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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 | ||
30 | Contents: | |
31 | --------- | |
32 | 1. Introduction | |
33 | 1.1 PCC interface | |
34 | 1.1.1 Get Average Frequency | |
35 | 1.1.2 Set Desired Frequency | |
36 | 1.2 Platforms affected | |
37 | 2. Driver and /sys details | |
38 | 2.1 scaling_available_frequencies | |
39 | 2.2 cpuinfo_transition_latency | |
40 | 2.3 cpuinfo_cur_freq | |
41 | 2.4 related_cpus | |
42 | 3. Caveats | |
43 | ||
44 | 1. Introduction: | |
45 | ---------------- | |
46 | Processor Clocking Control (PCC) is an interface between the platform | |
47 | firmware and OSPM. It is a mechanism for coordinating processor | |
48 | performance (ie: frequency) between the platform firmware and the OS. | |
49 | ||
50 | The PCC driver (pcc-cpufreq) allows OSPM to take advantage of the PCC | |
51 | interface. | |
52 | ||
53 | OS utilizes the PCC interface to inform platform firmware what frequency the | |
54 | OS wants for a logical processor. The platform firmware attempts to achieve | |
55 | the requested frequency. If the request for the target frequency could not be | |
56 | satisfied by platform firmware, then it usually means that power budget | |
57 | conditions are in place, and "power capping" is taking place. | |
58 | ||
59 | 1.1 PCC interface: | |
60 | ------------------ | |
61 | The complete PCC specification is available here: | |
62 | http://www.acpica.org/download/Processor-Clocking-Control-v1p0.pdf | |
63 | ||
64 | PCC relies on a shared memory region that provides a channel for communication | |
65 | between the OS and platform firmware. PCC also implements a "doorbell" that | |
66 | is used by the OS to inform the platform firmware that a command has been | |
67 | sent. | |
68 | ||
69 | The ACPI PCCH() method is used to discover the location of the PCC shared | |
70 | memory region. The shared memory region header contains the "command" and | |
71 | "status" interface. PCCH() also contains details on how to access the platform | |
72 | doorbell. | |
73 | ||
74 | The following commands are supported by the PCC interface: | |
75 | * Get Average Frequency | |
76 | * Set Desired Frequency | |
77 | ||
78 | The ACPI PCCP() method is implemented for each logical processor and is | |
79 | used to discover the offsets for the input and output buffers in the shared | |
80 | memory region. | |
81 | ||
82 | When PCC mode is enabled, the platform will not expose processor performance | |
83 | or throttle states (_PSS, _TSS and related ACPI objects) to OSPM. Therefore, | |
84 | the native P-state driver (such as acpi-cpufreq for Intel, powernow-k8 for | |
85 | AMD) will not load. | |
86 | ||
87 | However, OSPM remains in control of policy. The governor (eg: "ondemand") | |
88 | computes the required performance for each processor based on server workload. | |
89 | The PCC driver fills in the command interface, and the input buffer and | |
90 | communicates the request to the platform firmware. The platform firmware is | |
91 | responsible for delivering the requested performance. | |
92 | ||
93 | Each PCC command is "global" in scope and can affect all the logical CPUs in | |
94 | the system. Therefore, PCC is capable of performing "group" updates. With PCC | |
95 | the OS is capable of getting/setting the frequency of all the logical CPUs in | |
96 | the system with a single call to the BIOS. | |
97 | ||
98 | 1.1.1 Get Average Frequency: | |
99 | ---------------------------- | |
100 | This command is used by the OSPM to query the running frequency of the | |
101 | processor since the last time this command was completed. The output buffer | |
102 | indicates the average unhalted frequency of the logical processor expressed as | |
103 | a percentage of the nominal (ie: maximum) CPU frequency. The output buffer | |
104 | also signifies if the CPU frequency is limited by a power budget condition. | |
105 | ||
106 | 1.1.2 Set Desired Frequency: | |
107 | ---------------------------- | |
108 | This command is used by the OSPM to communicate to the platform firmware the | |
109 | desired frequency for a logical processor. The output buffer is currently | |
110 | ignored by OSPM. The next invocation of "Get Average Frequency" will inform | |
111 | OSPM if the desired frequency was achieved or not. | |
112 | ||
113 | 1.2 Platforms affected: | |
114 | ----------------------- | |
115 | The 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 | |
118 | to deliver the requested processor performance | |
119 | ||
120 | Currently, certain HP ProLiant platforms implement the PCC interface. On those | |
121 | platforms PCC is the "default" choice. | |
122 | ||
123 | However, it is possible to disable this interface via a BIOS setting. In | |
124 | such an instance, as is also the case on platforms where the PCC interface | |
125 | is not implemented, the PCC driver will fail to load silently. | |
126 | ||
127 | 2. Driver and /sys details: | |
128 | --------------------------- | |
129 | When the driver loads, it merely prints the lowest and the highest CPU | |
130 | frequencies supported by the platform firmware. | |
131 | ||
132 | The PCC driver loads with a message such as: | |
133 | pcc-cpufreq: (v1.00.00) driver loaded with frequency limits: 1600 MHz, 2933 | |
134 | MHz | |
135 | ||
136 | This means that the OPSM can request the CPU to run at any frequency in | |
137 | between the limits (1600 MHz, and 2933 MHz) specified in the message. | |
138 | ||
139 | Internally, there is no need for the driver to convert the "target" frequency | |
140 | to a corresponding P-state. | |
141 | ||
142 | The VERSION number for the driver will be of the format v.xy.ab. | |
143 | eg: 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 | ||
150 | The 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 | ||
153 | 2.1 scaling_available_frequencies: | |
154 | ---------------------------------- | |
155 | scaling_available_frequencies is not created in /sys. No intermediate | |
156 | frequencies need to be listed because the BIOS will try to achieve any | |
157 | frequency, within limits, requested by the governor. A frequency does not have | |
158 | to be strictly associated with a P-state. | |
159 | ||
160 | 2.2 cpuinfo_transition_latency: | |
161 | ------------------------------- | |
162 | The cpuinfo_transition_latency field is 0. The PCC specification does | |
163 | not include a field to expose this value currently. | |
164 | ||
165 | 2.3 cpuinfo_cur_freq: | |
166 | --------------------- | |
167 | A) Often cpuinfo_cur_freq will show a value different than what is declared | |
168 | in the scaling_available_frequencies or scaling_cur_freq, or scaling_max_freq. | |
169 | This is due to "turbo boost" available on recent Intel processors. If certain | |
170 | conditions are met the BIOS can achieve a slightly higher speed than requested | |
171 | by OSPM. An example: | |
172 | ||
173 | scaling_cur_freq : 2933000 | |
174 | cpuinfo_cur_freq : 3196000 | |
175 | ||
176 | B) There is a round-off error associated with the cpuinfo_cur_freq value. | |
177 | Since the driver obtains the current frequency as a "percentage" (%) of the | |
178 | nominal frequency from the BIOS, sometimes, the values displayed by | |
179 | scaling_cur_freq and cpuinfo_cur_freq may not match. An example: | |
180 | ||
181 | scaling_cur_freq : 1600000 | |
182 | cpuinfo_cur_freq : 1583000 | |
183 | ||
184 | In this example, the nominal frequency is 2933 MHz. The driver obtains the | |
185 | current frequency, cpuinfo_cur_freq, as 54% of the nominal frequency: | |
186 | ||
187 | 54% of 2933 MHz = 1583 MHz | |
188 | ||
189 | Nominal frequency is the maximum frequency of the processor, and it usually | |
190 | corresponds to the frequency of the P0 P-state. | |
191 | ||
192 | 2.4 related_cpus: | |
193 | ----------------- | |
194 | The related_cpus field is identical to affected_cpus. | |
195 | ||
196 | affected_cpus : 4 | |
197 | related_cpus : 4 | |
198 | ||
199 | Currently, the PCC driver does not evaluate _PSD. The platforms that support | |
200 | PCC do not implement SW_ALL. So OSPM doesn't need to perform any coordination | |
201 | to ensure that the same frequency is requested of all dependent CPUs. | |
202 | ||
203 | 3. Caveats: | |
204 | ----------- | |
205 | The "cpufreq_stats" module in its present form cannot be loaded and | |
206 | expected to work with the PCC driver. Since the "cpufreq_stats" module | |
207 | provides information wrt each P-state, it is not applicable to the PCC driver. |