linux/Documentation/cpu-freq/pcc-cpufreq.txt
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   1/*
   2 *  pcc-cpufreq.txt - PCC interface documentation
   3 *
   4 *  Copyright (C) 2009 Red Hat, Matthew Garrett <mjg@redhat.com>
   5 *  Copyright (C) 2009 Hewlett-Packard Development Company, L.P.
   6 *      Nagananda Chumbalkar <nagananda.chumbalkar@hp.com>
   7 *
   8 * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   9 *
  10 *  This program is free software; you can redistribute it and/or modify
  11 *  it under the terms of the GNU General Public License as published by
  12 *  the Free Software Foundation; version 2 of the License.
  13 *
  14 *  This program is distributed in the hope that it will be useful, but
  15 *  WITHOUT ANY WARRANTY; without even the implied warranty of
  16 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE, GOOD TITLE or NON
  17 *  INFRINGEMENT. See the GNU General Public License for more details.
  18 *
  19 *  You should have received a copy of the GNU General Public License along
  20 *  with this program; if not, write to the Free Software Foundation, Inc.,
  21 *  675 Mass Ave, Cambridge, MA 02139, USA.
  22 *
  23 * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  24 */
  25
  26
  27                        Processor Clocking Control Driver
  28                        ---------------------------------
  29
  30Contents:
  31---------
  321.      Introduction
  331.1     PCC interface
  341.1.1   Get Average Frequency
  351.1.2   Set Desired Frequency
  361.2     Platforms affected
  372.      Driver and /sys details
  382.1     scaling_available_frequencies
  392.2     cpuinfo_transition_latency
  402.3     cpuinfo_cur_freq
  412.4     related_cpus
  423.      Caveats
  43
  441. Introduction:
  45----------------
  46Processor Clocking Control (PCC) is an interface between the platform
  47firmware and OSPM. It is a mechanism for coordinating processor
  48performance (ie: frequency) between the platform firmware and the OS.
  49
  50The PCC driver (pcc-cpufreq) allows OSPM to take advantage of the PCC
  51interface.
  52
  53OS utilizes the PCC interface to inform platform firmware what frequency the
  54OS wants for a logical processor. The platform firmware attempts to achieve
  55the requested frequency. If the request for the target frequency could not be
  56satisfied by platform firmware, then it usually means that power budget
  57conditions are in place, and "power capping" is taking place.
  58
  591.1 PCC interface:
  60------------------
  61The complete PCC specification is available here:
  62http://www.acpica.org/download/Processor-Clocking-Control-v1p0.pdf
  63
  64PCC relies on a shared memory region that provides a channel for communication
  65between the OS and platform firmware. PCC also implements a "doorbell" that
  66is used by the OS to inform the platform firmware that a command has been
  67sent.
  68
  69The ACPI PCCH() method is used to discover the location of the PCC shared
  70memory region. The shared memory region header contains the "command" and
  71"status" interface. PCCH() also contains details on how to access the platform
  72doorbell.
  73
  74The following commands are supported by the PCC interface:
  75* Get Average Frequency
  76* Set Desired Frequency
  77
  78The ACPI PCCP() method is implemented for each logical processor and is
  79used to discover the offsets for the input and output buffers in the shared
  80memory region.
  81
  82When PCC mode is enabled, the platform will not expose processor performance
  83or throttle states (_PSS, _TSS and related ACPI objects) to OSPM. Therefore,
  84the native P-state driver (such as acpi-cpufreq for Intel, powernow-k8 for
  85AMD) will not load.
  86
  87However, OSPM remains in control of policy. The governor (eg: "ondemand")
  88computes the required performance for each processor based on server workload.
  89The PCC driver fills in the command interface, and the input buffer and
  90communicates the request to the platform firmware. The platform firmware is
  91responsible for delivering the requested performance.
  92
  93Each PCC command is "global" in scope and can affect all the logical CPUs in
  94the system. Therefore, PCC is capable of performing "group" updates. With PCC
  95the OS is capable of getting/setting the frequency of all the logical CPUs in
  96the system with a single call to the BIOS.
  97
  981.1.1 Get Average Frequency:
  99----------------------------
 100This command is used by the OSPM to query the running frequency of the
 101processor since the last time this command was completed. The output buffer
 102indicates the average unhalted frequency of the logical processor expressed as
 103a percentage of the nominal (ie: maximum) CPU frequency. The output buffer
 104also signifies if the CPU frequency is limited by a power budget condition.
 105
 1061.1.2 Set Desired Frequency:
 107----------------------------
 108This command is used by the OSPM to communicate to the platform firmware the
 109desired frequency for a logical processor. The output buffer is currently
 110ignored by OSPM. The next invocation of "Get Average Frequency" will inform
 111OSPM if the desired frequency was achieved or not.
 112
 1131.2 Platforms affected:
 114-----------------------
 115The PCC driver will load on any system where the platform firmware:
 116* supports the PCC interface, and the associated PCCH() and PCCP() methods
 117* assumes responsibility for managing the hardware clocking controls in order
 118to deliver the requested processor performance
 119
 120Currently, certain HP ProLiant platforms implement the PCC interface. On those
 121platforms PCC is the "default" choice.
 122
 123However, it is possible to disable this interface via a BIOS setting. In
 124such an instance, as is also the case on platforms where the PCC interface
 125is not implemented, the PCC driver will fail to load silently.
 126
 1272. Driver and /sys details:
 128---------------------------
 129When the driver loads, it merely prints the lowest and the highest CPU
 130frequencies supported by the platform firmware.
 131
 132The PCC driver loads with a message such as:
 133pcc-cpufreq: (v1.00.00) driver loaded with frequency limits: 1600 MHz, 2933
 134MHz
 135
 136This means that the OPSM can request the CPU to run at any frequency in
 137between the limits (1600 MHz, and 2933 MHz) specified in the message.
 138
 139Internally, there is no need for the driver to convert the "target" frequency
 140to a corresponding P-state.
 141
 142The VERSION number for the driver will be of the format v.xy.ab.
 143eg: 1.00.02
 144   ----- --
 145    |    |
 146    |    -- this will increase with bug fixes/enhancements to the driver
 147    |-- this is the version of the PCC specification the driver adheres to
 148
 149
 150The following is a brief discussion on some of the fields exported via the
 151/sys filesystem and how their values are affected by the PCC driver:
 152
 1532.1 scaling_available_frequencies:
 154----------------------------------
 155scaling_available_frequencies is not created in /sys. No intermediate
 156frequencies need to be listed because the BIOS will try to achieve any
 157frequency, within limits, requested by the governor. A frequency does not have
 158to be strictly associated with a P-state.
 159
 1602.2 cpuinfo_transition_latency:
 161-------------------------------
 162The cpuinfo_transition_latency field is 0. The PCC specification does
 163not include a field to expose this value currently.
 164
 1652.3 cpuinfo_cur_freq:
 166---------------------
 167A) Often cpuinfo_cur_freq will show a value different than what is declared
 168in the scaling_available_frequencies or scaling_cur_freq, or scaling_max_freq.
 169This is due to "turbo boost" available on recent Intel processors. If certain
 170conditions are met the BIOS can achieve a slightly higher speed than requested
 171by OSPM. An example:
 172
 173scaling_cur_freq        : 2933000
 174cpuinfo_cur_freq        : 3196000
 175
 176B) There is a round-off error associated with the cpuinfo_cur_freq value.
 177Since the driver obtains the current frequency as a "percentage" (%) of the
 178nominal frequency from the BIOS, sometimes, the values displayed by
 179scaling_cur_freq and cpuinfo_cur_freq may not match. An example:
 180
 181scaling_cur_freq        : 1600000
 182cpuinfo_cur_freq        : 1583000
 183
 184In this example, the nominal frequency is 2933 MHz. The driver obtains the
 185current frequency, cpuinfo_cur_freq, as 54% of the nominal frequency:
 186
 187        54% of 2933 MHz = 1583 MHz
 188
 189Nominal frequency is the maximum frequency of the processor, and it usually
 190corresponds to the frequency of the P0 P-state.
 191
 1922.4 related_cpus:
 193-----------------
 194The related_cpus field is identical to affected_cpus.
 195
 196affected_cpus   : 4
 197related_cpus    : 4
 198
 199Currently, the PCC driver does not evaluate _PSD. The platforms that support
 200PCC do not implement SW_ALL. So OSPM doesn't need to perform any coordination
 201to ensure that the same frequency is requested of all dependent CPUs.
 202
 2033. Caveats:
 204-----------
 205The "cpufreq_stats" module in its present form cannot be loaded and
 206expected to work with the PCC driver. Since the "cpufreq_stats" module
 207provides information wrt each P-state, it is not applicable to the PCC driver.
 208
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