rtc: ds1307: rx8130: honor Voltage Loss Flag when reading the time

When voltage dropped since the RTC was last set the reported time is not
reliable. In this case return an error indicator instead of a bogus
time.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-ds1307.c

@@ -194,6 +194,20 @@ static int ds1307_get_time(struct device *dev, struct rtc_time *t)
 	const struct chip_desc *chip = &chips[ds1307->type];
 	u8 regs[7];
 
+	if (ds1307->type == rx_8130) {
+		unsigned int regflag;
+		ret = regmap_read(ds1307->regmap, RX8130_REG_FLAG, &regflag);
+		if (ret) {
+			dev_err(dev, "%s error %d\n", "read", ret);
+			return ret;
+		}
+
+		if (regflag & RX8130_REG_FLAG_VLF) {
+			dev_warn_once(dev, "oscillator failed, set time!\n");
+			return -EINVAL;
+		}
+	}
+
 	/* read the RTC date and time registers all at once */
 	ret = regmap_bulk_read(ds1307->regmap, chip->offset, regs,
 			       sizeof(regs));
@@ -293,6 +307,17 @@ static int ds1307_set_time(struct device *dev, struct rtc_time *t)
 		dev_err(dev, "%s error %d\n", "write", result);
 		return result;
 	}
+
+	if (ds1307->type == rx_8130) {
+		/* clear Voltage Loss Flag as data is available now */
+		result = regmap_write(ds1307->regmap, RX8130_REG_FLAG,
+				      ~(u8)RX8130_REG_FLAG_VLF);
+		if (result) {
+			dev_err(dev, "%s error %d\n", "write", result);
+			return result;
+		}
+	}
+
 	return 0;
 }