linux-kernel - Re: [PATCH v4 7/7] clocksource: mips-gic-timer: Mark GIC timer as unstable if ref clock changes

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Message-ID: <20200521142219.ejsl4y5eaqqfsp5k@mobilestation>
Date:   Thu, 21 May 2020 17:22:19 +0300
From:   Serge Semin <Sergey.Semin@...kalelectronics.ru>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
CC:     Serge Semin <fancer.lancer@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        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>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@...r.kernel.org>,
        <linux-rtc@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Paul Cercueil <paul@...pouillou.net>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Randy Dunlap <rdunlap@...radead.org>,
        Claudiu Beznea <claudiu.beznea@...rochip.com>,
        Maarten ter Huurne <maarten@...ewalker.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 7/7] clocksource: mips-gic-timer: Mark GIC timer as
 unstable if ref clock changes

On Thu, May 21, 2020 at 11:09:50AM +0200, Geert Uytterhoeven wrote:
> Hi Serge,
> 
> On Thu, May 21, 2020 at 2:54 AM Serge Semin
> <Sergey.Semin@...kalelectronics.ru> wrote:
> > Currently clocksource framework doesn't support the clocks with variable
> > frequency. Since MIPS GIC timer ticks rate might be unstable on some
> > platforms, we must make sure that it justifies the clocksource
> > requirements. MIPS GIC timer is incremented with the CPU cluster reference
> > clocks rate. So in case if CPU frequency changes, the MIPS GIC tick rate
> > changes synchronously. Due to this the clocksource subsystem can't rely on
> > the timer to measure system clocks anymore. This commit marks the MIPS GIC
> > based clocksource as unstable if reference clock (normally it's a CPU
> > reference clocks) rate changes. The clocksource will execute a watchdog
> > thread, which lowers the MIPS GIC timer rating to zero and fallbacks to a
> > new stable one.
> >
> > Note we don't need to set the CLOCK_SOURCE_MUST_VERIFY flag to the MIPS
> > GIC clocksource since normally the timer is stable. The only reason why
> > it gets unstable is due to the ref clock rate change, which event we
> > detect here in the driver by means of the clocks event notifier.
> >
> > Signed-off-by: Serge Semin <Sergey.Semin@...kalelectronics.ru>
> 
> Thanks for your patch!
> 
> > --- a/drivers/clocksource/mips-gic-timer.c
> > +++ b/drivers/clocksource/mips-gic-timer.c
> > @@ -24,6 +24,9 @@
> >  static DEFINE_PER_CPU(struct clock_event_device, gic_clockevent_device);
> >  static int gic_timer_irq;
> >  static unsigned int gic_frequency;
> > +static bool __read_mostly gic_clock_unstable;
> > +
> > +static void git_clocksource_unstable(char *reason);
> 
> gic_clocksource_unstable? (everywhere)

This is the most used word lately. So my hands write git everywhere by itself.)
Thanks for noticing this.

-Sergey

> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 
> -- 
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org
> 
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds