rtc: wilco-ec: Handle reading invalid times

If the RTC HW returns an invalid time, the rtc_year_days()
call would crash. This patch adds error logging in this
situation, and removes the tm_yday and tm_wday calculations.
These fields should not be relied upon by userspace
according to man rtc, and thus we don't need to calculate
them.

Signed-off-by: Nick Crews <ncrews@chromium.org>
Reviewed-by: Daniel Campello <campello@chromium.org>
Link: https://lore.kernel.org/r/20191004142608.170159-1-ncrews@chromium.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-wilco-ec.c

@@ -110,10 +110,12 @@ static int wilco_ec_rtc_read(struct device *dev, struct rtc_time *tm)
 	tm->tm_mday	= rtc.day;
 	tm->tm_mon	= rtc.month - 1;
 	tm->tm_year	= rtc.year + (rtc.century * 100) - 1900;
-	tm->tm_yday	= rtc_year_days(tm->tm_mday, tm->tm_mon, tm->tm_year);
+	/* Ignore other tm fields, man rtc says userspace shouldn't use them. */
 
-	/* Don't compute day of week, we don't need it. */
-	tm->tm_wday = -1;
+	if (rtc_valid_tm(tm)) {
+		dev_err(dev, "Time from RTC is invalid: %ptRr\n", tm);
+		return -EIO;
+	}
 
 	return 0;
 }