Hi
I have discovered that the accuracy of the wallclock (xtime) on ppc is
not very good.
I am running a custom telco board based on a 8266, and the main busclock
is derived off of a T1 reference clock.
I was noticing a huge number of logentries fron OpenNTPD about
adjustiing the clock, so I started to check.
The drift of the walltime was a little over 7 seconds in 15 hours (7
seconds slow) (equals about 130us per s)
When I checked around, I found that xtime is updated in kernel/time.c
and is updated by time_nsec nanoseconds per timer interrupt.
This value is hardcoded (through some obscure macros) to be 999848
ns/timertick. Tis is 152ns slow per timertick. This works out to be
152us per second, almos the value I found.
I have tried a few workarounds, and made one that modifies time_nsec
before update_wall_time_one_tick() is called.
With this fix (which is 8260 specific at the time) I have less than 1 ms
drift after 5 days.
The fix works by calculating how many ns and fractions of ns there is in
one timer-tick (one jiffie).
Then, at each timer interrupt, I accumulate the ns fractions, and add
whole ns to the timer when needed.
This is tested on 8266 under 80.00MHz and 82.575360MHz main busclock. It
also works correct if HZ=1024 instead of 1000
It is not meant as a patch to be added to the kernel right now, but more
of a possible solution to a problem that it doesn't seem anybody has
noticed before.
Index: arch/ppc/kernel/time.c
===================================================================
--- arch/ppc/kernel/time.c (revision 20)
+++ arch/ppc/kernel/time.c (working copy)
@@ -119,6 +119,11 @@
EXPORT_SYMBOL(profile_pc);
#endif
+
+#ifdef CONFIG_8260
+extern unsigned long get_ns_delta(void); /*
arch/ppc/syslib/m8260_setup.c */
+extern unsigned long tick_nsec; /* kernel/time.c */
+#endif
/*
* timer_interrupt - gets called when the decrementer overflows,
* with interrupts disabled.
@@ -148,6 +153,10 @@
/* We are in an interrupt, no need to save/restore flags
*/
write_seqlock(&xtime_lock);
tb_last_stamp = jiffy_stamp;
+
+#ifdef CONFIG_8260
+ tick_nsec = get_ns_delta();
+#endif
do_timer(regs);
/*
@@ -179,7 +188,7 @@
write_sequnlock(&xtime_lock);
}
if ( !disarm_decr[smp_processor_id()] )
- set_dec(next_dec);
+ set_dec(next_dec-1);
last_jiffy_stamp(cpu) = jiffy_stamp;
if (ppc_md.heartbeat && !ppc_md.heartbeat_count--)
Index: arch/ppc/syslib/m8260_setup.c
===================================================================
--- arch/ppc/syslib/m8260_setup.c (revision 20)
+++ arch/ppc/syslib/m8260_setup.c (working copy)
@@ -31,6 +31,8 @@
#include "cpm2_pic.h"
+#include <linux/module.h>
+
unsigned char __res[sizeof(bd_t)];
extern void cpm2_reset(void);
@@ -82,16 +84,63 @@
/* The decrementer counts at the system (internal) clock frequency
* divided by four.
*/
+unsigned long tb_time_nsec;
+unsigned long tb_frac_nsec;
+unsigned long tb_frac_limit;
+
+extern uint32_t __div64_32(uint64_t *dividend, uint32_t divisor);
+
static void __init
m8260_calibrate_decr(void)
+{
+ bd_t *binfo = (bd_t *)__res;
+ int freq, divisor, tick_freq;
+ uint64_t tmp;
+ printk("Calibrating Decrementer\n");
+
+ freq = binfo->bi_busfreq;
+ divisor = 4;
+
+ /* get rounded off decrementer frequency */
+ tick_freq = (freq + 3) / 4;
+
+ /* get number of timebase ticks per jiffy (timerint) */
+ tb_ticks_per_jiffy = (tick_freq + (HZ/2)) / HZ;
+ tb_to_us = mulhwu_scale_factor(tick_freq, 1000000);
+
+ /* calculate timer interrupt interval in nano seconds */
+ tmp = 1000000000ULL * (u64)tb_ticks_per_jiffy;
+ __div64_32(&tmp, tick_freq);
+ tb_time_nsec = tmp;
+
+ /* calculate fractions of nanosecons per timer interrupt */
+ tb_frac_limit = 100000UL;
+ tmp = 1000000000ULL * (u64)tb_ticks_per_jiffy * tb_frac_limit;
+ __div64_32(&tmp, tick_freq);
+ tb_frac_nsec = (u32)(tmp - (u64)tb_time_nsec * (u64)tb_frac_limit);
+
+ printk("busfreq = %u\n", freq);
+ printk("tb_ticks_per_jiffy = %u\ntb_to_us = %u\n",
tb_ticks_per_jiffy, tb_to_us);
+ printk("tick_freq = %u\ntime_nsec = %u\ntb_frac_nsec = %u\n",
tick_freq, tb_time_nsec, tb_frac_nsec);
+}
+
+unsigned long get_ns_delta(void)
{
- bd_t *binfo = (bd_t *)__res;
- int freq, divisor;
+ static uint32_t frac = 0;
- freq = binfo->bi_busfreq;
- divisor = 4;
- tb_ticks_per_jiffy = freq / HZ / divisor;
- tb_to_us = mulhwu_scale_factor(freq / divisor, 1000000);
+ unsigned long delta_ns;
+
+ delta_ns = tb_time_nsec;
+
+ /* adjust for inaccuracy in calculation */
+ frac += tb_frac_nsec;
+ if (frac >= tb_frac_limit)
+ {
+ delta_ns += frac / tb_frac_limit;
+ frac %= tb_frac_limit;
+ }
+
+ return delta_ns;
}
/* The 8260 has an internal 1-second timer update register that
Rune Torgersen
System Developer
Innovative Systems LLC
1000 Innovative Drive
Mitchell, SD 57301
Ph: 605-995-6120
www.innovsys.com
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