[RFC PATCH v3 08/10] timekeeping: Add absolute reference for feed-forward clock discipline

[David Woodhouse <dwmw2@infradead.org>]

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

Add timekeeping_set_reference() which allows an external clock source
(such as a hypervisor vmclock) to provide an absolute time reference.
The reference defines a linear counter-to-time mapping that the kernel
uses to set both the frequency and phase of the system clock.

When timekeeping_set_reference() is called:
 - tick_length is computed from the reference period and set via
   ntp_set_tick_length()
 - the phase delta is set via ntp_set_time_offset()

So the NTP state is entirely consistent, and the existing time_offset
skew mechanism then converges the clock to the reference, with the
ntp_error and time_offset accumulators staying accurate throughout.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Assisted-by: Kiro:claude-opus-4.6-1m
---
 include/linux/timekeeping_reference.h | 19 ++++++++++++++
 kernel/time/ntp.c                     | 14 ++++++++++
 kernel/time/ntp_internal.h            |  1 +
 kernel/time/timekeeping.c             | 38 +++++++++++++++++++++++++++
 4 files changed, 72 insertions(+)
 create mode 100644 include/linux/timekeeping_reference.h

diff --git a/include/linux/timekeeping_reference.h b/include/linux/timekeeping_reference.h
new file mode 100644
index 000000000000..4c1d8a6c02f1
--- /dev/null
+++ b/include/linux/timekeeping_reference.h
@@ -0,0 +1,19 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_TIMEKEEPING_REFERENCE_H
+#define _LINUX_TIMEKEEPING_REFERENCE_H
+
+#include <linux/clocksource_ids.h>
+#include <linux/types.h>
+
+struct tk_reference {
+	enum clocksource_ids	cs_id;
+	u64			counter_value;
+	u64			time_sec;
+	u64			time_frac_sec;
+	u64			period_frac_sec;
+	u8			period_shift;
+};
+
+int timekeeping_set_reference(const struct tk_reference *ref);
+
+#endif
diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index 4494f258dd86..3dc098695665 100644
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -403,6 +403,20 @@ void ntp_set_time_offset(unsigned int tkid, s64 offset_ns)
 	ntpdata->time_adjust = 0;
 }
 
+void ntp_set_tick_length(unsigned int tkid, u64 tick_length)
+{
+	struct ntp_data *ntpdata = &tk_ntp_data[tkid];
+	u64 base;
+
+	/* Compute the nominal second length (without frequency adjustment) */
+	base = (u64)(ntpdata->tick_usec * NSEC_PER_USEC * USER_HZ)
+		<< NTP_SCALE_SHIFT;
+	base += ntpdata->ntp_tick_adj;
+
+	ntpdata->time_freq = (s64)(tick_length * NTP_INTERVAL_FREQ - base);
+	ntp_update_frequency(ntpdata);
+}
+
 /**
  * ntp_get_next_leap - Returns the next leapsecond in CLOCK_REALTIME ktime_t
  * @tkid:	Timekeeper ID
diff --git a/kernel/time/ntp_internal.h b/kernel/time/ntp_internal.h
index 639860ff2baf..14ca8bc08120 100644
--- a/kernel/time/ntp_internal.h
+++ b/kernel/time/ntp_internal.h
@@ -9,6 +9,7 @@ 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 void ntp_set_tick_length(unsigned int tkid, u64 tick_length);
 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,
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 27b2a093b138..5c4b377505bc 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -2324,6 +2324,44 @@ static __always_inline void timekeeping_apply_adjustment(struct timekeeper *tk,
  * Adjust the timekeeper's multiplier to the correct frequency
  * and also to reduce the accumulated error value.
  */
+
+#include <linux/timekeeping_reference.h>
+
+
+int timekeeping_set_reference(const struct tk_reference *ref)
+{
+	struct timekeeper *tk = &tk_core.timekeeper;
+	u64 new_tl, delta, ref_frac;
+	s64 ref_err;
+	unsigned long flags;
+
+	raw_spin_lock_irqsave(&tk_core.lock, flags);
+
+	if (tk->cs_id != ref->cs_id) {
+		raw_spin_unlock_irqrestore(&tk_core.lock, flags);
+		return -ENODEV;
+	}
+
+	new_tl = mul_u64_u64_shr(ref->period_frac_sec,
+			(u64)tk->cycle_interval * NSEC_PER_SEC,
+			32 + ref->period_shift);
+	ntp_set_tick_length(tk->id, new_tl);
+
+	/* Compute phase offset at cycle_last and set time_offset to slew */
+	delta = tk->tkr_mono.cycle_last - ref->counter_value;
+	ref_frac = mul_u64_u64_shr(delta, ref->period_frac_sec,
+				   ref->period_shift) + ref->time_frac_sec;
+	ref_err = (s64)mul_u64_u64_shr(ref_frac,
+			(u64)NSEC_PER_SEC << tk->tkr_mono.shift, 64) -
+		  (s64)tk->tkr_mono.xtime_nsec;
+	ntp_set_time_offset(tk->id, ref_err >> tk->tkr_mono.shift);
+	tk->ntp_error = 0;
+
+	raw_spin_unlock_irqrestore(&tk_core.lock, flags);
+	return 0;
+}
+EXPORT_SYMBOL_GPL(timekeeping_set_reference);
+
 static void timekeeping_adjust(struct timekeeper *tk, s64 offset)
 {
 	u64 ntp_tl = ntp_tick_length(tk->id);
-- 
2.54.0