rtc: pcf8523: don't return invalid date when battery is low

The RTC_VL_READ ioctl reports the low battery condition. Still,
pcf8523_rtc_read_time() happily returns invalid dates in this case.
Check the battery health on pcf8523_rtc_read_time() to avoid that.
Reported-by: Erik Čuk <erik.cuk@domel.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-pcf8523.c

@@ -85,6 +85,18 @@ static int pcf8523_write(struct i2c_client *client, u8 reg, u8 value)
 	return 0;
 }
 
+static int pcf8523_voltage_low(struct i2c_client *client)
+{
+	u8 value;
+	int err;
+
+	err = pcf8523_read(client, REG_CONTROL3, &value);
+	if (err < 0)
+		return err;
+
+	return !!(value & REG_CONTROL3_BLF);
+}
+
 static int pcf8523_select_capacitance(struct i2c_client *client, bool high)
 {
 	u8 value;
@@ -167,6 +179,14 @@ static int pcf8523_rtc_read_time(struct device *dev, struct rtc_time *tm)
 	struct i2c_msg msgs[2];
 	int err;
 
+	err = pcf8523_voltage_low(client);
+	if (err < 0) {
+		return err;
+	} else if (err > 0) {
+		dev_err(dev, "low voltage detected, time is unreliable\n");
+		return -EINVAL;
+	}
+
 	msgs[0].addr = client->addr;
 	msgs[0].flags = 0;
 	msgs[0].len = 1;
@@ -251,17 +271,13 @@ static int pcf8523_rtc_ioctl(struct device *dev, unsigned int cmd,
 			     unsigned long arg)
 {
 	struct i2c_client *client = to_i2c_client(dev);
-	u8 value;
-	int ret = 0, err;
+	int ret;
 
 	switch (cmd) {
 	case RTC_VL_READ:
-		err = pcf8523_read(client, REG_CONTROL3, &value);
-		if (err < 0)
-			return err;
-
-		if (value & REG_CONTROL3_BLF)
-			ret = 1;
+		ret = pcf8523_voltage_low(client);
+		if (ret < 0)
+			return ret;
 
 		if (copy_to_user((void __user *)arg, &ret, sizeof(int)))
 			return -EFAULT;