allow opting out of WATCHDOG_RESET() from timer interrupt

Having WATCHDOG_RESET() called automatically from the timer interrupt
runs counter to the idea of a watchdog device - if the board runs into
an infinite loops with interrupts still enabled, the watchdog will
never fire.

When using CONFIG_(SPL_)WDT, the watchdog_reset function is a lot more
complicated than just poking a few SOC-specific registers - it
involves accessing all kinds of global data, and if the interrupt
happens at the wrong time (say, in the middle of an WATCHDOG_RESET()
call from ordinary code), that can end up corrupting said global data.

Allow the board to opt out of calling WATCHDOG_RESET() from the timer
interrupt handler by setting CONFIG_SYS_WATCHDOG_FREQ to 0 - as that
setting is currently nonsensical (it would be compile-time
divide-by-zero), it cannot affect any existing boards.

Add documentation for both the existing and extended meaning of
CONFIG_SYS_WATCHDOG_FREQ.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>

README

@@ -747,6 +747,15 @@ The following options need to be configured:
 		SoC, then define this variable and provide board
 		specific code for the "hw_watchdog_reset" function.
 
+		CONFIG_SYS_WATCHDOG_FREQ
+		Some platforms automatically call WATCHDOG_RESET()
+		from the timer interrupt handler every
+		CONFIG_SYS_WATCHDOG_FREQ interrupts. If not set by the
+		board configuration file, a default of CONFIG_SYS_HZ/2
+		(i.e. 500) is used. Setting CONFIG_SYS_WATCHDOG_FREQ
+		to 0 disables calling WATCHDOG_RESET() from the timer
+		interrupt.
+
 - Real-Time Clock:
 
 		When CONFIG_CMD_DATE is selected, the type of the RTC

arch/m68k/lib/time.c

@@ -71,7 +71,7 @@ void dtimer_interrupt(void *not_used)
 		timestamp++;
 
 		#if defined(CONFIG_WATCHDOG) || defined (CONFIG_HW_WATCHDOG)
-		if ((timestamp % (CONFIG_SYS_WATCHDOG_FREQ)) == 0) {
+		if (CONFIG_SYS_WATCHDOG_FREQ && (timestamp % (CONFIG_SYS_WATCHDOG_FREQ)) == 0) {
 			WATCHDOG_RESET ();
 		}
 		#endif    /* CONFIG_WATCHDOG || CONFIG_HW_WATCHDOG */

arch/powerpc/lib/interrupts.c

@@ -80,7 +80,7 @@ void timer_interrupt(struct pt_regs *regs)
 	timestamp++;
 
 #if defined(CONFIG_WATCHDOG) || defined (CONFIG_HW_WATCHDOG)
-	if ((timestamp % (CONFIG_SYS_WATCHDOG_FREQ)) == 0)
+	if (CONFIG_SYS_WATCHDOG_FREQ && (timestamp % (CONFIG_SYS_WATCHDOG_FREQ)) == 0)
 		WATCHDOG_RESET ();
 #endif    /* CONFIG_WATCHDOG || CONFIG_HW_WATCHDOG */
 

drivers/timer/mpc83xx_timer.c

@@ -175,7 +175,7 @@ void timer_interrupt(struct pt_regs *regs)
 	priv->timestamp++;
 
 #if defined(CONFIG_WATCHDOG) || defined(CONFIG_HW_WATCHDOG)
-	if ((priv->timestamp % (CONFIG_SYS_WATCHDOG_FREQ)) == 0)
+	if (CONFIG_SYS_WATCHDOG_FREQ && (priv->timestamp % (CONFIG_SYS_WATCHDOG_FREQ)) == 0)
 		WATCHDOG_RESET();
 #endif    /* CONFIG_WATCHDOG || CONFIG_HW_WATCHDOG */