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-/*
- * 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>
- *
- * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
- *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation; version 2 of the License.
- *
- * This program is distributed in the hope that it will be useful, but
- * WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE, GOOD TITLE or NON
- * INFRINGEMENT. See the GNU General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License along
- * with this program; if not, write to the Free Software Foundation, Inc.,
- * 675 Mass Ave, Cambridge, MA 02139, USA.
- *
- * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
- */
-
-
- 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:
-http://www.acpica.org/download/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.