ARM: 7653/2: do not scale loops_per_jiffy when using a constant delay clock

[Will Deacon]

On Thu, Mar 07, 2013 at 06:37:30AM +0000, Nicolas Pitre wrote:
> On Thu, 7 Mar 2013, Will Deacon wrote:
> 
> > On the topic of this patch: I still think that we should revert it and
> > require cpufreq drivers to pass CPUFREQ_CONST_LOOPS in their flags (I guess
> > the cpu0 platform data may need extending to take some flags).
> 
> I must disagree.  The constness here is a property of the timer source 
> used to implement one of the udelay() providers.  Constness has no 
> relation with cpufreq which may or may not impact a given udelay 
> implementation.
> 
> > Longer term, we might want to assess the binding between timer-based 
> > delays and loops_per_jiffy, but that's an entirely new problem.
> 
> Actually I do think that the lpj should always be scaled regardless, as 
> this has meaning only for a CPU loop.  This is even more important if 
> there is the possibility for switching between different 
> implementations. The local timer based udelay implementation should 
> simply never factor in lpj into its delay evaluation.
> 
> So the current patch is a stop gap to fix the problem today.  When 
> something better is proposed we can remove this fix.

Ok, how about a simple change like the one below?

Will

--->8

diff --git a/arch/arm/include/asm/delay.h b/arch/arm/include/asm/delay.h
index 720799f..06777b9 100644
--- a/arch/arm/include/asm/delay.h
+++ b/arch/arm/include/asm/delay.h
@@ -24,7 +24,7 @@ extern struct arm_delay_ops {
        void (*delay)(unsigned long);
        void (*const_udelay)(unsigned long);
        void (*udelay)(unsigned long);
-       bool const_clock;
+       unsigned long lpj;
 } arm_delay_ops;
 
 #define __delay(n)             arm_delay_ops.delay(n)
diff --git a/arch/arm/lib/delay.c b/arch/arm/lib/delay.c
index 6b93f6a..0d90ed8 100644
--- a/arch/arm/lib/delay.c
+++ b/arch/arm/lib/delay.c
@@ -58,7 +58,7 @@ static void __timer_delay(unsigned long cycles)
 static void __timer_const_udelay(unsigned long xloops)
 {
        unsigned long long loops = xloops;
-       loops *= loops_per_jiffy;
+       loops *= arm_delay_ops.lpj;
        __timer_delay(loops >> UDELAY_SHIFT);
 }
 
@@ -73,11 +73,13 @@ void __init register_current_timer_delay(const struct delay_timer *timer)
                pr_info("Switching to timer-based delay loop\n");
                delay_timer                     = timer;
                lpj_fine                        = timer->freq / HZ;
-               loops_per_jiffy                 = lpj_fine;
+
+               /* cpufreq may scale loops_per_jiffy, so keep a private copy */
+               arm_delay_ops.lpj               = lpj_fine;
                arm_delay_ops.delay             = __timer_delay;
                arm_delay_ops.const_udelay      = __timer_const_udelay;
                arm_delay_ops.udelay            = __timer_udelay;
-               arm_delay_ops.const_clock       = true;
+
                delay_calibrated                = true;
        } else {
                pr_info("Ignoring duplicate/late registration of read_current_timer delay\n");