ACPICA: Add root node optimization to internal get namespace node function.

Detect a request for the root node (a lone backslash) up front
before invoking a full namespace lookup.
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/acpica/nsutils.c

@@ -671,6 +671,8 @@ acpi_ns_get_node(struct acpi_namespace_node *prefix_node,
 
 	ACPI_FUNCTION_TRACE_PTR(ns_get_node, ACPI_CAST_PTR(char, pathname));
 
+	/* Simplest case is a null pathname */
+
 	if (!pathname) {
 		*return_node = prefix_node;
 		if (!prefix_node) {
@@ -679,6 +681,13 @@ acpi_ns_get_node(struct acpi_namespace_node *prefix_node,
 		return_ACPI_STATUS(AE_OK);
 	}
 
+	/* Quick check for a reference to the root */
+
+	if (ACPI_IS_ROOT_PREFIX(pathname[0]) && (!pathname[1])) {
+		*return_node = acpi_gbl_root_node;
+		return_ACPI_STATUS(AE_OK);
+	}
+
 	/* Convert path to internal representation */
 
 	status = acpi_ns_internalize_name(pathname, &internal_path);