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 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
  13
  14 #include <linux/rtc.h>
  15
  16 /* IMPORTANT: the RTC only stores whole seconds. It is arbitrary
  17  * whether it stores the most close value or the value with partial
  18  * seconds truncated. However, it is important that we use it to store
  19  * the truncated value. This is because otherwise it is necessary,
  20  * in an rtc sync function, to read both xtime.tv_sec and
  21  * xtime.tv_nsec. On some processors (i.e. ARM), an atomic read
  22  * of >32bits is not possible. So storing the most close value would
  23  * slow down the sync API. So here we have the truncated value and
  24  * the best guess is to add 0.5s.
  25  */
  26
  27 static int __init rtc_hctosys(void)
  28 {
  29         int err = -ENODEV;
  30         struct rtc_time tm;
  31         struct timespec64 tv64 = {
  32                 .tv_nsec = NSEC_PER_SEC >> 1,
  33         };
  34         struct rtc_device *rtc = rtc_class_open(CONFIG_RTC_HCTOSYS_DEVICE);
  35
  36         if (rtc == NULL) {
  37                 pr_info("unable to open rtc device (%s)\n",
  38                         CONFIG_RTC_HCTOSYS_DEVICE);
  39                 goto err_open;
  40         }
  41
  42         err = rtc_read_time(rtc, &tm);
  43         if (err) {
  44                 dev_err(rtc->dev.parent,
  45                         "hctosys: unable to read the hardware clock\n");
  46                 goto err_read;
  47
  48         }
  49
  50         tv64.tv_sec = rtc_tm_to_time64(&tm);
  51
  52         err = do_settimeofday64(&tv64);
  53
  54         dev_info(rtc->dev.parent,
  55                 "setting system clock to "
  56                 "%d-%02d-%02d %02d:%02d:%02d UTC (%lld)\n",
  57                 tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday,
  58                 tm.tm_hour, tm.tm_min, tm.tm_sec,
  59                 (long long) tv64.tv_sec);
  60
  61 err_read:
  62         rtc_class_close(rtc);
  63
  64 err_open:
  65         rtc_hctosys_ret = err;
  66
  67         return err;
  68 }
  69
  70 late_initcall(rtc_hctosys);