Re: [non-pretimeout,4/7] Watchdog: introduce ARM SBSA watchdog driver

[Fu Wei]

Hi Guenter,

you always can provide help very quickly, thank you very much :-)

On 23 June 2015 at 23:21, Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> On Tue, Jun 23, 2015 at 09:26:35PM +0800, Fu Wei wrote:
>> Hi Guenter,
> [ ...]
>
>> >
>> >> + *       When the first timeout occurs, WS0(SPI or LPI) is triggered,
>> >> + *       the second timeout period(as long as the first timeout period) starts.
>> >
>> > no longer accurate if WOR is used for the second period.
>> >
>> >> + *       In WS0 interrupt routine, panic() will be called for collecting
>> >> + *       crashdown info.
>> >> + *       If system can not recover from WS0 interrupt routine, then second
>> >> + *       timeout occurs, WS1(reset or higher level interrupt) is triggered.
>> >> + *       The two timeout period can be set by WOR(32bit).
>> >
>> > The second timeout period is determined by ...
>> >
>> >> + *       WOR gives a maximum watch period of around 10s at the maximum
>> >> + *       system counter frequency.
>> >> + *       The System Counter shall run at maximum of 400MHz.
>> >
>> > "... at the maximum system counter frequency of 400 MHz.", and drop the
>> > last sentence.
>>
>> For the second timeout period,  I have discussed with a kdump developers,
>> (1)10s maybe not good enough for all the case of panic + kdump, so
>> maybe we still need to use WCV in the second timeout period
>> (2)in the second timeout period, maybe we need to programme WCV for
>> two reason: a, trigger WS1 to reboot system ASAP; b, feed the watchdog
>> without cleanning WS0 flag.
>>
>> WHY we want to feed the watchdog (keepalive) without cleanning WS0 flag??
>> REASON:
>> (1)if the system context is large, we may need to feed the dog until
>> we get all the things backed up.
>> (2)if system goes wrong,  WS0 triggered, then panic--> kdump. if we
>> feed the dog by WRR or programming WOR, WS0 flag will be cleaned. Once
>> system goes wrong again, then panic again.....
>> So this system will be in a panic--kdump--panic--kdump loop, have not
>> chance to reset.
>>
>> So if we are in the second timeout period, we may need to always programme WCV.
>>
> The crashdump kernel is supposed to reload the watchdog driver, which will ping
> the watchdog. If it isn't able to do that in 10 seconds, something is wrong.

yes, 10s maybe not enough for all case.
When I tested kdump on arm64, sometimes , it took 20s. So I am
thinking : can we make the max value of pretimeout > 10s in this
driver.


>
>> >> +
>> >> +     status = readl_relaxed(gwdt->control_base + SBSA_GWDT_WCS);
>> >> +     if (status & SBSA_GWDT_WCS_WS1) {
>> >> +             dev_warn(dev, "System reset by WDT(WCV: %llx)\n",
>> >> +                      sbsa_gwdt_get_wcv(wdd));
>> >
>> > WCV here only tells us how many clock cycles were executed since the
>> > system started (or something like that). So I still don't understand
>> > why it is valuable to print that number.
>>
>> this number provides the time of system reset, I thinks that may help
>> admin to analyse the system failure.
>>
> It doesn't mean anything to anyone but you since it is not in a well defined
> time scale.

maybe I should convert it to second?
I think the original value is better?

> Also, I would be somewhat surprised if WCV would retain its value
> on reset. Much more likely it is the time (in clock cycles) since reset.

yes, It has been mentioned in SBSA:
---------------------
If WS0 is asserted and a timeout refresh occurs then the following must occur:
ï If the system is compliant to SBSA level 0 or level 1 then it is
IMPLEMENTATION DEFINED as to whether the
   compare value is loaded with the sum of the zero-extended watchdog
offset register and the current
  generic timer system count value, or whether it retains its current value.
ï If the system is compliant to SBSA level 2 or higher the compare
value must retain its current value. This
   means that the compare value records the time that WS1 is asserted.
---------------------

Hope I understand it correctly. please let me know , if I
misunderstand something, thanks

>
> Guenter



-- 
Best regards,

Fu Wei
Software Engineer
Red Hat Software (Beijing) Co.,Ltd.Shanghai Branch
Ph: +86 21 61221326(direct)
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