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

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 2/8/20 1:05 AM, Thomas Gleixner wrote:
> Hans,
> 
> Hans de Goede <hdegoede@redhat.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?

That indeed looks nicer, I've prepared (and tested) a v4 with the
suggested change, I'll send out v4 right after this email.

Regards,

Hans