[PATCH 3/3] clocksource/drivers/nxp-stm: Increase min_delta to 20us for PREEMPT_RT

[Enric Balletbo i Serra via B4 Relay]

From: Enric Balletbo i Serra <eballetb at redhat.com>

Under bus contention on PREEMPT_RT kernels, the 2us min_delta is too
small and the counter can advance past the programmed comparator value
before the overshoot check runs. While the -ETIME fallback handles this
case, repeated retries add latency. Increase min_delta to 20us to reduce
collision probability.

Cc: stable at vger.kernel.org
Signed-off-by: Enric Balletbo i Serra <eballetb at redhat.com>
---
 drivers/clocksource/timer-nxp-stm.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/clocksource/timer-nxp-stm.c b/drivers/clocksource/timer-nxp-stm.c
index ed7662a4e2bc..a0761d7f8b01 100644
--- a/drivers/clocksource/timer-nxp-stm.c
+++ b/drivers/clocksource/timer-nxp-stm.c
@@ -335,14 +335,14 @@ static int nxp_stm_clockevent_starting_cpu(unsigned int cpu)
 	/*
 	 * The timings measurement show reading the counter register
 	 * and writing to the comparator register takes as a maximum
-	 * value 1100 ns at 133MHz rate frequency. The timer must be
-	 * set above this value and to be secure we set the minimum
-	 * value equal to 2000ns, so 2us.
+	 * value 1100 ns at 133MHz rate frequency. However, under bus
+	 * contention on PREEMPT_RT, 2us is too small and causes missed
+	 * matches resulting in 32s wraparounds. Increase to 20us.
 	 *
-	 * minimum ticks = (rate / MICRO) * 2
+	 * minimum ticks = (rate / MICRO) * 20
 	 */
 	clockevents_config_and_register(&stm_timer->ced, stm_timer->rate,
-					(stm_timer->rate / MICRO) * 2, 0x7FFFFFFFU);
+					(stm_timer->rate / MICRO) * 20, 0x7FFFFFFFU);
 
 	return 0;
 }

-- 
2.52.0