thermal: Add cooling device's statistics in sysfs

This extends the sysfs interface for thermal cooling devices and exposes
some pretty useful statistics. These statistics have proven to be quite
useful specially while doing benchmarks related to the task scheduler,
where we want to make sure that nothing has disrupted the test,
specially the cooling device which may have put constraints on the CPUs.
The information exposed here tells us to what extent the CPUs were
constrained by the thermal framework.

The write-only "reset" file is used to reset the statistics.

The read-only "time_in_state_ms" file shows the time (in msec) spent by the
device in the respective cooling states, and it prints one line per
cooling state.

The read-only "total_trans" file shows single positive integer value
showing the total number of cooling state transitions the device has
gone through since the time the cooling device is registered or the time
when statistics were reset last.

The read-only "trans_table" file shows a two dimensional matrix, where
an entry <i,j> (row i, column j) represents the number of transitions
from State_i to State_j.

This is how the directory structure looks like for a single cooling
device:

$ ls -R /sys/class/thermal/cooling_device0/
/sys/class/thermal/cooling_device0/:
cur_state  max_state  power  stats  subsystem  type  uevent

/sys/class/thermal/cooling_device0/power:
autosuspend_delay_ms  runtime_active_time  runtime_suspended_time
control               runtime_status

/sys/class/thermal/cooling_device0/stats:
reset  time_in_state_ms  total_trans  trans_table

This is tested on ARM 64-bit Hisilicon hikey620 board running Ubuntu and
ARM 64-bit Hisilicon hikey960 board running Android.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>

Documentation/thermal/sysfs-api.txt

@@ -255,6 +255,7 @@ temperature) and throttle appropriate devices.
 2. sysfs attributes structure
 
 RO	read only value
+WO	write only value
 RW	read/write value
 
 Thermal sysfs attributes will be represented under /sys/class/thermal.
@@ -286,6 +287,11 @@ Thermal cooling device sys I/F, created once it's registered:
     |---type:			Type of the cooling device(processor/fan/...)
     |---max_state:		Maximum cooling state of the cooling device
     |---cur_state:		Current cooling state of the cooling device
+    |---stats:			Directory containing cooling device's statistics
+    |---stats/reset:		Writing any value resets the statistics
+    |---stats/time_in_state_ms:	Time (msec) spent in various cooling states
+    |---stats/total_trans:	Total number of times cooling state is changed
+    |---stats/trans_table:	Cooing state transition table
 
 
 Then next two dynamic attributes are created/removed in pairs. They represent
@@ -490,6 +496,31 @@ cur_state
 	- cur_state == max_state means the maximum cooling.
 	RW, Required
 
+stats/reset
+	Writing any value resets the cooling device's statistics.
+	WO, Required
+
+stats/time_in_state_ms:
+	The amount of time spent by the cooling device in various cooling
+	states. The output will have "<state> <time>" pair in each line, which
+	will mean this cooling device spent <time> msec of time at <state>.
+	Output will have one line for each of the supported states.  usertime
+	units here is 10mS (similar to other time exported in /proc).
+	RO, Required
+
+stats/total_trans:
+	A single positive value showing the total number of times the state of a
+	cooling device is changed.
+	RO, Required
+
+stats/trans_table:
+	This gives fine grained information about all the cooling state
+	transitions. The cat output here is a two dimensional matrix, where an
+	entry <i,j> (row i, column j) represents the number of transitions from
+	State_i to State_j. If the transition table is bigger than PAGE_SIZE,
+	reading this will return an -EFBIG error.
+	RO, Required
+
 3. A simple implementation
 
 ACPI thermal zone may support multiple trip points like critical, hot,