linux-kernel - Re: [PATCH 1/2] x86/amd_node: Fix AMD root device caching

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Message-ID:
 <IA2PR22MB5609866886BA191F64D5049D90E0A@IA2PR22MB5609.namprd22.prod.outlook.com>
Date: Tue, 7 Oct 2025 17:37:56 +0000
From: Filip Barczyk <filip.barczyk@...o.net>
To: Yazen Ghannam <yazen.ghannam@....com>, "Mario Limonciello (AMD)
 (kernel.org)" <superm1@...nel.org>
CC: "x86@...nel.org" <x86@...nel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] x86/amd_node: Fix AMD root device caching

On Tue, Oct 01, 2025 at 03:46 PM +0200, Yazen Ghannam wrote:
> On Tue, Sep 30, 2025 at 01:07:47PM -0500, Mario Limonciello (AMD) (kernel.org) wrote:
> >
> >
> > On 9/30/2025 11:45 AM, Yazen Ghannam wrote:
> > > Recent AMD node rework removed the "search and count" method of caching
> > > AMD root devices. This depended on the value from a Data Fabric register
> > > that was expected to hold the PCI bus of one of the root devices
> > > attached to that fabric.
> > >
> > > However, this expectation is incorrect. The register, when read from PCI
> > > config space, returns the bitwise-OR of the buses of all attached root
> > > devices.
> > >
> > > This behavior is benign on AMD reference design boards, since the bus
> > > numbers are aligned. This results in a bitwise-OR value matching one of
> > > the buses. For example, 0x00 | 0x40 | 0xA0 | 0xE0 = 0xE0.
> > >
> > > This behavior breaks on boards where the bus numbers are not exactly
> > > aligned. For example, 0x00 | 0x07 | 0xE0 | 0x15 = F.
> > >
> > > The bus numbering style in the reference boards is not a requirement.
> > > The numbering found in other boards is not incorrect. Therefore, the
> > > root device caching method needs to be adjusted.
> > >
> > > Go back to the "search and count" method used before the recent rework.
> > > Search for root devices using PCI class code rather than fixed PCI IDs.
> > >
> > > This keeps the goal of the rework (remove dependency on PCI IDs) while
> > > being able to support various board designs.
> > >
> > > Fixes: 40a5f6ffdfc8 ("x86/amd_nb: Simplify root device search")
> >
> > Was this a publicly reported failure?
> >
> > If so is there a link to LKML or a Bugzilla with the details of the failure
> > you can include here?
> >
> No, it was reported off-list.
> Thanks,
> Yazen

I confirm that an issue when EDAC fails to enumerate DIMMs on Dell PowerEdge R7725, 2x EPYC 9475F @ kernel 6.14.3 is fixed with this patch.
Thanks,
Filip
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