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

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Date:   Thu, 19 Jul 2018 12:33:40 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Pavel Tatashin <pasha.tatashin@...cle.com>
Cc:     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, tglx@...utronix.de, 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 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.