1 files changed, 13 insertions, 0 deletions

diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index 88517dcfe0ca..58c0f60b132f 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -72,6 +72,7 @@ static DEFINE_SPINLOCK(hash_lock);
 
 static const struct k_clock * const posix_clocks[];
 static const struct k_clock *clockid_to_kclock(const clockid_t id);
+static const struct k_clock clock_realtime, clock_monotonic;
 
 /*
  * we assume that the new SIGEV_THREAD_ID shares no bits with the other
@@ -750,6 +751,18 @@ static void common_hrtimer_arm(struct k_itimer *timr, ktime_t expires,
         enum hrtimer_mode mode;
 
         mode = absolute ? HRTIMER_MODE_ABS : HRTIMER_MODE_REL;
+        /*
+         * Posix magic: Relative CLOCK_REALTIME timers are not affected by
+         * clock modifications, so they become CLOCK_MONOTONIC based under the
+         * hood. See hrtimer_init(). Update timr->kclock, so the generic
+         * functions which use timr->kclock->clock_get() work.
+         *
+         * Note: it_clock stays unmodified, because the next timer_set() might
+         * use ABSTIME, so it needs to switch back.
+         */
+        if (timr->it_clock == CLOCK_REALTIME)
+                timr->kclock = absolute ? &clock_realtime : &clock_monotonic;
+
         hrtimer_init(&timr->it.real.timer, timr->it_clock, mode);
         timr->it.real.timer.function = posix_timer_fn;