clocksource/drivers/timer-tegra186: Add WDIOC_GETTIMELEFT support

This change adds support for WDIOC_GETTIMELEFT so userspace
programs can get the number of seconds before system reset by
the watchdog timer via ioctl.

Signed-off-by: Pohsun Su <pohsuns@nvidia.com>
Signed-off-by: Robert Lin <robelin@nvidia.com>
Link: https://lore.kernel.org/r/20250507044311.3751033-2-robelin@nvidia.com
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

drivers/clocksource/timer-tegra186.c

@@ -1,8 +1,9 @@
 // SPDX-License-Identifier: GPL-2.0-only
 /*
- * Copyright (c) 2019-2020 NVIDIA Corporation. All rights reserved.
+ * Copyright (c) 2019-2025 NVIDIA Corporation. All rights reserved.
  */
 
+#include <linux/bitfield.h>
 #include <linux/clocksource.h>
 #include <linux/module.h>
 #include <linux/interrupt.h>
@@ -29,6 +30,7 @@
 
 #define TMRSR 0x004
 #define  TMRSR_INTR_CLR BIT(30)
+#define  TMRSR_PCV GENMASK(28, 0)
 
 #define TMRCSSR 0x008
 #define  TMRCSSR_SRC_USEC (0 << 0)
@@ -45,6 +47,9 @@
 #define  WDTCR_TIMER_SOURCE_MASK 0xf
 #define  WDTCR_TIMER_SOURCE(x) ((x) & 0xf)
 
+#define WDTSR 0x004
+#define  WDTSR_CURRENT_EXPIRATION_COUNT GENMASK(14, 12)
+
 #define WDTCMDR 0x008
 #define  WDTCMDR_DISABLE_COUNTER BIT(1)
 #define  WDTCMDR_START_COUNTER BIT(0)
@@ -234,12 +239,69 @@ static int tegra186_wdt_set_timeout(struct watchdog_device *wdd,
 	return 0;
 }
 
+static unsigned int tegra186_wdt_get_timeleft(struct watchdog_device *wdd)
+{
+	struct tegra186_wdt *wdt = to_tegra186_wdt(wdd);
+	u32 expiration, val;
+	u64 timeleft;
+
+	if (!watchdog_active(&wdt->base)) {
+		/* return zero if the watchdog timer is not activated. */
+		return 0;
+	}
+
+	/*
+	 * Reset occurs on the fifth expiration of the
+	 * watchdog timer and so when the watchdog timer is configured,
+	 * the actual value programmed into the counter is 1/5 of the
+	 * timeout value. Once the counter reaches 0, expiration count
+	 * will be increased by 1 and the down counter restarts.
+	 * Hence to get the time left before system reset we must
+	 * combine 2 parts:
+	 * 1. value of the current down counter
+	 * 2. (number of counter expirations remaining) * (timeout/5)
+	 */
+
+	/* Get the current number of counter expirations. Should be a
+	 * value between 0 and 4
+	 */
+	val = readl_relaxed(wdt->regs + WDTSR);
+	expiration = FIELD_GET(WDTSR_CURRENT_EXPIRATION_COUNT, val);
+	if (WARN_ON_ONCE(expiration > 4))
+		return 0;
+
+	/* Get the current counter value in microsecond. */
+	val = readl_relaxed(wdt->tmr->regs + TMRSR);
+	timeleft = FIELD_GET(TMRSR_PCV, val);
+
+	/*
+	 * Calculate the time remaining by adding the time for the
+	 * counter value to the time of the counter expirations that
+	 * remain.
+	 */
+	timeleft += (((u64)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.
+	 */
+	timeleft = DIV_ROUND_CLOSEST_ULL(timeleft, USEC_PER_SEC);
+
+	if (WARN_ON_ONCE(timeleft > U32_MAX))
+		return U32_MAX;
+
+	return lower_32_bits(timeleft);
+}
+
 static const struct watchdog_ops tegra186_wdt_ops = {
 	.owner = THIS_MODULE,
 	.start = tegra186_wdt_start,
 	.stop = tegra186_wdt_stop,
 	.ping = tegra186_wdt_ping,
 	.set_timeout = tegra186_wdt_set_timeout,
+	.get_timeleft = tegra186_wdt_get_timeleft,
 };
 
 static struct tegra186_wdt *tegra186_wdt_create(struct tegra186_timer *tegra,