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

[Thomas Gleixner]

Hans,

Hans de Goede <hdegoede@xxxxxxxxxx> 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