linux-kernel - RE: [PATCH] clocksource: arch_arm_timer: Fix timecounter initialization

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Message-ID: <bce193f3fc264abf82ff52ef69f0b86a@BY2PR03MB505.namprd03.prod.outlook.com>
Date:	Thu, 12 Jun 2014 07:45:36 +0000
From:	"Li.Xiubo@...escale.com" <Li.Xiubo@...escale.com>
To:	Stephen Boyd <sboyd@...eaurora.org>
CC:	Marc Zyngier <Marc.Zyngier@....com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: RE: [PATCH] clocksource: arch_arm_timer: Fix timecounter
 initialization

> >  drivers/clocksource/arm_arch_timer.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/clocksource/arm_arch_timer.c
> b/drivers/clocksource/arm_arch_timer.c
> > index 5163ec1..6c3cfd8 100644
> > --- a/drivers/clocksource/arm_arch_timer.c
> > +++ b/drivers/clocksource/arm_arch_timer.c
> > @@ -426,7 +426,7 @@ struct timecounter *arch_timer_get_timecounter(void)
> >
> >  static void __init arch_counter_register(unsigned type)
> >  {
> > -	u64 start_count;
> > +	u64 start_count, start_ns;
> >
> >  	/* Register the CP15 based counter if we have one */
> >  	if (type & ARCH_CP15_TIMER)
> > @@ -438,7 +438,8 @@ static void __init arch_counter_register(unsigned type)
> >  	clocksource_register_hz(&clocksource_counter, arch_timer_rate);
> >  	cyclecounter.mult = clocksource_counter.mult;
> >  	cyclecounter.shift = clocksource_counter.shift;
> > -	timecounter_init(&timecounter, &cyclecounter, start_count);
> > +	start_ns = cyclecounter_cyc2ns(&cyclecounter, start_count);
> > +	timecounter_init(&timecounter, &cyclecounter, start_ns);
> >
> >  	/* 56 bits minimum, so we assume worst case rollover */
> >  	sched_clock_register(arch_timer_read_counter, 56, arch_timer_rate);
> 
> This doesn't make much sense. We're converting start_count, which could
> be a very large number, into nanoseconds. It looks like we're assuming
> the counter always starts at 0 which is not always true if you sit in a
> bootloader for a long time. 

Yes, the counter here may usually start counting at bootloader time from zero.


> Perhaps it would be better to just do
> 
>     timecounter_init(&timecounter, &cyclecounter, 0);
> 
> or
> 
>     timecounter_init(&timecounter, &cyclecounter,
> ktime_to_ns(ktime_get_real()));
> 

Here the ktime_get_real() will always return 0, before setting the system clock,
Like:

"rtc-ds3232 1-0068: setting system clock to 2014-06-12 13:01:24 UTC (1402578084)"

And if so, why not just set it to 0 ? And by the way, 0 is must here ?

Thanks,
BRs
Xiubo
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