About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / cpu-freq / pcc-cpufreq.txt




Custom Search

Based on kernel version 3.15.4. Page generated on 2014-07-07 09:00 EST.

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.
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.