[Xen-devel] [PATCH 2/4] x86/time: reduce rounding errors in calculations

[Jan Beulich <jbeulich@xxxxxxxx>]

Plain (unsigned) integer division simply truncates the results. The
overall errors are smaller though if we use proper rounding. (Extend
this to the purely cosmetic aspect of time.c's freq_string(), which
before this change I've frequently observed to report e.g. NN.999MHz
HPET clock speeds.)

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
We could switch at least the first div/rem pair in calibrate_APIC_clock()
to use do_div(), but this would imply switching bus_freq (and then also
result) to unsigned int (as the divisor has to be 32-bit). While I think
there's pretty little risk for bus frequencies to go beyond 4GHz in the
next so many years, I still wasn't certain enough this would be a welcome
change.

--- a/xen/arch/x86/apic.c
+++ b/xen/arch/x86/apic.c
@@ -1261,7 +1261,9 @@ static int __init calibrate_APIC_clock(v
     /* set up multipliers for accurate timer code */
     bus_freq   = result*HZ;
     bus_cycle  = (u32) (1000000000000LL/bus_freq); /* in pico seconds */
+    bus_cycle += (1000000000000UL % bus_freq) * 2 > bus_freq;
     bus_scale  = (1000*262144)/bus_cycle;
+    bus_scale += ((1000 * 262144) % bus_cycle) * 2 > bus_cycle;
 
     apic_printk(APIC_VERBOSE, "..... bus_scale = %#x\n", bus_scale);
     /* reset APIC to zero timeout value */
--- a/xen/arch/x86/hpet.c
+++ b/xen/arch/x86/hpet.c
@@ -799,9 +799,9 @@ u64 __init hpet_setup(void)
     hpet_resume(hpet_boot_cfg);
 
     hpet_rate = 1000000000000000ULL; /* 10^15 */
-    (void)do_div(hpet_rate, hpet_period);
+    last = do_div(hpet_rate, hpet_period);
 
-    return hpet_rate;
+    return hpet_rate + (last * 2 > hpet_period);
 }
 
 void hpet_resume(u32 *boot_cfg)
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -275,7 +275,10 @@ static char *freq_string(u64 freq)
 {
     static char s[20];
     unsigned int x, y;
-    y = (unsigned int)do_div(freq, 1000000) / 1000;
+
+    if ( do_div(freq, 1000) > 500 )
+        ++freq;
+    y = (unsigned int)do_div(freq, 1000);
     x = (unsigned int)freq;
     snprintf(s, sizeof(s), "%u.%03uMHz", x, y);
     return s;


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