On 7/17/2012 3:11 PM, Shilimkar, Santosh wrote:

Thanks for the detailed explanation. CPU clock detection is indeed the
nit way to skip the calibration overhead and this was one of the comment
when I tried to push the skipping of calibration for secondary CPUs.

Looks like you have a working patch for the clock detection. Will
you able to post that patch so that this long pending calibration
for secondary CPUs gets optimized.

Something like this should work (not even build tested, can be applied
on top of Will's v2 patchset):

diff --git a/arch/arm/lib/delay.c b/arch/arm/lib/delay.c
index e1030e1..736dcea 100644
--- a/arch/arm/lib/delay.c
+++ b/arch/arm/lib/delay.c

@@ -25,6 +25,8 @@
 #include <linux/module.h>
 #include <linux/timex.h>
 
+static unsigned long lpj_early;
+
 /*
  * Default to the loop-based delay implementation.
  */

@@ -59,8 +61,22 @@ void __init init_current_timer_delay(unsigned long freq)
 {
 	pr_info("Switching to timer-based delay loop\n");
 	lpj_fine			= freq / HZ;
+	lpj_early			= lpj_fine;
+	loops_per_jiffy			= lpj_fine;
 	arm_delay_ops.delay		= __timer_delay;
 	arm_delay_ops.const_udelay	= __timer_const_udelay;
 	arm_delay_ops.udelay		= __timer_udelay;
 }
+
+void __cpuinit calibrate_delay_early(unsigned long cpu_freq)
+{
+	lpj_early = (cpu_freq + HZ/2) / HZ;
+	loops_per_jiffy = lpj_early;
+	pr_info("Calibrating delay using CPU frequency.. %lu Hz\n", cpu_freq);
+}
+
+unsigned long __cpuinit calibrate_delay_is_known(void)
+{
+	return lpj_early; /* this function works for both UP/SMP cases */
+}
 #endif

And change your ->timer() func (called via time_init) to make use of it:

        unsigned long freq;

        /* For UP/SMP systems */
        freq = get_CPU_frequency();
        calibrate_delay_early(freq);

#ifdef CONFIG_SMP
        /* For SMP systems */
        freq = get_Timer_frequency();
        init_current_timer_delay(freq);
#endif

The way to detect CPU clock speed can vary depending on your systems,
so hard to be generalized.  In my case, I have full-blown clock tree
described for my SoCs, and the CPU clock speed can be easily obtained
by clk_get_rate("xxx").

Hope this helps!
-- 
Shinya Kuribayashi
Renesas Electronics