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2024-10-12 a5969cabbb4660eab42b6ef0412cbbd1200cf14d 
 kernel/Documentation/admin-guide/pm/intel_pstate.rst
....@@ -1,10 +1,13 @@
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+.. SPDX-License-Identifier: GPL-2.0
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+.. include:: <isonum.txt>
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+
14 ===============================================
25 ``intel_pstate`` CPU Performance Scaling Driver
36 ===============================================
47 
5
-::
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+:Copyright: |copy| 2017 Intel Corporation
69 
7
- Copyright (c) 2017 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
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+:Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
811 
912 
1013 General Information
....@@ -20,11 +23,10 @@
2023 
2124 For the processors supported by ``intel_pstate``, the P-state concept is broader
2225 than just an operating frequency or an operating performance point (see the
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-`LinuxCon Europe 2015 presentation by Kristen Accardi <LCEU2015_>`_ for more
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+LinuxCon Europe 2015 presentation by Kristen Accardi [1]_ for more
2427 information about that).  For this reason, the representation of P-states used
2528 by ``intel_pstate`` internally follows the hardware specification (for details
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-refer to `Intel® 64 and IA-32 Architectures Software Developer’s Manual
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-Volume 3: System Programming Guide <SDM_>`_).  However, the ``CPUFreq`` core
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+refer to Intel Software Developer’s Manual [2]_).  However, the ``CPUFreq`` core
2830 uses frequencies for identifying operating performance points of CPUs and
2931 frequencies are involved in the user space interface exposed by it, so
3032 ``intel_pstate`` maps its internal representation of P-states to frequencies too
....@@ -52,17 +54,21 @@
5254 Operation Modes
5355 ===============
5456 
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-``intel_pstate`` can operate in three different modes: in the active mode with
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-or without hardware-managed P-states support and in the passive mode.  Which of
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-them will be in effect depends on what kernel command line options are used and
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-on the capabilities of the processor.
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+``intel_pstate`` can operate in two different modes, active or passive.  In the
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+active mode, it uses its own internal performance scaling governor algorithm or
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+allows the hardware to do preformance scaling by itself, while in the passive
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+mode it responds to requests made by a generic ``CPUFreq`` governor implementing
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+a certain performance scaling algorithm.  Which of them will be in effect
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+depends on what kernel command line options are used and on the capabilities of
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+the processor.
5964 
6065 Active Mode
6166 -----------
6267 
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-This is the default operation mode of ``intel_pstate``.  If it works in this
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-mode, the ``scaling_driver`` policy attribute in ``sysfs`` for all ``CPUFreq``
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-policies contains the string "intel_pstate".
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+This is the default operation mode of ``intel_pstate`` for processors with
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+hardware-managed P-states (HWP) support.  If it works in this mode, the
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+``scaling_driver`` policy attribute in ``sysfs`` for all ``CPUFreq`` policies
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+contains the string "intel_pstate".
6672 
6773 In this mode the driver bypasses the scaling governors layer of ``CPUFreq`` and
6874 provides its own scaling algorithms for P-state selection.  Those algorithms
....@@ -117,7 +123,9 @@
117123 internal P-state selection logic is expected to focus entirely on performance.
118124 
119125 This will override the EPP/EPB setting coming from the ``sysfs`` interface
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-(see `Energy vs Performance Hints`_ below).
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+(see `Energy vs Performance Hints`_ below).  Moreover, any attempts to change
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+the EPP/EPB to a value different from 0 ("performance") via ``sysfs`` in this
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+configuration will be rejected.
121129 
122130 Also, in this configuration the range of P-states available to the processor's
123131 internal P-state selection logic is always restricted to the upper boundary
....@@ -136,12 +144,13 @@
136144 Active Mode Without HWP
137145 ~~~~~~~~~~~~~~~~~~~~~~~
138146 
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-This is the default operation mode for processors that do not support the HWP
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-feature.  It also is used by default with the ``intel_pstate=no_hwp`` argument
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-in the kernel command line.  However, in this mode ``intel_pstate`` may refuse
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-to work with the given processor if it does not recognize it.  [Note that
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-``intel_pstate`` will never refuse to work with any processor with the HWP
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-feature enabled.]
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+This operation mode is optional for processors that do not support the HWP
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+feature or when the ``intel_pstate=no_hwp`` argument is passed to the kernel in
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+the command line.  The active mode is used in those cases if the
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+``intel_pstate=active`` argument is passed to the kernel in the command line.
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+In this mode ``intel_pstate`` may refuse to work with processors that are not
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+recognized by it.  [Note that ``intel_pstate`` will never refuse to work with
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+any processor with the HWP feature enabled.]
145154 
146155 In this mode ``intel_pstate`` registers utilization update callbacks with the
147156 CPU scheduler in order to run a P-state selection algorithm, either
....@@ -186,10 +195,15 @@
186195 Passive Mode
187196 ------------
188197 
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-This mode is used if the ``intel_pstate=passive`` argument is passed to the
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-kernel in the command line (it implies the ``intel_pstate=no_hwp`` setting too).
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-Like in the active mode without HWP support, in this mode ``intel_pstate`` may
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-refuse to work with the given processor if it does not recognize it.
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+This is the default operation mode of ``intel_pstate`` for processors without
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+hardware-managed P-states (HWP) support.  It is always used if the
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+``intel_pstate=passive`` argument is passed to the kernel in the command line
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+regardless of whether or not the given processor supports HWP.  [Note that the
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+``intel_pstate=no_hwp`` setting causes the driver to start in the passive mode
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+if it is not combined with ``intel_pstate=active``.]  Like in the active mode
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+without HWP support, in this mode ``intel_pstate`` may refuse to work with
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+processors that are not recognized by it if HWP is prevented from being enabled
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+through the kernel command line.
193207 
194208 If the driver works in this mode, the ``scaling_driver`` policy attribute in
195209 ``sysfs`` for all ``CPUFreq`` policies contains the string "intel_cpufreq".
....@@ -310,10 +324,9 @@
310324 
311325 For this reason, there is a list of supported processors in ``intel_pstate`` and
312326 the driver initialization will fail if the detected processor is not in that
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-list, unless it supports the `HWP feature <Active Mode_>`_.  [The interface to
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-obtain all of the information listed above is the same for all of the processors
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-supporting the HWP feature, which is why they all are supported by
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-``intel_pstate``.]
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+list, unless it supports the HWP feature.  [The interface to obtain all of the
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+information listed above is the same for all of the processors supporting the
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+HWP feature, which is why ``intel_pstate`` works with all of them.]
317330 
318331 
319332 User Space Interface in ``sysfs``
....@@ -417,11 +430,16 @@
417430 	as well as the per-policy ones) are then reset to their default
418431 	values, possibly depending on the target operation mode.]
419432 
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-	That only is supported in some configurations, though (for example, if
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-	the `HWP feature is enabled in the processor <Active Mode With HWP_>`_,
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-	the operation mode of the driver cannot be changed), and if it is not
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-	supported in the current configuration, writes to this attribute will
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-	fail with an appropriate error.
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+``energy_efficiency``
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+	This attribute is only present on platforms with CPUs matching the Kaby
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+	Lake or Coffee Lake desktop CPU model. By default, energy-efficiency
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+	optimizations are disabled on these CPU models if HWP is enabled.
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+	Enabling energy-efficiency optimizations may limit maximum operating
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+	frequency with or without the HWP feature.  With HWP enabled, the
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+	optimizations are done only in the turbo frequency range.  Without it,
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+	they are done in the entire available frequency range.  Setting this
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+	attribute to "1" enables the energy-efficiency optimizations and setting
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+	to "0" disables them.
425443 
426444 Interpretation of Policy Attributes
427445 -----------------------------------
....@@ -465,6 +483,13 @@
465483 	policy for the time interval between the last two invocations of the
466484 	driver's utilization update callback by the CPU scheduler for that CPU.
467485 
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+One more policy attribute is present if the HWP feature is enabled in the
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+processor:
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+
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+``base_frequency``
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+	Shows the base frequency of the CPU. Any frequency above this will be
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+	in the turbo frequency range.
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+
468493 The meaning of these attributes in the `passive mode <Passive Mode_>`_ is the
469494 same as for other scaling drivers.
470495 
....@@ -488,15 +513,23 @@
488513 
489514  2. Each individual CPU is affected by its own per-policy limits (that is, it
490515     cannot be requested to run faster than its own per-policy maximum and it
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-    cannot be requested to run slower than its own per-policy minimum).
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+    cannot be requested to run slower than its own per-policy minimum). The
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+    effective performance depends on whether the platform supports per core
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+    P-states, hyper-threading is enabled and on current performance requests
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+    from other CPUs. When platform doesn't support per core P-states, the
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+    effective performance can be more than the policy limits set on a CPU, if
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+    other CPUs are requesting higher performance at that moment. Even with per
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+    core P-states support, when hyper-threading is enabled, if the sibling CPU
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+    is requesting higher performance, the other siblings will get higher
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+    performance than their policy limits.
492525 
493526  3. The global and per-policy limits can be set independently.
494527 
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-If the `HWP feature is enabled in the processor <Active Mode With HWP_>`_, the
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-resulting effective values are written into its registers whenever the limits
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-change in order to request its internal P-state selection logic to always set
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-P-states within these limits.  Otherwise, the limits are taken into account by
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-scaling governors (in the `passive mode <Passive Mode_>`_) and by the driver
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+In the `active mode with the HWP feature enabled <Active Mode With HWP_>`_, the
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+resulting effective values are written into hardware registers whenever the
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+limits change in order to request its internal P-state selection logic to always
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+set P-states within these limits.  Otherwise, the limits are taken into account
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+by scaling governors (in the `passive mode <Passive Mode_>`_) and by the driver
500533 every time before setting a new P-state for a CPU.
501534 
502535 Additionally, if the ``intel_pstate=per_cpu_perf_limits`` command line argument
....@@ -507,12 +540,11 @@
507540 Energy vs Performance Hints
508541 ---------------------------
509542 
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-If ``intel_pstate`` works in the `active mode with the HWP feature enabled
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-<Active Mode With HWP_>`_ in the processor, additional attributes are present
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-in every ``CPUFreq`` policy directory in ``sysfs``.  They are intended to allow
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-user space to help ``intel_pstate`` to adjust the processor's internal P-state
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-selection logic by focusing it on performance or on energy-efficiency, or
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-somewhere between the two extremes:
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+If the hardware-managed P-states (HWP) is enabled in the processor, additional
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+attributes, intended to allow user space to help ``intel_pstate`` to adjust the
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+processor's internal P-state selection logic by focusing it on performance or on
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+energy-efficiency, or somewhere between the two extremes, are present in every
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+``CPUFreq`` policy directory in ``sysfs``.  They are :
516548 
517549 ``energy_performance_preference``
518550 	Current value of the energy vs performance hint for the given policy
....@@ -531,7 +563,11 @@
531563 Strings written to the ``energy_performance_preference`` attribute are
532564 internally translated to integer values written to the processor's
533565 Energy-Performance Preference (EPP) knob (if supported) or its
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-Energy-Performance Bias (EPB) knob.
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+Energy-Performance Bias (EPB) knob. It is also possible to write a positive
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+integer value between 0 to 255, if the EPP feature is present. If the EPP
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+feature is not present, writing integer value to this attribute is not
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+supported. In this case, user can use the
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+"/sys/devices/system/cpu/cpu*/power/energy_perf_bias" interface.
535571 
536572 [Note that tasks may by migrated from one CPU to another by the scheduler's
537573 load-balancing algorithm and if different energy vs performance hints are
....@@ -546,9 +582,9 @@
546582 
547583 On the majority of systems supported by ``intel_pstate``, the ACPI tables
548584 provided by the platform firmware contain ``_PSS`` objects returning information
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-that can be used for CPU performance scaling (refer to the `ACPI specification`_
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-for details on the ``_PSS`` objects and the format of the information returned
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-by them).
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+that can be used for CPU performance scaling (refer to the ACPI specification
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+[3]_ for details on the ``_PSS`` objects and the format of the information
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+returned by them).
552588 
553589 The information returned by the ACPI ``_PSS`` objects is used by the
554590 ``acpi-cpufreq`` scaling driver.  On systems supported by ``intel_pstate``
....@@ -612,11 +648,13 @@
612648 	Do not register ``intel_pstate`` as the scaling driver even if the
613649 	processor is supported by it.
614650 
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+``active``
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+	Register ``intel_pstate`` in the `active mode <Active Mode_>`_ to start
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+	with.
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+
615655 ``passive``
616656 	Register ``intel_pstate`` in the `passive mode <Passive Mode_>`_ to
617657 	start with.
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-
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-	This option implies the ``no_hwp`` one described below.
620658 
621659 ``force``
622660 	Register ``intel_pstate`` as the scaling driver instead of
....@@ -632,13 +670,12 @@
632670 	driver is used instead of ``acpi-cpufreq``.
633671 
634672 ``no_hwp``
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-	Do not enable the `hardware-managed P-states (HWP) feature
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-	<Active Mode With HWP_>`_ even if it is supported by the processor.
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+	Do not enable the hardware-managed P-states (HWP) feature even if it is
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+	supported by the processor.
637675 
638676 ``hwp_only``
639677 	Register ``intel_pstate`` as the scaling driver only if the
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-	`hardware-managed P-states (HWP) feature <Active Mode With HWP_>`_ is
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-	supported by the processor.
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+	hardware-managed P-states (HWP) feature is supported by the processor.
642679 
643680 ``support_acpi_ppc``
644681 	Take ACPI ``_PPC`` performance limits into account.
....@@ -685,7 +722,7 @@
685722 
686723 The ``ftrace`` interface can be used for low-level diagnostics of
687724 ``intel_pstate``.  For example, to check how often the function to set a
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-P-state is called, the ``ftrace`` filter can be set to to
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+P-state is called, the ``ftrace`` filter can be set to
689726 :c:func:`intel_pstate_set_pstate`::
690727 
691728  # cd /sys/kernel/debug/tracing/
....@@ -713,6 +750,14 @@
713750            <idle>-0     [000] ..s.  2537.654843: intel_pstate_set_pstate <-intel_pstate_timer_func
714751 
715752 
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-.. _LCEU2015: http://events.linuxfoundation.org/sites/events/files/slides/LinuxConEurope_2015.pdf
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-.. _SDM: http://www.intel.com/content/www/us/en/architecture-and-technology/64-ia-32-architectures-software-developer-system-programming-manual-325384.html
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-.. _ACPI specification: http://www.uefi.org/sites/default/files/resources/ACPI_6_1.pdf
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+References
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+==========
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+
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+.. [1] Kristen Accardi, *Balancing Power and Performance in the Linux Kernel*,
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+       https://events.static.linuxfound.org/sites/events/files/slides/LinuxConEurope_2015.pdf
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+
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+.. [2] *Intel® 64 and IA-32 Architectures Software Developer’s Manual Volume 3: System Programming Guide*,
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+       https://www.intel.com/content/www/us/en/architecture-and-technology/64-ia-32-architectures-software-developer-system-programming-manual-325384.html
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+
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+.. [3] *Advanced Configuration and Power Interface Specification*,
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+       https://uefi.org/sites/default/files/resources/ACPI_6_3_final_Jan30.pdf