Re: [PATCH v4 3/4] pps: Always use ktime_get_snapshot_id() for pps_get_ts()

[Rodolfo Giometti <giometti@enneenne.com>]

On Sat, 2026-08-29 at 21:57 +0100, David Woodhouse wrote:
> Rather than using that more accurate timestamp *only* in the case where
> CONFIG_NTP_PPS is enabled, do so unconditionally.
>
>  static inline void pps_get_ts(struct pps_event_time *ts)
>  {
> -#ifdef CONFIG_NTP_PPS
>  	struct system_time_snapshot snap;
>  
>  	ktime_get_snapshot_id(CLOCK_REALTIME, &snap);
>  	ts->ts_real = ktime_to_timespec64(snap.systime);
> +#ifdef CONFIG_NTP_PPS
>  	ts->ts_raw = ktime_to_timespec64(snap.monoraw);
> -#else
> -	ktime_get_real_ts64(&ts->ts_real);
>  #endif
>  }

Why are you removing ktime_get_real_ts64()?

The commit message says you are using the more accurate timestamp
unconditionally. What the diff does is delete the !CONFIG_NTP_PPS
branch. Those are not the same thing, and the second one changes an
ABI: ts_real reaches userspace through PPS_FETCH on /dev/ppsN and
through /sys/class/pps/ppsX/assert, documented in
Documentation/ABI/testing/sysfs-pps and unchanged since 2008. Until
now it came from ktime_get_real_ts64(), the same clock userspace reads
with clock_gettime(CLOCK_REALTIME). After your patch it does not.

That needs a good reason and the commit message does not give one.
Give me that first. Until then the details do not matter.

Separately: are we sure that calling ktime_get_snapshot_id() does not
introduce much larger delays than ktime_get_real_ts64()? pps_get_ts()
runs in hard IRQ, and in pps-gpio it is the first statement of the
handler. Whatever it costs sits between the edge and the timestamp,
and that is the one thing PPS has to keep short.

I would like to see that measured on something other than an x86 VM
with a TSC. A small 32-bit ARM board is what I worry about.

Rodolfo