[PATCH v2 3/3] clocksource/drivers/nxp-stm: Increase min_delta to 20us for PREEMPT_RT

[Enric Balletbo i Serra]

Under heavy bus contention on PREEMPT_RT kernels, a 2us min_delta is too
tight and causes ~32-second timer soft lockups.

Increase min_delta from 2us to 20us (~2,660 cycles at 133 MHz). This
provides sufficient margin under severe bus contention to ensure the
channel-enable write completes well before the counter reaches the match
value, resolving the 32-second soft lockup.

Fixes: cec32ac75827 ("clocksource/drivers/nxp-timer: Add the System Timer Module for the s32gx platforms")

Signed-off-by: Enric Balletbo i Serra <eballetb at redhat.com>
---
 drivers/clocksource/timer-nxp-stm.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/clocksource/timer-nxp-stm.c b/drivers/clocksource/timer-nxp-stm.c
index ed7662a4e2bc..a1a0e23d3342 100644
--- a/drivers/clocksource/timer-nxp-stm.c
+++ b/drivers/clocksource/timer-nxp-stm.c
@@ -335,14 +335,18 @@ 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 heavy
+	 * bus contention on PREEMPT_RT, 2us is too tight and can allow
+	 * the counter to pass the match value before the channel-enable
+	 * write takes effect, leading to 32s wraparounds.
 	 *
-	 * minimum ticks = (rate / MICRO) * 2
+	 * Increase to 20us to guarantee the comparator channel is fully
+	 * enabled before the match value is reached.
+	 *
+	 * 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;
 }

-- 
2.56.0