cpufreq: Avoid unnecessary frequency updates due to mismatch

For some platforms, the frequency returned by hardware may be slightly
different from what is provided in the frequency table. For example,
hardware may return 499 MHz instead of 500 MHz. In such cases it is
better to avoid getting into unnecessary frequency updates, as we may
end up switching policy->cur between the two and sending unnecessary
pre/post update notifications, etc.

This patch has chosen allows the hardware frequency and table frequency
to deviate by 1 MHz for now, we may want to increase it a bit later on
if someone still complains.

Reported-by: Rex-BC Chen <rex-bc.chen@mediatek.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Tested-by: Jia-wei Chang <jia-wei.chang@mediatek.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/cpufreq/cpufreq.c

@@ -28,6 +28,7 @@
 #include <linux/suspend.h>
 #include <linux/syscore_ops.h>
 #include <linux/tick.h>
+#include <linux/units.h>
 #include <trace/events/power.h>
 
 static LIST_HEAD(cpufreq_policy_list);
@@ -1708,6 +1709,16 @@ static unsigned int cpufreq_verify_current_freq(struct cpufreq_policy *policy, b
 		return new_freq;
 
 	if (policy->cur != new_freq) {
+		/*
+		 * For some platforms, the frequency returned by hardware may be
+		 * slightly different from what is provided in the frequency
+		 * table, for example hardware may return 499 MHz instead of 500
+		 * MHz. In such cases it is better to avoid getting into
+		 * unnecessary frequency updates.
+		 */
+		if (abs(policy->cur - new_freq) < HZ_PER_MHZ)
+			return policy->cur;
+
 		cpufreq_out_of_sync(policy, new_freq);
 		if (update)
 			schedule_work(&policy->update);