rtc: pcf8523: properly handle oscillator stop bit

The time and date register of the pcf8223 are undefined after a power
reset. Properly handle the OS bit and return -EINVAL when that bit is set.

It is properly removed when setting the time.

This solves an issue where the time and date may be valid for
rtc_valid_tm() but is not the current time.
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/rtc-pcf8523.c

@@ -178,28 +178,8 @@ static int pcf8523_rtc_read_time(struct device *dev, struct rtc_time *tm)
 	if (err < 0)
 		return err;
 
-	if (regs[0] & REG_SECONDS_OS) {
-		/*
-		 * If the oscillator was stopped, try to clear the flag. Upon
-		 * power-up the flag is always set, but if we cannot clear it
-		 * the oscillator isn't running properly for some reason. The
-		 * sensible thing therefore is to return an error, signalling
-		 * that the clock cannot be assumed to be correct.
-		 */
-
-		regs[0] &= ~REG_SECONDS_OS;
-
-		err = pcf8523_write(client, REG_SECONDS, regs[0]);
-		if (err < 0)
-			return err;
-
-		err = pcf8523_read(client, REG_SECONDS, &regs[0]);
-		if (err < 0)
-			return err;
-
 	if (regs[0] & REG_SECONDS_OS)
-			return -EAGAIN;
-	}
+		return -EINVAL;
 
 	tm->tm_sec = bcd2bin(regs[0] & 0x7f);
 	tm->tm_min = bcd2bin(regs[1] & 0x7f);
@@ -235,6 +215,7 @@ static int pcf8523_rtc_set_time(struct device *dev, struct rtc_time *tm)
 		return err;
 
 	regs[0] = REG_SECONDS;
+	/* This will purposely overwrite REG_SECONDS_OS */
 	regs[1] = bin2bcd(tm->tm_sec);
 	regs[2] = bin2bcd(tm->tm_min);
 	regs[3] = bin2bcd(tm->tm_hour);