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 --- drivers/rtc/lib.c | 24 +++++++++++++++++++----- 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/drivers/rtc/lib.c b/drivers/rtc/lib.c index fe361652727a..13b5b1f20465 100644 --- 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, struct rtc_time *tm) * thus, is slightly different from [1]. */

- udays = ((u32) days) + 719468; + udays = days + 719468;

u32tmp = 4 * udays + 3; century = u32tmp / 146097;

From: Alexandre Mergnat amergnat@baylibre.com

commit fe9f5f96cfe8b82d0f24cbfa93718925560f4f8d upstream.

The comparison

rtc->start_secs > rtc->range_max

has a signed left-hand side and an unsigned right-hand side. So the comparison might become true for negative start_secs which is interpreted as a (possibly very large) positive value.

As a negative value can never be bigger than an unsigned value the correct representation of the (mathematical) comparison

rtc->start_secs > rtc->range_max

in C is:

rtc->start_secs >= 0 && rtc->start_secs > rtc->range_max

Use that to fix the offset calculation currently used in the rtc-mt6397 driver.

Fixes: 989515647e783 ("rtc: Add one offset seconds to expand RTC range") 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-v4-2-2b2f7e3f9349@baylibre.com Signed-off-by: Alexandre Belloni alexandre.belloni@bootlin.com Signed-off-by: Uwe Kleine-König u.kleine-koenig@baylibre.com --- drivers/rtc/class.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/rtc/class.c b/drivers/rtc/class.c index b88cd4fb295b..b1a2be1f9e3b 100644 --- a/drivers/rtc/class.c +++ b/drivers/rtc/class.c @@ -326,7 +326,7 @@ static void rtc_device_get_offset(struct rtc_device *rtc) * * Otherwise the offset seconds should be 0. */ - if (rtc->start_secs > rtc->range_max || + if ((rtc->start_secs >= 0 && rtc->start_secs > rtc->range_max) || rtc->start_secs + range_secs - 1 < rtc->range_min) rtc->offset_secs = rtc->start_secs - rtc->range_min; else if (rtc->start_secs > rtc->range_min)