linux-kernel - Re: [PATCH v3 3/3] x86/tsc_msr: Make MSR derived TSC frequency more accurate

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Message-ID: <87eev67xkc.fsf@nanos.tec.linutronix.de>
Date:   Sat, 08 Feb 2020 01:05:39 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Hans de Goede <hdegoede@...hat.com>,
        Andy Shevchenko <andy@...radead.org>,
        Ingo Molnar <mingo@...hat.com>
Cc:     Hans de Goede <hdegoede@...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>,
        Rahul Tanwar <rahul.tanwar@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Gayatri Kammela <gayatri.kammela@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v3 3/3] x86/tsc_msr: Make MSR derived TSC frequency more accurate

Hans,

Hans de Goede <hdegoede@...hat.com> writes:
> @@ -120,11 +180,23 @@ unsigned long cpu_khz_from_msr(void)
>  	rdmsr(MSR_FSB_FREQ, lo, hi);
>  	index = lo & freq_desc->mask;
>  
> -	/* Map CPU reference clock freq ID(0-7) to CPU reference clock freq(KHz) */
> -	freq = freq_desc->freqs[index];
> -
> -	/* TSC frequency = maximum resolved freq * maximum resolved bus ratio */
> -	res = freq * ratio;
> +	/*
> +	 * Note this also catches cases where the index points to an unpopulated
> +	 * part of muldiv, in that case the else will set freq and res to 0.
> +	 */
> +	if (freq_desc->muldiv[index].divider) {
> +		freq = DIV_ROUND_CLOSEST(TSC_REFERENCE_KHZ *
> +					   freq_desc->muldiv[index].multiplier,
> +					 freq_desc->muldiv[index].divider);
> +		/* Multiply by ratio before the divide for better accuracy */
> +		res = DIV_ROUND_CLOSEST(TSC_REFERENCE_KHZ *
> +					   freq_desc->muldiv[index].multiplier *
> +					   ratio,
> +					freq_desc->muldiv[index].divider);

What about:

        struct muldiv *md = &freq_desc->muldiv[index];

        if (md->divider) {
		tscref = TSC_REFERENCE_KHZ * md->multiplier;
        	freq = DIV_ROUND_CLOSEST(tscref, md->divider);
		/*
                 * Multiplying by ratio before the division has better
                 * accuracy than just calculating freq * ratio
                 */
                res = DIV_ROUND_CLOSEST(tscref * ratio, md->divider);

Hmm?

Thanks,

        tglx