watchdog: at91sam9_wdt: increase security margin on watchdog counter reset

Try to reset the watchdog counter 4 or 2 times more often than actually
requested, to avoid spurious watchdog reset.
If this is not possible because of the min_heartbeat value, reset it at
the min_heartbeat period.

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

1 files changed, 16 insertions, 9 deletions

diff --git a/drivers/watchdog/at91sam9_wdt.c b/drivers/watchdog/at91sam9_wdt.c
index ab0c4e0165d1..489729b26298 100644
--- a/drivers/watchdog/at91sam9_wdt.c
+++ b/drivers/watchdog/at91sam9_wdt.c
@@ -158,6 +158,7 @@ static int at91_wdt_init(struct platform_device *pdev, struct at91wdt *wdt)
         int err;
         u32 mask = wdt->mr_mask;
         unsigned long min_heartbeat = 1;
+        unsigned long max_heartbeat;
         struct device *dev = &pdev->dev;
 
         tmp = wdt_read(wdt, AT91_WDT_MR);
@@ -181,23 +182,29 @@ static int at91_wdt_init(struct platform_device *pdev, struct at91wdt *wdt)
         if (delta < value)
                 min_heartbeat = ticks_to_hz_roundup(value - delta);
 
-        wdt->heartbeat = ticks_to_hz_rounddown(value);
-        if (!wdt->heartbeat) {
+        max_heartbeat = ticks_to_hz_rounddown(value);
+        if (!max_heartbeat) {
                 dev_err(dev,
                         "heartbeat is too small for the system to handle it correctly\n");
                 return -EINVAL;
         }
 
-        if (wdt->heartbeat < min_heartbeat + 4) {
+        /*
+         * Try to reset the watchdog counter 4 or 2 times more often than
+         * actually requested, to avoid spurious watchdog reset.
+         * If this is not possible because of the min_heartbeat value, reset
+         * it at the min_heartbeat period.
+         */
+        if ((max_heartbeat / 4) >= min_heartbeat)
+                wdt->heartbeat = max_heartbeat / 4;
+        else if ((max_heartbeat / 2) >= min_heartbeat)
+                wdt->heartbeat = max_heartbeat / 2;
+        else
                 wdt->heartbeat = min_heartbeat;
+
+        if (max_heartbeat < min_heartbeat + 4)
                 dev_warn(dev,
                          "min heartbeat and max heartbeat might be too close for the system to handle it correctly\n");
-                if (wdt->heartbeat < 4)
-                        dev_warn(dev,
-                                 "heartbeat might be too small for the system to handle it correctly\n");
-        } else {
-                wdt->heartbeat -= 4;
-        }
 
         if ((tmp & AT91_WDT_WDFIEN) && wdt->irq) {
                 err = request_irq(wdt->irq, wdt_interrupt,