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

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Message-ID: <20251021122909.GKaPd8ldoGqAf5JPfQ@fat_crate.local>
Date: Tue, 21 Oct 2025 14:29:09 +0200
From: Borislav Petkov <bp@...en8.de>
To: Yazen Ghannam <yazen.ghannam@....com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
	Mario Limonciello <mario.limonciello@....com>,
	Filip Barczyk <filip.barczyk@...o.net>
Subject: Re: [PATCH 1/2] x86/amd_node: Fix AMD root device caching

On Tue, Sep 30, 2025 at 04:45:45PM +0000, 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 = 0x1F.

Do I see it correctly that one of the root device's PCI bus is always 0x0 so
you can simply read that one and you can keep the current code?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette