linux-kernel - Re: [PATCH v1] watchdog: sama5d4

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Date:	Thu, 3 Mar 2016 04:10:15 -0800
From:	Guenter Roeck <linux@...ck-us.net>
To:	Romain Izard <romain.izard.pro@...il.com>,
	linux-kernel@...r.kernel.org, linux-watchdog@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Wim Van Sebroeck <wim@...ana.be>
Cc:	Wenyou Yang <wenyou.yang@...el.com>,
	Nicolas Ferre <nicolas.ferre@...el.com>
Subject: Re: [PATCH v1] watchdog: sama5d4_wdt: Reset delay on start

On 03/03/2016 02:29 AM, Romain Izard wrote:
> If the internal counter is not refreshed when the watchdog is started
> for the first time, the watchdog will trigger very rapidly. For example,
> opening /dev/watchdog without writing in it will immediately trigger a
> reboot, instead of waiting for the delay to expire.
>
> To avoid this problem, reload the timer on opening the watchdog device.
>
> Command: "while sleep 5; do echo 1; done > /dev/watchdog"
> Before: system reset
> After: the watchdog runs correctly
>
> Signed-off-by: Romain Izard <romain.izard.pro@...il.com>

Subject might better read "ping watchdog on start" or similar.

Does the watchdog have to be pinged before it is enabled ?
I am a bit concerned that there may still be a 125 uS window
during which the system could restart.

Thanks,
Guenter

> ---
>   drivers/watchdog/sama5d4_wdt.c | 3 +++
>   1 file changed, 3 insertions(+)
>
> diff --git a/drivers/watchdog/sama5d4_wdt.c b/drivers/watchdog/sama5d4_wdt.c
> index a49634cdc1cc..e162fe140ae1 100644
> --- a/drivers/watchdog/sama5d4_wdt.c
> +++ b/drivers/watchdog/sama5d4_wdt.c
> @@ -15,6 +15,7 @@
>   #include <linux/platform_device.h>
>   #include <linux/reboot.h>
>   #include <linux/watchdog.h>
> +#include <linux/delay.h>
>
>   #include "at91sam9_wdt.h"
>
> @@ -58,6 +59,8 @@ static int sama5d4_wdt_start(struct watchdog_device *wdd)
>   	reg = wdt_read(wdt, AT91_WDT_MR);
>   	reg &= ~AT91_WDT_WDDIS;
>   	wdt_write(wdt, AT91_WDT_MR, reg);
> +	udelay(125); /* > 4 cycles at 32,768 Hz */
> +	wdt_write(wdt, AT91_WDT_CR, AT91_WDT_KEY | AT91_WDT_WDRSTT);
>
>   	return 0;
>   }
>