Re: [PATCH] watchdog: rti_wdt: Fix calculation and evaluation of preset heartbeat

[Guenter Roeck <linux@roeck-us.net>]

On Mon, Feb 21, 2022 at 05:22:38PM +0100, Jan Kiszka wrote:
> From: Jan Kiszka <jan.kiszka@siemens.com>
> 
> This ensures that the same value is read back as was eventually
> programmed when using seconds as accuracy. Even then, comparing the more
> precise heartbeat_ms against heartbeat in seconds will almost never
> provide a match and will needlessly raise a warning. Fix by comparing
> apples to apples.
> 
> Tested in combination with U-Boot as watchdog starter.
> 
> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

Reviewed-by: Guenter Roeck <linux@roeck-us.net>

> ---
> 
> To fully work, it also needs a U-Boot patch to add the same 10% margin 
> to the clock frequency:
> 
> https://patchwork.ozlabs.org/project/uboot/patch/9b3603a3-1bf4-eb57-c378-20d5c2accc34@siemens.com/
> 
>  drivers/watchdog/rti_wdt.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/watchdog/rti_wdt.c b/drivers/watchdog/rti_wdt.c
> index 117bc2a8eb0a..3ecd14e180fb 100644
> --- a/drivers/watchdog/rti_wdt.c
> +++ b/drivers/watchdog/rti_wdt.c
> @@ -252,6 +252,7 @@ static int rti_wdt_probe(struct platform_device *pdev)
>  	}
>  
>  	if (readl(wdt->base + RTIDWDCTRL) == WDENABLE_KEY) {
> +		int preset_heartbeat;
>  		u32 time_left_ms;
>  		u64 heartbeat_ms;
>  		u32 wsize;
> @@ -262,11 +263,12 @@ static int rti_wdt_probe(struct platform_device *pdev)
>  		heartbeat_ms <<= WDT_PRELOAD_SHIFT;
>  		heartbeat_ms *= 1000;
>  		do_div(heartbeat_ms, wdt->freq);
> -		if (heartbeat_ms != heartbeat * 1000)
> +		preset_heartbeat = heartbeat_ms + 500;
> +		preset_heartbeat /= 1000;
> +		if (preset_heartbeat != heartbeat)
>  			dev_warn(dev, "watchdog already running, ignoring heartbeat config!\n");
>  
> -		heartbeat = heartbeat_ms;
> -		heartbeat /= 1000;
> +		heartbeat = preset_heartbeat;
>  
>  		wsize = readl(wdt->base + RTIWWDSIZECTRL);
>  		ret = rti_wdt_setup_hw_hb(wdd, wsize);