alarmtimer: Implement remaining callback

Preparatory change to utilize the common posix timer mechanisms.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: John Stultz <john.stultz@linaro.org>
Link: http://lkml.kernel.org/r/20170530211657.592676753@linutronix.de

kernel/time/alarmtimer.c

@@ -560,6 +560,18 @@ static int alarm_timer_forward(struct k_itimer *timr, ktime_t now)
 	return (int) alarm_forward(alarm, timr->it_interval, now);
 }
 
+/**
+ * alarm_timer_remaining - Posix timer callback to retrieve remaining time
+ * @timr:	Pointer to the posixtimer data struct
+ * @now:	Current time to calculate against
+ */
+static ktime_t alarm_timer_remaining(struct k_itimer *timr, ktime_t now)
+{
+	struct alarm *alarm = &timr->it.alarm.alarmtimer;
+
+	return ktime_sub(now, alarm->node.expires);
+}
+
 /**
  * alarm_clock_getres - posix getres interface
  * @which_clock: clockid
@@ -881,15 +893,16 @@ static int alarm_timer_nsleep(const clockid_t which_clock, int flags,
 }
 
 const struct k_clock alarm_clock = {
-	.clock_getres	= alarm_clock_getres,
-	.clock_get	= alarm_clock_get,
-	.timer_create	= alarm_timer_create,
-	.timer_set	= alarm_timer_set,
-	.timer_del	= alarm_timer_del,
-	.timer_get	= alarm_timer_get,
-	.timer_rearm	= alarm_timer_rearm,
-	.timer_forward	= alarm_timer_forward,
-	.nsleep		= alarm_timer_nsleep,
+	.clock_getres		= alarm_clock_getres,
+	.clock_get		= alarm_clock_get,
+	.timer_create		= alarm_timer_create,
+	.timer_set		= alarm_timer_set,
+	.timer_del		= alarm_timer_del,
+	.timer_get		= alarm_timer_get,
+	.timer_rearm		= alarm_timer_rearm,
+	.timer_forward		= alarm_timer_forward,
+	.timer_remaining	= alarm_timer_remaining,
+	.nsleep			= alarm_timer_nsleep,
 };
 #endif /* CONFIG_POSIX_TIMERS */