DMI: add dmi_match

Add a wrapper for testing system_info which will handle also NULL
system infos.

This will be used by the ata PIIX driver.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Alexandru Romanescu <a_romanescu@yahoo.co.uk>
Cc: Tejun Heo <tj@kernel.org>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

drivers/firmware/dmi_scan.c

@@ -582,3 +582,19 @@ int dmi_walk(void (*decode)(const struct dmi_header *))
 	return 0;
 }
 EXPORT_SYMBOL_GPL(dmi_walk);
+
+/**
+ * dmi_match - compare a string to the dmi field (if exists)
+ *
+ * Returns true if the requested field equals to the str (including NULL).
+ */
+bool dmi_match(enum dmi_field f, const char *str)
+{
+	const char *info = dmi_get_system_info(f);
+
+	if (info == NULL || str == NULL)
+		return info == str;
+
+	return !strcmp(info, str);
+}
+EXPORT_SYMBOL_GPL(dmi_match);