[PATCH] ARM: Don't ever downscale loops_per_jiffy in SMP systems

[Doug Anderson]

Nicolas,

On Thu, May 8, 2014 at 6:37 PM, Nicolas Pitre <nicolas.pitre at linaro.org> wrote:
> On Thu, 8 May 2014, Russell King - ARM Linux wrote:
>
>> If you're in a preempt or SMP environment, provide a timer for udelay().
>> IF you're in an environment with IRQs which can take a long time, use
>> a timer for udelay().  If you're in an environment where the CPU clock
>> can change unexpectedly, use a timer for udelay().
>
> Longer delays are normally not a problem.  If they are, then simply
> disabling IRQs may solve it if absolutely required.  With much shorter
> delays than expected this is another story.
>
> What about the following:
>
> diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
> index 7c4fada440..10030cc5a0 100644
> --- a/arch/arm/kernel/smp.c
> +++ b/arch/arm/kernel/smp.c
> @@ -682,6 +682,15 @@ static int cpufreq_callback(struct notifier_block *nb,
>                         cpufreq_scale(per_cpu(l_p_j_ref, cpu),
>                                         per_cpu(l_p_j_ref_freq, cpu),
>                                         freq->new);
> +               /*
> +                * Another CPU might have called udelay() just before LPJ
> +                * and a shared CPU clock is increased.  That other CPU still
> +                * looping on the old LPJ value would return significantly
> +                * sooner than expected.  The actual fix is to provide a
> +                * timer based udelay() implementation instead.
> +                */
> +               if (freq->old < freq->new)
> +                       pr_warn_once("*** udelay() on SMP is racy and may be much shorter than expected ***\n");

I would be OK with that.  At least someone would have a clue what to do.

-Doug