[PATCH 1/2] watchdog: realtek-otto: prevent PHASE2 underflows

[Sander Vanheule]

For small pretimeout values, ((timeout - pretimeout) / tick) might be
rounded up to the same value as (timeout / tick). As a result, the
number of PHASE2 ticks may be zero, causing an underflow when
subtracting 1 to configure the hardware. While this results in a
longer-than-expected time to system reset, the duration of PHASE1 and
minimum ping interval for the watchdog would still be correct.

As the watchdog core ensures pretimeout is strictly less than timeout,
ceil(timeout / tick) is strictly greater than floor(pretimeout / tick)
and the number of PHASE1 ticks cannot be 0. So instead of rounding up
the number of PHASE1 ticks, we can round down the number of PHASE2
ticks, maintaining the current behavior while avoiding underflows.

The original helper function is now inlined, as it doesn't save any
duplication anymore.

Signed-off-by: Sander Vanheule <sander@xxxxxxxxxxxxx>
---
 drivers/watchdog/realtek_otto_wdt.c | 12 +++---------
 1 file changed, 3 insertions(+), 9 deletions(-)

diff --git a/drivers/watchdog/realtek_otto_wdt.c b/drivers/watchdog/realtek_otto_wdt.c
index 2c30ddd574c5..1495e8a8cbb5 100644
--- a/drivers/watchdog/realtek_otto_wdt.c
+++ b/drivers/watchdog/realtek_otto_wdt.c
@@ -114,12 +114,6 @@ static int otto_wdt_tick_ms(struct otto_wdt_ctrl *ctrl, int prescale)
  * the value stored in those fields. This means each phase will run for at least
  * one tick, so small values need to be clamped to correctly reflect the timeout.
  */
-static inline unsigned int div_round_ticks(unsigned int val, unsigned int tick_duration,
-		unsigned int min_ticks)
-{
-	return max(min_ticks, DIV_ROUND_UP(val, tick_duration));
-}
-
 static int otto_wdt_determine_timeouts(struct watchdog_device *wdev, unsigned int timeout,
 		unsigned int pretimeout)
 {
@@ -140,9 +134,9 @@ static int otto_wdt_determine_timeouts(struct watchdog_device *wdev, unsigned in
 			return -EINVAL;
 
 		tick_ms = otto_wdt_tick_ms(ctrl, prescale);
-		total_ticks = div_round_ticks(timeout_ms, tick_ms, 2);
-		phase1_ticks = div_round_ticks(timeout_ms - pretimeout_ms, tick_ms, 1);
-		phase2_ticks = total_ticks - phase1_ticks;
+		total_ticks = max(2, DIV_ROUND_UP(timeout_ms, tick_ms));
+		phase2_ticks = max(1, pretimeout_ms / tick_ms);
+		phase1_ticks = total_ticks - phase2_ticks;
 
 		prescale_next++;
 	} while (phase1_ticks > OTTO_WDT_PHASE_TICKS_MAX
-- 
2.54.0