[PATCH] Documentation/cpu-freq: add intel-pstate.txt

From: Ramkumar Ramachandra
Date: Fri Jan 03 2014 - 06:31:41 EST

The Intel P-state driver is currently undocumented. Add some
documentation based on the cover-letter sent with the original series.

Cc: Dirk Brandewie <dirk.brandewie@xxxxxxxxx>
Cc: Rafael J. Wysocki <rjw@xxxxxxxxxxxxx>
Cc: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
Signed-off-by: Ramkumar Ramachandra <artagnon@xxxxxxxxx>
I was looking for documentation on my cpufreq driver, intel_pstate,
and found nothing: so, here's a small start. Although the original
message said SandyBridge only, I noticed that
/sys/devices/system/cpu/cpu0/cpufreq/scaling_driver says intel_pstate
on my IvyBridge; so I figured subsequent patches introduced support
for SandyBridge+.

I noticed two "policies" in the code: CPUFREQ_POLICY_PERFORMANCE and
CPUFREQ_POLICY_POWERSAVE, but I have no idea how to switch from one
to the other. The userspace tool cpupower seems to be too tightly
tied to the acpi-cpufreq driver, and I'm not able to find any others.

Also, how well is the driver performing? I seem to be getting
terrible battery life on my 2013 MBP.


Documentation/cpu-freq/intel-pstate.txt | 40 +++++++++++++++++++++++++++++++++
1 file changed, 40 insertions(+)
create mode 100644 Documentation/cpu-freq/intel-pstate.txt

diff --git a/Documentation/cpu-freq/intel-pstate.txt b/Documentation/cpu-freq/intel-pstate.txt
new file mode 100644
index 0000000..fda74ab
--- /dev/null
+++ b/Documentation/cpu-freq/intel-pstate.txt
@@ -0,0 +1,40 @@
+Intel P-state driver
+This driver implements a scaling driver with an internal governor for
+Intel Core processors. The driver follows the same model as the
+Transmeta scaling driver (longrun.c) and implements the setpolicy()
+instead of target(). Scaling drivers that implement setpolicy() are
+assmuned to implement internal governors by the cpufreq core. All the
+logic for selecting the current P state is contained within the driver
+no external governor is used by the cpufreq core.
+Only Intel SandyBridge+ processors are supported.
+New sysfs files for controlling P state selection have been added to
+ max_perf_pct: limits the maximum P state that will be requested by
+ the driver stated as a percentage of the avail performance.
+ min_perf_pct: limits the minimum P state that will be requested by
+ the driver stated as a percentage of the avail performance.
+ no_turbo: limits the driver to selecting P states below the turbo
+ frequency range.
+The units for these for these files are purposely abstract and stated
+in terms of available performance and not frequency. In idea that
+frequency can be set to a single frequency is a fiction for Intel Core
+processors. Even if the scaling driver selects a single P state the
+actual frequency the processor will run at is selected by the
+processor itself.
+New debugfs files have also been added to /sys/kernel/debug/pstate_snb/
+ deadband
+ d_gain_pct
+ i_gain_pct
+ p_gain_pct
+ sample_rate_ms
+ setpoint

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