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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: AW: [BUG] omap_wdt.c: Watchdog not serviced by kernel
Date: Fri, 17 Sep 2021 08:08:18 -0700	[thread overview]
Message-ID: <06eba290-1794-854a-f999-86668211dcaf@roeck-us.net> (raw)
In-Reply-To: <cd9db72b69964157b3e36e22ab1e542f@chdua14.duagon.ads>

On 9/17/21 8:00 AM, Walter Stoll wrote:
>> -----Ursprüngliche Nachricht-----
>> Von: Guenter Roeck <groeck7@gmail.com> Im Auftrag von Guenter Roeck
>> Gesendet: Freitag, 17. September 2021 15:47
>> An: Walter Stoll <Walter.Stoll@duagon.com>; wim@linux-watchdog.org
>> Cc: linux-watchdog@vger.kernel.org
>> Betreff: Re: [BUG] omap_wdt.c: Watchdog not serviced by kernel
>>
>> 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);
>>>
> 
> Hello Guenter
> 
> Thank you very much for your fast response. I checked your fix with all
> combinations of early_enable and CONFIG_WATCHDOG_HANDLE_BOOT_ENABLED.
> 
Excellent.

>  From my point of view, this works exactly as expected. Any chance to get
> that mainline ?
> 

Submit a patch, I'll review it, and Wim will pick it up for v5.16.

Guenter

      reply	other threads:[~2021-09-17 15:08 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
2021-09-17 15:00   ` AW: " Walter Stoll
2021-09-17 15:08     ` Guenter Roeck [this message]

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