.. SPDX-License-Identifier: GPL-2.0
.. include:: <isonum.txt>

===============================================
``intel_pstate`` CPU Performance Scaling Driver
===============================================

:Copyright: |copy| 2017 Intel Corporation

:Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>


General Information
===================

``intel_pstate`` is a part of the
:doc:`CPU performance scaling subsystem <cpufreq>` in the Linux kernel
(``CPUFreq``).  It is a scaling driver for the Sandy Bridge and later
generations of Intel processors.  Note, however, that some of those processors
may not be supported.  [To understand ``intel_pstate`` it is necessary to know
how ``CPUFreq`` works in general, so this is the time to read
Documentation/admin-guide/pm/cpufreq.rst if you have not done that yet.]

For the processors supported by ``intel_pstate``, the P-state concept is broader
than just an operating frequency or an operating performance point (see the
LinuxCon Europe 2015 presentation by Kristen Accardi [1]_ for more
information about that).  For this reason, the representation of P-states used
by ``intel_pstate`` internally follows the hardware specification (for details
refer to Intel Software Developer’s Manual [2]_).  However, the ``CPUFreq`` core
uses frequencies for identifying operating performance points of CPUs and
frequencies are involved in the user space interface exposed by it, so
``intel_pstate`` maps its internal representation of P-states to frequencies too
(fortunately, that mapping is unambiguous).  At the same time, it would not be
practical for ``intel_pstate`` to supply the ``CPUFreq`` core with a table of
available frequencies due to the possible size of it, so the driver does not do
that.  Some functionality of the core is limited by that.

Since the hardware P-state selection interface used by ``intel_pstate`` is
available at the logical CPU level, the driver always works with individual
CPUs.  Consequently, if ``intel_pstate`` is in use, every ``CPUFreq`` policy
object corresponds to one logical CPU and ``CPUFreq`` policies are effectively
equivalent to CPUs.  In particular, this means that they become "inactive" every
time the corresponding CPU is taken offline and need to be re-initialized when
it goes back online.

``intel_pstate`` is not modular, so it cannot be unloaded, which means that the
only way to pass early-configuration-time parameters to it is via the kernel
command line.  However, its configuration can be adjusted via ``sysfs`` to a
great extent.  In some configurations it even is possible to unregister it via
``sysfs`` which allows another ``CPUFreq`` scaling driver to be loaded and
registered (see `below <status_attr_>`_).


Operation Modes
===============

``intel_pstate`` can operate in two different modes, active or passive.  In the
active mode, it uses its own internal performance scaling governor algorithm or
allows the hardware to do performance scaling by itself, while in the passive
mode it responds to requests made by a generic ``CPUFreq`` governor implementing
a certain performance scaling algorithm.  Which of them will be in effect
depends on what kernel command line options are used and on the capabilities of
the processor.

Active Mode
-----------

This is the default operation mode of ``intel_pstate`` for processors with
hardware-managed P-states (HWP) support.  If it works in this mode, the
``scaling_driver`` policy attribute in ``sysfs`` for all ``CPUFreq`` policies
contains the string "intel_pstate".

In this mode the driver bypasses the scaling governors layer of ``CPUFreq`` and
provides its own scaling algorithms for P-state selection.  Those algorithms
can be applied to ``CPUFreq`` policies in the same way as generic scaling
governors (that is, through the ``scaling_governor`` policy attribute in
``sysfs``).  [Note that different P-state selection algorithms may be chosen for
different policies, but that is not recommended.]

They are not generic scaling governors, but their names are the same as the
names of some of those governors.  Moreover, confusingly enough, they generally
do not work in the same way as the generic governors they share the names with.
For example, the ``powersave`` P-state selection algorithm provided by
``intel_pstate`` is not a counterpart of the generic ``powersave`` governor
(roughly, it corresponds to the ``schedutil`` and ``ondemand`` governors).

There are two P-state selection algorithms provided by ``intel_pstate`` in the
active mode: ``powersave`` and ``performance``.  The way they both operate
depends on whether or not the hardware-managed P-states (HWP) feature has been
enabled in the processor and possibly on the processor model.

Which of the P-state selection algorithms is used by default depends on the
:c:macro:`CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE` kernel configuration option.
Namely, if that option is set, the ``performance`` algorithm will be used by
default, and the other one will be used by default if it is not set.

Active Mode With HWP
~~~~~~~~~~~~~~~~~~~~

If the processor supports the HWP feature, it will be enabled during the
processor initialization and cannot be disabled after that.  It is possible
to avoid enabling it by passing the ``intel_pstate=no_hwp`` argument to the
kernel in the command line.

If the HWP feature has been enabled, ``intel_pstate`` relies on the processor to
select P-states by itself, but still it can give hints to the processor's
internal P-state selection logic.  What those hints are depends on which P-state
selection algorithm has been applied to the given policy (or to the CPU it
corresponds to).

Even though the P-state selection is carried out by the processor automatically,
``intel_pstate`` registers utilization update callbacks with the CPU scheduler
in this mode.  However, they are not used for running a P-state selection
algorithm, but for periodic updates of the current CPU frequency information to
be made available from the ``scaling_cur_freq`` policy attribute in ``sysfs``.

HWP + ``performance``
.....................

In this configuration ``intel_pstate`` will write 0 to the processor's
Energy-Performance Preference (EPP) knob (if supported) or its
Energy-Performance Bias (EPB) knob (otherwise), which means that the processor's
internal P-state selection logic is expected to focus entirely on performance.

This will override the EPP/EPB setting coming from the ``sysfs`` interface
(see `Energy vs Performance Hints`_ below).  Moreover, any attempts to change
the EPP/EPB to a value different from 0 ("performance") via ``sysfs`` in this
configuration will be rejected.

Also, in this configuration the range of P-states available to the processor's
internal P-state selection logic is always restricted to the upper boundary
(that is, the maximum P-state that the driver is allowed to use).

HWP + ``powersave``
...................

In this configuration ``intel_pstate`` will set the processor's
Energy-Performance Preference (EPP) knob (if supported) or its
Energy-Performance Bias (EPB) knob (otherwise) to whatever value it was
previously set to via ``sysfs`` (or whatever default value it was
set to by the platform firmware).  This usually causes the processor's
internal P-state selection logic to be less performance-focused.

Active Mode Without HWP
~~~~~~~~~~~~~~~~~~~~~~~

This operation mode is optional for processors that do not support the HWP
feature or when the ``intel_pstate=no_hwp`` argument is passed to the kernel in
the command line.  The active mode is used in those cases if the
``intel_pstate=active`` argument is passed to the kernel in the command line.
In this mode ``intel_pstate`` may refuse to work with processors that are not
recognized by it.  [Note that ``intel_pstate`` will never refuse to work with
any processor with the HWP feature enabled.]

In this mode ``intel_pstate`` registers utilization update callbacks with the
CPU scheduler in order to run a P-state selection algorithm, either
``powersave`` or ``performance``, depending on the ``scaling_governor`` policy
setting in ``sysfs``.  The current CPU frequency information to be made
available from the ``scaling_cur_freq`` policy attribute in ``sysfs`` is
periodically updated by those utilization update callbacks too.

``performance``
...............

Without HWP, this P-state selection algorithm is always the same regardless of
the processor model and platform configuration.

It selects the maximum P-state it is allowed to use, subject to limits set via
``sysfs``, every time the driver configuration for the given CPU is updated
(e.g. via ``sysfs``).

This is the default P-state selection algorithm if the
:c:macro:`CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE` kernel configuration option
is set.

``powersave``
.............

Without HWP, this P-state selection algorithm is similar to the algorithm
implemented by the generic ``schedutil`` scaling governor except that the
utilization metric used by it is based on numbers coming from feedback
registers of the CPU.  It generally selects P-states proportional to the
current CPU utilization.

This algorithm is run by the driver's utilization update callback for the
given CPU when it is invoked by the CPU scheduler, but not more often than
every 10 ms.  Like in the ``performance`` case, the hardware configuration
is not touched if the new P-state turns out to be the same as the current
one.

This is the default P-state selection algorithm if the
:c:macro:`CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE` kernel configuration option
is not set.

Passive Mode
------------

This is the default operation mode of ``intel_pstate`` for processors without
hardware-managed P-states (HWP) support.  It is always used if the
``intel_pstate=passive`` argument is passed to the kernel in the command line
regardless of whether or not the given processor supports HWP.  [Note that the
``intel_pstate=no_hwp`` setting causes the driver to start in the passive mode
if it is not combined with ``intel_pstate=active``.]  Like in the active mode
without HWP support, in this mode ``intel_pstate`` may refuse to work with
processors that are not recognized by it if HWP is prevented from being enabled
through the kernel command line.

If the driver works in this mode, the ``scaling_driver`` policy attribute in
``sysfs`` for all ``CPUFreq`` policies contains the string "intel_cpufreq".
Then, the driver behaves like a regular ``CPUFreq`` scaling driver.  That is,
it is invoked by generic scaling governors when necessary to talk to the
hardware in order to change the P-state of a CPU (in particular, the
``schedutil`` governor can invoke it directly from scheduler context).

While in this mode, ``intel_pstate`` can be used with all of the (generic)
scaling governors listed by the ``scaling_available_governors`` policy attribute
in ``sysfs`` (and the P-state selection algorithms described above are not
used).  Then, it is responsible for the configuration of policy objects
corresponding to CPUs and provides the ``CPUFreq`` core (and the scaling
governors attached to the policy objects) with accurate information on the
maximum and minimum operating frequencies supported by the hardware (including
the so-called "turbo" frequency ranges).  In other words, in the passive mode
the entire range of available P-states is exposed by ``intel_pstate`` to the
``CPUFreq`` core.  However, in this mode the driver does not register
utilization update callbacks with the CPU scheduler and the ``scaling_cur_freq``
information comes from the ``CPUFreq`` core (and is the last frequency selected
by the current scaling governor for the given policy).


.. _turbo:

Turbo P-states Support
======================

In the majority of cases, the entire range of P-states available to
``intel_pstate`` can be divided into two sub-ranges that correspond to
different types of processor behavior, above and below a boundary that
will be referred to as the "turbo threshold" in what follows.

The P-states above the turbo threshold are referred to as "turbo P-states" and
the whole sub-range of P-states they belong to is referred to as the "turbo
range".  These names are related to the Turbo Boost technology allowing a
multicore processor to opportunistically increase the P-state of one or more
cores if there is enough power to do that and if that is not going to cause the
thermal envelope of the processor package to be exceeded.

Specifically, if software sets the P-state of a CPU core within the turbo range
(that is, above the turbo threshold), the processor is permitted to take over
performance scaling control for that core and put it into turbo P-states of its
choice going forward.  However, that permission is interpreted differently by
different processor generations.  Namely, the Sandy Bridge generation of
processors will never use any P-states above the last one set by software for
the given core, even if it is within the turbo range, whereas all of the later
processor generations will take it as a license to use any P-states from the
turbo range, even above the one set by software.  In other words, on those
processors setting any P-state from the turbo range will enable the processor
to put the given core into all turbo P-states up to and including the maximum
supported one as it sees fit.

One important property of turbo P-states is that they are not sustainable.  More
precisely, there is no guarantee that any CPUs will be able to stay in any of
those states indefinitely, because the power distribution within the processor
package may change over time  or the thermal envelope it was designed for might
be exceeded if a turbo P-state was used for too long.

In turn, the P-states below the turbo threshold generally are sustainable.  In
fact, if one of them is set by software, the processor is not expected to change
it to a lower one unless in a thermal stress or a power limit violation
situation (a higher P-state may still be used if it is set for another CPU in
the same package at the same time, for example).

Some processors allow multiple cores to be in turbo P-states at the same time,
but the maximum P-state that can be set for them generally depends on the number
of cores running concurrently.  The maximum turbo P-state that can be set for 3
cores at the same time usually is lower than the analogous maximum P-state for
2 cores, which in turn usually is lower than the maximum turbo P-state that can
be set for 1 core.  The one-core maximum turbo P-state is thus the maximum
supported one overall.

The maximum supported turbo P-state, the turbo threshold (the maximum supported
non-turbo P-state) and the minimum supported P-state are specific to the
processor model and can be determined by reading the processor's model-specific
registers (MSRs).  Moreover, some processors support the Configurable TDP
(Thermal Design Power) feature and, when that feature is enabled, the turbo
threshold effectively becomes a configurable value that can be set by the
platform firmware.

Unlike ``_PSS`` objects in the ACPI tables, ``intel_pstate`` always exposes
the entire range of available P-states, including the whole turbo range, to the
``CPUFreq`` core and (in the passive mode) to generic scaling governors.  This
generally causes turbo P-states to be set more often when ``intel_pstate`` is
used relative to ACPI-based CPU performance scaling (see `below <acpi-cpufreq_>`_
for more information).

Moreover, since ``intel_pstate`` always knows what the real turbo threshold is
(even if the Configurable TDP feature is enabled in the processor), its
``no_turbo`` attribute in ``sysfs`` (described `below <no_turbo_attr_>`_) should
work as expected in all cases (that is, if set to disable turbo P-states, it
always should prevent ``intel_pstate`` from using them).


Processor Support
=================

To handle a given processor ``intel_pstate`` requires a number of different
pieces of information on it to be known, including:

 * The minimum supported P-state.

 * The maximum supported `non-turbo P-state <turbo_>`_.

 * Whether or not turbo P-states are supported at all.

 * The maximum supported `one-core turbo P-state <turbo_>`_ (if turbo P-states
   are supported).

 * The scaling formula to translate the driver's internal representation
   of P-states into frequencies and the other way around.

Generally, ways to obtain that information are specific to the processor model
or family.  Although it often is possible to obtain all of it from the processor
itself (using model-specific registers), there are cases in which hardware
manuals need to be consulted to get to it too.

For this reason, there is a list of supported processors in ``intel_pstate`` and
the driver initialization will fail if the detected processor is not in that
list, unless it supports the HWP feature.  [The interface to obtain all of the
information listed above is the same for all of the processors supporting the
HWP feature, which is why ``intel_pstate`` works with all of them.]


User Space Interface in ``sysfs``
=================================

Global Attributes
-----------------

``intel_pstate`` exposes several global attributes (files) in ``sysfs`` to
control its functionality at the system level.  They are located in the
``/sys/devices/system/cpu/intel_pstate/`` directory and affect all CPUs.

Some of them are not present if the ``intel_pstate=per_cpu_perf_limits``
argument is passed to the kernel in the command line.

``max_perf_pct``
        Maximum P-state the driver is allowed to set in percent of the
        maximum supported performance level (the highest supported `turbo
        P-state <turbo_>`_).

        This attribute will not be exposed if the
        ``intel_pstate=per_cpu_perf_limits`` argument is present in the kernel
        command line.

``min_perf_pct``
        Minimum P-state the driver is allowed to set in percent of the
        maximum supported performance level (the highest supported `turbo
        P-state <turbo_>`_).

        This attribute will not be exposed if the
        ``intel_pstate=per_cpu_perf_limits`` argument is present in the kernel
        command line.

``num_pstates``
        Number of P-states supported by the processor (between 0 and 255
        inclusive) including both turbo and non-turbo P-states (see
        `Turbo P-states Support`_).

        This attribute is present only if the value exposed by it is the same
        for all of the CPUs in the system.

        The value of this attribute is not affected by the ``no_turbo``
        setting described `below <no_turbo_attr_>`_.

        This attribute is read-only.

``turbo_pct``
        Ratio of the `turbo range <turbo_>`_ size to the size of the entire
        range of supported P-states, in percent.

        This attribute is present only if the value exposed by it is the same
        for all of the CPUs in the system.

        This attribute is read-only.

.. _no_turbo_attr:

``no_turbo``
        If set (equal to 1), the driver is not allowed to set any turbo P-states
        (see `Turbo P-states Support`_).  If unset (equal to 0, which is the
        default), turbo P-states can be set by the driver.
        [Note that ``intel_pstate`` does not support the general ``boost``
        attribute (supported by some other scaling drivers) which is replaced
        by this one.]

        This attribute does not affect the maximum supported frequency value
        supplied to the ``CPUFreq`` core and exposed via the policy interface,
        but it affects the maximum possible value of per-policy P-state limits
        (see `Interpretation of Policy Attributes`_ below for details).

``hwp_dynamic_boost``
        This attribute is only present if ``intel_pstate`` works in the
        `active mode with the HWP feature enabled <Active Mode With HWP_>`_ in
        the processor.  If set (equal to 1), it causes the minimum P-state limit
        to be increased dynamically for a short time whenever a task previously
        waiting on I/O is selected to run on a given logical CPU (the purpose
        of this mechanism is to improve performance).

        This setting has no effect on logical CPUs whose minimum P-state limit
        is directly set to the highest non-turbo P-state or above it.

.. _status_attr:

``status``
        Operation mode of the driver: "active", "passive" or "off".

        "active"
                The driver is functional and in the `active mode
                <Active Mode_>`_.

        "passive"
                The driver is functional and in the `passive mode
                <Passive Mode_>`_.

        "off"
                The driver is not functional (it is not registered as a scaling
                driver with the ``CPUFreq`` core).

        This attribute can be written to in order to change the driver's
        operation mode or to unregister it.  The string written to it must be
        one of the possible values of it and, if successful, the write will
        cause the driver to switch over to the operation mode represented by
        that string - or to be unregistered in the "off" case.  [Actually,
        switching over from the active mode to the passive mode or the other
        way around causes the driver to be unregistered and registered again
        with a different set of callbacks, so all of its settings (the global
        as well as the per-policy ones) are then reset to their default
        values, possibly depending on the target operation mode.]

``energy_efficiency``
        This attribute is only present on platforms with CPUs matching the Kaby
        Lake or Coffee Lake desktop CPU model. By default, energy-efficiency
        optimizations are disabled on these CPU models if HWP is enabled.
        Enabling energy-efficiency optimizations may limit maximum operating
        frequency with or without the HWP feature.  With HWP enabled, the
        optimizations are done only in the turbo frequency range.  Without it,
        they are done in the entire available frequency range.  Setting this
        attribute to "1" enables the energy-efficiency optimizations and setting
        to "0" disables them.

Interpretation of Policy Attributes
-----------------------------------

The interpretation of some ``CPUFreq`` policy attributes described in
Documentation/admin-guide/pm/cpufreq.rst is special with ``intel_pstate``
as the current scaling driver and it generally depends on the driver's
`operation mode <Operation Modes_>`_.

First of all, the values of the ``cpuinfo_max_freq``, ``cpuinfo_min_freq`` and
``scaling_cur_freq`` attributes are produced by applying a processor-specific
multiplier to the internal P-state representation used by ``intel_pstate``.
Also, the values of the ``scaling_max_freq`` and ``scaling_min_freq``
attributes are capped by the frequency corresponding to the maximum P-state that
the driver is allowed to set.

If the ``no_turbo`` `global attribute <no_turbo_attr_>`_ is set, the driver is
not allowed to use turbo P-states, so the maximum value of ``scaling_max_freq``
and ``scaling_min_freq`` is limited to the maximum non-turbo P-state frequency.
Accordingly, setting ``no_turbo`` causes ``scaling_max_freq`` and
``scaling_min_freq`` to go down to that value if they were above it before.
However, the old values of ``scaling_max_freq`` and ``scaling_min_freq`` will be
restored after unsetting ``no_turbo``, unless these attributes have been written
to after ``no_turbo`` was set.

If ``no_turbo`` is not set, the maximum possible value of ``scaling_max_freq``
and ``scaling_min_freq`` corresponds to the maximum supported turbo P-state,
which also is the value of ``cpuinfo_max_freq`` in either case.

Next, the following policy attributes have special meaning if
``intel_pstate`` works in the `active mode <Active Mode_>`_:

``scaling_available_governors``
        List of P-state selection algorithms provided by ``intel_pstate``.

``scaling_governor``
        P-state selection algorithm provided by ``intel_pstate`` currently in
        use with the given policy.

``scaling_cur_freq``
        Frequency of the average P-state of the CPU represented by the given
        policy for the time interval between the last two invocations of the
        driver's utilization update callback by the CPU scheduler for that CPU.

One more policy attribute is present if the HWP feature is enabled in the
processor:

``base_frequency``
        Shows the base frequency of the CPU. Any frequency above this will be
        in the turbo frequency range.

The meaning of these attributes in the `passive mode <Passive Mode_>`_ is the
same as for other scaling drivers.

Additionally, the value of the ``scaling_driver`` attribute for ``intel_pstate``
depends on the operation mode of the driver.  Namely, it is either
"intel_pstate" (in the `active mode <Active Mode_>`_) or "intel_cpufreq" (in the
`passive mode <Passive Mode_>`_).

Coordination of P-State Limits
------------------------------

``intel_pstate`` allows P-state limits to be set in two ways: with the help of
the ``max_perf_pct`` and ``min_perf_pct`` `global attributes
<Global Attributes_>`_ or via the ``scaling_max_freq`` and ``scaling_min_freq``
``CPUFreq`` policy attributes.  The coordination between those limits is based
on the following rules, regardless of the current operation mode of the driver:

 1. All CPUs are affected by the global limits (that is, none of them can be
    requested to run faster than the global maximum and none of them can be
    requested to run slower than the global minimum).

 2. Each individual CPU is affected by its own per-policy limits (that is, it
    cannot be requested to run faster than its own per-policy maximum and it
    cannot be requested to run slower than its own per-policy minimum). The
    effective performance depends on whether the platform supports per core
    P-states, hyper-threading is enabled and on current performance requests
    from other CPUs. When platform doesn't support per core P-states, the
    effective performance can be more than the policy limits set on a CPU, if
    other CPUs are requesting higher performance at that moment. Even with per
    core P-states support, when hyper-threading is enabled, if the sibling CPU
    is requesting higher performance, the other siblings will get higher
    performance than their policy limits.

 3. The global and per-policy limits can be set independently.

In the `active mode with the HWP feature enabled <Active Mode With HWP_>`_, the
resulting effective values are written into hardware registers whenever the
limits change in order to request its internal P-state selection logic to always
set P-states within these limits.  Otherwise, the limits are taken into account
by scaling governors (in the `passive mode <Passive Mode_>`_) and by the driver
every time before setting a new P-state for a CPU.

Additionally, if the ``intel_pstate=per_cpu_perf_limits`` command line argument
is passed to the kernel, ``max_perf_pct`` and ``min_perf_pct`` are not exposed
at all and the only way to set the limits is by using the policy attributes.


Energy vs Performance Hints
---------------------------

If the hardware-managed P-states (HWP) is enabled in the processor, additional
attributes, intended to allow user space to help ``intel_pstate`` to adjust the
processor's internal P-state selection logic by focusing it on performance or on
energy-efficiency, or somewhere between the two extremes, are present in every
``CPUFreq`` policy directory in ``sysfs``.  They are :

``energy_performance_preference``
        Current value of the energy vs performance hint for the given policy
        (or the CPU represented by it).

        The hint can be changed by writing to this attribute.

``energy_performance_available_preferences``
        List of strings that can be written to the
        ``energy_performance_preference`` attribute.

        They represent different energy vs performance hints and should be
        self-explanatory, except that ``default`` represents whatever hint
        value was set by the platform firmware.

Strings written to the ``energy_performance_preference`` attribute are
internally translated to integer values written to the processor's
Energy-Performance Preference (EPP) knob (if supported) or its
Energy-Performance Bias (EPB) knob. It is also possible to write a positive
integer value between 0 to 255, if the EPP feature is present. If the EPP
feature is not present, writing integer value to this attribute is not
supported. In this case, user can use the
"/sys/devices/system/cpu/cpu*/power/energy_perf_bias" interface.

[Note that tasks may by migrated from one CPU to another by the scheduler's
load-balancing algorithm and if different energy vs performance hints are
set for those CPUs, that may lead to undesirable outcomes.  To avoid such
issues it is better to set the same energy vs performance hint for all CPUs
or to pin every task potentially sensitive to them to a specific CPU.]

.. _acpi-cpufreq:

``intel_pstate`` vs ``acpi-cpufreq``
====================================

On the majority of systems supported by ``intel_pstate``, the ACPI tables
provided by the platform firmware contain ``_PSS`` objects returning information
that can be used for CPU performance scaling (refer to the ACPI specification
[3]_ for details on the ``_PSS`` objects and the format of the information
returned by them).

The information returned by the ACPI ``_PSS`` objects is used by the
``acpi-cpufreq`` scaling driver.  On systems supported by ``intel_pstate``
the ``acpi-cpufreq`` driver uses the same hardware CPU performance scaling
interface, but the set of P-states it can use is limited by the ``_PSS``
output.

On those systems each ``_PSS`` object returns a list of P-states supported by
the corresponding CPU which basically is a subset of the P-states range that can
be used by ``intel_pstate`` on the same system, with one exception: the whole
`turbo range <turbo_>`_ is represented by one item in it (the topmost one).  By
convention, the frequency returned by ``_PSS`` for that item is greater by 1 MHz
than the frequency of the highest non-turbo P-state listed by it, but the
corresponding P-state representation (following the hardware specification)
returned for it matches the maximum supported turbo P-state (or is the
special value 255 meaning essentially "go as high as you can get").

The list of P-states returned by ``_PSS`` is reflected by the table of
available frequencies supplied by ``acpi-cpufreq`` to the ``CPUFreq`` core and
scaling governors and the minimum and maximum supported frequencies reported by
it come from that list as well.  In particular, given the special representation
of the turbo range described above, this means that the maximum supported
frequency reported by ``acpi-cpufreq`` is higher by 1 MHz than the frequency
of the highest supported non-turbo P-state listed by ``_PSS`` which, of course,
affects decisions made by the scaling governors, except for ``powersave`` and
``performance``.

For example, if a given governor attempts to select a frequency proportional to
estimated CPU load and maps the load of 100% to the maximum supported frequency
(possibly multiplied by a constant), then it will tend to choose P-states below
the turbo threshold if ``acpi-cpufreq`` is used as the scaling driver, because
in that case the turbo range corresponds to a small fraction of the frequency
band it can use (1 MHz vs 1 GHz or more).  In consequence, it will only go to
the turbo range for the highest loads and the other loads above 50% that might
benefit from running at turbo frequencies will be given non-turbo P-states
instead.

One more issue related to that may appear on systems supporting the
`Configurable TDP feature <turbo_>`_ allowing the platform firmware to set the
turbo threshold.  Namely, if that is not coordinated with the lists of P-states
returned by ``_PSS`` properly, there may be more than one item corresponding to
a turbo P-state in those lists and there may be a problem with avoiding the
turbo range (if desirable or necessary).  Usually, to avoid using turbo
P-states overall, ``acpi-cpufreq`` simply avoids using the topmost state listed
by ``_PSS``, but that is not sufficient when there are other turbo P-states in
the list returned by it.

Apart from the above, ``acpi-cpufreq`` works like ``intel_pstate`` in the
`passive mode <Passive Mode_>`_, except that the number of P-states it can set
is limited to the ones listed by the ACPI ``_PSS`` objects.


Kernel Command Line Options for ``intel_pstate``
================================================

Several kernel command line options can be used to pass early-configuration-time
parameters to ``intel_pstate`` in order to enforce specific behavior of it.  All
of them have to be prepended with the ``intel_pstate=`` prefix.

``disable``
        Do not register ``intel_pstate`` as the scaling driver even if the
        processor is supported by it.

``active``
        Register ``intel_pstate`` in the `active mode <Active Mode_>`_ to start
        with.

``passive``
        Register ``intel_pstate`` in the `passive mode <Passive Mode_>`_ to
        start with.

``force``
        Register ``intel_pstate`` as the scaling driver instead of
        ``acpi-cpufreq`` even if the latter is preferred on the given system.

        This may prevent some platform features (such as thermal controls and
        power capping) that rely on the availability of ACPI P-states
        information from functioning as expected, so it should be used with
        caution.

        This option does not work with processors that are not supported by
        ``intel_pstate`` and on platforms where the ``pcc-cpufreq`` scaling
        driver is used instead of ``acpi-cpufreq``.

``no_hwp``
        Do not enable the hardware-managed P-states (HWP) feature even if it is
        supported by the processor.

``hwp_only``
        Register ``intel_pstate`` as the scaling driver only if the
        hardware-managed P-states (HWP) feature is supported by the processor.

``support_acpi_ppc``
        Take ACPI ``_PPC`` performance limits into account.

        If the preferred power management profile in the FADT (Fixed ACPI
        Description Table) is set to "Enterprise Server" or "Performance
        Server", the ACPI ``_PPC`` limits are taken into account by default
        and this option has no effect.

``per_cpu_perf_limits``
        Use per-logical-CPU P-State limits (see `Coordination of P-state
        Limits`_ for details).


Diagnostics and Tuning
======================

Trace Events
------------

There are two static trace events that can be used for ``intel_pstate``
diagnostics.  One of them is the ``cpu_frequency`` trace event generally used
by ``CPUFreq``, and the other one is the ``pstate_sample`` trace event specific
to ``intel_pstate``.  Both of them are triggered by ``intel_pstate`` only if
it works in the `active mode <Active Mode_>`_.

The following sequence of shell commands can be used to enable them and see
their output (if the kernel is generally configured to support event tracing)::

 # cd /sys/kernel/debug/tracing/
 # echo 1 > events/power/pstate_sample/enable
 # echo 1 > events/power/cpu_frequency/enable
 # cat trace
 gnome-terminal--4510  [001] ..s.  1177.680733: pstate_sample: core_busy=107 scaled=94 from=26 to=26 mperf=1143818 aperf=1230607 tsc=29838618 freq=2474476
 cat-5235  [002] ..s.  1177.681723: cpu_frequency: state=2900000 cpu_id=2

If ``intel_pstate`` works in the `passive mode <Passive Mode_>`_, the
``cpu_frequency`` trace event will be triggered either by the ``schedutil``
scaling governor (for the policies it is attached to), or by the ``CPUFreq``
core (for the policies with other scaling governors).

``ftrace``
----------

The ``ftrace`` interface can be used for low-level diagnostics of
``intel_pstate``.  For example, to check how often the function to set a
P-state is called, the ``ftrace`` filter can be set to
:c:func:`intel_pstate_set_pstate`::

 # cd /sys/kernel/debug/tracing/
 # cat available_filter_functions | grep -i pstate
 intel_pstate_set_pstate
 intel_pstate_cpu_init
 ...
 # echo intel_pstate_set_pstate > set_ftrace_filter
 # echo function > current_tracer
 # cat trace | head -15
 # tracer: function
 #
 # entries-in-buffer/entries-written: 80/80   #P:4
 #
 #                              _-----=> irqs-off
 #                             / _----=> need-resched
 #                            | / _---=> hardirq/softirq
 #                            || / _--=> preempt-depth
 #                            ||| /     delay
 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
 #              | |       |   ||||       |         |
             Xorg-3129  [000] ..s.  2537.644844: intel_pstate_set_pstate <-intel_pstate_timer_func
  gnome-terminal--4510  [002] ..s.  2537.649844: intel_pstate_set_pstate <-intel_pstate_timer_func
      gnome-shell-3409  [001] ..s.  2537.650850: intel_pstate_set_pstate <-intel_pstate_timer_func
           <idle>-0     [000] ..s.  2537.654843: intel_pstate_set_pstate <-intel_pstate_timer_func


References
==========

.. [1] Kristen Accardi, *Balancing Power and Performance in the Linux Kernel*,
       https://events.static.linuxfound.org/sites/events/files/slides/LinuxConEurope_2015.pdf

.. [2] *Intel® 64 and IA-32 Architectures Software Developer’s Manual Volume 3: System Programming Guide*,
       https://www.intel.com/content/www/us/en/architecture-and-technology/64-ia-32-architectures-software-developer-system-programming-manual-325384.html

.. [3] *Advanced Configuration and Power Interface Specification*,
       https://uefi.org/sites/default/files/resources/ACPI_6_3_final_Jan30.pdf