rtc: Limit RTC PIE frequency

The RTC pie hrtimer is self rearming. We really need to limit the
frequency to something sensible. Thus limit it to the 8192Hz max
value from the rtc man documentation

Cc: Willy Tarreau <w@1wt.eu>
Cc: stable@kernel.org
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
[jstultz: slightly reworked to use RTC_MAX_FREQ value]
Signed-off-by: NJohn Stultz <john.stultz@linaro.org>

drivers/rtc/interface.c

@@ -708,7 +708,7 @@ int rtc_irq_set_freq(struct rtc_device *rtc, struct rtc_task *task, int freq)
 	int err = 0;
 	unsigned long flags;
 
-	if (freq <= 0)
+	if (freq <= 0 || freq > RTC_MAX_FREQ)
 		return -EINVAL;
 retry:
 	spin_lock_irqsave(&rtc->irq_task_lock, flags);

include/linux/rtc.h

@@ -97,6 +97,9 @@ struct rtc_pll_info {
 #define RTC_AF 0x20	/* Alarm interrupt */
 #define RTC_UF 0x10	/* Update interrupt for 1Hz RTC */
 
+
+#define RTC_MAX_FREQ	8192
+
 #ifdef __KERNEL__
 
 #include <linux/types.h>