6 files changed, 45 insertions, 15 deletions
diff --git a/drivers/rtc/Makefile b/drivers/rtc/Makefile
index 7230014c92af..0bf1fc02b82c 100644
--- a/drivers/rtc/Makefile
+++ b/drivers/rtc/Makefile
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0
 #
 # Makefile for RTC class/drivers.
 #
diff --git a/drivers/rtc/class.c b/drivers/rtc/class.c
index 2ed970d61da1..722d683e0b0f 100644
--- a/drivers/rtc/class.c
+++ b/drivers/rtc/class.c
@@ -161,6 +161,9 @@ static struct rtc_device *rtc_allocate_device(void)
 
 	device_initialize(&rtc->dev);
 
+	/* Drivers can revise this default after allocating the device. */
+	rtc->set_offset_nsec =  NSEC_PER_SEC / 2;
+
 	rtc->irq_freq = 1;
 	rtc->max_user_freq = 64;
 	rtc->dev.class = rtc_class;
diff --git a/drivers/rtc/rtc-core.h b/drivers/rtc/rtc-core.h
index ecab76a3207c..513b9bedd2c8 100644
--- a/drivers/rtc/rtc-core.h
+++ b/drivers/rtc/rtc-core.h
@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: GPL-2.0 */
 #ifdef CONFIG_RTC_INTF_DEV
 
 extern void __init rtc_dev_init(void);
diff --git a/drivers/rtc/rtc-efi-platform.c b/drivers/rtc/rtc-efi-platform.c
index 1a7f1d1bc174..6c037dc4e3dc 100644
--- a/drivers/rtc/rtc-efi-platform.c
+++ b/drivers/rtc/rtc-efi-platform.c
@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0
 /*
  * Moved from arch/ia64/kernel/time.c
  *
diff --git a/drivers/rtc/rtc-sa1100.h b/drivers/rtc/rtc-sa1100.h
index 2c79c0c57822..cc724f5b07bc 100644
--- a/drivers/rtc/rtc-sa1100.h
+++ b/drivers/rtc/rtc-sa1100.h
@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: GPL-2.0 */
 #ifndef __RTC_SA1100_H__
 #define __RTC_SA1100_H__
 
diff --git a/drivers/rtc/systohc.c b/drivers/rtc/systohc.c
index b4a68ffcd06b..0c177647ea6c 100644
--- a/drivers/rtc/systohc.c
+++ b/drivers/rtc/systohc.c
@@ -10,6 +10,7 @@
 /**
  * rtc_set_ntp_time - Save NTP synchronized time to the RTC
  * @now: Current time of day
+ * @target_nsec: pointer for desired now->tv_nsec value
  *
  * Replacement for the NTP platform function update_persistent_clock64
  * that stores time for later retrieval by rtc_hctosys.
@@ -18,30 +19,52 @@
  * possible at all, and various other -errno for specific temporary failure
  * cases.
  *
+ * -EPROTO is returned if now.tv_nsec is not close enough to *target_nsec.
+ (
  * If temporary failure is indicated the caller should try again 'soon'
  */
-int rtc_set_ntp_time(struct timespec64 now)
+int rtc_set_ntp_time(struct timespec64 now, unsigned long *target_nsec)
 {
 	struct rtc_device *rtc;
 	struct rtc_time tm;
+	struct timespec64 to_set;
 	int err = -ENODEV;
-
-	if (now.tv_nsec < (NSEC_PER_SEC >> 1))
-		rtc_time64_to_tm(now.tv_sec, &tm);
-	else
-		rtc_time64_to_tm(now.tv_sec + 1, &tm);
+	bool ok;
 
 	rtc = rtc_class_open(CONFIG_RTC_SYSTOHC_DEVICE);
-	if (rtc) {
-		/* rtc_hctosys exclusively uses UTC, so we call set_time here,
-		 * not set_mmss. */
-		if (rtc->ops &&
-		    (rtc->ops->set_time ||
-		     rtc->ops->set_mmss64 ||
-		     rtc->ops->set_mmss))
-			err = rtc_set_time(rtc, &tm);
-		rtc_class_close(rtc);
+	if (!rtc)
+		goto out_err;
+
+	if (!rtc->ops || (!rtc->ops->set_time && !rtc->ops->set_mmss64 &&
+			  !rtc->ops->set_mmss))
+		goto out_close;
+
+	/* Compute the value of tv_nsec we require the caller to supply in
+	 * now.tv_nsec.  This is the value such that (now +
+	 * set_offset_nsec).tv_nsec == 0.
+	 */
+	set_normalized_timespec64(&to_set, 0, -rtc->set_offset_nsec);
+	*target_nsec = to_set.tv_nsec;
+
+	/* The ntp code must call this with the correct value in tv_nsec, if
+	 * it does not we update target_nsec and return EPROTO to make the ntp
+	 * code try again later.
+	 */
+	ok = rtc_tv_nsec_ok(rtc->set_offset_nsec, &to_set, &now);
+	if (!ok) {
+		err = -EPROTO;
+		goto out_close;
 	}
 
+	rtc_time64_to_tm(to_set.tv_sec, &tm);
+
+	/* rtc_hctosys exclusively uses UTC, so we call set_time here, not
+	 * set_mmss.
+	 */
+	err = rtc_set_time(rtc, &tm);
+
+out_close:
+	rtc_class_close(rtc);
+out_err:
 	return err;
 }