Re: [PATCH 1/2] dt-bindings: PCI: Add linux,pci-domain-reuse property

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] dt-bindings: PCI: redundant linux,pci-domain-reuse OS-policy property
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commit 4d7d1f09f7c3c39f32142c31f5a0c2f23d8b30fc
Author: Cui Chao <cuichao1753@phytium.com.cn>

dt-bindings: PCI: Add linux,pci-domain-reuse property

This commit adds an optional boolean property linux,pci-domain-reuse to the
generic PCI host bridge Device Tree binding so that multiple host bridges with
disjoint resources can share the same software PCI domain number.

> diff --git a/Documentation/devicetree/bindings/pci/host-generic-pci.yaml b/Documentation/devicetree/bindings/pci/host-generic-pci.yaml
> index 420d551e9af97..1b439e2f38af7 100644
> --- a/Documentation/devicetree/bindings/pci/host-generic-pci.yaml
> +++ b/Documentation/devicetree/bindings/pci/host-generic-pci.yaml
> @@ -114,6 +114,16 @@ properties:
>        Requests with Address Type (AT).
>      type: boolean
>  
> +  linux,pci-domain-reuse:
> +    description:
> +      Boolean indicating that this host bridge is allowed to reuse the same
> +      "linux,pci-domain" value as another host bridge on the platform. Host
> +      bridges reusing the same domain must have disjoint ECAM, bus-range,
> +      memory, and IO windows so that (domain, bus) tuples remain globally
> +      unique. A duplicate domain on a host bridge without this property
> +      remains an error.
> +    type: boolean

[Severity: Medium]
Is a new Device Tree property needed here when the hardware topology of
multiple host bridges sharing a domain with disjoint bus ranges is already
fully described by setting the same linux,pci-domain alongside disjoint
bus-range, reg, and ranges properties?

In the PCI core, pci_register_host_bridge() in drivers/pci/probe.c already
tracks per-domain bus number resources via get_pci_domain_busn_res() and
pci_bus_insert_busn_res(), and rejects conflicting (domain, busnr) tuples:

drivers/pci/probe.c:pci_register_host_bridge() {
    ...
	b = pci_find_bus(pci_domain_nr(bus), bridge->busnr);
	if (b) {
		/* Ignore it if we already got here via a different bridge */
		dev_dbg(&b->dev, "bus already known\n");
		err = -EEXIST;
		goto free;
	}
    ...
}

The only reason duplicate linux,pci-domain values currently fail in Device
Tree is that of_pci_bus_find_domain_nr() in drivers/pci/pci.c unconditionally
allocates static DT domain numbers from pci_domain_nr_static_ida:

drivers/pci/pci.c:of_pci_bus_find_domain_nr() {
    ...
		domain_nr = of_get_pci_domain_nr(parent->of_node);
		if (domain_nr >= 0)
			return ida_alloc_range(&pci_domain_nr_static_ida,
					       domain_nr, domain_nr,
					       GFP_KERNEL);
    ...
}

Could the kernel distinguish valid multi-bridge domain sharing (disjoint
bus-range) from accidental DT duplication (overlapping bus-range) directly
using existing DT properties, rather than introducing a Linux-specific policy
property in the binding ABI?

Also, if a binding property were used, shouldn't it live in
Documentation/devicetree/bindings/pci/pci-host-bridge.yaml where
linux,pci-domain is defined (and parsed by generic OF PCI code in
drivers/pci/pci.c), rather than only in host-generic-pci.yaml?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261102095150.2370959-1-cuichao1753@phytium.com.cn?part=1