[CPUFREQ] Introduce /sys/devices/system/cpu/cpu*/cpufreq/cpuinfo_transition_latency

It's not only useful for the ondemand and conservative governors, but
also for userspace daemons to know about the HW transition latency of
the CPU.
It is especially useful for userspace to know about this value when
the ondemand or conservative governors are run. The sampling rate
control value depends on it and for userspace being able to set sane
tuning values there it has to know about the transition latency.
Signed-off-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Dave Jones <davej@redhat.com>

Documentation/cpu-freq/user-guide.txt

@@ -152,6 +152,18 @@ cpuinfo_min_freq :		this file shows the minimum operating
 				frequency the processor can run at(in kHz) 
 cpuinfo_max_freq :		this file shows the maximum operating
 				frequency the processor can run at(in kHz) 
+cpuinfo_transition_latency	The time it takes on this CPU to
+				switch between two frequencies in nano
+				seconds. If unknown or known to be
+				that high that the driver does not
+				work with the ondemand governor, -1
+				(CPUFREQ_ETERNAL) will be returned.
+				Using this information can be useful
+				to choose an appropriate polling
+				frequency for a kernel governor or
+				userspace daemon. Make sure to not
+				switch the frequency too often
+				resulting in performance loss.
 scaling_driver :		this file shows what cpufreq driver is
 				used to set the frequency on this CPU
 

drivers/cpufreq/cpufreq.c

@@ -452,6 +452,7 @@ static ssize_t show_##file_name				\
 
 show_one(cpuinfo_min_freq, cpuinfo.min_freq);
 show_one(cpuinfo_max_freq, cpuinfo.max_freq);
+show_one(cpuinfo_transition_latency, cpuinfo.transition_latency);
 show_one(scaling_min_freq, min);
 show_one(scaling_max_freq, max);
 show_one(scaling_cur_freq, cur);
@@ -659,6 +660,7 @@ __ATTR(_name, 0644, show_##_name, store_##_name)
 define_one_ro0400(cpuinfo_cur_freq);
 define_one_ro(cpuinfo_min_freq);
 define_one_ro(cpuinfo_max_freq);
+define_one_ro(cpuinfo_transition_latency);
 define_one_ro(scaling_available_governors);
 define_one_ro(scaling_driver);
 define_one_ro(scaling_cur_freq);
@@ -672,6 +674,7 @@ define_one_rw(scaling_setspeed);
 static struct attribute *default_attrs[] = {
 	&cpuinfo_min_freq.attr,
 	&cpuinfo_max_freq.attr,
+	&cpuinfo_transition_latency.attr,
 	&scaling_min_freq.attr,
 	&scaling_max_freq.attr,
 	&affected_cpus.attr,