[PATCH v3 1/2] watchdog: dw_wdt: pat the watchdog before enabling it

[Doug Anderson <dianders@chromium.org>]

On some dw_wdt implementations the "top" register may be initted to 0
at bootup.  In such a case, each "pat" of the watchdog will reset the
timer to 0xffff.  That's pretty short.

The input clock of the wdt can be any of a wide range of values.  On
an rk3288 system, I've seen the wdt clock be 24.75 MHz.  That means
each tick is ~40ns and we'll count to 0xffff in ~2.6ms.

Because of the above two facts, it's a really good idea to pat the
watchdog after initting the "top" register properly and before
enabling the watchdog.  If you don't then there's no way we'll get the
next heartbeat in time.

Jisheng Zhang fixed this problem on some dw_wdt versions by using the
TOP_INIT feature.  However, the dw_wdt on rk3288 doesn't have TOP_INIT
so it's a good idea to also pat the watchdog manually.

Signed-off-by: Doug Anderson <dianders@chromium.org>
---
Changes in v3:
- Better comments now that we know what's going on.
- Don't typo dw_wdt as dw_mmc.

Changes in v2:
- Add comment about why TOP_INIT doesn't work on rk3288; move pat
  to right next to the attempt to use TOP_INIT.

 drivers/watchdog/dw_wdt.c | 26 +++++++++++++++++++-------
 1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/drivers/watchdog/dw_wdt.c b/drivers/watchdog/dw_wdt.c
index b34a2e4..3dde6de 100644
--- a/drivers/watchdog/dw_wdt.c
+++ b/drivers/watchdog/dw_wdt.c
@@ -96,6 +96,12 @@ static inline void dw_wdt_set_next_heartbeat(void)
 	dw_wdt.next_heartbeat = jiffies + dw_wdt_get_top() * HZ;
 }
 
+static void dw_wdt_keepalive(void)
+{
+	writel(WDOG_COUNTER_RESTART_KICK_VALUE, dw_wdt.regs +
+	       WDOG_COUNTER_RESTART_REG_OFFSET);
+}
+
 static int dw_wdt_set_top(unsigned top_s)
 {
 	int i, top_val = DW_WDT_MAX_TOP;
@@ -110,21 +116,27 @@ static int dw_wdt_set_top(unsigned top_s)
 			break;
 		}
 
-	/* Set the new value in the watchdog. */
+	/*
+	 * Set the new value in the watchdog.  Some versions of dw_wdt
+	 * have have TOPINIT in the TIMEOUT_RANGE register (as per
+	 * CP_WDT_DUAL_TOP in WDT_COMP_PARAMS_1).  On those we
+	 * effectively get a pat of the watchdog right here.
+	 */
 	writel(top_val | top_val << WDOG_TIMEOUT_RANGE_TOPINIT_SHIFT,
 		dw_wdt.regs + WDOG_TIMEOUT_RANGE_REG_OFFSET);
 
+	/*
+	 * Add an explicit pat to handle versions of the watchdog that
+	 * don't have TOPINIT.  This won't hurt on versions that have
+	 * it.
+	 */
+	dw_wdt_keepalive();
+
 	dw_wdt_set_next_heartbeat();
 
 	return dw_wdt_top_in_seconds(top_val);
 }
 
-static void dw_wdt_keepalive(void)
-{
-	writel(WDOG_COUNTER_RESTART_KICK_VALUE, dw_wdt.regs +
-	       WDOG_COUNTER_RESTART_REG_OFFSET);
-}
-
 static int dw_wdt_restart_handle(struct notifier_block *this,
 				unsigned long mode, void *cmd)
 {
-- 
2.2.0.rc0.207.ga3a616c