diff --git a/drivers/serial/cpm_uart/cpm_uart_cpm2.c b/drivers/serial/cpm_uart/cpm_uart_cpm2.c
index a9802e76b5fa..722eac18f382 100644
--- a/drivers/serial/cpm_uart/cpm_uart_cpm2.c
+++ b/drivers/serial/cpm_uart/cpm_uart_cpm2.c
@@ -61,7 +61,7 @@ void __iomem *cpm_uart_map_pram(struct uart_cpm_port *port,
 	void __iomem *pram;
 	unsigned long offset;
 	struct resource res;
-	unsigned long len;
+	resource_size_t len;
 
 	/* Don't remap parameter RAM if it has already been initialized
 	 * during console setup.
@@ -74,7 +74,7 @@ void __iomem *cpm_uart_map_pram(struct uart_cpm_port *port,
 	if (of_address_to_resource(np, 1, &res))
 		return NULL;
 
-	len = 1 + res.end - res.start;
+	len = resource_size(&res);
 	pram = ioremap(res.start, len);
 	if (!pram)
 		return NULL;