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

[Ghennadi Procopciuc <ghennadi.procopciuc@oss.nxp.com>]

On 9/13/2026 4:36 PM, Juhee Kang wrote:
> 
> 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@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@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@redhat.com>
> Signed-off-by: Juhee Kang <juhee.kang@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;
>  }
>

One optional nit: a short comment above the block would help future
readers understand why it runs only once, e.g.:

/*
 * The STM instances are independent counters running at the same
 * rate. Only one of them can back sched_clock, use the first one
 * probed.
 */

With or without it:

Reviewed-by: Ghennadi Procopciuc <ghennadi.procopciuc@oss.nxp.com>

-- 
Regards,
Ghennadi