cpufreq: intel_pstate: Document the current behavior and user interface

Add a document describing the current behavior and user space
interface of the intel_pstate driver in the RST format and
drop the existing outdated intel_pstate.txt document.

Also update admin-guide/pm/cpufreq.rst with proper RST references
to the new intel_pstate.rst document.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

1 files changed, 0 insertions, 281 deletions

diff --git a/Documentation/cpu-freq/intel-pstate.txt b/Documentation/cpu-freq/intel-pstate.txt
deleted file mode 100644
index 3fdcdfd968ba..000000000000
--- a/Documentation/cpu-freq/intel-pstate.txt
+++ /dev/null
@@ -1,281 +0,0 @@
-Intel P-State driver
---------------------
-
-This driver provides an interface to control the P-State selection for the
-SandyBridge+ Intel processors.
-
-The following document explains P-States:
-http://events.linuxfoundation.org/sites/events/files/slides/LinuxConEurope_2015.pdf
-As stated in the document, P-State doesn’t exactly mean a frequency. However, for
-the sake of the relationship with cpufreq, P-State and frequency are used
-interchangeably.
-
-Understanding the cpufreq core governors and policies are important before
-discussing more details about the Intel P-State driver. Based on what callbacks
-a cpufreq driver provides to the cpufreq core, it can support two types of
-drivers:
-- with target_index() callback: In this mode, the drivers using cpufreq core
-simply provide the minimum and maximum frequency limits and an additional
-interface target_index() to set the current frequency. The cpufreq subsystem
-has a number of scaling governors ("performance", "powersave", "ondemand",
-etc.). Depending on which governor is in use, cpufreq core will call for
-transitions to a specific frequency using target_index() callback.
-- setpolicy() callback: In this mode, drivers do not provide target_index()
-callback, so cpufreq core can't request a transition to a specific frequency.
-The driver provides minimum and maximum frequency limits and callbacks to set a
-policy. The policy in cpufreq sysfs is referred to as the "scaling governor".
-The cpufreq core can request the driver to operate in any of the two policies:
-"performance" and "powersave". The driver decides which frequency to use based
-on the above policy selection considering minimum and maximum frequency limits.
-
-The Intel P-State driver falls under the latter category, which implements the
-setpolicy() callback. This driver decides what P-State to use based on the
-requested policy from the cpufreq core. If the processor is capable of
-selecting its next P-State internally, then the driver will offload this
-responsibility to the processor (aka HWP: Hardware P-States). If not, the
-driver implements algorithms to select the next P-State.
-
-Since these policies are implemented in the driver, they are not same as the
-cpufreq scaling governors implementation, even if they have the same name in
-the cpufreq sysfs (scaling_governors). For example the "performance" policy is
-similar to cpufreq’s "performance" governor, but "powersave" is completely
-different than the cpufreq "powersave" governor. The strategy here is similar
-to cpufreq "ondemand", where the requested P-State is related to the system load.
-
-Sysfs Interface
-
-In addition to the frequency-controlling interfaces provided by the cpufreq
-core, the driver provides its own sysfs files to control the P-State selection.
-These files have been added to /sys/devices/system/cpu/intel_pstate/.
-Any changes made to these files are applicable to all CPUs (even in a
-multi-package system, Refer to later section on placing "Per-CPU limits").
-
-      max_perf_pct: Limits the maximum P-State that will be requested by
-      the driver. It states it as a percentage of the available performance. The
-      available (P-State) performance may be reduced by the no_turbo
-      setting described below.
-
-      min_perf_pct: Limits the minimum P-State that will be requested by
-      the driver. It states it as a percentage of the max (non-turbo)
-      performance level.
-
-      no_turbo: Limits the driver to selecting P-State below the turbo
-      frequency range.
-
-      turbo_pct: Displays the percentage of the total performance that
-      is supported by hardware that is in the turbo range. This number
-      is independent of whether turbo has been disabled or not.
-
-      num_pstates: Displays the number of P-States that are supported
-      by hardware. This number is independent of whether turbo has
-      been disabled or not.
-
-For example, if a system has these parameters:
-	Max 1 core turbo ratio: 0x21 (Max 1 core ratio is the maximum P-State)
-	Max non turbo ratio: 0x17
-	Minimum ratio : 0x08 (Here the ratio is called max efficiency ratio)
-
-Sysfs will show :
-	max_perf_pct:100, which corresponds to 1 core ratio
-	min_perf_pct:24, max_efficiency_ratio / max 1 Core ratio
-	no_turbo:0, turbo is not disabled
-	num_pstates:26 = (max 1 Core ratio - Max Efficiency Ratio + 1)
-	turbo_pct:39 = (max 1 core ratio - max non turbo ratio) / num_pstates
-
-Refer to "Intel® 64 and IA-32 Architectures Software Developer’s Manual
-Volume 3: System Programming Guide" to understand ratios.
-
-There is one more sysfs attribute in /sys/devices/system/cpu/intel_pstate/
-that can be used for controlling the operation mode of the driver:
-
-      status: Three settings are possible:
-      "off"     - The driver is not in use at this time.
-      "active"  - The driver works as a P-state governor (default).
-      "passive" - The driver works as a regular cpufreq one and collaborates
-                  with the generic cpufreq governors (it sets P-states as
-                  requested by those governors).
-      The current setting is returned by reads from this attribute.  Writing one
-      of the above strings to it changes the operation mode as indicated by that
-      string, if possible.  If HW-managed P-states (HWP) are enabled, it is not
-      possible to change the driver's operation mode and attempts to write to
-      this attribute will fail.
-
-cpufreq sysfs for Intel P-State
-
-Since this driver registers with cpufreq, cpufreq sysfs is also presented.
-There are some important differences, which need to be considered.
-
-scaling_cur_freq: This displays the real frequency which was used during
-the last sample period instead of what is requested. Some other cpufreq driver,
-like acpi-cpufreq, displays what is requested (Some changes are on the
-way to fix this for acpi-cpufreq driver). The same is true for frequencies
-displayed at /proc/cpuinfo.
-
-scaling_governor: This displays current active policy. Since each CPU has a
-cpufreq sysfs, it is possible to set a scaling governor to each CPU. But this
-is not possible with Intel P-States, as there is one common policy for all
-CPUs. Here, the last requested policy will be applicable to all CPUs. It is
-suggested that one use the cpupower utility to change policy to all CPUs at the
-same time.
-
-scaling_setspeed: This attribute can never be used with Intel P-State.
-
-scaling_max_freq/scaling_min_freq: This interface can be used similarly to
-the max_perf_pct/min_perf_pct of Intel P-State sysfs. However since frequencies
-are converted to nearest possible P-State, this is prone to rounding errors.
-This method is not preferred to limit performance.
-
-affected_cpus: Not used
-related_cpus: Not used
-
-For contemporary Intel processors, the frequency is controlled by the
-processor itself and the P-State exposed to software is related to
-performance levels.  The idea that frequency can be set to a single
-frequency is fictional for Intel Core processors. Even if the scaling
-driver selects a single P-State, the actual frequency the processor
-will run at is selected by the processor itself.
-
-Per-CPU limits
-
-The kernel command line option "intel_pstate=per_cpu_perf_limits" forces
-the intel_pstate driver to use per-CPU performance limits.  When it is set,
-the sysfs control interface described above is subject to limitations.
-- The following controls are not available for both read and write
-	/sys/devices/system/cpu/intel_pstate/max_perf_pct
-	/sys/devices/system/cpu/intel_pstate/min_perf_pct
-- The following controls can be used to set performance limits, as far as the
-architecture of the processor permits:
-	/sys/devices/system/cpu/cpu*/cpufreq/scaling_max_freq
-	/sys/devices/system/cpu/cpu*/cpufreq/scaling_min_freq
-	/sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
-- User can still observe turbo percent and number of P-States from
-	/sys/devices/system/cpu/intel_pstate/turbo_pct
-	/sys/devices/system/cpu/intel_pstate/num_pstates
-- User can read write system wide turbo status
-	/sys/devices/system/cpu/no_turbo
-
-Support of energy performance hints
-It is possible to provide hints to the HWP algorithms in the processor
-to be more performance centric to more energy centric. When the driver
-is using HWP, two additional cpufreq sysfs attributes are presented for
-each logical CPU.
-These attributes are:
-	- energy_performance_available_preferences
-	- energy_performance_preference
-
-To get list of supported hints:
-$ cat energy_performance_available_preferences
-    default performance balance_performance balance_power power
-
-The current preference can be read or changed via cpufreq sysfs
-attribute "energy_performance_preference". Reading from this attribute
-will display current effective setting. User can write any of the valid
-preference string to this attribute. User can always restore to power-on
-default by writing "default".
-
-Since threads can migrate to different CPUs, this is possible that the
-new CPU may have different energy performance preference than the previous
-one. To avoid such issues, either threads can be pinned to specific CPUs
-or set the same energy performance preference value to all CPUs.
-
-Tuning Intel P-State driver
-
-When the performance can be tuned using PID (Proportional Integral
-Derivative) controller, debugfs files are provided for adjusting performance.
-They are presented under:
-/sys/kernel/debug/pstate_snb/
-
-The PID tunable parameters are:
-      deadband
-      d_gain_pct
-      i_gain_pct
-      p_gain_pct
-      sample_rate_ms
-      setpoint
-
-To adjust these parameters, some understanding of driver implementation is
-necessary. There are some tweeks described here, but be very careful. Adjusting
-them requires expert level understanding of power and performance relationship.
-These limits are only useful when the "powersave" policy is active.
-
--To make the system more responsive to load changes, sample_rate_ms can
-be adjusted  (current default is 10ms).
--To make the system use higher performance, even if the load is lower, setpoint
-can be adjusted to a lower number. This will also lead to faster ramp up time
-to reach the maximum P-State.
-If there are no derivative and integral coefficients, The next P-State will be
-equal to:
-	current P-State - ((setpoint - current cpu load) * p_gain_pct)
-
-For example, if the current PID parameters are (Which are defaults for the core
-processors like SandyBridge):
-      deadband = 0
-      d_gain_pct = 0
-      i_gain_pct = 0
-      p_gain_pct = 20
-      sample_rate_ms = 10
-      setpoint = 97
-
-If the current P-State = 0x08 and current load = 100, this will result in the
-next P-State = 0x08 - ((97 - 100) * 0.2) = 8.6 (rounded to 9). Here the P-State
-goes up by only 1. If during next sample interval the current load doesn't
-change and still 100, then P-State goes up by one again. This process will
-continue as long as the load is more than the setpoint until the maximum P-State
-is reached.
-
-For the same load at setpoint = 60, this will result in the next P-State
-= 0x08 - ((60 - 100) * 0.2) = 16
-So by changing the setpoint from 97 to 60, there is an increase of the
-next P-State from 9 to 16. So this will make processor execute at higher
-P-State for the same CPU load. If the load continues to be more than the
-setpoint during next sample intervals, then P-State will go up again till the
-maximum P-State is reached. But the ramp up time to reach the maximum P-State
-will be much faster when the setpoint is 60 compared to 97.
-
-Debugging Intel P-State driver
-
-Event tracing
-To debug P-State transition, the Linux event tracing interface can be used.
-There are two specific events, which can be enabled (Provided the kernel
-configs related to event tracing are enabled).
-
-# 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
-
-
-Using ftrace
-
-If function level tracing is required, the Linux ftrace interface can be used.
-For example if we want to check how often a function to set a P-State is
-called, we can set ftrace filter to 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