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

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Message-ID: <4f9ed3dd-cf02-eed7-7fdd-38f8dc092eee@linaro.org>
Date:   Fri, 8 Dec 2017 08:52:12 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Benjamin Gaignard <benjamin.gaignard@...aro.org>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ludovic Barre <ludovic.barre@...com>,
        Julien Thierry <julien.thierry@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Arnd Bergmann <arnd@...db.de>, devicetree@...r.kernel.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 4/6] clocksource: stm32: only use 32 bits timers

On 07/12/2017 21:36, Benjamin Gaignard wrote:
> 2017-12-07 17:49 GMT+01:00 Daniel Lezcano <daniel.lezcano@...aro.org>:
>> On 07/12/2017 17:33, Benjamin Gaignard wrote:
>>> 2017-12-07 16:27 GMT+01:00 Daniel Lezcano <daniel.lezcano@...aro.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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