mirror of
https://github.com/AuxXxilium/linux_dsm_epyc7002.git
synced 2024-12-24 03:22:23 +07:00
790d849bf8
The cpufreq documentation specifies
policy->cpuinfo.transition_latency the time it takes on this CPU to
switch between two frequencies in
nanoseconds (if appropriate, else
specify CPUFREQ_ETERNAL)
currently pcc-cpufreq does not expose the value and sets it to zero. I
changed the pcc-cpufreq driver and it's documentation to conform to the
default value specified in Documentation/cpu-freq/cpu-drivers.txt
Signed-off-by: Jacob Tanenbaum <jtanenba@redhat.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
208 lines
8.3 KiB
Plaintext
208 lines
8.3 KiB
Plaintext
/*
|
|
* 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
|
|
* the Free Software Foundation; version 2 of the License.
|
|
*
|
|
* This program is distributed in the hope that it will be useful, but
|
|
* WITHOUT ANY WARRANTY; without even the implied warranty of
|
|
* 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 CPUFREQ_ETERNAL. 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.
|