208 lines
8.3 KiB
Plaintext
208 lines
8.3 KiB
Plaintext
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/*
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* pcc-cpufreq.txt - PCC interface documentation
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*
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* Copyright (C) 2009 Red Hat, Matthew Garrett <mjg@redhat.com>
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* Copyright (C) 2009 Hewlett-Packard Development Company, L.P.
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* Nagananda Chumbalkar <nagananda.chumbalkar@hp.com>
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*
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* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; version 2 of the License.
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*
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* This program is distributed in the hope that it will be useful, but
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* WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE, GOOD TITLE or NON
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* INFRINGEMENT. See the GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License along
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* with this program; if not, write to the Free Software Foundation, Inc.,
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* 675 Mass Ave, Cambridge, MA 02139, USA.
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*
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* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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*/
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Processor Clocking Control Driver
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---------------------------------
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Contents:
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---------
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1. Introduction
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1.1 PCC interface
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1.1.1 Get Average Frequency
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1.1.2 Set Desired Frequency
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1.2 Platforms affected
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2. Driver and /sys details
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2.1 scaling_available_frequencies
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2.2 cpuinfo_transition_latency
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2.3 cpuinfo_cur_freq
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2.4 related_cpus
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3. Caveats
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1. Introduction:
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----------------
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Processor Clocking Control (PCC) is an interface between the platform
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firmware and OSPM. It is a mechanism for coordinating processor
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performance (ie: frequency) between the platform firmware and the OS.
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The PCC driver (pcc-cpufreq) allows OSPM to take advantage of the PCC
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interface.
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OS utilizes the PCC interface to inform platform firmware what frequency the
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OS wants for a logical processor. The platform firmware attempts to achieve
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the requested frequency. If the request for the target frequency could not be
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satisfied by platform firmware, then it usually means that power budget
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conditions are in place, and "power capping" is taking place.
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1.1 PCC interface:
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------------------
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The complete PCC specification is available here:
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http://www.acpica.org/download/Processor-Clocking-Control-v1p0.pdf
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PCC relies on a shared memory region that provides a channel for communication
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between the OS and platform firmware. PCC also implements a "doorbell" that
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is used by the OS to inform the platform firmware that a command has been
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sent.
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The ACPI PCCH() method is used to discover the location of the PCC shared
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memory region. The shared memory region header contains the "command" and
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"status" interface. PCCH() also contains details on how to access the platform
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doorbell.
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The following commands are supported by the PCC interface:
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* Get Average Frequency
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* Set Desired Frequency
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The ACPI PCCP() method is implemented for each logical processor and is
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used to discover the offsets for the input and output buffers in the shared
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memory region.
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When PCC mode is enabled, the platform will not expose processor performance
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or throttle states (_PSS, _TSS and related ACPI objects) to OSPM. Therefore,
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the native P-state driver (such as acpi-cpufreq for Intel, powernow-k8 for
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AMD) will not load.
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However, OSPM remains in control of policy. The governor (eg: "ondemand")
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computes the required performance for each processor based on server workload.
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The PCC driver fills in the command interface, and the input buffer and
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communicates the request to the platform firmware. The platform firmware is
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responsible for delivering the requested performance.
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Each PCC command is "global" in scope and can affect all the logical CPUs in
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the system. Therefore, PCC is capable of performing "group" updates. With PCC
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the OS is capable of getting/setting the frequency of all the logical CPUs in
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the system with a single call to the BIOS.
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1.1.1 Get Average Frequency:
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----------------------------
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This command is used by the OSPM to query the running frequency of the
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processor since the last time this command was completed. The output buffer
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indicates the average unhalted frequency of the logical processor expressed as
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a percentage of the nominal (ie: maximum) CPU frequency. The output buffer
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also signifies if the CPU frequency is limited by a power budget condition.
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1.1.2 Set Desired Frequency:
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----------------------------
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This command is used by the OSPM to communicate to the platform firmware the
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desired frequency for a logical processor. The output buffer is currently
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ignored by OSPM. The next invocation of "Get Average Frequency" will inform
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OSPM if the desired frequency was achieved or not.
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1.2 Platforms affected:
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-----------------------
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The PCC driver will load on any system where the platform firmware:
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* supports the PCC interface, and the associated PCCH() and PCCP() methods
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* assumes responsibility for managing the hardware clocking controls in order
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to deliver the requested processor performance
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Currently, certain HP ProLiant platforms implement the PCC interface. On those
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platforms PCC is the "default" choice.
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However, it is possible to disable this interface via a BIOS setting. In
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such an instance, as is also the case on platforms where the PCC interface
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is not implemented, the PCC driver will fail to load silently.
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2. Driver and /sys details:
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---------------------------
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When the driver loads, it merely prints the lowest and the highest CPU
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frequencies supported by the platform firmware.
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The PCC driver loads with a message such as:
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pcc-cpufreq: (v1.00.00) driver loaded with frequency limits: 1600 MHz, 2933
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MHz
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This means that the OPSM can request the CPU to run at any frequency in
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between the limits (1600 MHz, and 2933 MHz) specified in the message.
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Internally, there is no need for the driver to convert the "target" frequency
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to a corresponding P-state.
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The VERSION number for the driver will be of the format v.xy.ab.
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eg: 1.00.02
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----- --
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| -- this will increase with bug fixes/enhancements to the driver
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|-- this is the version of the PCC specification the driver adheres to
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The following is a brief discussion on some of the fields exported via the
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/sys filesystem and how their values are affected by the PCC driver:
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2.1 scaling_available_frequencies:
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----------------------------------
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scaling_available_frequencies is not created in /sys. No intermediate
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frequencies need to be listed because the BIOS will try to achieve any
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frequency, within limits, requested by the governor. A frequency does not have
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to be strictly associated with a P-state.
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2.2 cpuinfo_transition_latency:
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-------------------------------
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The cpuinfo_transition_latency field is 0. The PCC specification does
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not include a field to expose this value currently.
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2.3 cpuinfo_cur_freq:
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---------------------
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A) Often cpuinfo_cur_freq will show a value different than what is declared
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in the scaling_available_frequencies or scaling_cur_freq, or scaling_max_freq.
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This is due to "turbo boost" available on recent Intel processors. If certain
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conditions are met the BIOS can achieve a slightly higher speed than requested
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by OSPM. An example:
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scaling_cur_freq : 2933000
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cpuinfo_cur_freq : 3196000
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B) There is a round-off error associated with the cpuinfo_cur_freq value.
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Since the driver obtains the current frequency as a "percentage" (%) of the
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nominal frequency from the BIOS, sometimes, the values displayed by
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scaling_cur_freq and cpuinfo_cur_freq may not match. An example:
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scaling_cur_freq : 1600000
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cpuinfo_cur_freq : 1583000
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In this example, the nominal frequency is 2933 MHz. The driver obtains the
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current frequency, cpuinfo_cur_freq, as 54% of the nominal frequency:
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54% of 2933 MHz = 1583 MHz
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Nominal frequency is the maximum frequency of the processor, and it usually
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corresponds to the frequency of the P0 P-state.
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2.4 related_cpus:
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-----------------
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The related_cpus field is identical to affected_cpus.
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affected_cpus : 4
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related_cpus : 4
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Currently, the PCC driver does not evaluate _PSD. The platforms that support
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PCC do not implement SW_ALL. So OSPM doesn't need to perform any coordination
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to ensure that the same frequency is requested of all dependent CPUs.
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3. Caveats:
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-----------
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The "cpufreq_stats" module in its present form cannot be loaded and
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expected to work with the PCC driver. Since the "cpufreq_stats" module
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provides information wrt each P-state, it is not applicable to the PCC driver.
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