[PATCH 2/2] clocksource/drivers/timer-tegra186: Simplify calculating timeleft

[Guenter Roeck <linux@roeck-us.net>]

It is not necessary to use 64-bit operations to calculate the
remaining watchdog timeout. Simplify to use 32-bit operations,
and add comments explaining why there will be no overflow.

Cc: Pohsun Su <pohsuns@nvidia.com>
Cc: Robert Lin <robelin@nvidia.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
---
 drivers/clocksource/timer-tegra186.c | 25 +++++++++++++++----------
 1 file changed, 15 insertions(+), 10 deletions(-)

diff --git a/drivers/clocksource/timer-tegra186.c b/drivers/clocksource/timer-tegra186.c
index 7b506de65438..6ed319bb4e06 100644
--- a/drivers/clocksource/timer-tegra186.c
+++ b/drivers/clocksource/timer-tegra186.c
@@ -231,7 +231,7 @@ static unsigned int tegra186_wdt_get_timeleft(struct watchdog_device *wdd)
 {
 	struct tegra186_wdt *wdt = to_tegra186_wdt(wdd);
 	u32 expiration, val;
-	u64 timeleft;
+	u32 timeleft;
 
 	if (!watchdog_active(&wdt->base)) {
 		/* return zero if the watchdog timer is not activated. */
@@ -266,21 +266,26 @@ static unsigned int tegra186_wdt_get_timeleft(struct watchdog_device *wdd)
 	 * Calculate the time remaining by adding the time for the
 	 * counter value to the time of the counter expirations that
 	 * remain.
+	 * Note: Since wdt->base.timeout is bound to 255, the maximum
+	 * value added to timeleft is
+	 *   255 * (1,000,000 / 5) * 4
+	 * = 255 * 200,000 * 4
+	 * = 204,000,000
+	 * TMRSR_PCV is a 29-bit field.
+	 * Its maximum value is 0x1fffffff = 536,870,911.
+	 * 204,000,000 + 536,870,911 = 740,870,911 = 0x2C28CAFF.
+	 * timeleft can therefore not overflow, and 64-bit calculations
+	 * are not necessary.
 	 */
-	timeleft += ((u64)wdt->base.timeout * (USEC_PER_SEC / 5)) * (4 - expiration);
+	timeleft += (wdt->base.timeout * (USEC_PER_SEC / 5)) * (4 - expiration);
 
 	/*
 	 * Convert the current counter value to seconds,
-	 * rounding up to the nearest second. Cast u64 to
-	 * u32 under the assumption that no overflow happens
-	 * when coverting to seconds.
+	 * rounding to the nearest second.
 	 */
-	timeleft = DIV_ROUND_CLOSEST_ULL(timeleft, USEC_PER_SEC);
+	timeleft = DIV_ROUND_CLOSEST(timeleft, USEC_PER_SEC);
 
-	if (WARN_ON_ONCE(timeleft > U32_MAX))
-		return U32_MAX;
-
-	return lower_32_bits(timeleft);
+	return timeleft;
 }
 
 static const struct watchdog_ops tegra186_wdt_ops = {
-- 
2.45.2