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

[Geert Uytterhoeven]

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)

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