of/pci: Add of_pci_get_max_link_speed() to parse max-link-speed from DT

This new helper function could be used by host drivers to get the limitaion
of max link speed provided by DT. If the property isn't assigned or is
invalid, it will return -EINVAL to the caller.

Signed-off-by: Shawn Lin <[e-mail]>
Signed-off-by: Bjorn Helgaas <[e-mail]>
Acked-by: Rob Herring <[e-mail]>

drivers/of/of_pci.c

···
 EXPORT_SYMBOL_GPL(of_get_pci_domain_nr);
 
 /**
+ * This function will try to find the limitation of link speed by finding
+ * a property called "max-link-speed" of the given device node.
+ *
+ * @node: device tree node with the max link speed information
+ *
+ * Returns the associated max link speed from DT, or a negative value if the
+ * required property is not found or is invalid.
+ */
+int of_pci_get_max_link_speed(struct device_node *node)
+{
+	u32 max_link_speed;
+
+	if (of_property_read_u32(node, "max-link-speed", &max_link_speed) ||
+	    max_link_speed > 4)
+		return -EINVAL;
+
+	return max_link_speed;
+}
+EXPORT_SYMBOL_GPL(of_pci_get_max_link_speed);
+
+/**
  * of_pci_check_probe_only - Setup probe only mode if linux,pci-probe-only
  *                           is present and valid
  */

include/linux/of_pci.h

···
 int of_irq_parse_and_map_pci(const struct pci_dev *dev, u8 slot, u8 pin);
 int of_pci_parse_bus_range(struct device_node *node, struct resource *res);
 int of_get_pci_domain_nr(struct device_node *node);
+int of_pci_get_max_link_speed(struct device_node *node);
 void of_pci_check_probe_only(void);
 int of_pci_map_rid(struct device_node *np, u32 rid,
 		   const char *map_name, const char *map_mask_name,
···
 static inline int of_pci_map_rid(struct device_node *np, u32 rid,
 			const char *map_name, const char *map_mask_name,
 			struct device_node **target, u32 *id_out)
+{
+	return -EINVAL;
+}
+
+static inline int
+of_pci_get_max_link_speed(struct device_node *node)
 {
 	return -EINVAL;
 }