Documentation: admin-guide: pm: Document uncore frequency scaling

.. SPDX-License-Identifier: GPL-2.0

.. include:: <isonum.txt>

==============================

Intel Uncore Frequency Scaling

==============================

:Copyright: |copy| 2022 Intel Corporation

:Author: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>

Introduction

------------

The uncore can consume significant amount of power in Intel's Xeon servers based

on the workload characteristics. To optimize the total power and improve overall

performance, SoCs have internal algorithms for scaling uncore frequency. These

algorithms monitor workload usage of uncore and set a desirable frequency.

It is possible that users have different expectations of uncore performance and

want to have control over it. The objective is similar to allowing users to set

the scaling min/max frequencies via cpufreq sysfs to improve CPU performance.

Users may have some latency sensitive workloads where they do not want any

change to uncore frequency. Also, users may have workloads which require

different core and uncore performance at distinct phases and they may want to

use both cpufreq and the uncore scaling interface to distribute power and

improve overall performance.

Sysfs Interface

---------------

To control uncore frequency, a sysfs interface is provided in the directory:

`/sys/devices/system/cpu/intel_uncore_frequency/`.

There is one directory for each package and die combination as the scope of

uncore scaling control is per die in multiple die/package SoCs or per

package for single die per package SoCs. The name represents the

scope of control. For example: 'package_00_die_00' is for package id 0 and

die 0.

Each package_*_die_* contains the following attributes:

``initial_max_freq_khz``

	Out of reset, this attribute represent the maximum possible frequency.

	This is a read-only attribute. If users adjust max_freq_khz,

	they can always go back to maximum using the value from this attribute.

``initial_min_freq_khz``

	Out of reset, this attribute represent the minimum possible frequency.

	This is a read-only attribute. If users adjust min_freq_khz,

	they can always go back to minimum using the value from this attribute.

``max_freq_khz``

	This attribute is used to set the maximum uncore frequency.

``min_freq_khz``

	This attribute is used to set the minimum uncore frequency.

``current_freq_khz``

	This attribute is used to get the current uncore frequency.

   cpufreq_drivers

   intel_epb

   intel-speed-select

   intel_uncore_frequency_scaling