Samsung based S3C2440A chipset - watchdog timer issue

Samsung based S3C2440A chipset - watchdog timer issue

[Suniel Mahesh]

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.

Thanks-- 
Suniel Mahesh

Re: Samsung based S3C2440A chipset - watchdog timer issue

[Krzysztof Kozlowski]

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).

Best regards,
Krzysztof

Re: Samsung based S3C2440A chipset - watchdog timer issue

[Guenter Roeck]

On 9/19/19 12:40 AM, Suniel Mahesh 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 ?
> 

The driver does not detect if the watchdog is not running when probed.
What u-boot does is irrelevant. To start the watchdog at boot, you would
have to provide the tmr_atboot=1 module parameter. Otherwise, it is explicitly
disabled, as shown in the code, and opening the watchdog device would start
the watchdog.

Having said that, the code is less than perfect. Initializing the timeout
in the probe function should be handled differently, and the kernel should
be informed that/if the watchdog is started at boot. It should also be
possible to detect if the watchdog is running instead of relying on tmr_atboot,
and inform the kernel accordingly. But those are not bugs, just possible
improvements.

Guenter

> 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.
> 
> Thanks--
> Suniel Mahesh
> 

Re: Samsung based S3C2440A chipset - watchdog timer issue

[Guenter Roeck]

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