linux-kernel - Re: [PATCH v2] x86: Reduce clock calibration time during slave cpu startup

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Message-ID: <20110805104635.GB13055@elte.hu>
Date:	Fri, 5 Aug 2011 12:46:35 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jack Steiner <steiner@....com>
Cc:	tglx@...utronix.de, davej@...hat.com, yinghan@...gle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86: Reduce clock calibration time during slave cpu
 startup


* Jack Steiner <steiner@....com> wrote:

> +/*
> + * Check if another cpu is in the same socket and has already been calibrated.
> + * If found, use the previous value. This assumes all cores in the same physical
> + * socket have the same core frequency.
> +
> +unsigned long __cpuinit calibrate_delay_is_known(void)
> +{
> +	int i, cpu = smp_processor_id();
> +
> +	if (!tsc_disabled && !cpu_has(&cpu_data(cpu), X86_FEATURE_CONSTANT_TSC))
> +		return 0;
> +
> +	for_each_online_cpu(i)
> +		if (cpu_data(i).phys_proc_id == cpu_data(cpu).phys_proc_id)
> +			return cpu_data(i).loops_per_jiffy;

Hm, why do we have to make such an assumption? Cannot we query the 
core frequency?

Thanks,

	Ingo
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