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>

1 files changed, 11 insertions, 0 deletions

diff --git a/drivers/rtc/rtc-pcf8523.c b/drivers/rtc/rtc-pcf8523.c
index e7ebcc0b7e59..988566caaaa6 100644
--- a/drivers/rtc/rtc-pcf8523.c
+++ b/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;