clockevents: Provide support for clocksource coupled comparators

Some clockevent devices are coupled to the system clocksource by
implementing a less than or equal comparator which compares the programmed
absolute expiry time against the underlying time counter.

The timekeeping core provides a function to convert and absolute
CLOCK_MONOTONIC based expiry time to a absolute clock cycles time which can
be directly fed into the comparator. That spares two time reads in the next
event progamming path, one to convert the absolute nanoseconds time to a
delta value and the other to convert the delta value back to a absolute
time value suitable for the comparator.

Provide a new clocksource callback which takes the absolute cycle value and
wire it up in clockevents_program_event(). Similar to clocksources allow
architectures to inline the rearm operation.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260224163430.010425428@kernel.org

2 files changed, 43 insertions, 5 deletions

diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
index b51bc5625129..e1968ab8b37f 100644
--- a/kernel/time/Kconfig
+++ b/kernel/time/Kconfig
@@ -50,6 +50,10 @@ config GENERIC_CLOCKEVENTS_MIN_ADJUST
 config GENERIC_CLOCKEVENTS_COUPLED
 	bool
 
+config GENERIC_CLOCKEVENTS_COUPLED_INLINE
+	select GENERIC_CLOCKEVENTS_COUPLED
+	bool
+
 # Generic update of CMOS clock
 config GENERIC_CMOS_UPDATE
 	bool
diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
index 5abaeef08e6a..83712aa1d385 100644
--- a/kernel/time/clockevents.c
+++ b/kernel/time/clockevents.c
@@ -292,6 +292,38 @@ static int clockevents_program_min_delta(struct clock_event_device *dev)
 
 #endif /* CONFIG_GENERIC_CLOCKEVENTS_MIN_ADJUST */
 
+#ifdef CONFIG_GENERIC_CLOCKEVENTS_COUPLED
+#ifdef CONFIG_GENERIC_CLOCKEVENTS_COUPLED_INLINE
+#include <asm/clock_inlined.h>
+#else
+static __always_inline void
+arch_inlined_clockevent_set_next_coupled(u64 u64 cycles, struct clock_event_device *dev) { }
+#endif
+
+static inline bool clockevent_set_next_coupled(struct clock_event_device *dev, ktime_t expires)
+{
+	u64 cycles;
+
+	if (unlikely(!(dev->features & CLOCK_EVT_FEAT_CLOCKSOURCE_COUPLED)))
+		return false;
+
+	if (unlikely(!ktime_expiry_to_cycles(dev->cs_id, expires, &cycles)))
+		return false;
+
+	if (IS_ENABLED(CONFIG_GENERIC_CLOCKEVENTS_COUPLED_INLINE))
+		arch_inlined_clockevent_set_next_coupled(cycles, dev);
+	else
+		dev->set_next_coupled(cycles, dev);
+	return true;
+}
+
+#else
+static inline bool clockevent_set_next_coupled(struct clock_event_device *dev, ktime_t expires)
+{
+	return false;
+}
+#endif
+
 /**
  * clockevents_program_event - Reprogram the clock event device.
  * @dev:	device to program
@@ -300,11 +332,10 @@ static int clockevents_program_min_delta(struct clock_event_device *dev)
  *
  * Returns 0 on success, -ETIME when the event is in the past.
  */
-int clockevents_program_event(struct clock_event_device *dev, ktime_t expires,
-			      bool force)
+int clockevents_program_event(struct clock_event_device *dev, ktime_t expires, bool force)
 {
-	unsigned long long clc;
 	int64_t delta;
+	u64 cycles;
 	int rc;
 
 	if (WARN_ON_ONCE(expires < 0))
@@ -323,6 +354,9 @@ int clockevents_program_event(struct clock_event_device *dev, ktime_t expires,
 	if (unlikely(dev->features & CLOCK_EVT_FEAT_HRTIMER))
 		return dev->set_next_ktime(expires, dev);
 
+	if (likely(clockevent_set_next_coupled(dev, expires)))
+		return 0;
+
 	delta = ktime_to_ns(ktime_sub(expires, ktime_get()));
 	if (delta <= 0)
 		return force ? clockevents_program_min_delta(dev) : -ETIME;
@@ -330,8 +364,8 @@ int clockevents_program_event(struct clock_event_device *dev, ktime_t expires,
 	delta = min(delta, (int64_t) dev->max_delta_ns);
 	delta = max(delta, (int64_t) dev->min_delta_ns);
 
-	clc = ((unsigned long long) delta * dev->mult) >> dev->shift;
-	rc = dev->set_next_event((unsigned long) clc, dev);
+	cycles = ((u64)delta * dev->mult) >> dev->shift;
+	rc = dev->set_next_event((unsigned long) cycles, dev);
 
 	return (rc && force) ? clockevents_program_min_delta(dev) : rc;
 }