Legacy Documentation of CPU Performance Scaling Drivers — The Linux Kernel 5.18.5-gnu-1 documentation

# Legacy Documentation of CPU Performance Scaling Drivers

Included below are historic documents describing assorted CPU performance scaling drivers. They are reproduced verbatim, with the original white space formatting and indentation preserved, except for the added leading space character in every line of text.

## AMD PowerNow! Drivers

    PowerNow! and Cool'n'Quiet are AMD names for frequency
    management capabilities in AMD processors. As the hardware
    implementation changes in new generations of the processors,
    there is a different cpu-freq driver for each generation.

    Note that the driver's will not load on the "wrong" hardware,
    so it is safe to try each driver in turn when in doubt as to
    which is the correct driver.

    Note that the functionality to change frequency (and voltage)
    is not available in all processors. The drivers will refuse
    to load on processors without this capability. The capability
    is detected with the cpuid instruction.

    The drivers use BIOS supplied tables to obtain frequency and
    voltage information appropriate for a particular platform.
    Frequency transitions will be unavailable if the BIOS does
    not supply these tables.

    6th Generation: powernow-k6

    7th Generation: powernow-k7: Athlon, Duron, Geode.

    8th Generation: powernow-k8: Athlon, Athlon 64, Opteron, Sempron.
    Documentation on this functionality in 8th generation processors
    is available in the "BIOS and Kernel Developer's Guide", publication
    26094, in chapter 9, available for download from www.amd.com.

    BIOS supplied data, for powernow-k7 and for powernow-k8, may be
    from either the PSB table or from ACPI objects. The ACPI support
    is only available if the kernel config sets CONFIG_ACPI_PROCESSOR.
    The powernow-k8 driver will attempt to use ACPI if so configured,
    and fall back to PST if that fails.
    The powernow-k7 driver will try to use the PSB support first, and
    fall back to ACPI if the PSB support fails. A module parameter,
    acpi_force, is provided to force ACPI support to be used instead
    of PSB support.

## `cpufreq-nforce2`

    The cpufreq-nforce2 driver changes the FSB on nVidia nForce2 platforms.

    This works better than on other platforms, because the FSB of the CPU
    can be controlled independently from the PCI/AGP clock.

    The module has two options:

           fid:     multiplier * 10 (for example 8.5 = 85)
           min_fsb: minimum FSB

    If not set, fid is calculated from the current CPU speed and the FSB.
    min_fsb defaults to FSB at boot time - 50 MHz.

    IMPORTANT: The available range is limited downwards!
               Also the minimum available FSB can differ, for systems
               booting with 200 MHz, 150 should always work.

## `pcc-cpufreq`

    /*
     *  pcc-cpufreq.txt - PCC interface documentation
     *
     *  Copyright (C) 2009 Red Hat, Matthew Garrett <mjg@redhat.com>
     *  Copyright (C) 2009 Hewlett-Packard Development Company, L.P.
     *      Nagananda Chumbalkar <nagananda.chumbalkar@hp.com>
     */

                           Processor Clocking Control Driver
                           ---------------------------------

    Contents:
    ---------
    1.     Introduction
    1.1    PCC interface
    1.1.1  Get Average Frequency
    1.1.2  Set Desired Frequency
    1.2    Platforms affected
    2.     Driver and /sys details
    2.1    scaling_available_frequencies
    2.2    cpuinfo_transition_latency
    2.3    cpuinfo_cur_freq
    2.4    related_cpus
    3.     Caveats

    1. Introduction:
    ----------------
    Processor Clocking Control (PCC) is an interface between the platform
    firmware and OSPM. It is a mechanism for coordinating processor
    performance (ie: frequency) between the platform firmware and the OS.

    The PCC driver (pcc-cpufreq) allows OSPM to take advantage of the PCC
    interface.

    OS utilizes the PCC interface to inform platform firmware what frequency the
    OS wants for a logical processor. The platform firmware attempts to achieve
    the requested frequency. If the request for the target frequency could not be
    satisfied by platform firmware, then it usually means that power budget
    conditions are in place, and "power capping" is taking place.

    1.1 PCC interface:
    ------------------
    The complete PCC specification is available here:
    https://acpica.org/sites/acpica/files/Processor-Clocking-Control-v1p0.pdf

    PCC relies on a shared memory region that provides a channel for communication
    between the OS and platform firmware. PCC also implements a "doorbell" that
    is used by the OS to inform the platform firmware that a command has been
    sent.

    The ACPI PCCH() method is used to discover the location of the PCC shared
    memory region. The shared memory region header contains the "command" and
    "status" interface. PCCH() also contains details on how to access the platform
    doorbell.

    The following commands are supported by the PCC interface:
    * Get Average Frequency
    * Set Desired Frequency

    The ACPI PCCP() method is implemented for each logical processor and is
    used to discover the offsets for the input and output buffers in the shared
    memory region.

    When PCC mode is enabled, the platform will not expose processor performance
    or throttle states (_PSS, _TSS and related ACPI objects) to OSPM. Therefore,
    the native P-state driver (such as acpi-cpufreq for Intel, powernow-k8 for
    AMD) will not load.

    However, OSPM remains in control of policy. The governor (eg: "ondemand")
    computes the required performance for each processor based on server workload.
    The PCC driver fills in the command interface, and the input buffer and
    communicates the request to the platform firmware. The platform firmware is
    responsible for delivering the requested performance.

    Each PCC command is "global" in scope and can affect all the logical CPUs in
    the system. Therefore, PCC is capable of performing "group" updates. With PCC
    the OS is capable of getting/setting the frequency of all the logical CPUs in
    the system with a single call to the BIOS.

    1.1.1 Get Average Frequency:
    ----------------------------
    This command is used by the OSPM to query the running frequency of the
    processor since the last time this command was completed. The output buffer
    indicates the average unhalted frequency of the logical processor expressed as
    a percentage of the nominal (ie: maximum) CPU frequency. The output buffer
    also signifies if the CPU frequency is limited by a power budget condition.

    1.1.2 Set Desired Frequency:
    ----------------------------
    This command is used by the OSPM to communicate to the platform firmware the
    desired frequency for a logical processor. The output buffer is currently
    ignored by OSPM. The next invocation of "Get Average Frequency" will inform
    OSPM if the desired frequency was achieved or not.

    1.2 Platforms affected:
    -----------------------
    The PCC driver will load on any system where the platform firmware:
    * supports the PCC interface, and the associated PCCH() and PCCP() methods
    * assumes responsibility for managing the hardware clocking controls in order
    to deliver the requested processor performance

    Currently, certain HP ProLiant platforms implement the PCC interface. On those
    platforms PCC is the "default" choice.

    However, it is possible to disable this interface via a BIOS setting. In
    such an instance, as is also the case on platforms where the PCC interface
    is not implemented, the PCC driver will fail to load silently.

    2. Driver and /sys details:
    ---------------------------
    When the driver loads, it merely prints the lowest and the highest CPU
    frequencies supported by the platform firmware.

    The PCC driver loads with a message such as:
    pcc-cpufreq: (v1.00.00) driver loaded with frequency limits: 1600 MHz, 2933
    MHz

    This means that the OPSM can request the CPU to run at any frequency in
    between the limits (1600 MHz, and 2933 MHz) specified in the message.

    Internally, there is no need for the driver to convert the "target" frequency
    to a corresponding P-state.

    The VERSION number for the driver will be of the format v.xy.ab.
    eg: 1.00.02
       ----- --
        |    |
        |    -- this will increase with bug fixes/enhancements to the driver
        |-- this is the version of the PCC specification the driver adheres to

    The following is a brief discussion on some of the fields exported via the
    /sys filesystem and how their values are affected by the PCC driver:

    2.1 scaling_available_frequencies:
    ----------------------------------
    scaling_available_frequencies is not created in /sys. No intermediate
    frequencies need to be listed because the BIOS will try to achieve any
    frequency, within limits, requested by the governor. A frequency does not have
    to be strictly associated with a P-state.

    2.2 cpuinfo_transition_latency:
    -------------------------------
    The cpuinfo_transition_latency field is 0. The PCC specification does
    not include a field to expose this value currently.

    2.3 cpuinfo_cur_freq:
    ---------------------
    A) Often cpuinfo_cur_freq will show a value different than what is declared
    in the scaling_available_frequencies or scaling_cur_freq, or scaling_max_freq.
    This is due to "turbo boost" available on recent Intel processors. If certain
    conditions are met the BIOS can achieve a slightly higher speed than requested
    by OSPM. An example:

    scaling_cur_freq       : 2933000
    cpuinfo_cur_freq       : 3196000

    B) There is a round-off error associated with the cpuinfo_cur_freq value.
    Since the driver obtains the current frequency as a "percentage" (%) of the
    nominal frequency from the BIOS, sometimes, the values displayed by
    scaling_cur_freq and cpuinfo_cur_freq may not match. An example:

    scaling_cur_freq       : 1600000
    cpuinfo_cur_freq       : 1583000

    In this example, the nominal frequency is 2933 MHz. The driver obtains the
    current frequency, cpuinfo_cur_freq, as 54% of the nominal frequency:

           54% of 2933 MHz = 1583 MHz

    Nominal frequency is the maximum frequency of the processor, and it usually
    corresponds to the frequency of the P0 P-state.

    2.4 related_cpus:
    -----------------
    The related_cpus field is identical to affected_cpus.

    affected_cpus  : 4
    related_cpus   : 4

    Currently, the PCC driver does not evaluate _PSD. The platforms that support
    PCC do not implement SW_ALL. So OSPM doesn't need to perform any coordination
    to ensure that the same frequency is requested of all dependent CPUs.

    3. Caveats:
    -----------
    The "cpufreq_stats" module in its present form cannot be loaded and
    expected to work with the PCC driver. Since the "cpufreq_stats" module
    provides information wrt each P-state, it is not applicable to the PCC driver.