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SubjectRe: [RFC][PATCH] tsc_khz= boot option to avoid TSC calibration variance
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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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