Re: [PATCH v8 4/6] clocksource: stm32: only use 32 bits timers

[Daniel Lezcano <daniel.lezcano-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>]

On 07/12/2017 21:36, Benjamin Gaignard wrote:
> 2017-12-07 17:49 GMT+01:00 Daniel Lezcano <daniel.lezcano-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>:
>> On 07/12/2017 17:33, Benjamin Gaignard wrote:
>>> 2017-12-07 16:27 GMT+01:00 Daniel Lezcano <daniel.lezcano-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>:
>>>> On 14/11/2017 09:52, Benjamin Gaignard wrote:
>>>>> The clock driving counters is at 90MHz so the maximum period
>>>>> for 16 bis counters is around 750 ms which is a short period
>>>>> for a clocksource.
>>>>
>>>> Isn't it 728us ?
>>>
>>> yes it is: 2^16 / 90.000.000 => 728us
>>
>> Ok, now I can do the connection with the previous patch.
>>
>> So, the real issue of all this is the 16bits clocksource is wrapping up
>> every 728us, hence the clockevent periodically expires every ~728us to
>> keep the timekeeping consistent. Unfortunately, the kernel has a too
>> high overhead for this as the system is consistently processing this
>> timer leading to a CPU time resource starvation.
>>
>> Is that correct ?
> 
> Yes that is correct

Oh man. That was unclear since the beginning, we are not talking about
inaccurate clocksource or whatever but just these 16bits timers can't
work on Linux.


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