linux-kernel - Re: [RFC][PATCH] tsc_khz= boot option to avoid TSC calibration variance

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Date:	Fri, 08 May 2009 12:02:52 -0700
From:	john stultz <johnstul@...ibm.com>
To:	George Spelvin <linux@...izon.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, tglx@...utronix.de,
	ulrich.windl@...uni-regensburg.de, williams@...hat.com,
	zippel@...ux-m68k.org
Subject: Re: [RFC][PATCH] tsc_khz= boot option to avoid TSC calibration 
 variance

On Fri, 2009-05-08 at 08:19 -0400, George Spelvin wrote:
> > To mitigate this, I wanted to provide a tsc_khz= boot option. This would
> > allow users to set the tsc_khz value at boot-up, assuming they are
> > within 1Mhz of the calibrated value (to protect against bad values).
> > Once the tsc_khz value is set in grub, the box will always boot with the
> > same value, so the NTP drift value prior to reboot will still be correct
> > after rebooting.
> 
> A run-time adjustable would be more convenient, but this is simple and works.
> 
> The 1 MHz tolerance, however, isn't implemented right.  I can't quote
> figure out what you were trying to do; was that (x + (x/2))/1000 trying
> to round the /1000 properly?  That should be (x + 1000/2)/1000 in that
> case, which I normally write as (x/500 + 1)/2;
> 
> But in any case, no equality comparison can possibly work; there's
> always a case where the measured value rounds to k and the correct
> value rounds to k+1.  Also, 1 MHz is a pretty wide tolerance on
> sub-GHz processors.  I'd suggest the following:

Ah. Indeed you're right. Thanks for catching this.
Your version is much better.

-john

> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index d57de05..d7ab640 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -825,15 +825,42 @@ static void __init init_tsc_clocksource(void)
>  	clocksource_register(&clocksource_tsc);
>  }
> 
> +unsigned long tsc_khz_specified;
> +static int __init tsc_khz_specified_setup(char *str)
> +{
> +	tsc_khz_specified = simple_strtoul(str, NULL, 0);
> +	return 1;
> +}
> +
> +__setup("tsc_khz=", tsc_khz_specified_setup);
> +
> +
>  void __init tsc_init(void)
>  {
>  	u64 lpj;
>  	int cpu;
> +	long difference;
> 
>  	if (!cpu_has_tsc)
>  		return;
> 
>  	tsc_khz = calibrate_tsc();
> +
> +	/*
> *	 * If the calibrated TSC freq and user specified TSC freq
> *	 * are close enough, pick the what the user told us.  Reject
> *	 * obviously bogus values to make the option safe to use.
> +	 */
> +	difference = tsc_khz - tsc_khz_specified;
> +	if (difference < 0)
> +		difference = -difference;
> +	if (difference <= tsc_khz >> 10) {	/* 1/1024 = 976 ppm */
> +		printk(KERN_INFO "Using user defined TSC freq: %lu.%03lu MHz\n",
> +			tsc_khz_specified/1000,
> +			tsc_khz_specified%1000);
> +		tsc_khz = tsc_khz_specified;
> +	}
> +
>  	cpu_khz = tsc_khz;
> 
>  	if (!tsc_khz) {

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