linux-kernel - Re: [PATCH 2/8] PCI: Add pci_find_common_upstream

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Date:   Mon, 2 Apr 2018 15:45:53 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     Christian König <christian.koenig@....com>,
        Christoph Hellwig <hch@...radead.org>,
        Will Davis <wdavis@...dia.com>, Joerg Roedel <joro@...tes.org>,
        linaro-mm-sig@...ts.linaro.org, amd-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-media@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH 2/8] PCI: Add pci_find_common_upstream_dev()

On Mon, Apr 02, 2018 at 01:32:37PM -0600, Logan Gunthorpe wrote:
> 
> 
> On 02/04/18 01:16 PM, Jerome Glisse wrote:
> > There isn't good API at the moment AFAIK, closest thing would either be
> > lookup_resource() or region_intersects(), but a more appropriate one can
> > easily be added, code to walk down the tree is readily available. More-
> > over this can be optimize like vma lookup are, even more as resource are
> > seldomly added so read side (finding a resource) can be heavily favor
> > over write side (adding|registering a new resource).
> 
> So someone needs to create a highly optimized tree that registers all
> physical address on the system and maps them to devices? That seems a
> long way from being realized. I'd hardly characterize that as "easily".
> If we can pass both devices to the API I'd suspect it would be preferred
> over the complicated tree. This, of course, depends on what users of the
> API need.

This tree already exist, it is all there upstream see kernel/resource.c
What is missing is something that take a single address and return the
device struct. There is function that take a range region_intersects()
or one that take the start address lookup_resource(). It isn't hard to
think that using roughly same code as region_intersects() an helper
that return the device for a resource can be added.

And yes currently this does not have a pointer back to the device that
own the resource but this can be added. It wasn't needed until now.

It can latter be optimize if device lookup shows as a bottleneck in perf
profile.


> 
> > cache coherency protocol (bit further than PCIE snoop). But also the
> > other direction the CPU access to device memory can also be cache coherent,
> > which is not the case in PCIE.
> 
> I was not aware that CAPI allows PCI device memory to be cache coherent.
> That sounds like it would be very tricky...

And yet CAPI, CCIX, Gen-Z, NVLink, ... are all inter-connect that aim at
achieving this cache coherency between multiple devices and CPUs.

Jérôme