[PATCH] cpufreq-stats driver documentation

Documentation for cpufreq stats.

Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Dave Jones <davej@redhat.com>

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diff --git a/Documentation/cpu-freq/cpufreq-stats.txt b/Documentation/cpu-freq/cpufreq-stats.txt
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+
+     CPU frequency and voltage scaling statictics in the Linux(TM) kernel
+
+
+             L i n u x    c p u f r e q - s t a t s   d r i v e r
+
+                       - information for users -
+
+
+             Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
+
+Contents
+1. Introduction
+2. Statistics Provided (with example)
+3. Configuring cpufreq-stats
+
+
+1. Introduction
+
+cpufreq-stats is a driver that provices CPU frequency statistics for each CPU.
+This statistics is provided in /sysfs as a bunch of read_only interfaces. This
+interface (when configured) will appear in a seperate directory under cpufreq
+in /sysfs (<sysfs root>/devices/system/cpu/cpuX/cpufreq/stats/) for each CPU.
+Various statistics will form read_only files under this directory.
+
+This driver is designed to be independent of any particular cpufreq_driver
+that may be running on your CPU. So, it will work with any cpufreq_driver.
+
+
+2. Statistics Provided (with example)
+
+cpufreq stats provides following statistics (explained in detail below).
+-  time_in_state
+-  total_trans
+-  trans_table
+
+All the statistics will be from the time the stats driver has been inserted 
+to the time when a read of a particular statistic is done. Obviously, stats 
+driver will not have any information about the the frequcny transitions before
+the stats driver insertion.
+
+--------------------------------------------------------------------------------
+<mysystem>:/sys/devices/system/cpu/cpu0/cpufreq/stats # ls -l
+total 0
+drwxr-xr-x  2 root root    0 May 14 16:06 .
+drwxr-xr-x  3 root root    0 May 14 15:58 ..
+-r--r--r--  1 root root 4096 May 14 16:06 time_in_state
+-r--r--r--  1 root root 4096 May 14 16:06 total_trans
+-r--r--r--  1 root root 4096 May 14 16:06 trans_table
+--------------------------------------------------------------------------------
+
+-  time_in_state
+This gives the amount of time spent in each of the frequencies supported by
+this CPU. The cat output will have "<frequency> <time>" pair in each line, which
+will mean this CPU spent <time> usertime units of time at <frequency>. Output
+will have one line for each of the supported freuencies. usertime units here 
+is 10mS (similar to other time exported in /proc).
+
+--------------------------------------------------------------------------------
+<mysystem>:/sys/devices/system/cpu/cpu0/cpufreq/stats # cat time_in_state 
+3600000 2089
+3400000 136
+3200000 34
+3000000 67
+2800000 172488
+--------------------------------------------------------------------------------
+
+
+-  total_trans
+This gives the total number of frequency transitions on this CPU. The cat 
+output will have a single count which is the total number of frequency
+transitions.
+
+--------------------------------------------------------------------------------
+<mysystem>:/sys/devices/system/cpu/cpu0/cpufreq/stats # cat total_trans
+20
+--------------------------------------------------------------------------------
+
+-  trans_table
+This will give a fine grained information about all the CPU frequency
+transitions. The cat output here is a two dimensional matrix, where an entry
+<i,j> (row i, column j) represents the count of number of transitions from 
+Freq_i to Freq_j. Freq_i is in descending order with increasing rows and 
+Freq_j is in descending order with increasing columns. The output here also 
+contains the actual freq values for each row and column for better readability.
+
+--------------------------------------------------------------------------------
+<mysystem>:/sys/devices/system/cpu/cpu0/cpufreq/stats # cat trans_table
+   From  :    To
+         :   3600000   3400000   3200000   3000000   2800000 
+  3600000:         0         5         0         0         0 
+  3400000:         4         0         2         0         0 
+  3200000:         0         1         0         2         0 
+  3000000:         0         0         1         0         3 
+  2800000:         0         0         0         2         0 
+--------------------------------------------------------------------------------
+
+
+3. Configuring cpufreq-stats
+
+To configure cpufreq-stats in your kernel
+Config Main Menu
+	Power management options (ACPI, APM)  --->
+		CPU Frequency scaling  --->
+			[*] CPU Frequency scaling
+			<*>   CPU frequency translation statistics 
+			[*]     CPU frequency translation statistics details
+
+
+"CPU Frequency scaling" (CONFIG_CPU_FREQ) should be enabled to configure
+cpufreq-stats.
+
+"CPU frequency translation statistics" (CONFIG_CPU_FREQ_STAT) provides the
+basic statistics which includes time_in_state and total_trans.
+
+"CPU frequency translation statistics details" (CONFIG_CPU_FREQ_STAT_DETAILS)
+provides fine grained cpufreq stats by trans_table. The reason for having a
+seperate config option for trans_table is:
+- trans_table goes against the traditional /sysfs rule of one value per
+  interface. It provides a whole bunch of value in a 2 dimensional matrix
+  form.
+
+Once these two options are enabled and your CPU supports cpufrequency, you
+will be able to see the CPU frequency statistics in /sysfs.
+
+
+
+