[RFC PATCH] x86/PCI: Use the only online node for root buses with no _PXM

[Ferran <ferran.duarri@me.com>]

When a host bridge has no _PXM and the Northbridge fallback finds
nothing either, pci_acpi_root_get_node() leaves every root bus at
NUMA_NO_NODE. With exactly one node online that isn't really unknown,
there's only one possible answer.

Seen on an ASRock B760M-ITX/D4 (i9-14900KF). nvidia-fs calls
pcibus_to_node() directly and warns "error retrieving numa node" four
times per boot, once for the GPU and once per NVMe. Rewriting the
per-device numa_node attribute from udev doesn't help, since
pcibus_to_node() reads the bus value set at enumeration.

Fall back to first_online_node when num_online_nodes() == 1. Systems
with more than one node are unaffected.

With this applied the warnings are gone over four boots. I didn't
observe pcibus_to_node() itself at the call site, there's no sysfs
file for it.

Assisted-by: Claude:claude-opus-5
Signed-off-by: Ferran Duarri <ferran.duarri@me.com>
---
Only user I've got is the out-of-tree nvidia-fs, hence the RFC. The
single-node condition is the same reasoning as Bjorn's 2019 "PCI: Warn
about host bridge device when its numa node is NO_NODE" change. Fine to
drop this if callers are expected to cope with NUMA_NO_NODE.

 arch/x86/pci/acpi.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/pci/acpi.c b/arch/x86/pci/acpi.c
index 7cd5388ed..2ae31c3e7 100644
--- a/arch/x86/pci/acpi.c
+++ b/arch/x86/pci/acpi.c
@@ -463,6 +463,13 @@ static int pci_acpi_root_get_node(struct acpi_pci_root *root)
 	if (node != NUMA_NO_NODE && !node_online(node))
 		node = NUMA_NO_NODE;
 
+	/*
+	 * One online node means there's only one possible answer, so don't
+	 * leave the bus at NUMA_NO_NODE just because firmware didn't say.
+	 */
+	if (node == NUMA_NO_NODE && num_online_nodes() == 1)
+		node = first_online_node;
+
 	return node;
 }