linux-kernel - Re: [PATCH v3 7/7] clocksource: mips-gic-timer: Set limitations on clocksource/sched-clocks usage

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Date:   Mon, 18 May 2020 15:59:16 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>
Cc:     Serge Semin <fancer.lancer@...il.com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Paul Burton <paulburton@...nel.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Arnd Bergmann <arnd@...db.de>,
        Rob Herring <robh+dt@...nel.org>, linux-mips@...r.kernel.org,
        linux-rtc@...r.kernel.org, devicetree@...r.kernel.org,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 7/7] clocksource: mips-gic-timer: Set limitations on
 clocksource/sched-clocks usage

On 16/05/2020 14:16, Serge Semin wrote:
> Hello Daniel,
> 
> Thanks for your comment. My response is below.
> 
> On Fri, May 15, 2020 at 07:10:04PM +0200, Daniel Lezcano wrote:
>> On Thu, May 07, 2020 at 12:41:07AM +0300, Serge Semin wrote:
>>> Currently neither clocksource nor scheduler clock kernel framework
>>> support the clocks with variable frequency. Needless to say how many
>>> problems may cause the sudden base clocks frequency change. In a
>>> simplest case the system time will either slow down or speed up.
>>> Since on CM2.5 and earlier MIPS GIC timer is synchronously clocked
>>> with CPU we must set some limitations on using it for these frameworks
>>> if CPU frequency may change. First of all it's not safe to have the
>>> MIPS GIC used for scheduler timings. So we shouldn't proceed with
>>> the clocks registration in the sched-subsystem. Secondly we must
>>> significantly decrease the MIPS GIC clocksource rating. This will let
>>> the system to use it only as a last resort.
>>>
>>> Note CM3.x-based systems may also experience the problems with MIPS GIC
>>> if the CPU-frequency change is activated for the whole CPU cluster
>>> instead of using the individual CPC core clocks divider.
>>
>> May be there is no alternative but the code looks a bit hacksih. Isn't possible
>> to do something with the sched_mark_unstable?
>>
>> Or just not use the timer at all ?
> 
> Not using the timer might be better, but not that good alternative either
> especially in our case due to very slow external timer. Me and Thomas
> Bogendoerfer discussed the similar commit I've provided to the csrc-r4k driver
> available on MIPS:
> https://lkml.org/lkml/2020/5/11/576
> 
> To cut it short, you are right. The solution with using clocksource_mark_unstable()
> is better alternative spied up in x86 tsc implementation. I'll use a similar
> approach here and submit the updated patch in v3.
> 
> Could you please proceed with the rest of the series review? I'd like to send
> the next version with as many comments taken into account as possible. The
> patchset has been submitted a while ago, but except Rob noone have had any
> comments.(

For me other patches are ok.

I can apply patches 1, 2, 4, 5, 6

Will remain patches 3 et 7


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