[CPUFREQ] Processor Clocking Control interface driver

/*

 *  pcc-cpufreq.txt - PCC interface documentation

 *

 *  Copyright (C) 2009 Red Hat, Matthew Garrett <mjg@redhat.com>

 *  Copyright (C) 2009 Hewlett-Packard Development Company, L.P.

 *      Nagananda Chumbalkar <nagananda.chumbalkar@hp.com>

 *

 * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 *

 *  This program is free software; you can redistribute it and/or modify

 *  it under the terms of the GNU General Public License as published by

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 *

 *  This program is distributed in the hope that it will be useful, but

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 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE, GOOD TITLE or NON

 *  INFRINGEMENT. See the GNU General Public License for more details.

 *

 *  You should have received a copy of the GNU General Public License along

 *  with this program; if not, write to the Free Software Foundation, Inc.,

 *  675 Mass Ave, Cambridge, MA 02139, USA.

 *

 * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 */

			Processor Clocking Control Driver

			---------------------------------

Contents:

---------

1.	Introduction

1.1	PCC interface

1.1.1   Get Average Frequency

1.1.2	Set Desired Frequency

1.2	Platforms affected

2.	Driver and /sys details

2.1	scaling_available_frequencies

2.2	cpuinfo_transition_latency

2.3	cpuinfo_cur_freq

2.4	related_cpus

3.	Caveats

1. Introduction:

----------------

Processor Clocking Control (PCC) is an interface between the platform

firmware and OSPM. It is a mechanism for coordinating processor

performance (ie: frequency) between the platform firmware and the OS.

The PCC driver (pcc-cpufreq) allows OSPM to take advantage of the PCC

interface.

OS utilizes the PCC interface to inform platform firmware what frequency the

OS wants for a logical processor. The platform firmware attempts to achieve

the requested frequency. If the request for the target frequency could not be

satisfied by platform firmware, then it usually means that power budget

conditions are in place, and "power capping" is taking place.

1.1 PCC interface:

------------------

The complete PCC specification is available here:

http://www.acpica.org/download/Processor-Clocking-Control-v1p0.pdf

PCC relies on a shared memory region that provides a channel for communication

between the OS and platform firmware. PCC also implements a "doorbell" that

is used by the OS to inform the platform firmware that a command has been

sent.

The ACPI PCCH() method is used to discover the location of the PCC shared

memory region. The shared memory region header contains the "command" and

"status" interface. PCCH() also contains details on how to access the platform

doorbell.

The following commands are supported by the PCC interface:

* Get Average Frequency

* Set Desired Frequency

The ACPI PCCP() method is implemented for each logical processor and is

used to discover the offsets for the input and output buffers in the shared

memory region.

When PCC mode is enabled, the platform will not expose processor performance

or throttle states (_PSS, _TSS and related ACPI objects) to OSPM. Therefore,

the native P-state driver (such as acpi-cpufreq for Intel, powernow-k8 for

AMD) will not load.

However, OSPM remains in control of policy. The governor (eg: "ondemand")

computes the required performance for each processor based on server workload.

The PCC driver fills in the command interface, and the input buffer and

communicates the request to the platform firmware. The platform firmware is

responsible for delivering the requested performance.

Each PCC command is "global" in scope and can affect all the logical CPUs in

the system. Therefore, PCC is capable of performing "group" updates. With PCC

the OS is capable of getting/setting the frequency of all the logical CPUs in

the system with a single call to the BIOS.

1.1.1 Get Average Frequency:

----------------------------

This command is used by the OSPM to query the running frequency of the

processor since the last time this command was completed. The output buffer

indicates the average unhalted frequency of the logical processor expressed as

a percentage of the nominal (ie: maximum) CPU frequency. The output buffer

also signifies if the CPU frequency is limited by a power budget condition.

1.1.2 Set Desired Frequency:

----------------------------

This command is used by the OSPM to communicate to the platform firmware the

desired frequency for a logical processor. The output buffer is currently

ignored by OSPM. The next invocation of "Get Average Frequency" will inform

OSPM if the desired frequency was achieved or not.

1.2 Platforms affected:

-----------------------

The PCC driver will load on any system where the platform firmware:

* supports the PCC interface, and the associated PCCH() and PCCP() methods

* assumes responsibility for managing the hardware clocking controls in order

to deliver the requested processor performance

Currently, certain HP ProLiant platforms implement the PCC interface. On those

platforms PCC is the "default" choice.

However, it is possible to disable this interface via a BIOS setting. In

such an instance, as is also the case on platforms where the PCC interface

is not implemented, the PCC driver will fail to load silently.

2. Driver and /sys details:

---------------------------

When the driver loads, it merely prints the lowest and the highest CPU

frequencies supported by the platform firmware.

The PCC driver loads with a message such as:

pcc-cpufreq: (v1.00.00) driver loaded with frequency limits: 1600 MHz, 2933

MHz

This means that the OPSM can request the CPU to run at any frequency in

between the limits (1600 MHz, and 2933 MHz) specified in the message.

Internally, there is no need for the driver to convert the "target" frequency

to a corresponding P-state.

The VERSION number for the driver will be of the format v.xy.ab.

eg: 1.00.02

   ----- --

    |    |

    |    -- this will increase with bug fixes/enhancements to the driver

    |-- this is the version of the PCC specification the driver adheres to

The following is a brief discussion on some of the fields exported via the

/sys filesystem and how their values are affected by the PCC driver:

2.1 scaling_available_frequencies:

----------------------------------

scaling_available_frequencies is not created in /sys. No intermediate

frequencies need to be listed because the BIOS will try to achieve any

frequency, within limits, requested by the governor. A frequency does not have

to be strictly associated with a P-state.

2.2 cpuinfo_transition_latency:

-------------------------------

The cpuinfo_transition_latency field is 0. The PCC specification does

not include a field to expose this value currently.

2.3 cpuinfo_cur_freq:

---------------------

A) Often cpuinfo_cur_freq will show a value different than what is declared

in the scaling_available_frequencies or scaling_cur_freq, or scaling_max_freq.

This is due to "turbo boost" available on recent Intel processors. If certain

conditions are met the BIOS can achieve a slightly higher speed than requested

by OSPM. An example:

scaling_cur_freq	: 2933000

cpuinfo_cur_freq	: 3196000

B) There is a round-off error associated with the cpuinfo_cur_freq value.

Since the driver obtains the current frequency as a "percentage" (%) of the

nominal frequency from the BIOS, sometimes, the values displayed by

scaling_cur_freq and cpuinfo_cur_freq may not match. An example:

scaling_cur_freq	: 1600000

cpuinfo_cur_freq	: 1583000

In this example, the nominal frequency is 2933 MHz. The driver obtains the

current frequency, cpuinfo_cur_freq, as 54% of the nominal frequency:

	54% of 2933 MHz = 1583 MHz

Nominal frequency is the maximum frequency of the processor, and it usually

corresponds to the frequency of the P0 P-state.

2.4 related_cpus:

-----------------

The related_cpus field is identical to affected_cpus.

affected_cpus	: 4

related_cpus	: 4

Currently, the PCC driver does not evaluate _PSD. The platforms that support

PCC do not implement SW_ALL. So OSPM doesn't need to perform any coordination

to ensure that the same frequency is requested of all dependent CPUs.

3. Caveats:

-----------

The "cpufreq_stats" module in its present form cannot be loaded and

expected to work with the PCC driver. Since the "cpufreq_stats" module

provides information wrt each P-state, it is not applicable to the PCC driver.

comment "CPUFreq processor drivers"

config X86_PCC_CPUFREQ

	tristate "Processor Clocking Control interface driver"

	depends on ACPI && ACPI_PROCESSOR

	help

	  This driver adds support for the PCC interface.

	  For details, take a look at:

	  <file:Documentation/cpu-freq/pcc-cpufreq.txt>.

	  To compile this driver as a module, choose M here: the

	  module will be called pcc-cpufreq.

	  If in doubt, say N.

config X86_ACPI_CPUFREQ

	tristate "ACPI Processor P-States driver"

	select CPU_FREQ_TABLE

obj-$(CONFIG_X86_POWERNOW_K8)		+= powernow-k8.o

obj-$(CONFIG_X86_ACPI_CPUFREQ)		+= acpi-cpufreq.o

obj-$(CONFIG_X86_PCC_CPUFREQ)		+= pcc-cpufreq.o

obj-$(CONFIG_X86_POWERNOW_K6)		+= powernow-k6.o

obj-$(CONFIG_X86_POWERNOW_K7)		+= powernow-k7.o

obj-$(CONFIG_X86_LONGHAUL)		+= longhaul.o

#endif

DEFINE_PER_CPU(struct acpi_processor *, processors);

EXPORT_PER_CPU_SYMBOL(processors);

struct acpi_processor_errata errata __read_mostly;

static int set_no_mwait(const struct dmi_system_id *id)

{