// SPDX-License-Identifier: GPL-2.0

  #include <linux/rtc.h>
  #include <linux/time.h>
  
  /**
   * 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.
   *
   * Returns 0 on successful RTC update, -ENODEV if a RTC update is not
   * 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, unsigned long *target_nsec)

  {
  	struct rtc_device *rtc;
  	struct rtc_time tm;

  	struct timespec64 to_set;

  	int err = -ENODEV;

  	bool ok;

  	rtc = rtc_class_open(CONFIG_RTC_SYSTOHC_DEVICE);

  	if (!rtc)
  		goto out_err;

  	if (!rtc->ops || !rtc->ops->set_time)

  		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);

  	err = rtc_set_time(rtc, &tm);
  
  out_close:
  	rtc_class_close(rtc);
  out_err:

  	return err;
  }