drivers: rtc: resolve year 2038 problem in rtc_to_tm

Our implementation of rtc_to_tm() cannot handle dates of more than
0x7fffffff seconds after 1970-01-01.

Adopt the Linux kernel implementation.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

drivers/rtc/Makefile

@@ -8,6 +8,7 @@ obj-$(CONFIG_DM_RTC) += rtc-uclass.o
 
 obj-$(CONFIG_RTC_AT91SAM9_RTT) += at91sam9_rtt.o
 obj-y += date.o
+obj-y += rtc-lib.o
 obj-$(CONFIG_RTC_DAVINCI) += davinci.o
 obj-$(CONFIG_RTC_DS1302) += ds1302.o
 obj-$(CONFIG_RTC_DS1306) += ds1306.o

drivers/rtc/date.c

@@ -19,10 +19,6 @@
 #define	days_in_year(a)		(leapyear(a) ? 366 : 365)
 #define	days_in_month(a)	(month_days[(a) - 1])
 
-static int month_days[12] = {
-	31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
-};
-
 static int month_offset[] = {
 	0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334
 };
@@ -65,45 +61,6 @@ int rtc_calc_weekday(struct rtc_time *tm)
 	return 0;
 }
 
-int rtc_to_tm(int tim, struct rtc_time *tm)
-{
-	register int i;
-	register long hms, day;
-
-	day = tim / SECDAY;
-	hms = tim % SECDAY;
-
-	/* Hours, minutes, seconds are easy */
-	tm->tm_hour = hms / 3600;
-	tm->tm_min = (hms % 3600) / 60;
-	tm->tm_sec = (hms % 3600) % 60;
-
-	/* Number of years in days */
-	for (i = STARTOFTIME; day >= days_in_year(i); i++)
-		day -= days_in_year(i);
-	tm->tm_year = i;
-
-	/* Number of months in days left */
-	if (leapyear(tm->tm_year))
-		days_in_month(FEBRUARY) = 29;
-	for (i = 1; day >= days_in_month(i); i++)
-		day -= days_in_month(i);
-	days_in_month(FEBRUARY) = 28;
-	tm->tm_mon = i;
-
-	/* Days are what is left over (+1) from all that */
-	tm->tm_mday = day + 1;
-
-	/* Zero unused fields */
-	tm->tm_yday = 0;
-	tm->tm_isdst = 0;
-
-	/*
-	 * Determine the day of week
-	 */
-	return rtc_calc_weekday(tm);
-}
-
 /*
  * Converts Gregorian date to seconds since 1970-01-01 00:00:00.
  * Assumes input in normal date format, i.e. 1980-12-31 23:59:59

drivers/rtc/i2c_rtc_emul.c

@@ -96,7 +96,9 @@ static int sandbox_i2c_rtc_get(struct udevice *dev, struct rtc_time *time)
 		now = plat->base_time;
 	}
 
-	return rtc_to_tm(now + plat->offset, time);
+	rtc_to_tm(now + plat->offset, time);
+
+	return 0;
 }
 
 static int sandbox_i2c_rtc_set(struct udevice *dev, const struct rtc_time *time)

drivers/rtc/rtc-lib.c

@@ -0,0 +1,73 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * rtc and date/time utility functions
+ *
+ * Copyright (C) 2005-06 Tower Technologies
+ * Author: Alessandro Zummo <a.zummo@towertech.it>
+ *
+ * U-Boot rtc_time differs from Linux rtc_time:
+ * - The year field takes the actual value, not year - 1900.
+ * - January is month 1.
+ */
+
+#include <common.h>
+#include <rtc.h>
+#include <linux/math64.h>
+
+static const unsigned char rtc_days_in_month[] = {
+	31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
+};
+
+#define LEAPS_THRU_END_OF(y) ((y) / 4 - (y) / 100 + (y) / 400)
+
+/*
+ * The number of days in the month.
+ */
+static int rtc_month_days(unsigned int month, unsigned int year)
+{
+	return rtc_days_in_month[month] + (is_leap_year(year) && month == 1);
+}
+
+/*
+ * rtc_to_tm - Converts u64 to rtc_time.
+ * Convert seconds since 01-01-1970 00:00:00 to Gregorian date.
+ */
+void rtc_to_tm(u64 time, struct rtc_time *tm)
+{
+	unsigned int month, year, secs, days;
+
+	days = div_u64_rem(time, 86400, &secs);
+
+	/* day of the week, 1970-01-01 was a Thursday */
+	tm->tm_wday = (days + 4) % 7;
+
+	year = 1970 + days / 365;
+	days -= (year - 1970) * 365
+		+ LEAPS_THRU_END_OF(year - 1)
+		- LEAPS_THRU_END_OF(1970 - 1);
+	while (days < 0) {
+		year -= 1;
+		days += 365 + is_leap_year(year);
+	}
+	tm->tm_year = year; /* Not year - 1900 */
+	tm->tm_yday = days + 1;
+
+	for (month = 0; month < 11; month++) {
+		int newdays;
+
+		newdays = days - rtc_month_days(month, year);
+		if (newdays < 0)
+			break;
+		days = newdays;
+	}
+	tm->tm_mon = month + 1; /* January = 1 */
+	tm->tm_mday = days + 1;
+
+	tm->tm_hour = secs / 3600;
+	secs -= tm->tm_hour * 3600;
+	tm->tm_min = secs / 60;
+	tm->tm_sec = secs - tm->tm_min * 60;
+
+	/* Zero unused fields */
+	tm->tm_isdst = 0;
+}

include/rtc.h

@@ -208,7 +208,18 @@ void rtc_write32(int reg, u32 value);
  * rtc_init() - Set up the real time clock ready for use
  */
 void rtc_init(void);
-#endif
+#endif /* CONFIG_DM_RTC */
+
+/**
+ * is_leap_year - Check if year is a leap year
+ *
+ * @year	Year
+ * @return	1 if leap year
+ */
+static inline bool is_leap_year(unsigned int year)
+{
+	return (!(year % 4) && (year % 100)) || !(year % 400);
+}
 
 /**
  * rtc_calc_weekday() - Work out the weekday from a time
@@ -231,9 +242,8 @@ int rtc_calc_weekday(struct rtc_time *time);
  *
  * @time_t:	Number of seconds since 1970-01-01 00:00:00
  * @time:	Place to put the broken-out time
- * @return 0 if OK, -EINVAL if the weekday could not be determined
  */
-int rtc_to_tm(int time_t, struct rtc_time *time);
+void rtc_to_tm(u64 time_t, struct rtc_time *time);
 
 /**
  * rtc_mktime() - Convert a broken-out time into a time_t value