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From: Guenter Roeck <linux@roeck-us.net>
To: Walter Stoll <Walter.Stoll@duagon.com>,
	"wim@linux-watchdog.org" <wim@linux-watchdog.org>
Cc: "linux-watchdog@vger.kernel.org" <linux-watchdog@vger.kernel.org>
Subject: Re: [BUG] omap_wdt.c: Watchdog not serviced by kernel
Date: Fri, 17 Sep 2021 06:47:10 -0700	[thread overview]
Message-ID: <49fe3ca1-2243-f007-92b2-3c2d3430417f@roeck-us.net> (raw)
In-Reply-To: <cb068bbba92347b2ab3190fda5d85ebf@chdua14.duagon.ads>

On 9/17/21 1:36 AM, Walter Stoll wrote:
> Effect observed
> ---------------
> 
> We use the watchdog timer on a AM335x controller. When U-Boot runs, we enable
> the watchdog and want the kernel to service the watchdog until userspace takes
> it over.
> 
> We compile the watchdog directly into the kernel and add the parameter
> "omap_wdt.early_enable=1" to the kernel command line. We furthermore set
> "CONFIG_WATCHDOG_HANDLE_BOOT_ENABLED=y" in the kernel configuration.
> 
> Our expectation is, that the watchdog is serviced by the kernel as long as
> userspace does not touch the device /dev/watchdog. However, this is not the
> case. The watchdog always expires. It is obviously not serviced by the kernel.
> 
> We observed the effect with kernel version v5.4.138-rt62. However, we think
> that the most recent kernel exhibits the same behavior because the structure of
> the sources in question (see below) did not change. This also holds for the non
> realtime kernel.
> 
> 
> Root cause
> ----------
> 
> The CONFIG_WATCHDOG_HANDLE_BOOT_ENABLED configuration option is not implemented
> in omap_wdt.c.
> 
> 
> Fix proposal
> ------------
> 
> Interestingly we found only one single driver that implements this featrue,
> namely the driver from STM, see
> https://elixir.bootlin.com/linux/v5.4.138/source/drivers/watchdog/stm32_iwdg.c#L274
> 
> This makes us wonder if there might be a good reason not to implement it???
> 
It is primarily a watchdog core feature. Handling running watchdogs in the core
used to be enabled by default. Not everyone was happy with that, so
WATCHDOG_HANDLE_BOOT_ENABLED was added to be able to _disable_ that functionality.
It was never intended to be a driver feature. The STM driver (mis)uses it
because it wants to be able to support WDOG_HW_RUNNING, but the HW has no means
to detect if it is running. That doesn't mean that other drivers need to do
the same.

> However we think this feature should be available. Our use case is to make
> software updates more robust. If an updated kernel hangs for whatever reason,
> then U-Boot gets the chance to boot the old one provided there is a reboot.
>   
> Based on the STM implementation, we created a patch (see below) which resolves
> the issue. The watchdog is now correctly handled by the kernel until userspace
> first accesses it.
> 
> diff --git a/drivers/watchdog/omap_wdt.c b/drivers/watchdog/omap_wdt.c
> index 9b91882fe3c4..94e2e1b494d2 100644
> --- a/drivers/watchdog/omap_wdt.c
> +++ b/drivers/watchdog/omap_wdt.c
> @@ -271,6 +271,11 @@ static int omap_wdt_probe(struct platform_device *pdev)
>   	if (!early_enable)
>   		omap_wdt_disable(wdev);
>   
> +	if (IS_ENABLED(CONFIG_WATCHDOG_HANDLE_BOOT_ENABLED)) {
> +		/* Make sure the watchdog is serviced */
> +		set_bit(WDOG_HW_RUNNING, &wdev->wdog.status);
> +	}
> +

No, this is wrong. The driver should set WDOG_HW_RUNNING if the watchdog is running,
independently of CONFIG_WATCHDOG_HANDLE_BOOT_ENABLED.

The omap_wdt driver has a boot option "early_enable" which does start the watchdog
during probe, but it doesn't set WDOG_HW_RUNNING (which is the real problem).
Plus, if early_enable is not set, the driver explicitly disables the watchdog
(see code above), and setting WDOG_HW_RUNNING would be wrong in that case.

The fix would be something like
	
         if (early_enable) {
                 omap_wdt_start(&wdev->wdog);
		set_bit(WDOG_HW_RUNNING, &wdev->wdog.status);
	} else {
		omap_wdt_disable(wdev);
	}

That needs to be ahead of watchdog_register_device(), and is independent
of CONFIG_WATCHDOG_HANDLE_BOOT_ENABLED.

Guenter

>   	ret = watchdog_register_device(&wdev->wdog);
>   	if (ret) {
>   		pm_runtime_disable(wdev->dev);
> 


  reply	other threads:[~2021-09-17 13:47 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-17  8:36 Walter Stoll
2021-09-17 13:47 ` Guenter Roeck [this message]
2021-09-17 15:00   ` AW: " Walter Stoll
2021-09-17 15:08     ` Guenter Roeck

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