2 files changed, 82 insertions, 0 deletions

diff --git a/drivers/rtc/rtc-lib.c b/drivers/rtc/rtc-lib.c
index ba795a4db1e9..7bbc26a34bd2 100644
--- a/drivers/rtc/rtc-lib.c
+++ b/drivers/rtc/rtc-lib.c
@@ -117,4 +117,85 @@ int rtc_tm_to_time(struct rtc_time *tm, unsigned long *time)
 }
 EXPORT_SYMBOL(rtc_tm_to_time);
 
+
+/* Merge the valid (i.e. non-negative) fields of alarm into the current
+ * time.  If the valid alarm fields are earlier than the equivalent
+ * fields in the time, carry one into the least significant invalid
+ * field, so that the alarm expiry is in the future.  It assumes that the
+ * least significant invalid field is more significant than the most
+ * significant valid field, and that the seconds field is valid.
+ *
+ * This is used by alarms that take relative (rather than absolute)
+ * times, and/or have a simple binary second counter instead of
+ * day/hour/minute/sec registers.
+ */
+void rtc_merge_alarm(struct rtc_time *now, struct rtc_time *alarm)
+{
+        int *alarmp = &alarm->tm_sec;
+        int *timep = &now->tm_sec;
+        int carry_into, i;
+
+        /* Ignore everything past the 6th element (tm_year). */
+        for (i = 5; i > 0; i--) {
+                if (alarmp[i] < 0)
+                        alarmp[i] = timep[i];
+                else
+                        break;
+        }
+
+        /* No carry needed if all fields are valid. */
+        if (i == 5)
+                return;
+
+        for (carry_into = i + 1; i >= 0; i--) {
+                if (alarmp[i] < timep[i])
+                        break;
+
+                if (alarmp[i] > timep[i])
+                        return;
+        }
+
+        switch (carry_into) {
+                case 1:
+                        alarm->tm_min++;
+
+                        if (alarm->tm_min < 60)
+                                return;
+
+                        alarm->tm_min = 0;
+                        /* fall-through */
+
+                case 2:
+                        alarm->tm_hour++;
+
+                        if (alarm->tm_hour < 60)
+                                return;
+
+                        alarm->tm_hour = 0;
+                        /* fall-through */
+
+                case 3:
+                        alarm->tm_mday++;
+
+                        if (alarm->tm_mday <= rtc_days_in_month[alarm->tm_mon])
+                                return;
+
+                        alarm->tm_mday = 1;
+                        /* fall-through */
+
+                case 4:
+                        alarm->tm_mon++;
+
+                        if (alarm->tm_mon <= 12)
+                                return;
+
+                        alarm->tm_mon = 1;
+                        /* fall-through */
+
+                case 5:
+                        alarm->tm_year++;
+        }
+}
+EXPORT_SYMBOL(rtc_merge_alarm);
+
 MODULE_LICENSE("GPL");
diff --git a/include/linux/rtc.h b/include/linux/rtc.h
index 09ff4c3e2713..5e22d4510d11 100644
--- a/include/linux/rtc.h
+++ b/include/linux/rtc.h
@@ -106,6 +106,7 @@ extern int rtc_year_days(unsigned int day, unsigned int month, unsigned int year
 extern int rtc_valid_tm(struct rtc_time *tm);
 extern int rtc_tm_to_time(struct rtc_time *tm, unsigned long *time);
 extern void rtc_time_to_tm(unsigned long time, struct rtc_time *tm);
+extern void rtc_merge_alarm(struct rtc_time *now, struct rtc_time *alarm);
 
 #include <linux/device.h>
 #include <linux/seq_file.h>