alarmtimer: Provide alarm_start_timer()

Alarm timers utilize hrtimers for normal operation and only switch to the
RTC on suspend. In order to catch already expired timers early and without
going through a timer interrupt cycle, provide a new start function which
internally uses hrtimer_start_range_ns_user().

If hrtimer_start_range_ns_user() detects an already expired timer, it does
not queue it. In that case remove the timer from the alarm base as well.

Return the status queued or not back to the caller to handle the early
expiry.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: John Stultz <jstultz@google.com>
Link: https://patch.msgid.link/20260408114952.332822525@kernel.org

1 files changed, 28 insertions, 0 deletions

diff --git a/kernel/time/alarmtimer.c b/kernel/time/alarmtimer.c
index e10021be190f..2348b0839114 100644
--- a/kernel/time/alarmtimer.c
+++ b/kernel/time/alarmtimer.c
@@ -369,6 +369,34 @@ void alarm_start_relative(struct alarm *alarm, ktime_t start)
 }
 EXPORT_SYMBOL_GPL(alarm_start_relative);
 
+/**
+ * alarm_start_timer - Sets an alarm to fire
+ * @alarm:	Pointer to alarm to set
+ * @expires:	Expiry time
+ * @relative:	True if @expires is relative
+ *
+ * Returns: True if the alarm was queued. False if it already expired
+ */
+bool alarm_start_timer(struct alarm *alarm, ktime_t expires, bool relative)
+{
+	struct alarm_base *base = &alarm_bases[alarm->type];
+
+	if (relative)
+		expires = ktime_add_safe(expires, base->get_ktime());
+
+	trace_alarmtimer_start(alarm, base->get_ktime());
+
+	guard(spinlock_irqsave)(&base->lock);
+	alarm->node.expires = expires;
+	alarmtimer_enqueue(base, alarm);
+	if (!hrtimer_start_range_ns_user(&alarm->timer, expires, 0, HRTIMER_MODE_ABS)) {
+		alarmtimer_dequeue(base, alarm);
+		return false;
+	}
+	return true;
+}
+EXPORT_SYMBOL_GPL(alarm_start_timer);
+
 void alarm_restart(struct alarm *alarm)
 {
 	struct alarm_base *base = &alarm_bases[alarm->type];