[Xen-devel] [PATCH 3/4] x86/APIC: reduce rounding errors in calculations

[Jan Beulich <jbeulich@xxxxxxxx>]

Dividing by HZ/10 just to subsequently multiply by HZ again in all uses
of the respective variable is pretty pointlessly introducing rounding
(really: truncation) errors. While transforming the respective
expressions it became apparent that "result" would be left unused except
for its use as function return value. As the sole caller of the function
doesn't look at the returned value, simply convert the function to have
"void" return type.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/apic.c
+++ b/xen/arch/x86/apic.c
@@ -1204,14 +1204,14 @@ static void wait_tick_pvh(void)
  * APIC irq that way.
  */
 
-static int __init calibrate_APIC_clock(void)
+static void __init calibrate_APIC_clock(void)
 {
     unsigned long long t1, t2;
-    unsigned long tt1, tt2, result;
+    unsigned long tt1, tt2;
     unsigned int i;
     unsigned long bus_freq; /* KAF: pointer-size avoids compile warns. */
     unsigned int bus_cycle; /* length of one bus cycle in pico-seconds */
-    const unsigned int LOOPS = HZ/10;
+#define LOOPS_FRAC 10U      /* measure for one tenth of a second */
 
     apic_printk(APIC_VERBOSE, "calibrating APIC timer ...\n");
 
@@ -1238,9 +1238,9 @@ static int __init calibrate_APIC_clock(v
     tt1 = apic_read(APIC_TMCCT);
 
     /*
-     * Let's wait LOOPS ticks:
+     * Let's wait HZ / LOOPS_FRAC ticks:
      */
-    for (i = 0; i < LOOPS; i++)
+    for (i = 0; i < HZ / LOOPS_FRAC; i++)
         if ( !xen_guest )
             wait_8254_wraparound();
         else
@@ -1249,17 +1249,16 @@ static int __init calibrate_APIC_clock(v
     tt2 = apic_read(APIC_TMCCT);
     t2 = rdtsc_ordered();
 
-    result = (tt1-tt2)*APIC_DIVISOR/LOOPS;
+    bus_freq = (tt1 - tt2) * APIC_DIVISOR * LOOPS_FRAC;
 
-    apic_printk(APIC_VERBOSE, "..... CPU clock speed is %ld.%04ld MHz.\n",
-                ((long)(t2 - t1) / LOOPS) / (1000000 / HZ),
-                ((long)(t2 - t1) / LOOPS) % (1000000 / HZ));
+    apic_printk(APIC_VERBOSE, "..... CPU clock speed is %lu.%04lu MHz.\n",
+                ((unsigned long)(t2 - t1) * LOOPS_FRAC) / 1000000,
+                ((unsigned long)(t2 - t1) * LOOPS_FRAC / 100) % 10000);
 
     apic_printk(APIC_VERBOSE, "..... host bus clock speed is %ld.%04ld MHz.\n",
-                result / (1000000 / HZ), result % (1000000 / HZ));
+                bus_freq / 1000000, (bus_freq / 100) % 10000);
 
     /* 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;
@@ -1269,7 +1268,7 @@ static int __init calibrate_APIC_clock(v
     /* reset APIC to zero timeout value */
     __setup_APIC_LVTT(0);
 
-    return result;
+#undef LOOPS_FRAC
 }
 
 void __init setup_boot_APIC_clock(void)


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