
> for them (tm) :) Note that Christian is doing this in PCIE only context
> and again dma_map_resource can easily figure that out if the address is
> a PCIE or something else. Note that the exporter export the CPU bus
> address. So again dma_map_resource has all the informations it will ever
> need, if the peer to peer is fundamentaly un-doable it can return dma
> error and it is up to the caller to handle this, just like GPU code do.
> 
> Do you claim that dma_map_resource is missing any information ?

Yes, that's what I said. All the existing ARM code appears to use it for
platform devices only. I suspect platform P2P is relatively trivial to
support on ARM. I think it's a big difference from using it for PCI P2P
or general P2P on any bus.

> I agree and understand that but for platform where such feature make sense
> this will work. For me it is PowerPC and x86 and given PowerPC has CAPI
> which has far more advance feature when it comes to peer to peer, i don't
> see something more basic not working. On x86, Intel is a bit of lone wolf,
> dunno if they gonna support this usecase pro-actively. AMD definitly will.

Well PowerPC doesn't even support P2P between root ports. And I fail to
see how CAPI applies unless/until we get this memory mapped into
userspace and the mappings need to be dynamically managed. Seems like
that's a long way away.

Logan