dt-bindings: PCI: pci-ep: Document 'linux,pci-domain' property

'linux,pci-domain' property provides the PCI domain number for the PCI
endpoint controllers in a SoC. If this property is not present, then an
unstable (across boots) unique number will be assigned.

Devicetrees can specify the domain number based on the actual hardware
instance of the PCI endpoint controllers in the SoC.

Link: https://lore.kernel.org/linux-pci/20240828-pci-qcom-hotplug-v4-4-263a385fbbcb@linaro.org
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>

Documentation/devicetree/bindings/pci/pci-ep.yaml

@@ -42,6 +42,17 @@ properties:
     default: 1
     maximum: 16
 
+  linux,pci-domain:
+    description:
+      If present this property assigns a fixed PCI domain number to a PCI
+      Endpoint Controller, otherwise an unstable (across boots) unique number
+      will be assigned. It is required to either not set this property at all
+      or set it for all PCI endpoint controllers in the system, otherwise
+      potentially conflicting domain numbers may be assigned to endpoint
+      controllers. The domain number for each endpoint controller in the system
+      must be unique.
+    $ref: /schemas/types.yaml#/definitions/uint32
+
 required:
   - compatible
 

Documentation/devicetree/bindings/pci/qcom,pcie-ep.yaml

@@ -280,4 +280,5 @@ examples:
         phy-names = "pciephy";
         max-link-speed = <3>;
         num-lanes = <2>;
+        linux,pci-domain = <0>;
     };