[PATCH 6.15 12/34] rtc: Make rtc_time64_to_tm() support dates before 1970

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

6.15-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Alexandre Mergnat <amergnat@baylibre.com>

commit 7df4cfef8b351fec3156160bedfc7d6d29de4cce upstream.

Conversion of dates before 1970 is still relevant today because these
dates are reused on some hardwares to store dates bigger than the
maximal date that is representable in the device's native format.
This prominently and very soon affects the hardware covered by the
rtc-mt6397 driver that can only natively store dates in the interval
1900-01-01 up to 2027-12-31. So to store the date 2028-01-01 00:00:00
to such a device, rtc_time64_to_tm() must do the right thing for
time=-2208988800.

Signed-off-by: Alexandre Mergnat <amergnat@baylibre.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/20250428-enable-rtc-v4-1-2b2f7e3f9349@baylibre.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/rtc/lib.c |   24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

--- a/drivers/rtc/lib.c
+++ b/drivers/rtc/lib.c
@@ -46,24 +46,38 @@ 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.)
+ *		Works for values since at least 1900
  * @tm:		Pointer to the struct rtc_time.
  */
 void rtc_time64_to_tm(time64_t time, struct rtc_time *tm)
 {
-	unsigned int secs;
-	int days;
+	int days, secs;
 
 	u64 u64tmp;
 	u32 u32tmp, udays, century, day_of_century, year_of_century, year,
 		day_of_year, month, day;
 	bool is_Jan_or_Feb, is_leap_year;
 
-	/* time must be positive */
+	/*
+	 * Get days and seconds while preserving the sign to
+	 * handle negative time values (dates before 1970-01-01)
+	 */
 	days = div_s64_rem(time, 86400, &secs);
 
+	/*
+	 * We need 0 <= secs < 86400 which isn't given for negative
+	 * values of time. Fixup accordingly.
+	 */
+	if (secs < 0) {
+		days -= 1;
+		secs += 86400;
+	}
+
 	/* 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 +107,7 @@ void rtc_time64_to_tm(time64_t time, str
 	 * thus, is slightly different from [1].
 	 */
 
-	udays		= ((u32) days) + 719468;
+	udays		= days + 719468;
 
 	u32tmp		= 4 * udays + 3;
 	century		= u32tmp / 146097;