drivers/rtc/hctosys.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * RTC subsystem, initialize system time on startup
   4  *
   5  * Copyright (C) 2005 Tower Technologies
   6  * Author: Alessandro Zummo <a.zummo@towertech.it>
   7  */
   8
   9 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
  10
  11 #include <linux/rtc.h>
  12
  13 /* IMPORTANT: the RTC only stores whole seconds. It is arbitrary
  14  * whether it stores the most close value or the value with partial
  15  * seconds truncated. However, it is important that we use it to store
  16  * the truncated value. This is because otherwise it is necessary,
  17  * in an rtc sync function, to read both xtime.tv_sec and
  18  * xtime.tv_nsec. On some processors (i.e. ARM), an atomic read
  19  * of >32bits is not possible. So storing the most close value would
  20  * slow down the sync API. So here we have the truncated value and
  21  * the best guess is to add 0.5s.
  22  */
  23
  24 static int __init rtc_hctosys(void)
  25 {
  26         int err = -ENODEV;
  27         struct rtc_time tm;
  28         struct timespec64 tv64 = {
  29                 .tv_nsec = NSEC_PER_SEC >> 1,
  30         };
  31         struct rtc_device *rtc = rtc_class_open(CONFIG_RTC_HCTOSYS_DEVICE);
  32
  33         if (!rtc) {
  34                 pr_info("unable to open rtc device (%s)\n",
  35                         CONFIG_RTC_HCTOSYS_DEVICE);
  36                 goto err_open;
  37         }
  38
  39         err = rtc_read_time(rtc, &tm);
  40         if (err) {
  41                 dev_err(rtc->dev.parent,
  42                         "hctosys: unable to read the hardware clock\n");
  43                 goto err_read;
  44         }
  45
  46         tv64.tv_sec = rtc_tm_to_time64(&tm);
  47
  48 #if BITS_PER_LONG == 32
  49         if (tv64.tv_sec > INT_MAX) {
  50                 err = -ERANGE;
  51                 goto err_read;
  52         }
  53 #endif
  54
  55         err = do_settimeofday64(&tv64);
  56
  57         dev_info(rtc->dev.parent, "setting system clock to %ptR UTC (%lld)\n",
  58                  &tm, (long long)tv64.tv_sec);
  59
  60 err_read:
  61         rtc_class_close(rtc);
  62
  63 err_open:
  64         rtc_hctosys_ret = err;
  65
  66         return err;
  67 }
  68
  69 late_initcall(rtc_hctosys);