linux-kernel - Re: [PATCH] x86, apic: Add apic calibration hook.

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Date:	Tue, 24 Aug 2010 10:06:04 -0700
From:	Alok Kataria <akataria@...are.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86, apic: Add apic calibration hook.

Hi Thomas,

Thanks for taking a look.

On Tue, 2010-08-24 at 06:37 -0700, Thomas Gleixner wrote:
> On Wed, 18 Aug 2010, Alok Kataria wrote:
> >  
> > +static void __init initialize_lapic_clkevt(long delta)
> > +{
> > +	/* Calculate the scaled math multiplication factor */
> > +	lapic_clockevent.mult = div_sc(delta, TICK_NSEC * LAPIC_CAL_LOOPS,
> > +				       lapic_clockevent.shift);
> > +	lapic_clockevent.max_delta_ns =
> > +		clockevent_delta2ns(0x7FFFFF, &lapic_clockevent);
> > +	lapic_clockevent.min_delta_ns =
> > +		clockevent_delta2ns(0xF, &lapic_clockevent);
> > +}
> > +
> >  static int __init calibrate_APIC_clock(void)
> >  {
> > +	apic_calibration_res = x86_init.timers.calibrate_apic();
> > +	if (apic_calibration_res) {
> > +		struct clock_event_device *levt = &__get_cpu_var(lapic_events);
> > +		long delta;
> > +
> > +		if (lapic_clockevent.mult)
> > +			return 0;
> 
>   Why is this necessary ? calibrate_APIC_clock() is only called once.
> 

Yes, though timer.calibrate_apci() could behave differently on different
platforms. For eg. the vmware_calibrate_apic doesn't initialize any of
the lapic_clockevent state and we need this to set it up in that case.

Thanks,
Alok

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