[PATCH v2 2/3] clocksource/drivers/nxp-stm: Fix missed comparator match in set_next_event

[Enric Balletbo i Serra]

The NXP STM generates an interrupt only on exact match between STM_CNT
and STM_CMP0. If the counter passes the comparator value before the
overshoot check runs, the match is missed and the hardware must count
through all 2^32 ticks (~32.2s at 133 MHz) before firing, causing soft
lockups.

The existing overshoot check:
    if (val > nxp_stm_clockevent_read_counter(stm_timer) + delta)
simplifies to checking if counter_before > counter_after, which only
detects a 32-bit counter wrap and misses the normal overshoot case where
the counter advances past val.

Use a signed comparison on the difference between the target and current
counter values instead, following the established pattern in
timer-imx-gpt. If the counter has reached or passed val, the signed
difference goes to zero or negative and we return -ETIME so the
clockevents layer retries immediately.

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 | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/drivers/clocksource/timer-nxp-stm.c b/drivers/clocksource/timer-nxp-stm.c
index ee249fed1b91..ed7662a4e2bc 100644
--- a/drivers/clocksource/timer-nxp-stm.c
+++ b/drivers/clocksource/timer-nxp-stm.c
@@ -247,7 +247,7 @@ static int nxp_stm_clockevent_shutdown(struct clock_event_device *ced)
 static int nxp_stm_clockevent_set_next_event(unsigned long delta, struct clock_event_device *ced)
 {
 	struct stm_timer *stm_timer = ced_to_stm(ced);
-	u32 val;
+	u32 val, now;
 
 	nxp_stm_clockevent_disable(stm_timer);
 
@@ -258,16 +258,12 @@ static int nxp_stm_clockevent_set_next_event(unsigned long delta, struct clock_e
 	writel(val, STM_CMP0(stm_timer->base));
 
 	/*
-	 * The counter is shared across the channels and can not be
-	 * stopped while we are setting the next event. If the delta
-	 * is very small it is possible the counter increases above
-	 * the computed 'val'. The min_delta value specified when
-	 * registering the clockevent will prevent that. The second
-	 * case is if the counter wraps while we compute the 'val' and
-	 * before writing the comparator register. We read the counter,
-	 * check if we are back in time and abort the timer with -ETIME.
+	 * If the counter reached or passed 'val' during or before the
+	 * write to the comparator, the hardware match will be missed,
+	 * resulting in a full 32-bit (32.2s) wraparound lockup.
 	 */
-	if (val > nxp_stm_clockevent_read_counter(stm_timer) + delta)
+	now = nxp_stm_clockevent_read_counter(stm_timer);
+	if ((int)(val - now) <= 0)
 		return -ETIME;
 
 	nxp_stm_clockevent_enable(stm_timer);

-- 
2.56.0