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

[Serge Semin]

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@xxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxx>
> 
> 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@xxxxxxxxxxxxxx
> 
> 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