linux-kernel - Re: [PATCH 3/3] x86/tsc_msr: Make MSR derived TSC frequency more accurate

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Message-ID: <8a8fd7a1-945e-4541-f0bc-387fae7c6822@redhat.com>
Date:   Thu, 30 Jan 2020 17:04:15 +0100
From:   Hans de Goede <hdegoede@...hat.com>
To:     David Laight <David.Laight@...LAB.COM>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Andy Shevchenko <andy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Vipul Kumar <vipulk0511@...il.com>,
        Vipul Kumar <vipul_kumar@...tor.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Srikanth Krishnakar <Srikanth_Krishnakar@...tor.com>,
        Cedric Hombourger <Cedric_Hombourger@...tor.com>,
        Len Brown <len.brown@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH 3/3] x86/tsc_msr: Make MSR derived TSC frequency more
 accurate

Hi,

On 30-01-2020 17:02, David Laight wrote:
> From: Hans de Goede
>> Sent: 30 January 2020 15:55
> ...
>>>> + * Moorefield (CHT MID) SDM MSR_FSB_FREQ frequencies simplified PLL model:
>>>> + * 0000:   100 *  5 /  6  =  83.3333 MHz
>>>> + * 0001:   100 *  1 /  1  = 100.0000 MHz
>>>> + * 0010:   100 *  4 /  3  = 133.3333 MHz
>>>> + * 0011:   100 *  1 /  1  = 100.0000 MHz
>>>
>>> Unless I'm going cross-eyed, that's 4 times the exact same table.
>>
>> Correct, except that the not listed values on the none Cherry Trail
>> table are undefined in the SDM, so we should probably deny them
>> (or as the old code was doing simply return 0).
>>
>> And at least the Moorefield (CHT MID) table is different for 0011, that
>> is again 100 MHz like 0001 instead of 116.6667 as it is for BYT and CHT.
>>
>> Note that the Merriefield (BYT MID) and Moorefield (CHT MID) values are
>> based on the old code I've not seen those values in the current latest
>> version of the SDM.
> 
> I wonder if Moorefield:11 is an old typo?

I have no idea. Andy if you can find any docs on the MSR_FSB_FREQ values
for Merriefield (BYT MID) and Moorefield (CHT MID) that would be great,
if not I suggest we stick with what we have.

Regards,

Hans