clocksource/drivers/timer-tegra186: Simplify calculating timeleft

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.

Signed-off-by: Guenter Roeck <[e-mail]>
Signed-off-by: Daniel Lezcano <[e-mail]>
Reviewed-by: Jon Hunter <[e-mail]>
Cc: Pohsun Su <[e-mail]>
Cc: Robert Lin <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/clocksource/timer-tegra186.c

···
 {
 	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. */
···
 	 * 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 = {