rtc: cros-ec: return -ETIME when refused to set alarms in the past

Since accessing a Chrome OS EC based rtc is a slow operation, there is a
race window where if the alarm is set for the next second and the second
ticks over right before calculating the alarm offset.

In this case the current driver is setting a 0-second alarm, which would
be considered as disabling alarms by the EC(EC_RTC_ALARM_CLEAR).

This breaks, e.g., hwclock which relies on RTC_UIE_ON ->
rtc_update_irq_enable(), which sets a 1-second alarm and expects it to
fire an interrupt.

So return -ETIME when the alarm is in the past, follow __rtc_set_alarm().

Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Tested-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-cros-ec.c

@@ -197,10 +197,10 @@ static int cros_ec_rtc_set_alarm(struct device *dev, struct rtc_wkalrm *alrm)
 		cros_ec_rtc->saved_alarm = (u32)alarm_time;
 	} else {
 		/* Don't set an alarm in the past. */
-		if ((u32)alarm_time < current_time)
-			alarm_offset = EC_RTC_ALARM_CLEAR;
-		else
-			alarm_offset = (u32)alarm_time - current_time;
+		if ((u32)alarm_time <= current_time)
+			return -ETIME;
+
+		alarm_offset = (u32)alarm_time - current_time;
 	}
 
 	ret = cros_ec_rtc_set(cros_ec, EC_CMD_RTC_SET_ALARM, alarm_offset);