drivers/rtc/hctosys.c

   1 /*
   2  * RTC subsystem, initialize system time on startup
   3  *
   4  * Copyright (C) 2005 Tower Technologies
   5  * Author: Alessandro Zummo <a.zummo@towertech.it>
   6  *
   7  * This program is free software; you can redistribute it and/or modify
   8  * it under the terms of the GNU General Public License version 2 as
   9  * published by the Free Software Foundation.
  10 */
  11
  12 #include <linux/rtc.h>
  13
  14 /* IMPORTANT: the RTC only stores whole seconds. It is arbitrary
  15  * whether it stores the most close value or the value with partial
  16  * seconds truncated. However, it is important that we use it to store
  17  * the truncated value. This is because otherwise it is necessary,
  18  * in an rtc sync function, to read both xtime.tv_sec and
  19  * xtime.tv_nsec. On some processors (i.e. ARM), an atomic read
  20  * of >32bits is not possible. So storing the most close value would
  21  * slow down the sync API. So here we have the truncated value and
  22  * the best guess is to add 0.5s.
  23  */
  24
  25 static int __init rtc_hctosys(void)
  26 {
  27         int err = -ENODEV;
  28         struct rtc_time tm;
  29         struct timespec64 tv64 = {
  30                 .tv_nsec = NSEC_PER_SEC >> 1,
  31         };
  32         struct rtc_device *rtc = rtc_class_open(CONFIG_RTC_HCTOSYS_DEVICE);
  33
  34         if (rtc == NULL) {
  35                 pr_err("%s: unable to open rtc device (%s)\n",
  36                         __FILE__, CONFIG_RTC_HCTOSYS_DEVICE);
  37                 goto err_open;
  38         }
  39
  40         err = rtc_read_time(rtc, &tm);
  41         if (err) {
  42                 dev_err(rtc->dev.parent,
  43                         "hctosys: unable to read the hardware clock\n");
  44                 goto err_read;
  45
  46         }
  47
  48         tv64.tv_sec = rtc_tm_to_time64(&tm);
  49
  50         err = do_settimeofday64(&tv64);
  51
  52         dev_info(rtc->dev.parent,
  53                 "setting system clock to "
  54                 "%d-%02d-%02d %02d:%02d:%02d UTC (%lld)\n",
  55                 tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday,
  56                 tm.tm_hour, tm.tm_min, tm.tm_sec,
  57                 (long long) tv64.tv_sec);
  58
  59 err_read:
  60         rtc_class_close(rtc);
  61
  62 err_open:
  63         rtc_hctosys_ret = err;
  64
  65         return err;
  66 }
  67
  68 late_initcall(rtc_hctosys);