[v2,2/2] ARM: delay: allow timer-based delay implementation to be selected

Commit Message

Shinya Kuribayashi July 19, 2012, 12:43 p.m. UTC

Hi Will,

Have not gone through your new mail yet, sorry..

On 7/17/2012 6:05 PM, Will Deacon wrote:
> I assume the reason you've done it like this is because your timer isn't up
> and running until after the delay loop has been calibrated? In which case,
> I'd really rather not duplicate the calibration here -- is there no way you
> can ensure the timer is available earlier? If not, then I'm not sure we
> should be using it as the delay source.

Of course not :-)  My timers get started and running from time_init()
point, before calibrate_delay().

And don't get me wrong, I just tried to provide a working example to
skip calibrate_delay() in response to Santosh request:

> 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.

... with keeping the following points in mind:

1. skip the calibration for secondary CPUs (that is, for SMP use)

2. can work without init_current_timer_delay() help; calculating lpj
   from the CPU clock speed, not from current timer

If I understand Santosh request correctly, he requested a patch that
could be tested on non-A15 SMP systems (may be A9).  We know that A15
comes with the architected timer and does not require any additional
patches apart from already provided ones.  Also he said he gave a try
to "the suggested change + two patches" and confirmed it worked.

As for lpj_early variable, I just wanted to reserve lpj_fine for real
"timer" use, and did not want to mix with CPU frequency thing.  That
being said, I have to admit that rewriting using lpj_fine looks much
simpler, and should have done so from the beginning:

Patch

diff --git a/arch/arm/lib/delay.c b/arch/arm/lib/delay.c
index 395d5fb..b9874a3 100644
--- a/arch/arm/lib/delay.c
+++ b/arch/arm/lib/delay.c
@@ -65,6 +65,13 @@  void __init init_current_timer_delay(unsigned long freq)
 	arm_delay_ops.udelay		= __timer_udelay;
 }
 
+void __init calibrate_delay_early(unsigned long rate)
+{
+	pr_info("Calibrating delay using CPU frequency.. %lu Hz\n", rate);
+	lpj_fine = (rate + (HZ/2)) / HZ;
+	loops_per_jiffy = lpj_fine;
+}
+
 unsigned long __cpuinit calibrate_delay_is_known(void)
 {
 	return lpj_fine;