When does RTC update the system time?

RTC-real time clock maintains the system's hardware time. When Linux is started, you need to use the RTC hardware clock to set the system time.

This process is implemented in the drivers/RTC/hctosys. c driver. This driver actually only has one init function and declares its own init function

Late_initcall to ensure that the RTC driver is running properly.

The init function reads the current hardware clock from the RTC device, and then calls do_settimeofday to rewrite the system clock.

27 static int _ init rtc_hctosys (void)
28 {
29 int err =-enodev;
30 struct rtc_time TM;
31 struct timespec TV = {
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_err ("% s: Unable to open RTC device (% s) \ n ",
38 _ file __, 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 err = rtc_valid_tm (& TM );
51 if (ERR ){
52 dev_err (RTC-> Dev. parent,
53 "hctosys: Invalid date/time \ n ");
54 goto err_invalid;
55}
56
57 rtc_tm_to_time (& TM, & TV. TV _sec );
58
59 do_settimeofday (& TV );
60
61 dev_info (RTC-> Dev. parent,
62 "setting system clock"
63 "% d-% 02d-% 02d % 02d: % 02d: % 02d UTC (% u) \ n ",
64 TM. tm_year + 1900, TM. tm_mon + 1, TM. tm_mday,
65 TM. tm_hour, TM. tm_min, TM. tm_sec,
66 (unsigned INT) TV. TV _sec );
67
68 err_invalid:
69 err_read:
70 rtc_class_close (RTC );
71
72 err_open:
73 rtc_hctosys_ret = err;
74
75 return err;
76}
77

78 late_initcall (rtc_hctosys );

34. Obtain the RTC Device

42. The read time from the RTC device. rtc_read_time is only the read time interface function of the RTC subsystem. The real read operation is implemented in a specific RTC hardware driver.

The returned value is in the TM structure. rtc_valid_tm verifies that the returned value is valid.

57 to UTC seconds format

59 set the system time

78 ensure that this initialization function is called at least after the RTC driver initialization