[PATCH v2 2/2] rtc: mt6397: Fix mt6357 RTC year offset handling for hwclock commands

[Alexandre Mergnat <amergnat@baylibre.com>]

The mt6357 RTC was failing when using the `hwclock -r --verbose` command,
despite reading correctly through sysfs. There is high chance for other
platform to be impacted by the year offset handling issue.

The hardware RTC registers store years relative to 1968, but the driver
wasn't consistently applying the offset when converting between
hardware and Linux time representation.

This inconsistency caused alarm rollover failures during device
registration, with the error "alarm rollover not handled -22" in the
logs, causing hwclock commands to fail.

The ioctl interface used by the hwclock command requires proper time
range validation that wasn't happening with the inconsistent year
offsets.

Fixes the issue by applying the year offset in all operations:
   - Adding (RTC_MIN_YEAR - RTC_BASE_YEAR) when reading from hardware
   - Subtracting the same offset when writing to hardware
   - Using the same logic for both regular time and alarm operations

With these changes, the hwclock command works correctly and time
values are consistently handled across all interfaces.

Signed-off-by: Alexandre Mergnat <amergnat@baylibre.com>
---
 drivers/rtc/rtc-mt6397.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/drivers/rtc/rtc-mt6397.c b/drivers/rtc/rtc-mt6397.c
index 692c00ff544b2..ba52e225dc8fa 100644
--- a/drivers/rtc/rtc-mt6397.c
+++ b/drivers/rtc/rtc-mt6397.c
@@ -77,7 +77,8 @@ static int __mtk_rtc_read_time(struct mt6397_rtc *rtc,
 	tm->tm_mday = data[RTC_OFFSET_DOM];
 	tm->tm_wday = data[RTC_OFFSET_DOW];
 	tm->tm_mon = data[RTC_OFFSET_MTH] & RTC_TC_MTH_MASK;
-	tm->tm_year = data[RTC_OFFSET_YEAR];
+	/* The RTC registers store years since 1968 (hardware's base year) */
+	tm->tm_year = data[RTC_OFFSET_YEAR] + (RTC_MIN_YEAR - RTC_BASE_YEAR);
 
 	ret = regmap_read(rtc->regmap, rtc->addr_base + RTC_TC_SEC, sec);
 exit:
@@ -119,7 +120,8 @@ static int mtk_rtc_set_time(struct device *dev, struct rtc_time *tm)
 	data[RTC_OFFSET_DOM] = tm->tm_mday;
 	data[RTC_OFFSET_DOW] = tm->tm_wday;
 	data[RTC_OFFSET_MTH] = tm->tm_mon;
-	data[RTC_OFFSET_YEAR] = tm->tm_year;
+	/* Convert from tm_year (years since 1900) to RTC register format (years since 1968) */
+	data[RTC_OFFSET_YEAR] = tm->tm_year - (RTC_MIN_YEAR - RTC_BASE_YEAR);
 
 	mutex_lock(&rtc->lock);
 	ret = regmap_bulk_write(rtc->regmap, rtc->addr_base + RTC_TC_SEC,
@@ -165,8 +167,8 @@ static int mtk_rtc_read_alarm(struct device *dev, struct rtc_wkalrm *alm)
 	tm->tm_hour = data[RTC_OFFSET_HOUR] & RTC_AL_HOU_MASK;
 	tm->tm_mday = data[RTC_OFFSET_DOM] & RTC_AL_DOM_MASK;
 	tm->tm_mon = data[RTC_OFFSET_MTH] & RTC_AL_MTH_MASK;
-	tm->tm_year = data[RTC_OFFSET_YEAR] & RTC_AL_YEA_MASK;
-
+	/* Apply the same offset conversion for alarm read */
+	tm->tm_year = (data[RTC_OFFSET_YEAR] & RTC_AL_YEA_MASK) + (RTC_MIN_YEAR - RTC_BASE_YEAR);
 	tm->tm_mon--;
 
 	return 0;
@@ -200,8 +202,9 @@ static int mtk_rtc_set_alarm(struct device *dev, struct rtc_wkalrm *alm)
 				(tm->tm_mday & RTC_AL_DOM_MASK));
 	data[RTC_OFFSET_MTH] = ((data[RTC_OFFSET_MTH] & ~(RTC_AL_MTH_MASK)) |
 				(tm->tm_mon & RTC_AL_MTH_MASK));
+	/* Convert alarm year using the same offset as in read/write time */
 	data[RTC_OFFSET_YEAR] = ((data[RTC_OFFSET_YEAR] & ~(RTC_AL_YEA_MASK)) |
-				(tm->tm_year & RTC_AL_YEA_MASK));
+				((tm->tm_year - (RTC_MIN_YEAR - RTC_BASE_YEAR)) & RTC_AL_YEA_MASK));
 
 	if (alm->enabled) {
 		ret = regmap_bulk_write(rtc->regmap,

-- 
2.25.1