driver core: Add class_attr_string for simple read-only string

Several drivers just export a static string as class attributes.

Use the new extensible attribute support to define a simple
CLASS_ATTR_STRING() macro for this.

This will allow to remove code from drivers in followon patches.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/base/class.c

@@ -490,6 +490,16 @@ void class_interface_unregister(struct class_interface *class_intf)
 	class_put(parent);
 }
 
+ssize_t show_class_attr_string(struct class *class, struct class_attribute *attr,
+                        	char *buf)
+{
+	struct class_attribute_string *cs;
+	cs = container_of(attr, struct class_attribute_string, attr);
+	return snprintf(buf, PAGE_SIZE, "%s\n", cs->str);
+}
+
+EXPORT_SYMBOL_GPL(show_class_attr_string);
+
 struct class_compat {
 	struct kobject *kobj;
 };