rtc: fix readback from /sys/class/rtc/rtc?/wakealarm

[safe/jmp/linux-2.6] / drivers / rtc / interface.c

diff --git a/drivers/rtc/interface.c b/drivers/rtc/interface.c
index 8adcab3..de0da54 100644 (file)
--- 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)