2 files changed, 34 insertions, 4 deletions

diff --git a/Documentation/cpu-freq/governors.txt b/Documentation/cpu-freq/governors.txt
index 5b0cfa67aff9..ce73f3eb5ddb 100644
--- a/Documentation/cpu-freq/governors.txt
+++ b/Documentation/cpu-freq/governors.txt
@@ -117,10 +117,28 @@ accessible parameters:
 sampling_rate: measured in uS (10^-6 seconds), this is how often you
 want the kernel to look at the CPU usage and to make decisions on
 what to do about the frequency.  Typically this is set to values of
-around '10000' or more.
-
-show_sampling_rate_(min|max): the minimum and maximum sampling rates
-available that you may set 'sampling_rate' to.
+around '10000' or more. It's default value is (cmp. with users-guide.txt):
+transition_latency * 1000
+The lowest value you can set is:
+transition_latency * 100 or it may get restricted to a value where it
+makes not sense for the kernel anymore to poll that often which depends
+on your HZ config variable (HZ=1000: max=20000us, HZ=250: max=5000).
+Be aware that transition latency is in ns and sampling_rate is in us, so you
+get the same sysfs value by default.
+Sampling rate should always get adjusted considering the transition latency
+To set the sampling rate 750 times as high as the transition latency
+in the bash (as said, 1000 is default), do:
+echo `$(($(cat cpuinfo_transition_latency) * 750 / 1000)) \
+    >ondemand/sampling_rate
+
+show_sampling_rate_(min|max): THIS INTERFACE IS DEPRECATED, DON'T USE IT.
+You can use wider ranges now and the general
+cpuinfo_transition_latency variable (cmp. with user-guide.txt) can be
+used to obtain exactly the same info:
+show_sampling_rate_min = transtition_latency * 500    / 1000
+show_sampling_rate_max = transtition_latency * 500000 / 1000
+(divided by 1000 is to illustrate that sampling rate is in us and
+transition latency is exported ns).
 
 up_threshold: defines what the average CPU usage between the samplings
 of 'sampling_rate' needs to be for the kernel to make a decision on
diff --git a/Documentation/cpu-freq/user-guide.txt b/Documentation/cpu-freq/user-guide.txt
index 917918f84fc7..75f41193f3e1 100644
--- a/Documentation/cpu-freq/user-guide.txt
+++ b/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