linux-kernel - Re: [PATCH v14 20/25] x86/tsc: calibrate tsc only once

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Message-ID: <alpine.DEB.2.21.1807191300150.1602@nanos.tec.linutronix.de>
Date:   Thu, 19 Jul 2018 13:01:22 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Pavel Tatashin <pasha.tatashin@...cle.com>,
        steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        linux@...linux.org.uk, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, john.stultz@...aro.org,
        sboyd@...eaurora.org, x86@...nel.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, hpa@...or.com, douly.fnst@...fujitsu.com,
        prarit@...hat.com, feng.tang@...el.com, pmladek@...e.com,
        gnomes@...rguk.ukuu.org.uk, linux-s390@...r.kernel.org,
        boris.ostrovsky@...cle.com, jgross@...e.com, pbonzini@...hat.com
Subject: Re: [PATCH v14 20/25] x86/tsc: calibrate tsc only once

On Thu, 19 Jul 2018, Peter Zijlstra wrote:
> On Tue, Jul 17, 2018 at 10:22:06PM -0400, Pavel Tatashin wrote:
> > During boot tsc is calibrated twice: once in tsc_early_delay_calibrate(),
> > and the second time in tsc_init().
> > 
> > Rename tsc_early_delay_calibrate() to tsc_early_init(), and rework it so
> > the calibration is done only early, and make tsc_init() to use the values
> > already determined in tsc_early_init().
> > 
> > Sometimes it is not possible to determine tsc early, as the subsystem that
> > is required is not yet initialized, in such case try again later in
> > tsc_init().
> 
> It might be nice to preserve some of the information tglx dug out during
> review of all this. Like the various methods of calibrate_*() and their
> dependencies.
> 
> And I note that this patch relies on the magic of native_calibrate_cpu()
> working really early and not exploding in the quick calibration run.
> This either wants fixing or documenting.
> 
> I think the initial idea was to only do the fast_calibrate (cpuid, msr
> and possibly the quick_pit) things early and delay the HPET/PMTIMER
> magic until later.

Yes. I really would prefer to have this as an explicit expressed mechanism
rather than relying on magic variables not being initialized.

Thanks

	tglx