diff --git a/drivers/rtc/rtc-at32ap700x.c b/drivers/rtc/rtc-at32ap700x.c
index e725d51..8dd0830 100644
--- a/drivers/rtc/rtc-at32ap700x.c
+++ b/drivers/rtc/rtc-at32ap700x.c
@@ -223,7 +223,7 @@
 	}
 
 	rtc->irq = irq;
-	rtc->regs = ioremap(regs->start, regs->end - regs->start + 1);
+	rtc->regs = ioremap(regs->start, resource_size(regs));
 	if (!rtc->regs) {
 		ret = -ENOMEM;
 		dev_dbg(&pdev->dev, "could not map I/O memory\n");