Re: [PATCH v3 1/3] clocksource/drivers/timer-tegra186: add WDIOC_GETTIMELEFT support

[Thierry Reding]

On Mon, Apr 07, 2025 at 06:23:21PM +0800, Robert Lin wrote:
> From: Pohsun Su <pohsuns@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> Signed-off-by: Robert Lin <robelin@xxxxxxxxxx>
> ---
>  drivers/clocksource/timer-tegra186.c | 53 +++++++++++++++++++++++++++-
>  1 file changed, 52 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/clocksource/timer-tegra186.c b/drivers/clocksource/timer-tegra186.c
> index ea742889ee06..fc2ca86c1436 100644
> --- a/drivers/clocksource/timer-tegra186.c
> +++ b/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>
> @@ -30,6 +31,7 @@
>  
>  #define TMRSR 0x004
>  #define  TMRSR_INTR_CLR BIT(30)
> +#define  TMRSR_PCV GENMASK(28, 0)
>  
>  #define TMRCSSR 0x008
>  #define  TMRCSSR_SRC_USEC (0 << 0)
> @@ -46,6 +48,9 @@
>  #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)
> @@ -235,12 +240,58 @@ 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 timeleft, expiration, val;
> +
> +	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);
> +
> +	/* Convert the current counter value to seconds, rounding up
> +	 * to the nearest second.
> +	 */
> +	val = readl_relaxed(wdt->tmr->regs + TMRSR);
> +	timeleft = FIELD_GET(TMRSR_PCV, val);
> +	timeleft = (timeleft + USEC_PER_SEC / 2) / USEC_PER_SEC;
> +
> +	/*
> +	 * Calculate the time remaining by adding the time for the
> +	 * counter value to the time of the counter expirations that
> +	 * remain. Do the multiplication first on purpose just to keep
> +	 * the precision due to the integer division.
> +	 */
> +	timeleft += wdt->base.timeout * (4 - expiration) / 5;

I'm a little worried that we might be getting a very skewed value here
due to the multiple rounds of rounding. Can we not compute timeleft in
microseconds until the very end and only convert to seconds then?

Thierry

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