rtc: bq32k: Fix handling of oscillator failure flag

While the oscillator failure flag is set, the RTC registers
should be considered invalid. bq32k_rtc_read_time() now
returns an error instead of an invalid time.

The failure flag is cleared the next time the clock is set.
Signed-off-by: Jan Östlund <jao@hms.se>
Signed-off-by: Daniel Romell <daro@hms.se>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/rtc-bq32k.c

@@ -93,6 +93,13 @@ static int bq32k_rtc_read_time(struct device *dev, struct rtc_time *tm)
 	if (error)
 		return error;
 
+	/*
+	 * In case of oscillator failure, the register contents should be
+	 * considered invalid. The flag is cleared the next time the RTC is set.
+	 */
+	if (regs.minutes & BQ32K_OF)
+		return -EINVAL;
+
 	tm->tm_sec = bcd2bin(regs.seconds & BQ32K_SECONDS_MASK);
 	tm->tm_min = bcd2bin(regs.minutes & BQ32K_MINUTES_MASK);
 	tm->tm_hour = bcd2bin(regs.cent_hours & BQ32K_HOURS_MASK);
@@ -204,13 +211,10 @@ static int bq32k_probe(struct i2c_client *client,
 
 	/* Check Oscillator Failure flag */
 	error = bq32k_read(dev, &reg, BQ32K_MINUTES, 1);
-	if (!error && (reg & BQ32K_OF)) {
-		dev_warn(dev, "Oscillator Failure. Check RTC battery.\n");
-		reg &= ~BQ32K_OF;
-		error = bq32k_write(dev, &reg, BQ32K_MINUTES, 1);
-	}
 	if (error)
 		return error;
+	if (reg & BQ32K_OF)
+		dev_warn(dev, "Oscillator Failure. Check RTC battery.\n");
 
 	if (client->dev.of_node)
 		trickle_charger_of_init(dev, client->dev.of_node);