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 <rmk+kernel@armlinux.org.uk>

2 files changed, 18 insertions, 2 deletions

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index cb57da499ebb..b525a3a8c30c 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
@@ -1610,6 +1612,17 @@ int proc_do_static_key(const struct ctl_table *table, int write,
 }
 
 static const 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 163e7a2033b6..0018ce6787e9 100644
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -482,6 +482,7 @@ out:
 	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);
@@ -632,7 +633,8 @@ 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);
@@ -672,7 +674,8 @@ void ntp_notify_cmos_timer(bool offset_set)
 	 * 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);
 }