1 files changed, 82 insertions, 1 deletions

diff --git a/drivers/rtc/interface.c b/drivers/rtc/interface.c
index 8adcab3c3653..de0da545c7a1 100644
--- a/drivers/rtc/interface.c
+++ b/drivers/rtc/interface.c
@@ -100,7 +100,7 @@ int rtc_set_mmss(struct rtc_device *rtc, unsigned long secs)
 }
 EXPORT_SYMBOL_GPL(rtc_set_mmss);
 
-int rtc_read_alarm(struct rtc_device *rtc, struct rtc_wkalrm *alarm)
+static int rtc_read_alarm_internal(struct rtc_device *rtc, struct rtc_wkalrm *alarm)
 {
         int err;
 
@@ -120,6 +120,87 @@ int rtc_read_alarm(struct rtc_device *rtc, struct rtc_wkalrm *alarm)
         mutex_unlock(&rtc->ops_lock);
         return err;
 }
+
+int rtc_read_alarm(struct rtc_device *rtc, struct rtc_wkalrm *alarm)
+{
+        int err;
+        struct rtc_time before, now;
+        int first_time = 1;
+
+        /* The lower level RTC driver may not be capable of filling
+         * in all fields of the rtc_time struct (eg. rtc-cmos),
+         * and so might instead return -1 in some fields.
+         * We deal with that here by grabbing a current RTC timestamp
+         * and using values from that for any missing (-1) values.
+         *
+         * But this can be racey, because some fields of the RTC timestamp
+         * may have wrapped in the interval since we read the RTC alarm,
+         * which would lead to us inserting inconsistent values in place
+         * of the -1 fields.
+         *
+         * Reading the alarm and timestamp in the reverse sequence
+         * would have the same race condition, and not solve the issue.
+         *
+         * So, we must first read the RTC timestamp,
+         * then read the RTC alarm value,
+         * and then read a second RTC timestamp.
+         *
+         * If any fields of the second timestamp have changed
+         * when compared with the first timestamp, then we know
+         * our timestamp may be inconsistent with that used by
+         * the low-level rtc_read_alarm_internal() function.
+         *
+         * So, when the two timestamps disagree, we just loop and do
+         * the process again to get a fully consistent set of values.
+         *
+         * This could all instead be done in the lower level driver,
+         * but since more than one lower level RTC implementation needs it,
+         * then it's probably best best to do it here instead of there..
+         */
+
+        /* Get the "before" timestamp */
+        err = rtc_read_time(rtc, &before);
+        if (err < 0)
+                return err;
+        do {
+                if (!first_time)
+                        memcpy(&before, &now, sizeof(struct rtc_time));
+                first_time = 0;
+
+                /* get the RTC alarm values, which may be incomplete */
+                err = rtc_read_alarm_internal(rtc, alarm);
+                if (err)
+                        return err;
+                if (!alarm->enabled)
+                        return 0;
+
+                /* get the "after" timestamp, to detect wrapped fields */
+                err = rtc_read_time(rtc, &now);
+                if (err < 0)
+                        return err;
+
+                /* note that tm_sec is a "don't care" value here: */
+        } while (   before.tm_min   != now.tm_min
+                 || before.tm_hour  != now.tm_hour
+                 || before.tm_mon   != now.tm_mon
+                 || before.tm_year  != now.tm_year
+                 || before.tm_isdst != now.tm_isdst);
+
+        /* Fill in any missing alarm fields using the timestamp */
+        if (alarm->time.tm_sec == -1)
+                alarm->time.tm_sec = now.tm_sec;
+        if (alarm->time.tm_min == -1)
+                alarm->time.tm_min = now.tm_min;
+        if (alarm->time.tm_hour == -1)
+                alarm->time.tm_hour = now.tm_hour;
+        if (alarm->time.tm_mday == -1)
+                alarm->time.tm_mday = now.tm_mday;
+        if (alarm->time.tm_mon == -1)
+                alarm->time.tm_mon = now.tm_mon;
+        if (alarm->time.tm_year == -1)
+                alarm->time.tm_year = now.tm_year;
+        return 0;
+}
 EXPORT_SYMBOL_GPL(rtc_read_alarm);
 
 int rtc_set_alarm(struct rtc_device *rtc, struct rtc_wkalrm *alarm)