Re: [PATCH v4 2/4] pps: Drop the !NO_HZ_COMMON dependency from NTP_PPS

[Rodolfo Giometti <giometti@enneenne.com>]

On Sat, 2026-08-29 at 21:57 +0100, David Woodhouse wrote:
> Whatever the original reasons were, the only *remaining* reason seems to
> have been that the accuracy of the time captured by pps_get_ts() was poor
> on tickless kernels due to the kernel's per-tick timekeeping mechanism.

"Whatever the original reasons were" and "seems to have been" is not
enough to drop a dependency that has been there for fifteen years. The
old comment is useless, I agree. But then we have to say what breaks
and what does not, not guess.

First a structural point. This is the only patch of the four that
applies to mainline, and it has no build dependency on the rest. It is
a three-line Kconfig delete that compiles on its own. That worries me:
a small "pps:" patch that applies cleanly is exactly what gets picked
up alone. Then NTP_PPS becomes selectable on tickless kernels without
1/4, and we are worse off than today. Reorder it last, or say in the
commit message that it must not be applied without 1/4.

> A recent change to ktime_get_snapshot_id() which is used by pps_get_ts()
> has fixed that problem, by applying a correction to the ::systime field

That "recent change" is 1/4 of this series, and it is in no tree yet.
Reading this, one assumes the groundwork already landed. Say "the
previous patch". Same wording in 3/4.

About the test. The pulse comes from 4/4, which derives it from the
same counter the timekeeping reads. No independent reference anywhere.

Before I ack this I want to see:

   - a real source, pps-gpio with a GPS receiver, where pulse and system
     clock are independent;
   - NO_HZ_FULL, not only NO_HZ_IDLE;
   - a run that goes through a long idle period, not just a busy system.

That is more work than a three-line delete suggests, I know. But those
three lines unlock a configuration people will run against real
receivers and then trust.

Rodolfo