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

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?

Thanks,

tglx