watchdog: honour hw_margin_ms DT property

Some watchdog devices, e.g. external gpio-triggered ones, must be
reset more often than once per second, which means that the current
rate-limiting logic in watchdog_reset() fails to keep the board alive.

gpio-wdt.txt in the linux source tree defines a "hw_margin_ms"
property used to specifiy the maximum time allowed between resetting
the device. Allow any watchdog device to specify such a property, and
then use a reset period of one quarter of that. We keep the current
default of resetting once every 1000ms.
Signed-off-by: NRasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: NStefan Roese <sr@denx.de>

drivers/watchdog/wdt-uclass.c

@@ -16,6 +16,12 @@ DECLARE_GLOBAL_DATA_PTR;
 
 #define WATCHDOG_TIMEOUT_SECS	(CONFIG_WATCHDOG_TIMEOUT_MSECS / 1000)
 
+/*
+ * Reset every 1000ms, or however often is required as indicated by a
+ * hw_margin_ms property.
+ */
+static ulong reset_period = 1000;
+
 int initr_watchdog(void)
 {
 	u32 timeout = WATCHDOG_TIMEOUT_SECS;
@@ -37,6 +43,9 @@ int initr_watchdog(void)
 	if (CONFIG_IS_ENABLED(OF_CONTROL) && !CONFIG_IS_ENABLED(OF_PLATDATA)) {
 		timeout = dev_read_u32_default(gd->watchdog_dev, "timeout-sec",
 					       WATCHDOG_TIMEOUT_SECS);
+		reset_period = dev_read_u32_default(gd->watchdog_dev,
+						    "hw_margin_ms",
+						    4 * reset_period) / 4;
 	}
 
 	wdt_start(gd->watchdog_dev, timeout * 1000, 0);
@@ -118,7 +127,7 @@ void watchdog_reset(void)
 	/* Do not reset the watchdog too often */
 	now = get_timer(0);
 	if (time_after(now, next_reset)) {
-		next_reset = now + 1000;	/* reset every 1000ms */
+		next_reset = now + reset_period;
 		wdt_reset(gd->watchdog_dev);
 	}
 }