linux-kernel - Re: [PATCH v3 06/12] x86/amd_nb: Use topology info to get AMD node count

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Message-ID: <20251022011610.60d0ba6e.michal.pecio@gmail.com>
Date: Wed, 22 Oct 2025 01:16:10 +0200
From: Michal Pecio <michal.pecio@...il.com>
To: yazen.ghannam@....com
Cc: Shyam-sundar.S-k@....com, bhelgaas@...gle.com, hdegoede@...hat.com,
 ilpo.jarvinen@...ux.intel.com, jdelvare@...e.com,
 linux-edac@...r.kernel.org, linux-hwmon@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
 linux@...ck-us.net, mario.limonciello@....com,
 naveenkrishna.chatradhi@....com, platform-driver-x86@...r.kernel.org,
 suma.hegde@....com, tony.luck@...el.com, x86@...nel.org
Subject: Re: [PATCH v3 06/12] x86/amd_nb: Use topology info to get AMD node
 count

> Currently, the total AMD node count is determined by searching and
> counting CPU/node devices using PCI IDs.
> 
> However, AMD node information is already available through topology
> CPUID/MSRs. The recent topology rework has made this info easier to
> access.
> 
> Replace the node counting code with a simple product of topology info.
> 
> Every node/northbridge is expected to have a 'misc' device. Clear
> everything out if a 'misc' device isn't found on a node.

Hi,

I have a weird/buggy AM3 machine (Asus M4A88TD-M EVO, Phenom 965) where
the kernel believes there are two packages and this assumption fails.

[    0.072051] CPU topo: Max. logical packages:   2
[    0.072052] CPU topo: Max. logical dies:       2
[    0.072052] CPU topo: Max. dies per package:   1
[    0.072057] CPU topo: Max. threads per core:   1
[    0.072058] CPU topo: Num. cores per package:     4
[    0.072059] CPU topo: Num. threads per package:   4

It's currently on v6.12 series and working OK, but I remember trying
v6.15 at one point and finding that EDAC and GART IOMMU were broken
because the NB driver failed to initialize. This fixed it:

--- a/arch/x86/kernel/cpu/topology.c
+++ b/arch/x86/kernel/cpu/topology.c
@@ -496,8 +496,8 @@ void __init topology_init_possible_cpus(void)
        total_cpus = allowed;
        set_nr_cpu_ids(allowed);
 
-       cnta = domain_weight(TOPO_PKG_DOMAIN);
-       cntb = domain_weight(TOPO_DIE_DOMAIN);
+       cnta = 1;
+       cntb = 1;
        __max_logical_packages = cnta;
        __max_dies_per_package = 1U << (get_count_order(cntb) - get_count_order(cnta));

It was a few weeks ago and the machine is currently back on v6.12,
but I'm almost sure I tracked it down to this exact code:

> +	amd_northbridges.num = amd_num_nodes();
> [...]
> +		/*
> +		 * Each Northbridge must have a 'misc' device.
> +		 * If not, then uninitialize everything.
> +		 */
> +		if (!node_to_amd_nb(i)->misc) {
> +			amd_northbridges.num = 0;
> +			kfree(nb);
> +			return -ENODEV;
> +		}

Thanks,
Michal