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

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Message-ID: <20200515171004.GA760381@linaro.org>
Date:   Fri, 15 May 2020 19:10:04 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>
Cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Serge Semin <fancer.lancer@...il.com>,
        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 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 ?