[tip: timers/core] alarmtimer: Provide alarm_start_timer()

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     183d00b727139cf3b4be78d66a5602ce71a3acec
Gitweb:        https://git.kernel.org/tip/183d00b727139cf3b4be78d66a5602ce71a3acec
Author:        Thomas Gleixner <tglx@xxxxxxxxxx>
AuthorDate:    Wed, 08 Apr 2026 13:54:11 +02:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxx>
CommitterDate: Fri, 01 May 2026 21:36:12 +02:00

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@xxxxxxxxxx>
Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Acked-by: John Stultz <jstultz@xxxxxxxxxx>
Link: https://patch.msgid.link/20260408114952.332822525@xxxxxxxxxx
---
 include/linux/alarmtimer.h |  6 ++++++
 kernel/time/alarmtimer.c   | 28 ++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/include/linux/alarmtimer.h b/include/linux/alarmtimer.h
index 3ffa534..14d729f 100644
--- a/include/linux/alarmtimer.h
+++ b/include/linux/alarmtimer.h
@@ -42,8 +42,14 @@ struct alarm {
 	void			*data;
 };
 
+static __always_inline ktime_t alarm_get_expires(struct alarm *alarm)
+{
+	return alarm->node.expires;
+}
+
 void alarm_init(struct alarm *alarm, enum alarmtimer_type type,
 		void (*function)(struct alarm *, ktime_t));
+bool alarm_start_timer(struct alarm *alarm, ktime_t expires, bool relative);
 void alarm_start(struct alarm *alarm, ktime_t start);
 void alarm_start_relative(struct alarm *alarm, ktime_t start);
 void alarm_restart(struct alarm *alarm);
diff --git a/kernel/time/alarmtimer.c b/kernel/time/alarmtimer.c
index e10021b..2348b08 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];