linux-kernel - Re: [PATCH] Whitelist AMD host bridge device(s) to enable P2P DMA

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Date:   Wed, 11 Aug 2021 16:50:28 -0400
From:   Felix Kuehling <felix.kuehling@....com>
To:     Alex Deucher <alexdeucher@...il.com>,
        Ramesh Errabolu <Ramesh.Errabolu@....com>
Cc:     amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        Maling list - DRI developers 
        <dri-devel@...ts.freedesktop.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Linux PCI <linux-pci@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Whitelist AMD host bridge device(s) to enable P2P DMA


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@....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.

Regards,
  Felix


>
> Alex
>
>> Signed-off-by: Ramesh Errabolu <Ramesh.Errabolu@....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
>>