diff --git a/drivers/rtc/rtc-m48t59.c b/drivers/rtc/rtc-m48t59.c
index 3978f4c..2836538 100644
--- a/drivers/rtc/rtc-m48t59.c
+++ b/drivers/rtc/rtc-m48t59.c
@@ -433,7 +433,7 @@
 
 	if (!m48t59->ioaddr) {
 		/* ioaddr not mapped externally */
-		m48t59->ioaddr = ioremap(res->start, res->end - res->start + 1);
+		m48t59->ioaddr = ioremap(res->start, resource_size(res));
 		if (!m48t59->ioaddr)
 			goto out;
 	}