rtc: Add API function to return alarm time bound by hardware limit

Add rtc_bound_alarmtime() to return the requested alarm timeout bound by
the maxmum alarm timeout that is supported by a given RTC.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230915152238.1144706-2-linux@roeck-us.net

include/linux/rtc.h

@@ -225,6 +225,23 @@ static inline bool is_leap_year(unsigned int year)
 	return (!(year % 4) && (year % 100)) || !(year % 400);
 }
 
+/**
+ * rtc_bound_alarmtime() - Return alarm time bound by rtc limit
+ * @rtc: Pointer to rtc device structure
+ * @requested: Requested alarm timeout
+ *
+ * Return: Alarm timeout bound by maximum alarm time supported by rtc.
+ */
+static inline ktime_t rtc_bound_alarmtime(struct rtc_device *rtc,
+					  ktime_t requested)
+{
+	if (rtc->alarm_offset_max &&
+	    rtc->alarm_offset_max * MSEC_PER_SEC < ktime_to_ms(requested))
+		return ms_to_ktime(rtc->alarm_offset_max * MSEC_PER_SEC);
+
+	return requested;
+}
+
 #define devm_rtc_register_device(device) \
 	__devm_rtc_register_device(THIS_MODULE, device)