[PATCH] clocksource/drivers/nxp-stm: Register sched_clock once

[Juhee Kang]

Each probe updates stm_sched_clock before calling sched_clock_register()
again. The call then subtracts the counter value saved during the
previous registration from the new STM's counter. If the new counter is
smaller, the subtraction wraps and the timestamp jumps forward.
(e.g. from 0.4 to 32.9 seconds below)

  [    0.403653] sched_clock: 32 bits at 132MHz, resolution 7ns, wraps every 16243448316ns
  [    0.404056] clocksource: timer at 40120000: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 14456668980 ns
  [   32.890509] sched_clock: 32 bits at 132MHz, resolution 7ns, wraps every 16243448316ns
  [   32.890882] clocksource: timer at 40124000: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 14456668980 ns

To fix this, register sched_clock once at the end of the first
successful probe. Clocksource and per-CPU clockevent registration
remain unchanged.

Fixes: cec32ac75827 ("clocksource/drivers/nxp-timer: Add the System Timer Module for the s32gx platforms")
Reported-by: Dustin Black <dblack at redhat.com>
Signed-off-by: Juhee Kang <juhee.kang at redhat.com>
---
 drivers/clocksource/timer-nxp-stm.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/clocksource/timer-nxp-stm.c b/drivers/clocksource/timer-nxp-stm.c
index 6fe098a4a33f..7572cd20732f 100644
--- a/drivers/clocksource/timer-nxp-stm.c
+++ b/drivers/clocksource/timer-nxp-stm.c
@@ -211,10 +211,6 @@ static int nxp_stm_clocksource_init(struct device *dev, struct stm_timer *stm_ti
 	if (ret)
 		return ret;
 
-	stm_sched_clock = stm_timer;
-
-	sched_clock_register(nxp_stm_read_sched_clock, 32, stm_timer->rate);
-
 	dev_dbg(dev, "Registered clocksource %s\n", name);
 
 	return 0;
@@ -473,6 +469,11 @@ static int nxp_stm_timer_probe(struct platform_device *pdev)
 			return ret;
 	}
 
+	if (!stm_sched_clock) {
+		stm_sched_clock = stm_timer;
+		sched_clock_register(nxp_stm_read_sched_clock, 32, stm_timer->rate);
+	}
+
 	return 0;
 }
 
-- 
2.55.0