arm64: PCI: Preserve firmware configuration when desired

If we must preserve the firmware resource assignments, claim the existing
resources rather than reassigning everything.

Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Benjamin Herrenschmidt <[email protected]>
[bhelgaas: commit log]
Signed-off-by: Bjorn Helgaas <[email protected]>
Acked-by: Lorenzo Pieralisi <[email protected]>
Acked-by: Ard Biesheuvel <[email protected]>

arch/arm64/kernel/pci.c

@@ -168,6 +168,7 @@ struct pci_bus *pci_acpi_scan_root(struct acpi_pci_root *root)
 	struct acpi_pci_generic_root_info *ri;
 	struct pci_bus *bus, *child;
 	struct acpi_pci_root_ops *root_ops;
+	struct pci_host_bridge *host;
 
 	ri = kzalloc(sizeof(*ri), GFP_KERNEL);
 	if (!ri)
@@ -193,6 +194,15 @@ struct pci_bus *pci_acpi_scan_root(struct acpi_pci_root *root)
 	if (!bus)
 		return NULL;
 
+	/* If we must preserve the resource configuration, claim now */
+	host = pci_find_host_bridge(bus);
+	if (host->preserve_config)
+		pci_bus_claim_resources(bus);
+
+	/*
+	 * Assign whatever was left unassigned. If we didn't claim above,
+	 * this will reassign everything.
+	 */
 	pci_assign_unassigned_root_bus_resources(bus);
 
 	list_for_each_entry(child, &bus->children, node)