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Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+===============================================
+``intel_pstate`` CPU Performance Scaling Driver
+===============================================
+
+::
+
+ Copyright (c) 2017 Intel Corp., 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 :doc:`cpufreq` 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 <LCEU2015_>`_ 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® 64 and IA-32 Architectures Software Developer’s Manual
+Volume 3: System Programming Guide <SDM_>`_).  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 three different modes: in the active mode with
+or without hardware-managed P-states support and in the passive mode.  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``.  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).
+
+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 is the default operation mode for processors that do not support the HWP
+feature.  It also is used by default with the ``intel_pstate=no_hwp`` argument
+in the kernel command line.  However, in this mode ``intel_pstate`` may refuse
+to work with the given processor if it does not recognize 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 mode is used if the ``intel_pstate=passive`` argument is passed to the
+kernel in the command line (it implies the ``intel_pstate=no_hwp`` setting too).
+Like in the active mode without HWP support, in this mode ``intel_pstate`` may
+refuse to work with the given processor if it does not recognize it.
+
+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 <Active Mode_>`_.  [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 they all are supported by
+``intel_pstate``.]
+
+
+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`_).
+
+	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 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 (equalt 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 attrubute 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.]
+
+	That only is supported in some configurations, though (for example, if
+	the `HWP feature is enabled in the processor <Active Mode With HWP_>`_,
+	the operation mode of the driver cannot be changed), and if it is not
+	supported in the current configuration, writes to this attribute will
+	fail with an appropriate error.
+
+Interpretation of Policy Attributes
+-----------------------------------
+
+The interpretation of some ``CPUFreq`` policy attributes described in
+:doc:`cpufreq` 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.
+
+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).
+
+ 3. The global and per-policy limits can be set independently.
+
+If the `HWP feature is enabled in the processor <Active Mode With HWP_>`_, the
+resulting effective values are written into its 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 ``intel_pstate`` works in the `active mode with the HWP feature enabled
+<Active Mode With HWP_>`_ in the processor, additional attributes are present
+in every ``CPUFreq`` policy directory in ``sysfs``.  They are 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:
+
+``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.
+
+[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`_
+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.
+
+``passive``
+	Register ``intel_pstate`` in the `passive mode <Passive Mode_>`_ to
+	start with.
+
+	This option implies the ``no_hwp`` one described below.
+
+``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
+	<Active Mode With HWP_>`_ 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 <Active Mode With HWP_>`_ 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 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
+
+
+.. _LCEU2015: http://events.linuxfoundation.org/sites/events/files/slides/LinuxConEurope_2015.pdf
+.. _SDM: http://www.intel.com/content/www/us/en/architecture-and-technology/64-ia-32-architectures-software-developer-system-programming-manual-325384.html
+.. _ACPI specification: http://www.uefi.org/sites/default/files/resources/ACPI_6_1.pdf