rtc: ds1307: handle oscillator stop flag (OSF) for ds1341

In using CONFIG_RTC_HCTOSYS, rtc_hctosys() will sync the RTC time to the
kernel time as long as rtc_read_time() succeeds. In some power loss
situations, our supercapacitor-backed DS1342 RTC comes up with either an
unpredictable future time or the default 01/01/00 from the datasheet.
The oscillator stop flag (OSF) is set in these scenarios due to the
power loss and can be used to determine the validity of the RTC data.

This change expands the oscillator stop flag (OSF) handling that has
already been implemented for some chips to the ds1341 chip (DS1341 and
DS1342 share a datasheet). This handling manages the validity of the RTC
data in .read_time and .set_time based on the OSF.

Signed-off-by: Meagan Lloyd <meaganlloyd@linux.microsoft.com>
Reviewed-by: Tyler Hicks <code@tyhicks.com>
Acked-by: Rodolfo Giometti <giometti@enneenne.com>
Link: https://lore.kernel.org/r/1749665656-30108-3-git-send-email-meaganlloyd@linux.microsoft.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Author: Meagan Lloyd

drivers/rtc/rtc-ds1307.c

@@ -279,6 +279,13 @@ static int ds1307_get_time(struct device *dev, struct rtc_time *t)
 		if (tmp & DS1340_BIT_OSF)
 			return -EINVAL;
 		break;
+	case ds_1341:
+		ret = regmap_read(ds1307->regmap, DS1337_REG_STATUS, &tmp);
+		if (ret)
+			return ret;
+		if (tmp & DS1337_BIT_OSF)
+			return -EINVAL;
+		break;
 	case ds_1388:
 		ret = regmap_read(ds1307->regmap, DS1388_REG_FLAG, &tmp);
 		if (ret)
@@ -377,6 +384,10 @@ static int ds1307_set_time(struct device *dev, struct rtc_time *t)
 		regmap_update_bits(ds1307->regmap, DS1340_REG_FLAG,
 				   DS1340_BIT_OSF, 0);
 		break;
+	case ds_1341:
+		regmap_update_bits(ds1307->regmap, DS1337_REG_STATUS,
+				   DS1337_BIT_OSF, 0);
+		break;
 	case ds_1388:
 		regmap_update_bits(ds1307->regmap, DS1388_REG_FLAG,
 				   DS1388_BIT_OSF, 0);