@@ -48,7 +48,7 @@ In addition to the frequency-controlling interfaces provided by the cpufreq
core, the driver provides its own sysfs files to control the P-State selection.
These files have been added to /sys/devices/system/cpu/intel_pstate/.
Any changes made to these files are applicable to all CPUs (even in a
-multi-package system).
+multi-package system, so they are global limits).
max_perf_pct: Limits the maximum P-State that will be requested by
the driver. It states it as a percentage of the available performance. The
@@ -88,7 +88,26 @@ Volume 3: System Programming Guide" to understand ratios.
cpufreq sysfs for Intel P-State
Since this driver registers with cpufreq, cpufreq sysfs is also presented.
-There are some important differences, which need to be considered.
+Under cpufreq sysfs (/sys/devices/system/cpu/cpufreq/) each logical CPU has
+its own policy. The policy limits for each CPU are still constrained by global
+limits using Intel P-State sysfs. So for example, if global limit is reducing
+max_perf_pct to 80%, the individual CPU can't exceed 80% of performance, but
+each CPU can add more restrictive limit (E.g. say 60% here). Same rule applies
+to min_perf_pct. Each CPU can request more than min_perf_pct performance.
+When performance limits are placed using cpufreq interface, the Intel P-State
+sysfs display of max_perf_pct and min_perf_pct accounts for those limits.
+Here
+ # cat /sys/devices/system/cpu/intel_pstate/max_perf_pct
+ Prints the least maximum performance set by individual
+ CPU if any. Otherwise it will print what user has
+ written to max_perf_pct
+ # cat /sys/devices/system/cpu/intel_pstate/min_perf_pct
+ Prints the maximum of minimum performances set by individual CPU
+ if any. Otherwise it will print what user has written to
+ min_perf_pct
+
+There are some important differences compared to acpi_cpufreq, which need to
+be considered.
scaling_cur_freq: This displays the real frequency which was used during
the last sample period instead of what is requested. Some other cpufreq driver,
Update documentation related to limiting performance via Intel_Pstate sysfs and cpufreq policy. Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com> --- Documentation/cpu-freq/intel-pstate.txt | 23 +++++++++++++++++++++-- 1 file changed, 21 insertions(+), 2 deletions(-)