linux-kernel - Re: [PATCH 2/2] ARM: omap: timers: reduce rating of gp

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Date:   Fri, 2 Dec 2016 08:47:14 -0800
From:   Tony Lindgren <tony@...mide.com>
To:     Grygorii Strashko <grygorii.strashko@...com>
Cc:     Keerthy <j-keerthy@...com>, t-kristo@...com, linux@...linux.org.uk,
        linux-omap@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Dave Gerlach <d-gerlach@...com>
Subject: Re: [PATCH 2/2] ARM: omap: timers: reduce rating of gp_timer
 clocksource

* Grygorii Strashko <grygorii.strashko@...com> [161129 08:43]:
> 
> 
> On 11/24/2016 12:19 AM, Keerthy wrote:
> > From: Grygorii Strashko <grygorii.strashko@...com>
> > 
> > Now ARM Global timer (rating 300) will not be selected as clocksource,
> > because it's initialized after OMAP GP Timer (rating 300) and
> > Timekeeping core will not allow to replace clocksource with new one if
> > both of them have the same rating.
> > 
> > Reduce rating of OMAP GP Timer (300->290) when it's used as
> > clocksource device - this will allow to select ARM Global timer (300)
> > as clocksource when enabled.
> > 
> > Signed-off-by: Grygorii Strashko <grygorii.strashko@...com>
> > Signed-off-by: Dave Gerlach <d-gerlach@...com>
> > Signed-off-by: Keerthy <j-keerthy@...com>
> 
> Unfortunately, this patch has dependency [1] and can't be used alone as
> it will cause ARM Global timer to be selected as clocksource
> always on am437x and this will kill cpuidle, because ARM Global timer
> is not in always_on domain.
> 
> The intention of enabling ARM Global timer is only for non-pm aware use
> cases for RT-kernel latency improvement - where deep cpuidle states are not
> enabled.

Yeah we need to fix up things to be able to change the clocksource
in addition to clockevent. However, currently only cpuidle_coupled
knows when the whole system is idle, so quite a bit of work is
needed to do that in a sane way.

What about the first patch in this series?

Regards,

Tony


> [1] https://patchwork.kernel.org/patch/8940051/
> 
> > ---
> >  arch/arm/mach-omap2/timer.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm/mach-omap2/timer.c b/arch/arm/mach-omap2/timer.c
> > index b2f2448..a0dbb0b 100644
> > --- a/arch/arm/mach-omap2/timer.c
> > +++ b/arch/arm/mach-omap2/timer.c
> > @@ -376,7 +376,7 @@ static cycle_t clocksource_read_cycles(struct clocksource *cs)
> >  }
> > 
> >  static struct clocksource clocksource_gpt = {
> > -	.rating		= 300,
> > +	.rating		= 290,
> >  	.read		= clocksource_read_cycles,
> >  	.mask		= CLOCKSOURCE_MASK(32),
> >  	.flags		= CLOCK_SOURCE_IS_CONTINUOUS,
> > 
> 
> -- 
> regards,
> -grygorii