linux-kernel - Re: [PATCH 7/7] watchdog: s3c2410: Let kernel kick watchdog

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Message-ID: <d07b0269-acff-b267-b90a-732502978712@roeck-us.net>
Date:   Sat, 30 Oct 2021 10:47:37 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Sam Protsenko <semen.protsenko@...aro.org>
Cc:     Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>,
        linux-watchdog@...r.kernel.org,
        devicetree <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
        Linux Samsung SOC <linux-samsung-soc@...r.kernel.org>
Subject: Re: [PATCH 7/7] watchdog: s3c2410: Let kernel kick watchdog

On 10/30/21 9:59 AM, Sam Protsenko wrote:
> On Sat, 30 Oct 2021 at 18:14, Guenter Roeck <linux@...ck-us.net> wrote:
>>
>> On 10/30/21 7:29 AM, Sam Protsenko wrote:
>>> On Fri, 29 Oct 2021 at 03:30, Guenter Roeck <linux@...ck-us.net> wrote:
>>>>
>>>> On 10/28/21 11:35 AM, Sam Protsenko wrote:
>>>>> When "tmr_atboot" module param is set, the watchdog is started in
>>>>> driver's probe. In that case, also set WDOG_HW_RUNNING bit to let
>>>>> watchdog core driver know it's running. This way wathcdog core can kick
>>>>> the watchdog for us (if CONFIG_WATCHDOG_HANDLE_BOOT_ENABLED option is
>>>>> enabled), until user space takes control.
>>>>>
>>>>> Signed-off-by: Sam Protsenko <semen.protsenko@...aro.org>
>>>>> ---
>>>>>     drivers/watchdog/s3c2410_wdt.c | 26 +++++++++++++++-----------
>>>>>     1 file changed, 15 insertions(+), 11 deletions(-)
>>>>>
>>>>> diff --git a/drivers/watchdog/s3c2410_wdt.c b/drivers/watchdog/s3c2410_wdt.c
>>>>> index ca082b1226e3..9af014ff1468 100644
>>>>> --- a/drivers/watchdog/s3c2410_wdt.c
>>>>> +++ b/drivers/watchdog/s3c2410_wdt.c
>>>>> @@ -732,6 +732,21 @@ static int s3c2410wdt_probe(struct platform_device *pdev)
>>>>>         wdt->wdt_device.bootstatus = s3c2410wdt_get_bootstatus(wdt);
>>>>>         wdt->wdt_device.parent = dev;
>>>>>
>>>>> +     /*
>>>>> +      * If "tmr_atboot" param is non-zero, start the watchdog right now. Also
>>>>> +      * set WDOG_HW_RUNNING bit, so that watchdog core can kick the watchdog.
>>>>> +      *
>>>>> +      * If we're not enabling the watchdog, then ensure it is disabled if it
>>>>> +      * has been left running from the bootloader or other source.
>>>>> +      */
>>>>> +     if (tmr_atboot && started == 0) {
>>>>> +             dev_info(dev, "starting watchdog timer\n");
>>>>> +             s3c2410wdt_start(&wdt->wdt_device);
>>>>> +             set_bit(WDOG_HW_RUNNING, &wdt->wdt_device.status);
>>>>> +     } else if (!tmr_atboot) {
>>>>> +             s3c2410wdt_stop(&wdt->wdt_device);
>>>>> +     }
>>>>> +
>>>>
>>>> This doesn't cover the case where the watchdog is already enabled by the BIOS.
>>>> In that case, WDOG_HW_RUNNING won't be set, and the watchdog will time out
>>>> if the userspace handler is not loaded fast enough. The code should consistently
>>>> set WDOG_HW_RUNNING if the watchdog is running.
>>>>
>>>
>>> As I understand, in the case when bootloader started the watchdog, the
>>> driver just stops it. You can see it in the code you replied to.
>>>
>>>       } else if (!tmr_atboot) {
>>>               s3c2410wdt_stop(&wdt->wdt_device);
>>>
>>> In other words, having "tmr_atboot" module param makes it irrelevant
>>> whether bootloader enabled WDT or no.
>>>
>>
>> Sure, but I am concerned about "if (tmr_atboot && started)", which doesn't
>> set WDOG_HW_RUNNING with your current code, and I was looking for something
>> like
>>
>>          if (tmr_atboot) {
>>                  if (!started) {
>>                          dev_info(dev, "starting watchdog timer\n");
>>                          s3c2410wdt_start(&wdt->wdt_device);
>>                  }
>>                  set_bit(WDOG_HW_RUNNING, &wdt->wdt_device.status);
>>          } else {
>>                  s3c2410wdt_stop(&wdt->wdt_device);
>>          }
>>
> 
> Wow, I really overlooked that case. Nice catch! Not having '} else {'
> section is vicious...
> 
> Though if started != 0, it means s3c2410wdt_set_heartbeat() failed to
> set wdd->timeout, and without that the watchdog core won't be able to
> calculate correctly ping interval in watchdog_next_keepalive(), and
> WDOG_HW_RUNNING bit won't do much good, right? So I'll probably just
> call s3c2410wdt_stop() in that case, to be on the safe side.
> 
> Also this 'started' variable name is misleading, I'll convert it to
> "bool timeout_ok" while at it.
> 

This driver is a mess :-(. "started" true means that the driver doesn't
work as currently written because there is no known valid timeout. In
reality, s3c2410wdt_set_heartbeat() should in that case select a valid
timeout and set it. On top of that, a timeout value out of range should
never be passed to it in the first place. The check for "if (timeout < 1)"
is, in that context, pointless. The range check should happen in
s3c2410wdt_max_timeout(). If that range check is invalid, ie if
s3c2410wdt_set_heartbeat() fails even though the timeout is in the range
of 1 ..s3c2410wdt_max_timeout(), s3c2410wdt_max_timeout() is buggy.

The simplest fix (kludge/hack) might be to fail driver installation if
s3c2410wdt_set_heartbeat() fails.

Guenter