rtc: rtc-at91rm9200: fix infinite wait for ACKUPD irq

The rtc user must wait at least 1 sec between each time/calandar update
(see atmel's datasheet chapter "Updating Time/Calendar").

Use the 1Hz interrupt to update the at91_rtc_upd_rdy flag and wait for
the at91_rtc_wait_upd_rdy event if the rtc is not ready.

This patch fixes a deadlock in an uninterruptible wait when the RTC is
updated more than once every second.  AFAICT the bug is here from the
beginning, but I think we should at least backport this fix to 3.10 and
the following longterm and stable releases.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Reported-by: Bryan Evenson <bevenson@melinkcorp.com>
Tested-by: Bryan Evenson <bevenson@melinkcorp.com>
Cc: Andrew Victor <linux@maxim.org.za>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: <stable@vger.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

@@ -48,6 +48,7 @@ struct at91_rtc_config {
 
 static const struct at91_rtc_config *at91_rtc_config;
 static DECLARE_COMPLETION(at91_rtc_updated);
+static DECLARE_COMPLETION(at91_rtc_upd_rdy);
 static unsigned int at91_alarm_year = AT91_RTC_EPOCH;
 static void __iomem *at91_rtc_regs;
 static int irq;
@@ -161,6 +162,8 @@ static int at91_rtc_settime(struct device *dev, struct rtc_time *tm)
 		1900 + tm->tm_year, tm->tm_mon, tm->tm_mday,
 		tm->tm_hour, tm->tm_min, tm->tm_sec);
 
+	wait_for_completion(&at91_rtc_upd_rdy);
+
 	/* Stop Time/Calendar from counting */
 	cr = at91_rtc_read(AT91_RTC_CR);
 	at91_rtc_write(AT91_RTC_CR, cr | AT91_RTC_UPDCAL | AT91_RTC_UPDTIM);
@@ -183,7 +186,9 @@ static int at91_rtc_settime(struct device *dev, struct rtc_time *tm)
 
 	/* Restart Time/Calendar */
 	cr = at91_rtc_read(AT91_RTC_CR);
+	at91_rtc_write(AT91_RTC_SCCR, AT91_RTC_SECEV);
 	at91_rtc_write(AT91_RTC_CR, cr & ~(AT91_RTC_UPDCAL | AT91_RTC_UPDTIM));
+	at91_rtc_write_ier(AT91_RTC_SECEV);
 
 	return 0;
 }
@@ -290,8 +295,10 @@ static irqreturn_t at91_rtc_interrupt(int irq, void *dev_id)
 	if (rtsr) {		/* this interrupt is shared!  Is it ours? */
 		if (rtsr & AT91_RTC_ALARM)
 			events |= (RTC_AF | RTC_IRQF);
-		if (rtsr & AT91_RTC_SECEV)
-			events |= (RTC_UF | RTC_IRQF);
+		if (rtsr & AT91_RTC_SECEV) {
+			complete(&at91_rtc_upd_rdy);
+			at91_rtc_write_idr(AT91_RTC_SECEV);
+		}
 		if (rtsr & AT91_RTC_ACKUPD)
 			complete(&at91_rtc_updated);
 
@@ -413,6 +420,11 @@ static int __init at91_rtc_probe(struct platform_device *pdev)
 		return PTR_ERR(rtc);
 	platform_set_drvdata(pdev, rtc);
 
+	/* enable SECEV interrupt in order to initialize at91_rtc_upd_rdy
+	 * completion.
+	 */
+	at91_rtc_write_ier(AT91_RTC_SECEV);
+
 	dev_info(&pdev->dev, "AT91 Real Time Clock driver.\n");
 	return 0;
 }