[PATCH v4 6/7] ntp: Convert adjtime() to use time_offset instead of tick_length inflation

[David Woodhouse <dwmw2@infradead.org>]

From: David Woodhouse <dwmw@amazon.co.uk>

The legacy adjtime() syscall (ADJ_ADJTIME/ADJ_OFFSET_SINGLESHOT) used
to slew the clock by inflating tick_length directly via time_adjust.
This was the last remaining user of tick_length != tick_length_base.

Convert it to fold time_adjust into time_offset each second (up to
MAX_TICKADJ per second, same rate limit as before). The existing
time_offset skew mechanism then delivers it via the per-tick ntp_error
transfer and mult adjustment.

Introduce ntp_set_time_offset() helper for setting time_offset from a
nanosecond value, and refactor ntp_update_offset() to use it. This
helper will also be used by the feed-forward reference clock API in a
subsequent commit.

This eliminates the last source of tick_length inflation, making
tick_length always equal tick_length_base.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Assisted-by: Kiro:claude-opus-4.6-1m
---
 kernel/time/ntp.c          | 52 ++++++++++++++++++++++++--------------
 kernel/time/ntp_internal.h |  1 +
 2 files changed, 34 insertions(+), 19 deletions(-)

diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index 3c1e287eb384..9470ac1f597b 100644
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -329,7 +329,7 @@ static void ntp_update_offset(struct ntp_data *ntpdata, long offset)
 
 	ntpdata->time_freq   = max(freq_adj, -MAXFREQ_SCALED);
 
-	ntpdata->time_offset = div_s64(offset64 << NTP_SCALE_SHIFT, NTP_INTERVAL_FREQ);
+	ntp_set_time_offset(ntpdata - tk_ntp_data, offset64);
 }
 
 static void __ntp_clear(struct ntp_data *ntpdata)
@@ -390,6 +390,24 @@ s64 ntp_drain_time_offset(unsigned int tkid, s64 amount)
 	return 0;
 }
 
+/**
+ * ntp_set_time_offset - Set the NTP time offset (phase correction)
+ * @tkid:	Timekeeper ID
+ * @offset_ns:	Desired offset in nanoseconds
+ *
+ * Converts nanoseconds to internal time_offset units and stores it.
+ * Also clears time_adjust since a new offset supersedes any pending
+ * adjtime() slew.
+ */
+void ntp_set_time_offset(unsigned int tkid, s64 offset_ns)
+{
+	struct ntp_data *ntpdata = &tk_ntp_data[tkid];
+
+	ntpdata->time_offset = div_s64((s64)offset_ns << NTP_SCALE_SHIFT,
+				       NTP_INTERVAL_FREQ);
+	ntpdata->time_adjust = 0;
+}
+
 /**
  * ntp_get_next_leap - Returns the next leapsecond in CLOCK_REALTIME ktime_t
  * @tkid:	Timekeeper ID
@@ -498,26 +516,22 @@ int second_overflow(unsigned int tkid, time64_t secs)
 	/* Check PPS signal */
 	pps_dec_valid(ntpdata);
 
-	if (!ntpdata->time_adjust)
-		goto out;
-
-	if (ntpdata->time_adjust > MAX_TICKADJ) {
-		ntpdata->time_adjust -= MAX_TICKADJ;
-		ntpdata->tick_length += MAX_TICKADJ_SCALED;
-		goto out;
-	}
-
-	if (ntpdata->time_adjust < -MAX_TICKADJ) {
-		ntpdata->time_adjust += MAX_TICKADJ;
-		ntpdata->tick_length -= MAX_TICKADJ_SCALED;
-		goto out;
+	/*
+	 * Fold any pending time_adjust (from adjtime()) into time_offset.
+	 * This used to inflate tick_length directly; now it uses the same
+	 * per-tick skew mechanism as NTP's time_offset. Rate-limited to
+	 * MAX_TICKADJ (500µs) per second.
+	 */
+	if (ntpdata->time_adjust) {
+		long adj = clamp(ntpdata->time_adjust,
+				 (long)-MAX_TICKADJ, (long)MAX_TICKADJ);
+
+		ntpdata->time_adjust -= adj;
+		ntpdata->time_offset += div_s64(
+			(s64)adj * NSEC_PER_USEC << NTP_SCALE_SHIFT,
+			NTP_INTERVAL_FREQ);
 	}
 
-	ntpdata->tick_length += (s64)(ntpdata->time_adjust * NSEC_PER_USEC / NTP_INTERVAL_FREQ)
-				<< NTP_SCALE_SHIFT;
-	ntpdata->time_adjust = 0;
-
-out:
 	return leap;
 }
 
diff --git a/kernel/time/ntp_internal.h b/kernel/time/ntp_internal.h
index 05e5dd5e1b70..639860ff2baf 100644
--- a/kernel/time/ntp_internal.h
+++ b/kernel/time/ntp_internal.h
@@ -8,6 +8,7 @@ extern void ntp_clear(unsigned int tkid);
 extern u64 ntp_tick_length(unsigned int tkid);
 extern s64 ntp_get_skew_delta(unsigned int tkid);
 extern s64 ntp_drain_time_offset(unsigned int tkid, s64 amount);
+extern void ntp_set_time_offset(unsigned int tkid, s64 offset_ns);
 extern ktime_t ntp_get_next_leap(unsigned int tkid);
 extern int second_overflow(unsigned int tkid, time64_t secs);
 extern int ntp_adjtimex(unsigned int tkid, struct __kernel_timex *txc, const struct timespec64 *ts,
-- 
2.54.0