diff options
author | Russell King <rmk+kernel@arm.linux.org.uk> | 2010-03-08 15:21:04 -0500 |
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committer | Russell King <rmk+kernel@arm.linux.org.uk> | 2010-03-08 15:21:04 -0500 |
commit | 988addf82e4c03739375279de73929580a2d4a6a (patch) | |
tree | 989ae1cd4e264bbad80c65f04480486246e7b9f3 /Documentation/cpu-freq/pcc-cpufreq.txt | |
parent | 004c1c7096659d352b83047a7593e91d8a30e3c5 (diff) | |
parent | 25cf84cf377c0aae5dbcf937ea89bc7893db5176 (diff) |
Merge branch 'origin' into devel-stable
Conflicts:
arch/arm/mach-mx2/devices.c
arch/arm/mach-mx2/devices.h
sound/soc/pxa/pxa-ssp.c
Diffstat (limited to 'Documentation/cpu-freq/pcc-cpufreq.txt')
-rw-r--r-- | Documentation/cpu-freq/pcc-cpufreq.txt | 207 |
1 files changed, 207 insertions, 0 deletions
diff --git a/Documentation/cpu-freq/pcc-cpufreq.txt b/Documentation/cpu-freq/pcc-cpufreq.txt new file mode 100644 index 000000000000..9e3c3b33514c --- /dev/null +++ b/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 | |||
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. | ||