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 <linux@roeck-us.net>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
Cc: Pohsun Su <pohsuns@nvidia.com>
Cc: Robert Lin <robelin@nvidia.com>
Link: https://lore.kernel.org/r/20250614175556.922159-2-linux@roeck-us.net

diff --git a/drivers/clocksource/timer-tegra186.c b/drivers/clocksource/timer-tegra186.c
index 76a5481c8eb689ab47c6fc03f98e7e02520bbaf6..d403a3fe25e76aba20c3217e4df18b0ba2c4dc78 100644 (file)
--- 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 = {