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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Feng Tang <feng.tang@intel.com>
Cc:     Bjorn Helgaas <helgaas@kernel.org>, Ingo Molnar <mingo@redhat.com>,
        Borislav Petkov <bp@alien8.de>, x86@kernel.org,
        linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
        Bjorn Helgaas <bhelgaas@google.com>,
        Kai-Heng Feng <kai.heng.feng@canonical.com>,
        rui.zhang@intel.com, len.brown@intel.com
Subject: Re: [PATCH] x86/PCI: Convert force_disable_hpet() to standard quirk
In-Reply-To: <20201127061131.GB105524@shbuild999.sh.intel.com>
References: <20201119181904.149129-1-helgaas@kernel.org> <87v9dtk3j4.fsf@nanos.tec.linutronix.de> <20201126012421.GA92582@shbuild999.sh.intel.com> <87eekfk8bd.fsf@nanos.tec.linutronix.de> <20201127061131.GB105524@shbuild999.sh.intel.com>
Date:   Mon, 30 Nov 2020 20:21:03 +0100
Message-ID: <87eekairc0.fsf@nanos.tec.linutronix.de>
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Feng,

On Fri, Nov 27 2020 at 14:11, Feng Tang wrote:
> On Fri, Nov 27, 2020 at 12:27:34AM +0100, Thomas Gleixner wrote:
>> On Thu, Nov 26 2020 at 09:24, Feng Tang wrote:
>> Yes, that can happen. But OTOH, we should start to think about the
>> requirements for using the TSC watchdog.
>> 
>> I'm inclined to lift that requirement when the CPU has:
>> 
>>     1) X86_FEATURE_CONSTANT_TSC
>>     2) X86_FEATURE_NONSTOP_TSC
>
>>     3) X86_FEATURE_NONSTOP_TSC_S3
> IIUC, this feature exists for several generations of Atom platforms,
> and it is always coupled with 1) and 2), so it could be skipped for
> the checking.

Yes, we can ignore that bit as it's not widely available and not
required to solve the problem.

>>     4) X86_FEATURE_TSC_ADJUST
>>     
>>     5) At max. 4 sockets
>> 
>> The only reason I hate to disable HPET upfront at least during boot is
>> that HPET is the best mechanism for the refined TSC calibration. PMTIMER
>> sucks because it's slow and wraps around pretty quick.
>> 
>> So we could do the following even on platforms where HPET stops in some
>> magic PC? state:
>> 
>>   - Register it during early boot as clocksource
>> 
>>   - Prevent the enablement as clockevent and the chardev hpet timer muck
>> 
>>   - Prevent the magic PC? state up to the point where the refined
>>     TSC calibration is finished.
>> 
>>   - Unregister it once the TSC has taken over as system clocksource and
>>     enable the magic PC? state in which HPET gets disfunctional.
>
> This looks reasonable to me. 
>
> I have thought about lowering the hpet rating to lower than PMTIMER, so it
> still contributes in early boot phase, and fades out after PMTIMER is
> initialised.

Not a good idea. pm_timer is initialized before the refined calibration
finishes.

Thanks,

        tglx