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