linux-kernel - Re: [PATCH 1/2] x86/tsc: Allow quick PIT calibration despite interruptions

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Message-ID: <alpine.DEB.2.21.1901292327090.1950@nanos.tec.linutronix.de>
Date:   Tue, 29 Jan 2019 23:28:45 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jan H. Schönherr <jan@...nhrr.de>
cc:     bp@...en8.de, mingo@...hat.com, x86@...nel.org,
        pmenzel@...gen.mpg.de, Thomas.Lendacky@....com, hpa@...or.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] x86/tsc: Allow quick PIT calibration despite
 interruptions

On Tue, 29 Jan 2019, Jan H. Schönherr wrote:

> Am 29.01.2019 um 11:23 schrieb Jan H. Schönherr:
> > +calibrate:
> > +	/*
> > +	 * Extrapolate the error and fail fast if the error will
> > +	 * never be below 500 ppm.
> > +	 */
> > +	if (d1 + d1 >= (delta * MAX_QUICK_PIT_ITERATIONS) >> 11) {
> > +		pr_info("Fast TSC calibration failed (wouldn't work)\n");
> > +		return 0;
> > +	}
> 
> I messed this check up. "delta" is not the actual tsc delta between the
> first two successful reads at this point.
> 
> (If it were, it might also correspond to more than one iteration; not
> sure if we care about that aspect, though.)

I rather go for correct and if only for the reason that me/you/whoelse
doesn't have to scratch the head for an hour when looking at this half a
year from now.

Thanks,

	tglx