Re: [PATCH v2 3/3] time: clocksource: Add a comment to CLOCK_SOURCE_SUSPEND_NONSTOP

[Xunlei Pang <pang.xunlei@linaro.org>]

Hi Thomas,

On 25 January 2015 at 01:07, Thomas Gleixner <tglx@linutronix.de> wrote:
> On Sat, 24 Jan 2015, Xunlei Pang wrote:
>
>> Before this, I tried to add some code to catch such problem at the
>> time of registering the clocksource, like using the
>> CLOCKSOURCE_MASK(), for example 64bit counter will never wrap for
>> us. But there may be other values like CLOCKSOURCE_MASK(56), I just
>> can't figure out exactly how to do this judge.
>
> I don't think there is a good way to do so. Registration time is the
> wrong place anyway because the problem depends on:
>
>  - The width of the counter
>  - The frequency of the counter
>
> The frequency of the counter might even change after registration. Now
> add the unknown duration of the suspend to the picture and you're
> completely lost.
>
> All we can do is provide information about the actual wraparound time,
> if the CLOCK_SOURCE_SUSPEND_NONSTOP flag is set and the wraparound
> time is less than some reasonable margin.
>

Yes, we can only deal with it approximately. How about this?

1) Add a new member about reference wraparound time(max system suspend
period allowed) to struct  clocksource. In
__clocksource_updatefreq_scale(), we can use  "sec" which already
applys 12.5% margin as its value.

2) Add a new tuneable sysctl threshold with a default time period
value(for example, 365 days)
    We can also printk its value when booting or changing its value to
notice people about this.

3) then,  in timekeeping_resume(), we can compare the reference
wraparound of the nonstop clocksource with the sysctl threshold to
decide if it is dependable to use.

Thanks,
Xunlei

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