linux-kernel - Re: [PATCH v1] watchdog: imx2_wdt: clear WDOG_HW

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Date:   Mon, 10 May 2021 20:59:25 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Robin Gong <yibin.gong@....com>, wim@...ux-watchdog.org,
        shawnguo@...nel.org, s.hauer@...gutronix.de, festevam@...il.com
Cc:     linux-imx@....com, kernel@...gutronix.de,
        linux-watchdog@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] watchdog: imx2_wdt: clear WDOG_HW_RUNNING before
 suspend

On 5/11/21 4:20 AM, Robin Gong wrote:
> Since watchdog_ping_work is not freezable so that it maybe scheduled before
> imx2_wdt_resume where watchdog clock enabled, hence, kernel will hang in
> imx2_wdt_ping without clock, and then watchdog reset happen. Prevent the
> above case by clearing WDOG_HW_RUNNING before suspend, and restore it with
> ping in imx2_wdt_resume.
> 
> Signed-off-by: Robin Gong <yibin.gong@....com>
> ---
>   drivers/watchdog/imx2_wdt.c | 8 ++++++++
>   1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/watchdog/imx2_wdt.c b/drivers/watchdog/imx2_wdt.c
> index b84f80f..d3751e9 100644
> --- a/drivers/watchdog/imx2_wdt.c
> +++ b/drivers/watchdog/imx2_wdt.c
> @@ -357,6 +357,13 @@ static int __maybe_unused imx2_wdt_suspend(struct device *dev)
>   		 */
>   		__imx2_wdt_set_timeout(wdog, IMX2_WDT_MAX_TIME);
>   		imx2_wdt_ping(wdog);
> +
> +		/*
> +		 * clear WDOG_HW_RUNNING to prevent watchdog_ping_work running
> +		 * before imx2_wdt_resume where clock enabled, otherwise kernel
> +		 * will hang and watchdog reset happen then.
> +		 */
> +		clear_bit(WDOG_HW_RUNNING, &wdog->status);
>   	}
>   
>   	clk_disable_unprepare(wdev->clk);
> @@ -386,6 +393,7 @@ static int __maybe_unused imx2_wdt_resume(struct device *dev)
>   	if (imx2_wdt_is_running(wdev)) {
>   		imx2_wdt_set_timeout(wdog, wdog->timeout);
>   		imx2_wdt_ping(wdog);
> +		set_bit(WDOG_HW_RUNNING, &wdog->status);
>   	}
>   
>   	return 0;
> 
I really don't like this; it depends on assumptions how the watchdog
core works, and it misuses WDOG_HW_RUNNING to indicate that the clock
is stopped (or that the driver is in suspend mode).

If ping doesn't work with the clock stopped, I think it would be much
better to have a flag indicating that the clock is stopped, and have
the ping function do nothing if it is called with stopped clock
(ie if suspended).

Guenter