| .. SPDX-License-Identifier: GPL-2.0 |
| |
| ======================================================= |
| Legacy Documentation of CPU Performance Scaling Drivers |
| ======================================================= |
| |
| Included below are historic documents describing assorted |
| :doc:`CPU performance scaling <cpufreq>` 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. |
