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

[Doug Anderson <dianders@chromium.org>]

Downscaling loops_per_jiffy on SMP ARM systems really doesn't work.
You could really only do this if:

* Each CPU is has independent frequency changes (changing one CPU
  doesn't affect another).
* We change the generic ARM udelay() code to actually look at percpu
  loops_per_jiffy.

I don't know of any ARM CPUs that are totally independent that don't
just use a timer-based delay anyway.  For those that don't have a
timer-based delay, we should be conservative and overestimate
loops_per_jiffy.

Note that on some systems you might sometimes see (in the extreme case
when we're all the way downclocked) a udelay(100) become a
udelay(1000) now.

Signed-off-by: Doug Anderson <dianders@chromium.org>
---
Note that I don't have an board that has cpufreq enabled upstream so
I'm relying on the testing I did on our local kernel-3.8.  Hopefully
someone out there can test using David's nifty udelay tests.  In order
to see this you'd need to make sure that you _don't_ have arch timers
enabled.  See:
* https://patchwork.kernel.org/patch/4124721/
* https://patchwork.kernel.org/patch/4124731/

 arch/arm/kernel/smp.c | 45 ++++++++++++++++++++++++++++-----------------
 1 file changed, 28 insertions(+), 17 deletions(-)

diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index 7c4fada..9d944f6 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -649,39 +649,50 @@ int setup_profiling_timer(unsigned int multiplier)
 
 #ifdef CONFIG_CPU_FREQ
 
-static DEFINE_PER_CPU(unsigned long, l_p_j_ref);
-static DEFINE_PER_CPU(unsigned long, l_p_j_ref_freq);
 static unsigned long global_l_p_j_ref;
 static unsigned long global_l_p_j_ref_freq;
+static unsigned long global_l_p_j_max_freq;
+
+/**
+ * cpufreq_callback - Adjust loops_per_jiffies when frequency changes
+ *
+ * When the CPU frequency changes we need to adjust loops_per_jiffies, which
+ * we assume scales linearly with frequency.
+ *
+ * This function is fairly castrated and only ever adjust loops_per_jiffies
+ * upward.  It also doesn't adjust the PER_CPU loops_per_jiffies.  Here's why:
+ * 1. The ARM udelay only ever looks at the global loops_per_jiffy not the
+ *    percpu one.  If your CPUs _are not_ changed in lockstep you could run
+ *    into problems by decreasing loops_per_jiffies since one of the other
+ *    processors might still be running slower.
+ * 2. The ARM udelay reads the loops_per_jiffy at the beginning of its loop and
+ *    no other times.  If your CPUs _are_ changed in lockstep you could run
+ *    into a race where one CPU has started its loop with old (slower)
+ *    loops_per_jiffy and then suddenly is running faster.
+ *
+ * Anyone who wants a good udelay() should be using a timer-based solution
+ * anyway.  If you don't have a timer solution, you just gotta be conservative.
+ */
 
 static int cpufreq_callback(struct notifier_block *nb,
 					unsigned long val, void *data)
 {
 	struct cpufreq_freqs *freq = data;
-	int cpu = freq->cpu;
 
 	if (freq->flags & CPUFREQ_CONST_LOOPS)
 		return NOTIFY_OK;
 
-	if (!per_cpu(l_p_j_ref, cpu)) {
-		per_cpu(l_p_j_ref, cpu) =
-			per_cpu(cpu_data, cpu).loops_per_jiffy;
-		per_cpu(l_p_j_ref_freq, cpu) = freq->old;
-		if (!global_l_p_j_ref) {
-			global_l_p_j_ref = loops_per_jiffy;
-			global_l_p_j_ref_freq = freq->old;
-		}
+	if (!global_l_p_j_ref) {
+		global_l_p_j_ref = loops_per_jiffy;
+		global_l_p_j_ref_freq = freq->old;
+		global_l_p_j_max_freq = freq->old;
 	}
 
-	if ((val == CPUFREQ_PRECHANGE  && freq->old < freq->new) ||
-	    (val == CPUFREQ_POSTCHANGE && freq->old > freq->new)) {
+	if (freq->new > global_l_p_j_max_freq) {
 		loops_per_jiffy = cpufreq_scale(global_l_p_j_ref,
 						global_l_p_j_ref_freq,
 						freq->new);
-		per_cpu(cpu_data, cpu).loops_per_jiffy =
-			cpufreq_scale(per_cpu(l_p_j_ref, cpu),
-					per_cpu(l_p_j_ref_freq, cpu),
-					freq->new);
+		global_l_p_j_max_freq = freq->new;
 	}
 	return NOTIFY_OK;
 }
-- 
1.9.1.423.g4596e3a