# Base git commit: e8f897f4afef
# (Linux 6.8)
# 
# Author:    Russell King (Sat 23 Sep 19:27:37 BST 2017)
# Committer: Russell King (Oracle) (Tue 26 Mar 12:20:58 GMT 2024)
# 
#   rtc: armada38x: provide set_offset_nsec
# 
#   Armada 388 resets the prescaler when the time register is written, so
#   it requires the current second as close to the start of that second as
#   possible.  The data sheet is a bit unclear, so this has been confirmed
#   on real hardware.
# 
#   Verified on Armada 388 based Clearfog.
# 
#   Signed-off-by: Russell King
# 
#   b23affb84e38c31b08aa3d33f364b04a21a0e936
#    drivers/rtc/rtc-armada38x.c | 10 ++++++++++
#    1 file changed, 10 insertions(+)
# 
# Author:    Russell King (Sun 24 Sep 11:32:52 BST 2017)
# Committer: Russell King (Oracle) (Tue 26 Mar 12:20:57 GMT 2024)
# 
#   rtc: pcf8523: provide set_offset_nsec
# 
#   When we set the time, we set the STOP bit, set the time, and then clear
#   the STOP bit.  Concerning the timing, the PCF8523 data sheet says:
# 
#   "The first increment of the time circuits is between 0.499878s and
#   0.500000s after STOP is released."
# 
#   However, practical measurement shows this is not the case - with the I2C
#   bus speed at 100kHz on iMX6, it takes about 5ms for the rtclib call for
#   setting the time to complete.  However, reading back when the second
#   actually flips shows that there's an additional 10ms which can't be
#   accounted for by the read - a read of the RTC takes 1.7 to 1.8 ms.
# 
#   Practical measurement shows that the first increment occurs about 515ms
#   after the write, which means we need to set the current second 485ms
#   after it has started.
# 
#   Signed-off-by: Russell King
# 
#   7c7477089ab723d46730f4608649a957025d39b9
#    drivers/rtc/rtc-pcf8523.c | 11 +++++++++++
#    1 file changed, 11 insertions(+)
# 
# Author:    Russell King (Sat 23 Sep 10:31:33 BST 2017)
# Committer: Russell King (Oracle) (Tue 26 Mar 12:20:56 GMT 2024)
# 
#   rtc: pre-register hook
# 
#   Add a pre-register hook to allow RTC drivers to adjust the default
#   set_offset_nsec value.
# 
#   Signed-off-by: Russell King
# 
#   6a19411ac6c3611b7067e3c222060ec02e59eab7
#    drivers/rtc/class.c | 3 +++
#    include/linux/rtc.h | 3 +++
#    2 files changed, 6 insertions(+)
# 
# Author:    Russell King (Thu 21 Sep 12:11:49 BST 2017)
# Committer: Russell King (Oracle) (Tue 26 Mar 12:20:54 GMT 2024)
# 
#   rtc: add procfs sysctl to control RTC updates
# 
#   Add a procfs sysctl to allow the RTC updates by the NTP code to be
#   disabled.
# 
#   This is necessary if one wishes to adjust the RTC trimming.  When
#   performing such an adjustment, one would want to compare the RTC
#   against a good reference.  One way to do that is to synchronise the
#   systems own time using NTP, but doing so causes the kernel to set
#   the RTC every 11 minutes.
# 
#   This repeated setting prevents measuring the long-term drift of the
#   RTC, and so prevents any attempt to trim the RTC for better accuracy.
# 
#   This sysctl defaults to enabled, so existing behaviour is preserved.
#   When one wishes to adjust the long-term drift, the RTC update can be
#   disabled via: echo 0 > /proc/sys/kernel/ntp_rtc_sync, the drift
#   measured, and the update subsequently re-enabled.  The next update
#   will be triggered by the next successful adjtimex call.
# 
#   Signed-off-by: Russell King
# 
#   937d31aacda28e352ad93c1afbd11a4d4f562ebe
#    kernel/sysctl.c   | 13 +++++++++++++
#    kernel/time/ntp.c |  7 +++++--
#    2 files changed, 18 insertions(+), 2 deletions(-)
# 
# Author:    Russell King (Sat 23 Sep 10:30:24 BST 2017)
# Committer: Russell King (Oracle) (Tue 26 Mar 12:20:53 GMT 2024)
# 
#   rtc: export RTC setting offset via sysfs
# 
#   Export the RTC time set offset via sysfs so the value can be read and
#   tweaked as necessary.  This is useful when determining the correct
#   setting for a RTC, as it eliminates the need to rebuild/install/reboot
#   from the test cycle.
# 
#   Signed-off-by: Russell King
# 
#   41afca166c28e4056e9e859102660d1b1df6aa38
#    drivers/rtc/sysfs.c | 26 ++++++++++++++++++++++++++
#    1 file changed, 26 insertions(+)
# 
diff --git a/drivers/rtc/class.c b/drivers/rtc/class.c
index 921ee1827974..40be7251f697 100644
--- a/drivers/rtc/class.c
+++ b/drivers/rtc/class.c
@@ -410,6 +410,9 @@ int __devm_rtc_register_device(struct module *owner, struct rtc_device *rtc)
 	if (!err && !rtc_valid_tm(&alrm.time))
 		rtc_initialize_alarm(rtc, &alrm);
 
+	if (rtc->ops->pre_register)
+		rtc->ops->pre_register(rtc);
+
 	rtc_dev_prepare(rtc);
 
 	err = cdev_device_add(&rtc->char_dev, &rtc->dev);
diff --git a/drivers/rtc/rtc-armada38x.c b/drivers/rtc/rtc-armada38x.c
index 569c1054d6b0..be216a32db20 100644
--- a/drivers/rtc/rtc-armada38x.c
+++ b/drivers/rtc/rtc-armada38x.c
@@ -447,6 +447,15 @@ static int armada38x_rtc_set_offset(struct device *dev, long offset)
 	return 0;
 }
 
+static void armada38x_rtc_pre_register(struct rtc_device *rtc)
+{
+	/*
+	 * The Armada 388 resets the prescaler when we write the time.
+	 * Validated on clearfog.
+	 */
+	rtc->set_offset_nsec = 0;
+}
+
 static const struct rtc_class_ops armada38x_rtc_ops = {
 	.read_time = armada38x_rtc_read_time,
 	.set_time = armada38x_rtc_set_time,
@@ -455,6 +464,7 @@ static const struct rtc_class_ops armada38x_rtc_ops = {
 	.alarm_irq_enable = armada38x_rtc_alarm_irq_enable,
 	.read_offset = armada38x_rtc_read_offset,
 	.set_offset = armada38x_rtc_set_offset,
+	.pre_register = armada38x_rtc_pre_register,
 };
 
 static const struct armada38x_rtc_data armada38x_data = {
diff --git a/drivers/rtc/rtc-pcf8523.c b/drivers/rtc/rtc-pcf8523.c
index d1efde3e7a80..3446b4b5a677 100644
--- a/drivers/rtc/rtc-pcf8523.c
+++ b/drivers/rtc/rtc-pcf8523.c
@@ -370,6 +370,16 @@ static int pcf8523_rtc_set_offset(struct device *dev, long offset)
 	return regmap_write(pcf8523->regmap, PCF8523_REG_OFFSET, value);
 }
 
+static void pcf8523_rtc_pre_register(struct rtc_device *rtc)
+{
+	/*
+	 * "The first increment of the time circuits is between 0.499878s
+	 * and 0.5s after STOP is released. Measurement on iMX6 show that
+	 * -485ms gives a time set within +/- 5ms.
+	 */
+	rtc->set_offset_nsec = -495 * NSEC_PER_MSEC;
+}
+
 static const struct rtc_class_ops pcf8523_rtc_ops = {
 	.read_time = pcf8523_rtc_read_time,
 	.set_time = pcf8523_rtc_set_time,
@@ -381,6 +391,7 @@ static const struct rtc_class_ops pcf8523_rtc_ops = {
 	.set_offset = pcf8523_rtc_set_offset,
 	.param_get = pcf8523_param_get,
 	.param_set = pcf8523_param_set,
+	.pre_register = pcf8523_rtc_pre_register,
 };
 
 static const struct regmap_config regmap_config = {
diff --git a/drivers/rtc/sysfs.c b/drivers/rtc/sysfs.c
index e3062c4d3f2c..2255e7c3da71 100644
--- a/drivers/rtc/sysfs.c
+++ b/drivers/rtc/sysfs.c
@@ -243,6 +243,31 @@ offset_store(struct device *dev, struct device_attribute *attr,
 }
 static DEVICE_ATTR_RW(offset);
 
+static ssize_t
+set_offset_nsec_show(struct device *dev, struct device_attribute *attr,
+		     char *buf)
+{
+	return sprintf(buf, "%ld\n", to_rtc_device(dev)->set_offset_nsec);
+}
+
+static ssize_t
+set_offset_nsec_store(struct device *dev, struct device_attribute *attr,
+		      const char *buf, size_t n)
+{
+	struct rtc_device *rtc = to_rtc_device(dev);
+	long val;
+	int err;
+
+	err = kstrtol(buf, 0, &val);
+	if (err)
+		return err;
+
+	rtc->set_offset_nsec = val;
+
+	return n;
+}
+static DEVICE_ATTR_RW(set_offset_nsec);
+
 static ssize_t
 range_show(struct device *dev, struct device_attribute *attr, char *buf)
 {
@@ -260,6 +285,7 @@ static struct attribute *rtc_attrs[] = {
 	&dev_attr_hctosys.attr,
 	&dev_attr_wakealarm.attr,
 	&dev_attr_offset.attr,
+	&dev_attr_set_offset_nsec.attr,
 	&dev_attr_range.attr,
 	NULL,
 };
diff --git a/include/linux/rtc.h b/include/linux/rtc.h
index 5f8e438a0312..6b4c676f0f90 100644
--- a/include/linux/rtc.h
+++ b/include/linux/rtc.h
@@ -44,6 +44,8 @@ static inline time64_t rtc_tm_sub(struct rtc_time *lhs, struct rtc_time *rhs)
 
 extern struct class *rtc_class;
 
+struct rtc_device;
+
 /*
  * For these RTC methods the device parameter is the physical device
  * on whatever bus holds the hardware (I2C, Platform, SPI, etc), which
@@ -68,6 +70,7 @@ struct rtc_class_ops {
 	int (*set_offset)(struct device *, long offset);
 	int (*param_get)(struct device *, struct rtc_param *param);
 	int (*param_set)(struct device *, struct rtc_param *param);
+	void (*pre_register)(struct rtc_device *);
 };
 
 struct rtc_device;
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 157f7ce2942d..06e2c420aa3d 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -90,6 +90,8 @@ EXPORT_SYMBOL_GPL(sysctl_long_vals);
 
 #if defined(CONFIG_SYSCTL)
 
+extern unsigned int sysctl_ntp_rtc_sync;
+
 /* Constants used for minimum and maximum */
 
 #ifdef CONFIG_PERF_EVENTS
@@ -1616,6 +1618,17 @@ int proc_do_static_key(struct ctl_table *table, int write,
 }
 
 static struct ctl_table kern_table[] = {
+#if defined(CONFIG_GENERIC_CMOS_UPDATE) || defined(CONFIG_RTC_SYSTOHC)
+	{
+		.procname	= "ntp_rtc_sync",
+		.data		= &sysctl_ntp_rtc_sync,
+		.maxlen		= sizeof(sysctl_ntp_rtc_sync),
+		.mode		= 0644,
+		.proc_handler	= proc_douintvec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_ONE,
+	},
+#endif
 	{
 		.procname	= "panic",
 		.data		= &panic_timeout,
diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index 406dccb79c2b..85a56b888671 100644
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -494,6 +494,7 @@ int second_overflow(time64_t secs)
 	return leap;
 }
 
+unsigned int sysctl_ntp_rtc_sync = true;
 #if defined(CONFIG_GENERIC_CMOS_UPDATE) || defined(CONFIG_RTC_SYSTOHC)
 static void sync_hw_clock(struct work_struct *work);
 static DECLARE_WORK(sync_work, sync_hw_clock);
@@ -635,7 +636,8 @@ static void sync_hw_clock(struct work_struct *work)
 	 * managed to schedule the work between the timer firing and the
 	 * work being able to rearm the timer. Wait for the timer to expire.
 	 */
-	if (!ntp_synced() || hrtimer_is_queued(&sync_hrtimer))
+	if (!ntp_synced() || !sysctl_ntp_rtc_sync ||
+	    hrtimer_is_queued(&sync_hrtimer))
 		return;
 
 	ktime_get_real_ts64(&now);
@@ -667,7 +669,8 @@ void ntp_notify_cmos_timer(void)
 	 * rearmed this queues the work immediately again. No big issue,
 	 * just a pointless work scheduled.
 	 */
-	if (ntp_synced() && !hrtimer_is_queued(&sync_hrtimer))
+	if (ntp_synced() && sysctl_ntp_rtc_sync &&
+	    !hrtimer_is_queued(&sync_hrtimer))
 		queue_work(system_freezable_power_efficient_wq, &sync_work);
 }