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calibrate_tsc should not use HZ

PATCH In review Jim Houston 2 participants 2 messages Oct 31, 2002

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Jim Houston

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#0 lore raw

PATCH 1/1 calibrate_tsc should not use HZ

Hi Everyone,

I noticed that the calibration time for calibrate_tsc() is 5 jiffies. This means the result with 1000 Hz jiffies may be less accurate then it was at 100 Hz. The attached patch removes this dependency.

Jim Houston - Concurrent Computer Corp.

arch/i386/kernel/timers/timer_tsc.c +7 -2
diff -X /usr1/jhouston/dontdiff -ur linux.orig/arch/i386/kernel/timers/timer_tsc.c linux.kdb/arch/i386/kernel/timers/timer_tsc.c
@@ -97,8 +97,13 @@
  * device.
  */
 
-#define CALIBRATE_LATCH	(5 * LATCH)
-#define CALIBRATE_TIME	(5 * 1000020/HZ)
+/*
+ * Pick the largest possible latch value (it is a 16 bit counter)
+ * and calculate the corresponding time.
+ */
+#define CALIBRATE_LATCH	(0xffff)	
+#define CALIBRATE_TIME	((int)((1000000LL*CALIBRATE_LATCH + \
+				CLOCK_TICK_RATE/2)/CLOCK_TICK_RATE)
 
 static unsigned long __init calibrate_tsc(void)
 {
#1 lore raw
Quoted 19 lines · Jim Houston

Hi Everyone,

I noticed that the calibration time for calibrate_tsc() is 5 jiffies. This means the result with 1000 Hz jiffies may be less accurate then it was at 100 Hz. The attached patch removes this dependency.

Jim Houston - Concurrent Computer Corp.

diff -X /usr1/jhouston/dontdiff -ur linux.orig/arch/i386/kernel/timers/timer_tsc.c linux.kdb/arch/i386/kernel/timers/timer_tsc.c

arch/i386/kernel/timers/timer_tsc.c Thu Oct 31 14:18:21 2002 renamed from arch/i386/kernel/timers/timer_tsc.c Wed Oct 23 00:54:19 2002 +0 -2
@@ -97,8 +97,13 @@
  * device.
  */
 
-#define CALIBRATE_LATCH        (5 * LATCH)
-#define CALIBRATE_TIME (5 * 1000020/HZ)

arch/i386/kernel/timers/timer_tsc.c Thu Oct 31 14:18:21 2002 line 109

Quoted 10 lines
arch/i386/kernel/timers/timer_tsc.c Thu Oct 31 14:18:21 2002 renamed from arch/i386/kernel/timers/timer_tsc.c Wed Oct 23 00:54:19 2002 +7 -0
+/*
+ * Pick the largest possible latch value (it is a 16 bit counter)
+ * and calculate the corresponding time.
+ */
+#define CALIBRATE_LATCH        (0xffff)
+#define CALIBRATE_TIME ((int)((1000000LL*CALIBRATE_LATCH + \
+                               CLOCK_TICK_RATE/2)/CLOCK_TICK_RATE)
 
 static unsigned long __init calibrate_tsc(void)
 {

And one step further, if you tack on a few zeros in the numerator, you can pick up a couple of more bits of precision in the tick rate, i.e.:

CLOCK_TICK_RATE is derived from 14.3181818Mhz/12 = 1.19318181667 so:
#define CALIBRATE_LATCH_1000 1193181817
#define CALIBRATE_TIME ((int)((1000000000LL*CALIBRATE_LATCH
+ \

CLOCK_TICK_RATE_1000/2)/CLOCK_TICK_RATE_1000)

Signature · 5 lines
George Anzinger   [email protected]
High-res-timers: 
http://sourceforge.net/projects/high-res-timers/
Preemption patch:
http://www.kernel.org/pub/linux/kernel/people/rml