From 7f3abae5b447a7f8458a0f58a003c11c46aade99 Mon Sep 17 00:00:00 2001 From: Guenter Roeck Date: Sat, 14 Jun 2025 10:55:56 -0700 Subject: [PATCH] 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 Signed-off-by: Daniel Lezcano Reviewed-by: Jon Hunter Cc: Pohsun Su Cc: Robert Lin Link: https://lore.kernel.org/r/20250614175556.922159-2-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 76a5481c8eb68..d403a3fe25e76 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 = { -- 2.47.3