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

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Date:   Thu, 19 Jul 2018 16:46:43 -0400
From:   Pavel Tatashin <pasha.tatashin@...cle.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     peterz@...radead.org, Steven Sistare <steven.sistare@...cle.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        linux@...linux.org.uk, schwidefsky@...ibm.com,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        John Stultz <john.stultz@...aro.org>, sboyd@...eaurora.org,
        x86@...nel.org, LKML <linux-kernel@...r.kernel.org>,
        mingo@...hat.com, hpa@...or.com, douly.fnst@...fujitsu.com,
        prarit@...hat.com, feng.tang@...el.com,
        Petr Mladek <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 07/19/2018 04:44 PM, Thomas Gleixner wrote:
> On Thu, 19 Jul 2018, Pavel Tatashin wrote:
> 
>> On Thu, Jul 19, 2018 at 12:49 PM Pavel Tatashin
>> <pasha.tatashin@...cle.com> wrote:
>>>
>>>> So create two functions. native_...early..() and native....(). The early
>>>> one does not contain the hpet/pmtimer stuff and it replaces the ops.pointer
>>>> with the late one which contains all of it.
>>>
>>> Good idea. Actually, the late one will contain only hpet/pmtimer and I
>>> will set it only if tsc frequency was not determined only.
>>
>> If we determined tsc early in boot using one of the quick methods:
>> from cpuid/msr/quick_pit, can we assume that frequencies of all other
>> CPUs will be determined the same way? Or do we still have to fallback
>> to PIT/HPET/PMTIMER? I wondering if we support heterogeneous
>> multi-socket platforms with different CPUs, because that the only
>> platforms where I see such scenario is possible.
> 
> The frequency for secondary CPUs is usually taken from the boot CPU and the
> only reason why recalibration can happen is when the CPU does not have a
> constant frequency TSC.
> 
> For that case the quick PIT + hpet/pmtimer calibration bundle is
> required. So yes, the early calibration might work with quick PIT (those
> CPUs definitely do not have MSR/CPUID based calibration), but the
> recalibration might fail the quick PIT calibration for various reasons.

OK, good, I implemented with this in-mind. I will send out a new series shortly.

Thank you,
Pavel

> 
> Thanks,
> 
> 	tglx
>