PCI: build a bus number resource tree for every domain

This adds get_pci_domain_busn_res(), which returns the root of the
bus number resource tree for a domain, creating it if necessary.
We will later populate the tree with the bus numbers used by host
bridges and P2P bridges in the domain.

[bhelgaas: changelog]
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/probe.c

@@ -20,6 +20,36 @@
 LIST_HEAD(pci_root_buses);
 EXPORT_SYMBOL(pci_root_buses);
 
+static LIST_HEAD(pci_domain_busn_res_list);
+
+struct pci_domain_busn_res {
+	struct list_head list;
+	struct resource res;
+	int domain_nr;
+};
+
+static struct resource *get_pci_domain_busn_res(int domain_nr)
+{
+	struct pci_domain_busn_res *r;
+
+	list_for_each_entry(r, &pci_domain_busn_res_list, list)
+		if (r->domain_nr == domain_nr)
+			return &r->res;
+
+	r = kzalloc(sizeof(*r), GFP_KERNEL);
+	if (!r)
+		return NULL;
+
+	r->domain_nr = domain_nr;
+	r->res.start = 0;
+	r->res.end = 0xff;
+	r->res.flags = IORESOURCE_BUS | IORESOURCE_PCI_FIXED;
+
+	list_add_tail(&r->list, &pci_domain_busn_res_list);
+
+	return &r->res;
+}
+
 static int find_anything(struct device *dev, void *data)
 {
 	return 1;