rtc-at91rm9200: avoid spurious irqs

This fixes kernel http://bugzilla.kernel.org/show_bug.cgi?id=11112 (bogus
RTC update IRQs reported) for rtc-at91rm9200 by scrubbing old IRQ status
before enabling IRQs.

It also removes nonfunctional periodic IRQ support from this driver;
only update IRQs are reported, or provided by the hardware.

I suspect some other RTCs probably have versions of #11112; it's easy to
overlook, since most non-RTC drivers don't care about spurious IRQs:
they're not reported to userspace.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Report-by: W Unruh <unruh@physics.ubc.ca>
Cc: Andrew Victor <avictor.za@gmail.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/rtc/rtc-at91rm9200.c

@@ -171,8 +171,10 @@ static int at91_rtc_setalarm(struct device *dev, struct rtc_wkalrm *alrm)
 		| BIN2BCD(tm.tm_mday) << 24
 		| AT91_RTC_DATEEN | AT91_RTC_MTHEN);
 
-	if (alrm->enabled)
+	if (alrm->enabled) {
+		at91_sys_write(AT91_RTC_SCCR, AT91_RTC_ALARM);
 		at91_sys_write(AT91_RTC_IER, AT91_RTC_ALARM);
+	}
 
 	pr_debug("%s(): %4d-%02d-%02d %02d:%02d:%02d\n", __func__,
 		at91_alarm_year, tm.tm_mon, tm.tm_mday, tm.tm_hour,
@@ -191,28 +193,22 @@ static int at91_rtc_ioctl(struct device *dev, unsigned int cmd,
 
 	pr_debug("%s(): cmd=%08x, arg=%08lx.\n", __func__, cmd, arg);
 
+	/* important:  scrub old status before enabling IRQs */
 	switch (cmd) {
 	case RTC_AIE_OFF:	/* alarm off */
 		at91_sys_write(AT91_RTC_IDR, AT91_RTC_ALARM);
 		break;
 	case RTC_AIE_ON:	/* alarm on */
+		at91_sys_write(AT91_RTC_SCCR, AT91_RTC_ALARM);
 		at91_sys_write(AT91_RTC_IER, AT91_RTC_ALARM);
 		break;
 	case RTC_UIE_OFF:	/* update off */
-	case RTC_PIE_OFF:	/* periodic off */
 		at91_sys_write(AT91_RTC_IDR, AT91_RTC_SECEV);
 		break;
 	case RTC_UIE_ON:	/* update on */
-	case RTC_PIE_ON:	/* periodic on */
+		at91_sys_write(AT91_RTC_SCCR, AT91_RTC_SECEV);
 		at91_sys_write(AT91_RTC_IER, AT91_RTC_SECEV);
 		break;
-	case RTC_IRQP_READ:	/* read periodic alarm frequency */
-		ret = put_user(AT91_RTC_FREQ, (unsigned long *) arg);
-		break;
-	case RTC_IRQP_SET:	/* set periodic alarm frequency */
-		if (arg != AT91_RTC_FREQ)
-			ret = -EINVAL;
-		break;
 	default:
 		ret = -ENOIOCTLCMD;
 		break;