powerpc/pseries: Perform proper max_bus_speed detection

On pseries machines the detection for max_bus_speed should be done
through an OpenFirmware property. This patch adds a function to perform
this detection and a hook to perform dynamic adding of the function only
for pseries. This is done by overwriting the weak
pcibios_root_bridge_prepare function which is called by
pci_create_root_bus().

From: Lucas Kannebley Tavares <lucaskt@linux.vnet.ibm.com>
Signed-off-by: Kleber Sacilotto de Souza <klebers@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/pseries/pci.c

@@ -108,3 +108,56 @@ static void fixup_winbond_82c105(struct pci_dev* dev)
 }
 DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_WINBOND, PCI_DEVICE_ID_WINBOND_82C105,
 			 fixup_winbond_82c105);
+
+int pseries_root_bridge_prepare(struct pci_host_bridge *bridge)
+{
+	struct device_node *dn, *pdn;
+	struct pci_bus *bus;
+	const uint32_t *pcie_link_speed_stats;
+
+	bus = bridge->bus;
+
+	dn = pcibios_get_phb_of_node(bus);
+	if (!dn)
+		return 0;
+
+	for (pdn = dn; pdn != NULL; pdn = of_get_next_parent(pdn)) {
+		pcie_link_speed_stats = (const uint32_t *) of_get_property(pdn,
+			"ibm,pcie-link-speed-stats", NULL);
+		if (pcie_link_speed_stats)
+			break;
+	}
+
+	of_node_put(pdn);
+
+	if (!pcie_link_speed_stats) {
+		pr_err("no ibm,pcie-link-speed-stats property\n");
+		return 0;
+	}
+
+	switch (pcie_link_speed_stats[0]) {
+	case 0x01:
+		bus->max_bus_speed = PCIE_SPEED_2_5GT;
+		break;
+	case 0x02:
+		bus->max_bus_speed = PCIE_SPEED_5_0GT;
+		break;
+	default:
+		bus->max_bus_speed = PCI_SPEED_UNKNOWN;
+		break;
+	}
+
+	switch (pcie_link_speed_stats[1]) {
+	case 0x01:
+		bus->cur_bus_speed = PCIE_SPEED_2_5GT;
+		break;
+	case 0x02:
+		bus->cur_bus_speed = PCIE_SPEED_5_0GT;
+		break;
+	default:
+		bus->cur_bus_speed = PCI_SPEED_UNKNOWN;
+		break;
+	}
+
+	return 0;
+}