rtc: pcf8523: refuse to write dates later than 2099

When the chip increments the YEAR register and it already holds
bin2bcd(99) it reads as 0 afterwards. With this behaviour the last valid
day (without trickery) that has a representation is 2099-12-31 23:59:59.
So refuse to write later dates.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/rtc-pcf8523.c

@@ -219,6 +219,17 @@ static int pcf8523_rtc_set_time(struct device *dev, struct rtc_time *tm)
 	u8 regs[8];
 	int err;
 
+	/*
+	 * The hardware can only store values between 0 and 99 in it's YEAR
+	 * register (with 99 overflowing to 0 on increment).
+	 * After 2100-02-28 we could start interpreting the year to be in the
+	 * interval [2100, 2199], but there is no path to switch in a smooth way
+	 * because the chip handles YEAR=0x00 (and the out-of-spec
+	 * YEAR=0xa0) as a leap year, but 2100 isn't.
+	 */
+	if (tm->tm_year < 100 || tm->tm_year >= 200)
+		return -EINVAL;
+
 	err = pcf8523_stop_rtc(client);
 	if (err < 0)
 		return err;