linux-kernel - Re: [PATCH v3] x86: tsc: make TSC calibration immune to interrupts

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Message-ID: <alpine.LFD.2.02.1105260021570.3078@ionos>
Date:	Thu, 26 May 2011 00:23:38 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Kasper Pedersen <kkp2010@...perkp.dk>
cc:	linux-kernel@...r.kernel.org,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	John Stultz <johnstul@...ibm.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Josh Triplett <josh@...htriplett.org>
Subject: Re: [PATCH v3] x86: tsc: make TSC calibration immune to interrupts

On Thu, 26 May 2011, Kasper Pedersen wrote:

> On 05/25/2011 12:51 PM, Thomas Gleixner wrote:
> >> The 8 additional samples costs us 28 microseconds in startup
> >> time.
> > 
> > That's a good reason to avoid the whole conditional thing and just do
> > the best of 5 always.
> 
> I do not mind removing it again.
> The additional 170us only happen when quick_pit_calibrate fails,
> and then native_calibrate_tsc burns 30ms extra anyway.

Yes, not worth the extra conditional.
 
> >> +
> >> +	*p = tp;
> > 
> > The value is completely uninteresting when we return ULLONG_MAX.
> > 
> 
> native_calibrate_tsc() depends on *p being written regardless of
> whether we can get a SMI-free reading:
> 
> 	/* We don't have an alternative source, disable TSC */
> 	if (!hpet && !ref1 && !ref2) {
> 		printk("TSC: No reference (HPET/PMTIMER) available\n");
> 		return 0;
> 	}
> 
> this works since acpi_pm_read_early() returns 0 when pmtmr_ioport 
> is 0, or CONFIG_X86_PM_TIMER is not set.
> 
> Without it we would report "calibration failed" rather than 
> "No reference" or "Using PIT" when there is neither pmtimer nor 
> hpet.

Fair enough. So a comment might be in order :) That code is fricking
complex just to figure out on which frequency that damn thing runs :(

Thanks,

	tglx
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