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calibrating MIPS counter frequency

To: linux-mips@oss.sgi.com
Subject: calibrating MIPS counter frequency
From: Jun Sun <jsun@mvista.com>
Date: Wed, 28 Nov 2001 14:25:47 -0800
Sender: owner-linux-mips@oss.sgi.com
I wrote a little routine to calibrate CPU counter if the frequency is not
given by the board setup routine.  See the patch below.

Not too much use of it right now - unless you are writing some generic MIPS
performance test program.  Then you should be grateful.  It removes the board
dependency.

In the future, I think mips counter frequency really should go to mips_cpu
structure.  If we always know the counter frequency, either by board setup
routine or runtime calibration, we can get rid of the gettimeoffset
calibration routines.

Feedbacks are welcome.

Jun
Calibrate CPU counter frequency if it is not specified by board setup
routine.  No usage for now.  In the future, it should be part of
mips_cpu structure and we can get rid of the run-time gettimeoffset
calibration routines.  Should also speed timer interrupt a little bit.

Jun Sun

diff -Nru linux-2.4.14/init/main.c.orig linux-2.4.14/init/main.c
--- linux-2.4.14/init/main.c.orig       Fri Oct 12 10:17:15 2001
+++ linux-2.4.14/init/main.c    Wed Nov 28 11:18:58 2001
@@ -539,6 +539,9 @@
  *     Activate the first processor.
  */
 
+#if defined(CONFIG_MIPS) && defined(CONFIG_NEW_TIME_C)
+extern void calibrate_mips_counter(void);
+#endif
 asmlinkage void __init start_kernel(void)
 {
        char * command_line;
@@ -581,6 +584,9 @@
        kmem_cache_init();
        sti();
        calibrate_delay();
+#if defined(CONFIG_MIPS) && defined(CONFIG_NEW_TIME_C)
+       calibrate_mips_counter();
+#endif
 #ifdef CONFIG_BLK_DEV_INITRD
        if (initrd_start && !initrd_below_start_ok &&
                        initrd_start < min_low_pfn << PAGE_SHIFT) {
diff -Nru linux-2.4.14/arch/mips/kernel/time.c.orig 
linux-2.4.14/arch/mips/kernel/time.c
--- linux-2.4.14/arch/mips/kernel/time.c.orig   Mon Nov 26 18:22:58 2001
+++ linux-2.4.14/arch/mips/kernel/time.c        Wed Nov 28 13:57:23 2001
@@ -27,6 +27,7 @@
 #include <asm/time.h>
 #include <asm/hardirq.h>
 #include <asm/div64.h>
+#include <asm/debug.h>
 
 /* This is for machines which generate the exact clock. */
 #define USECS_PER_JIFFY (1000000/HZ)
@@ -505,4 +506,47 @@
         * Determine the day of week
         */
        tm->tm_wday = (gday + 4) % 7; /* 1970/1/1 was Thursday */
+}
+
+/*
+ * calibrate the mips_counter_frequency
+ */
+#define                CALIBRATION_CYCLES              20
+void calibrate_mips_counter(void)
+{
+       volatile unsigned long ticks;
+       volatile unsigned long countStart, countEnd;
+
+       if (mips_counter_frequency) {
+               /* it is already specified by the board */
+               return;
+       }
+
+       if ((mips_cpu.options & MIPS_CPU_COUNTER) == 0) {
+               /* we don't have cpu counter */
+               return;
+       }
+
+       printk("calibrating MIPS CPU counter frequency ...");
+
+       /* wait for the change of jiffies */
+       ticks = jiffies;
+       while (ticks == jiffies);
+
+       /* read cpu counter */
+       countStart = read_32bit_cp0_register(CP0_COUNT);
+
+       /* loop for another n jiffies */
+       ticks += CALIBRATION_CYCLES + 1;
+       while (ticks != jiffies);
+
+       /* read counter again */
+       countEnd = read_32bit_cp0_register(CP0_COUNT);
+
+       /* assuming HZ is 10's multiple */
+       db_assert((HZ % CALIBRATION_CYCLES) == 0);
+
+       mips_counter_frequency = 
+               (countEnd - countStart) * (HZ / CALIBRATION_CYCLES);
+       printk(" %d Hz\n", mips_counter_frequency);
 }
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