[tip: timers/core] timekeeping: Prepare timekeeping_cycles_to_ns() for overflow safety

[tip-bot2 for Adrian Hunter]

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

Commit-ID:     e809a80aa0bcf802f99407c23fd6be6fd4eb250a
Gitweb:        https://git.kernel.org/tip/e809a80aa0bcf802f99407c23fd6be6fd4eb250a
Author:        Adrian Hunter <adrian.hunter@xxxxxxxxx>
AuthorDate:    Mon, 25 Mar 2024 08:40:20 +02:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Mon, 08 Apr 2024 15:03:08 +02:00

timekeeping: Prepare timekeeping_cycles_to_ns() for overflow safety

Open code clocksource_delta() in timekeeping_cycles_to_ns() so that
overflow safety can be added efficiently.

Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20240325064023.2997-17-adrian.hunter@xxxxxxxxx

---
 kernel/time/timekeeping.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 749387f..d174840 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -367,7 +367,17 @@ static void tk_setup_internals(struct timekeeper *tk, struct clocksource *clock)
 static inline u64 timekeeping_cycles_to_ns(const struct tk_read_base *tkr, u64 cycles)
 {
 	/* Calculate the delta since the last update_wall_time() */
-	u64 delta = clocksource_delta(cycles, tkr->cycle_last, tkr->mask);
+	u64 mask = tkr->mask, delta = (cycles - tkr->cycle_last) & mask;
+
+	if (IS_ENABLED(CONFIG_CLOCKSOURCE_VALIDATE_LAST_CYCLE)) {
+		/*
+		 * Handle clocksource inconsistency between CPUs to prevent
+		 * time from going backwards by checking for the MSB of the
+		 * mask being set in the delta.
+		 */
+		if (unlikely(delta & ~(mask >> 1)))
+			return tkr->xtime_nsec >> tkr->shift;
+	}
 
 	return ((delta * tkr->mult) + tkr->xtime_nsec) >> tkr->shift;
 }