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

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Date:   Thu, 30 Jan 2020 14:43:10 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Hans de Goede <hdegoede@...hat.com>
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,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 3/3] x86/tsc_msr: Make MSR derived TSC frequency more
 accurate

On Thu, Jan 30, 2020 at 12:52:55PM +0100, Hans de Goede wrote:

> This does not matter though, we can model the chain of PLLs as a single
> PLL with a quotient equal to the quotients of all PLLs in the chain
> multiplied.
> 
> So we can create a simplified model of the CPU clock setup using a
> reference clock of 100 MHz plus a quotient which gets us as close to the
> frequency from the DSM as possible.

s/DSM/SDM/ ?

> For the 83.3 MHz example from above this would give us 100 MHz * 5 / 6 =
> 83 and 1/3 MHz, which matches exactly what has been measured on actual hw.
> 
> This commit makes the tsc_msr.c code use a simplified PLL model with a
> reference clock of 100 MHz for all Bay and Cherry Trail models.


> + * Bay Trail SDM MSR_FSB_FREQ frequencies simplified PLL model:
> + *  000:   100 *  5 /  6  =  83.3333 MHz
> + *  001:   100 *  1 /  1  = 100.0000 MHz
> + *  010:   100 *  4 /  3  = 133.3333 MHz
> + *  011:   100 *  7 /  6  = 116.6667 MHz
> + *  100:   100 *  4 /  5  =  80.0000 MHz

> + * Cherry Trail 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 *  7 /  6  = 116.6667 MHz
> + * 0100:   100 *  4 /  5  =  80.0000 MHz
> + * 0101:   100 * 14 / 15  =  93.3333 MHz
> + * 0110:   100 *  9 / 10  =  90.0000 MHz
> + * 0111:   100 *  8 /  9  =  88.8889 MHz
> + * 1000:   100 *  7 /  8  =  87.5000 MHz

> + * Merriefield (BYT MID) SDM MSR_FSB_FREQ frequencies simplified PLL model:
> + * 0001:   100 *  1 /  1  = 100.0000 MHz
> + * 0010:   100 *  4 /  3  = 133.3333 MHz

> + * 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.

Do we want to use the Cherry Trail table (for being the most complete)
for all of them?