linux-kernel - Re: [PATCH v2 3/3] watchdog: qcom: register a restart notifier

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Date:	Wed, 24 Sep 2014 10:05:21 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Josh Cartwright <joshc@...eaurora.org>
Cc:	Wim Van Sebroeck <wim@...ana.be>, linux-watchdog@...r.kernel.org,
	linux-arm-msm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Kumar Gala <galak@...eaurora.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] watchdog: qcom: register a restart notifier

On Tue, Sep 23, 2014 at 06:04:38PM -0500, Josh Cartwright wrote:
> The WDT's BITE_TIME warm-reset behavior can be leveraged as a last
> resort mechanism for triggering chip reset.  Usually, other restart
> methods (such as PS_HOLD) are preferrable for issuing a more complete
> reset of the chip.  As such, keep the priority of the watchdog notifier
> low.
> 
> Signed-off-by: Josh Cartwright <joshc@...eaurora.org>
> ---
>  drivers/watchdog/qcom-wdt.c | 28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
> 
> diff --git a/drivers/watchdog/qcom-wdt.c b/drivers/watchdog/qcom-wdt.c
> index d5e46e2..eba92ef 100644
> --- a/drivers/watchdog/qcom-wdt.c
> +++ b/drivers/watchdog/qcom-wdt.c
> @@ -16,6 +16,7 @@
>  #include <linux/module.h>
>  #include <linux/of.h>
>  #include <linux/platform_device.h>
> +#include <linux/reboot.h>
>  #include <linux/watchdog.h>
>  
>  #define WDT_RST		0x0
> @@ -25,6 +26,7 @@
>  struct qcom_wdt {
>  	struct watchdog_device	wdd;
>  	struct clk		*clk;
> +	struct notifier_block	restart_nb;
>  	void __iomem		*base;
>  };
>  
> @@ -86,6 +88,23 @@ static const struct watchdog_info qcom_wdt_info = {
>  	.identity	= KBUILD_MODNAME,
>  };
>  
> +static int qcom_wdt_restart(struct notifier_block *nb, unsigned long action,
> +			    void *data)
> +{
> +	struct qcom_wdt *wdt = container_of(nb, struct qcom_wdt, restart_nb);
> +
> +	/*
> +	 * Trigger watchdog bite:
> +	 *    Setup BITE_TIME to be very low, and enable WDT.
> +	 *      0x31F3 => 390ms @ 32kHz, also value at reset

What happens if you set it to a lower value ? If you set it to, say,
0x100, would the timeout be ~8ms ? Wouldn't that make more sense ?
Or at least something like 0x800, which should make it around 64 ms,
or 0x400 for 32 ms.

[ In other words, I don't really see the point for a 390 ms wait here ]

> +	 */
> +	writel(0, wdt->base + WDT_EN);
> +	writel(1, wdt->base + WDT_RST);
> +	writel(0x31F3, wdt->base + WDT_BITE_TIME);
> +	writel(1, wdt->base + WDT_EN);

You should have some mdelay() here, larger than the expected timeout.

Thanks,
Guenter

> +	return NOTIFY_DONE;
> +}
> +
>  static int qcom_wdt_probe(struct platform_device *pdev)
>  {
>  	struct qcom_wdt *wdt;
> @@ -141,6 +160,14 @@ static int qcom_wdt_probe(struct platform_device *pdev)
>  		return ret;
>  	}
>  
> +	/*
> +	 * WDT restart notifier has priority 0 (use as a last resort)
> +	 */
> +	wdt->restart_nb.notifier_call = qcom_wdt_restart;
> +	ret = register_restart_handler(&wdt->restart_nb);
> +	if (ret)
> +		dev_err(&pdev->dev, "failed to setup restart handler\n");
> +
>  	platform_set_drvdata(pdev, wdt);
>  	return 0;
>  }
> @@ -149,6 +176,7 @@ static int qcom_wdt_remove(struct platform_device *pdev)
>  {
>  	struct qcom_wdt *wdt = platform_get_drvdata(pdev);
>  
> +	unregister_restart_handler(&wdt->restart_nb);
>  	watchdog_unregister_device(&wdt->wdd);
>  	clk_disable_unprepare(wdt->clk);
>  	return 0;
> -- 
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
> hosted by The Linux Foundation
> 
> --
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