[PATCH] Whitelist AMD host bridge device(s) to enable P2P DMA

[Errabolu, Ramesh]

I will investigate it further. I am using DKMS-5.11 branch. The codebase I am using to build has the right definition i.e. allow P2PDMA for Zen CPU's.

Regards,
Ramesh

-----Original Message-----
From: Alex Deucher <alexdeucher at gmail.com> 
Sent: Wednesday, August 11, 2021 4:07 PM
To: Kuehling, Felix <Felix.Kuehling at amd.com>
Cc: Errabolu, Ramesh <Ramesh.Errabolu at amd.com>; amd-gfx list <amd-gfx at lists.freedesktop.org>; Maling list - DRI developers <dri-devel at lists.freedesktop.org>; Bjorn Helgaas <bhelgaas at google.com>; Linux PCI <linux-pci at vger.kernel.org>; LKML <linux-kernel at vger.kernel.org>
Subject: Re: [PATCH] Whitelist AMD host bridge device(s) to enable P2P DMA

[CAUTION: External Email]

On Wed, Aug 11, 2021 at 4:50 PM Felix Kuehling <felix.kuehling at amd.com> wrote:
>
>
> Am 2021-08-11 um 3:29 p.m. schrieb Alex Deucher:
> > On Wed, Aug 11, 2021 at 3:11 PM Ramesh Errabolu <Ramesh.Errabolu at amd.com> wrote:
> >> Current implementation will disallow P2P DMA if the participating 
> >> devices belong to different root complexes. Implementation allows 
> >> this default behavior to be overridden for whitelisted devices. The 
> >> patch adds an AMD host bridge to be whitelisted
> > Why do we need this?  cpu_supports_p2pdma() should return true for 
> > all AMD Zen CPUs.
>
> This is part of our on-going work to get P2P support upstream. We want 
> to use pci_p2pdma_distance_many to determine whether P2P is possible 
> between a pair of devices. This whitelist is used in this function. 
> This will affect the P2P links reported in the topology and it will be 
> double-checked in the BO mapping function to ensure a peer mapping is legal.
>
> I think this change is a bit free of context at the moment, as we're 
> still working on a few other loose ends for P2P support in our 
> internal branch. I'm hoping we'll have a bigger patch series for 
> upstreamable KFD P2P support ready in a few weeks. I also think we'll 
> probably want to add a few more PCI IDs for other supported AMD root complexes.

We don't need to keep adding AMD root complexes.  You must be using an older kernel or something.  All root complexes on all Zen platforms support P2P DMA.  See:

commit dea286bb71baded7d2fb4f090e3b9fd2c1ccac58
Author: Logan Gunthorpe <logang at deltatee.com>
Date:   Wed Jul 29 17:18:44 2020 -0600

    PCI/P2PDMA: Allow P2PDMA on AMD Zen and newer CPUs

    Allow P2PDMA if the CPU vendor is AMD and family is 0x17 (Zen) or greater.

    [bhelgaas: commit log, simplify #if/#else/#endif]
    Link: https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fr%2F20200729231844.4653-1-logang%40deltatee.com&data=04%7C01%7CRamesh.Errabolu%40amd.com%7C7ffc96d473d4471889f808d95d0bf57b%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637643128220735445%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=vmwJP2VwKPrXwl3ngUfwN%2BJu3JWeMP0ZZMDh29evW%2Bg%3D&reserved=0
    Signed-off-by: Logan Gunthorpe <logang at deltatee.com>
    Signed-off-by: Bjorn Helgaas <bhelgaas at google.com>
    Reviewed-by: Alex Deucher <alexander.deucher at amd.com>
    Cc: Christian König <christian.koenig at amd.com>
    Cc: Huang Rui <ray.huang at amd.com>

Alex


>
> Regards,
>   Felix
>
>
> >
> > Alex
> >
> >> Signed-off-by: Ramesh Errabolu <Ramesh.Errabolu at amd.com>
> >> ---
> >>  drivers/pci/p2pdma.c | 2 ++
> >>  1 file changed, 2 insertions(+)
> >>
> >> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c index 
> >> 196382630363..7003bb9faf23 100644
> >> --- a/drivers/pci/p2pdma.c
> >> +++ b/drivers/pci/p2pdma.c
> >> @@ -305,6 +305,8 @@ static const struct pci_p2pdma_whitelist_entry {
> >>         {PCI_VENDOR_ID_INTEL,   0x2032, 0},
> >>         {PCI_VENDOR_ID_INTEL,   0x2033, 0},
> >>         {PCI_VENDOR_ID_INTEL,   0x2020, 0},
> >> +       /* AMD Host Bridge Devices */
> >> +       {PCI_VENDOR_ID_AMD,     0x1480, 0},
> >>         {}
> >>  };
> >>
> >> --
> >> 2.31.1
> >>