[PATCH v3 2/5] rtc: Add handling of pre-1970 dates in time conversion functions

[Alexandre Mergnat <amergnat@baylibre.com>]

Linux RTC subsystem's time conversion functions couldn't properly handle
dates before January 1, 1970 (negative time64_t values). This affected
offset calculations, causing incorrect time translations for RTCs with
pre-1970 base years like those using a 1900 epoch.

The original rtc_time64_to_tm() function produced incorrect dates for
pre-1970 inputs and rtc_valid_tm() rejected all years before 1970 as
invalid, even if they were within the hardware's capabilities. For
example, converting January 1, 1942 2:36:47 is equal to -883603393.
converting it back resulted in wildly incorrect values
=> January 2, 1942 1193025:5:3.

These issues made it impossible to correctly use RTCs with pre-1970 base
years, particularly affecting embedded systems using hardware like the
MT6357 RTC.

Modify rtc_time64_to_tm to implement special handling for negative
time values, properly calculating days and seconds for dates before
1970. It also removes the tm_year < 70 restriction in rtc_valid_tm to
allow pre-1970 dates to be validated correctly, ensuring accurate
conversion between hardware and system time across the full range of
RTC hardware capabilities.

Signed-off-by: Alexandre Mergnat <amergnat@baylibre.com>
---
 drivers/rtc/lib.c | 38 ++++++++++++++++++++++++++++++++------
 1 file changed, 32 insertions(+), 6 deletions(-)

diff --git a/drivers/rtc/lib.c b/drivers/rtc/lib.c
index fe361652727a3..2014a86499e02 100644
--- a/drivers/rtc/lib.c
+++ b/drivers/rtc/lib.c
@@ -46,7 +46,6 @@ EXPORT_SYMBOL(rtc_year_days);
  * rtc_time64_to_tm - converts time64_t to rtc_time.
  *
  * @time:	The number of seconds since 01-01-1970 00:00:00.
- *		(Must be positive.)
  * @tm:		Pointer to the struct rtc_time.
  */
 void rtc_time64_to_tm(time64_t time, struct rtc_time *tm)
@@ -59,11 +58,39 @@ void rtc_time64_to_tm(time64_t time, struct rtc_time *tm)
 		day_of_year, month, day;
 	bool is_Jan_or_Feb, is_leap_year;
 
-	/* time must be positive */
-	days = div_s64_rem(time, 86400, &secs);
+	bool is_negative = false;
+
+	/* Handle negative time values (dates before 1970-01-01) */
+	if (time < 0) {
+		/* Store that we had a negative value */
+		is_negative = true;
+
+		/* Convert to positive value for the algorithm, but
+		 * we'll subtract one more day to handle the boundary correctly
+		 */
+		time = -time;
+
+		/* Get days and seconds */
+		days = div_s64_rem(time, 86400, &secs);
+
+		/* If we have seconds, we need to adjust to the previous day */
+		if (secs > 0) {
+			days += 1;
+			secs = 86400 - secs;
+		}
+
+		/* Make days negative again */
+		days = -days;
+	} else {
+		/* Positive time value - normal case */
+		days = div_s64_rem(time, 86400, &secs);
+	}
 
 	/* day of the week, 1970-01-01 was a Thursday */
 	tm->tm_wday = (days + 4) % 7;
+	/* Ensure tm_wday is always positive */
+	if (tm->tm_wday < 0)
+		tm->tm_wday += 7;
 
 	/*
 	 * The following algorithm is, basically, Proposition 6.3 of Neri
@@ -93,7 +120,7 @@ void rtc_time64_to_tm(time64_t time, struct rtc_time *tm)
 	 * thus, is slightly different from [1].
 	 */
 
-	udays		= ((u32) days) + 719468;
+	udays		= days + 719468;
 
 	u32tmp		= 4 * udays + 3;
 	century		= u32tmp / 146097;
@@ -146,8 +173,7 @@ EXPORT_SYMBOL(rtc_time64_to_tm);
  */
 int rtc_valid_tm(struct rtc_time *tm)
 {
-	if (tm->tm_year < 70 ||
-	    tm->tm_year > (INT_MAX - 1900) ||
+	if (tm->tm_year > (INT_MAX - 1900) ||
 	    ((unsigned int)tm->tm_mon) >= 12 ||
 	    tm->tm_mday < 1 ||
 	    tm->tm_mday > rtc_month_days(tm->tm_mon,

-- 
2.25.1