rtc: pcf85063: fix time/date reading

Check if the RTC signals an invalid time/date (due to a battery power loss
for example). In this case ignore the time/date until it is really set again.
Signed-off-by: Juergen Borleis <jbe@pengutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/rtc-pcf85063.c

@@ -22,6 +22,7 @@
 #define PCF85063_REG_CTRL2		0x01
 
 #define PCF85063_REG_SC			0x04 /* datetime */
+#define PCF85063_REG_SC_OS		0x80
 #define PCF85063_REG_MN			0x05
 #define PCF85063_REG_HR			0x06
 #define PCF85063_REG_DM			0x07
@@ -62,6 +63,12 @@ static int pcf85063_get_datetime(struct i2c_client *client, struct rtc_time *tm)
 		return -EIO;
 	}
 
+	/* if the clock has lost its power it makes no sense to use its time */
+	if (regs[0] & PCF85063_REG_SC_OS) {
+		dev_warn(&client->dev, "Power loss detected, invalid time\n");
+		return -EINVAL;
+	}
+
 	tm->tm_sec = bcd2bin(regs[0] & 0x7F);
 	tm->tm_min = bcd2bin(regs[1] & 0x7F);
 	tm->tm_hour = bcd2bin(regs[2] & 0x3F); /* rtc hr 0-23 */