Re: [PATCH v4 0/4] Add ntp_error to clock snapshot, enable NTP_PPS on tickless kernel

[David Woodhouse <dwmw2@infradead.org>]

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On Tue, 2026-09-01 at 17:35 +0200, Rodolfo Giometti wrote:
> On Sat, 2026-08-29 at 21:56 +0100, David Woodhouse wrote:
> > With this change, CONFIG_NTP_PPS works correctly on a tickless kernel;
> > enable it. And change the non-CONFIG_NTP_PPS code path in pps_get_ts()
> > to use ktime_get_snapshot_id() too, for the more accurate data.
> 
> Thanks for respinning. The idea is good, the !NO_HZ_COMMON dependency
> has needed attention since 2011. Comments on 2/4 and 3/4 go in
> separate mails. Here the general ones.
> 
> The series does not apply to mainline (v7.1-13176-g840ef6c78e6a). It
> is written against some timekeeping rework that is not merged yet,
> and there is no base-commit: and no word about which tree to use.
> Please repost with "git format-patch --base=".

Oops, the dependencies actually landed in Linus's tree a lot more
recently than I thought; I thought it was so old that it wasn't worth
specifying a base. In fact although it *does* apply to mainline, it
needs a tree from mid-August or fresher (v7.2-1297-g3b4128b9f374).

> There is no changelog. Where are the v3 -> v4 notes? And why is this
> now PATCH and no longer RFC?

There were no substantive changes from RFC v3 to v4 (I rewrapped one
line which offended me while rebasing, I believe).

It's no longer RFC because nobody saw fit to comment :)

The evolution of this was in the timekeeping part, not on the PPS side.
If you really want to see it:

v1: https://lore.kernel.org/all/3616fc9718614bf11915569599038a5bcb268c02.camel@infradead.org/
v2: https://lore.kernel.org/all/3b10d2e91b18f49d8a3e6226b08ac8cd9cb49aa6.camel@infradead.org/
v3: https://lore.kernel.org/all/20260622211822.1056437-1-dwmw2@infradead.org/

> 3/4 does nothing at all without 1/4: it changes no timestamp value,
> only the cost. So the series has to go through tip/timers as a unit,
> not with the PPS bits going via Andrew separately.

Yes. Or we let the timekeeping part go through, and the PPS part can
come later. There's no rush.

> 1/4 itself is not mine to judge. It changes ::systime for every user of
> ktime_get_snapshot_id() and get_device_system_crosststamp(), not only
> PPS. Whether that is the right value for ptp4l and phc2sys is for
> Richard and the PTP people to say. I raise it only because I want that
> decision made explicitly, not inherited from a PPS series.
> 
> > Tested with a hack to make vmclock simulate a 1PPS signal, although there
> > are now better options for that. But it's enough to show that even the
> > tickless kernel converges to [...] the PPS signal and remains there
> > (tested with a periodic PTP_SYS_OFFSET_EXTENDED to compare with the
> > vmclock reference).
> 
> It is not enough. 4/4 takes the pulse from the same counter the
> timekeeping reads, so there is no independent reference in the test at
> all. Converging to +0ns against your own clock source proves very
> little, and it says nothing about hardpps() driven by a real pulse.

It's not about the counter being an independent reference. The point
here is that in a NO_HZ_FULL kernel, the CLOCK_REALTIME reading itself
sawtooths around where it should be, so even that perfect *dependent*
reference gives deltas of up to ±10ns for the same test.

You may be right that it doesn't actually test the pps_get_ts() path
though; the vmclock test builds its own pps_times. So it's proving the
*concept* but not the actual pps_get_ts() code path.

> You are asking me to undo something that has stood for fifteen years.
> I am not going to ack that on this evidence. :) What I want to see
> instead is in my reply to 2/4.

Ack. qv.

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