Re: Samsung based S3C2440A chipset - watchdog timer issue

From: Guenter Roeck <linux@roeck-us.net>
To: Krzysztof Kozlowski <krzk@kernel.org>,
	Suniel Mahesh <sunil.m@techveda.org>
Cc: kgene@kernel.org, wim@linux-watchdog.org, heiko@sntech.de,
	"linux-samsung-soc@vger.kernel.org" 
	<linux-samsung-soc@vger.kernel.org>,
	linux-watchdog@vger.kernel.org
Subject: Re: Samsung based S3C2440A chipset - watchdog timer issue
Date: Thu, 19 Sep 2019 06:26:26 -0700	[thread overview]
Message-ID: <19a79d6b-e373-3ac6-0339-2ef06f14818b@roeck-us.net> (raw)
In-Reply-To: <CAJKOXPdoYZTr_4ysVxN17bo8FZzQXRJS=ch4BAZzY+bLsLhS9Q@mail.gmail.com>

On 9/19/19 1:38 AM, Krzysztof Kozlowski wrote:
> On Thu, 19 Sep 2019 at 09:40, Suniel Mahesh <sunil.m@techveda.org> wrote:
>>
>> Hi,
>>
>> I am working on one of the Samsung based S3C2440A chipset based target .
>>
>> I have couple of questions and I request someone to shed some light on these: (Thank you)
>>
>> The watchdog driver in linux, looks like, it just configured WDT but didn't start it (code snippet included below).
>>
>> May i know the reasson why WDT is not started ? Is it because u-boot already started WDT, implies it is not required to do the
>> same once we jump into linux ? or is there any specific reason ?
>>
>> drivers/watchdog/s3c2410_wdt.c (line 53 and lines 616 - 625)
>>
>> #define S3C2410_WATCHDOG_ATBOOT         (0)
>> ....
>> static int tmr_atboot   = S3C2410_WATCHDOG_ATBOOT;
>> ...
>> ...
>> if (tmr_atboot && started == 0) {
>>                  dev_info(dev, "starting watchdog timer\n");
>>                  s3c2410wdt_start(&wdt->wdt_device);
>>          } else if (!tmr_atboot) {
>>                  /* if we're not enabling the watchdog, then ensure it is
>>                   * disabled if it has been left running from the bootloader
>>                   * or other source */
>>
>>                  s3c2410wdt_stop(&wdt->wdt_device);
>>          }
>> ...
>> ...
>>
>> Tried to start WDT in linux by assigning value 1 to S3C2410_WATCHDOG_ATBOOT. The target resets.
>>
>> please comment.
> 
> I think watchdog should not start during boot before user-space is
> brought up. Otherwise who will ping it? Usually watchdog is started by
> opening the watchdog device by user-space. If you need it to be
> running without user-space, there are special flags for this (see
> WDOG_HW_RUNNING and others).
> 
That is not entirely correct. There are use cases where the watchdog
is started on purpose and userspace has to open it within a specified
period of time to prevent the system from rebooting. Presumably this is
what the current driver tries to implement. WDOG_HW_RUNNING was added
to the core after the driver was written, so it is not surprising that
the driver does not support it. Patches welcome.

Thanks,
Guenter