[PATCH 2/8] Documentation: amd-pstate: grammar and sentence structure updates
Paolo Pisati
paolo.pisati at canonical.com
Mon Nov 28 16:47:49 UTC 2022
From: Jan Engelhardt <jengelh at inai.de>
BugLink: https://bugs.launchpad.net/bugs/1998106
Signed-off-by: Jan Engelhardt <jengelh at inai.de>
Reviewed-by: Randy Dunlap <rdunlap at infradead.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki at intel.com>
(cherry picked from commit 7000ef38052b219583f17a10ef2e7ca99b5e585e)
Signed-off-by: Paolo Pisati <paolo.pisati at canonical.com>
---
Documentation/admin-guide/pm/amd-pstate.rst | 135 ++++++++++----------
1 file changed, 67 insertions(+), 68 deletions(-)
diff --git a/Documentation/admin-guide/pm/amd-pstate.rst b/Documentation/admin-guide/pm/amd-pstate.rst
index 1923cb25073b..83b58eb4ab4d 100644
--- a/Documentation/admin-guide/pm/amd-pstate.rst
+++ b/Documentation/admin-guide/pm/amd-pstate.rst
@@ -19,7 +19,7 @@ Linux kernel. The new mechanism is based on Collaborative Processor
Performance Control (CPPC) which provides finer grain frequency management
than legacy ACPI hardware P-States. Current AMD CPU/APU platforms are using
the ACPI P-states driver to manage CPU frequency and clocks with switching
-only in 3 P-states. CPPC replaces the ACPI P-states controls, allows a
+only in 3 P-states. CPPC replaces the ACPI P-states controls and allows a
flexible, low-latency interface for the Linux kernel to directly
communicate the performance hints to hardware.
@@ -27,7 +27,7 @@ communicate the performance hints to hardware.
``ondemand``, etc. to manage the performance hints which are provided by
CPPC hardware functionality that internally follows the hardware
specification (for details refer to AMD64 Architecture Programmer's Manual
-Volume 2: System Programming [1]_). Currently ``amd-pstate`` supports basic
+Volume 2: System Programming [1]_). Currently, ``amd-pstate`` supports basic
frequency control function according to kernel governors on some of the
Zen2 and Zen3 processors, and we will implement more AMD specific functions
in future after we verify them on the hardware and SBIOS.
@@ -41,9 +41,9 @@ continuous, abstract, and unit-less performance value in a scale that is
not tied to a specific performance state / frequency. This is an ACPI
standard [2]_ which software can specify application performance goals and
hints as a relative target to the infrastructure limits. AMD processors
-provides the low latency register model (MSR) instead of AML code
+provide the low latency register model (MSR) instead of an AML code
interpreter for performance adjustments. ``amd-pstate`` will initialize a
-``struct cpufreq_driver`` instance ``amd_pstate_driver`` with the callbacks
+``struct cpufreq_driver`` instance, ``amd_pstate_driver``, with the callbacks
to manage each performance update behavior. ::
Highest Perf ------>+-----------------------+ +-----------------------+
@@ -91,26 +91,26 @@ AMD CPPC Performance Capability
Highest Performance (RO)
.........................
-It is the absolute maximum performance an individual processor may reach,
+This is the absolute maximum performance an individual processor may reach,
assuming ideal conditions. This performance level may not be sustainable
for long durations and may only be achievable if other platform components
-are in a specific state; for example, it may require other processors be in
+are in a specific state; for example, it may require other processors to be in
an idle state. This would be equivalent to the highest frequencies
supported by the processor.
Nominal (Guaranteed) Performance (RO)
......................................
-It is the maximum sustained performance level of the processor, assuming
-ideal operating conditions. In absence of an external constraint (power,
-thermal, etc.) this is the performance level the processor is expected to
+This is the maximum sustained performance level of the processor, assuming
+ideal operating conditions. In the absence of an external constraint (power,
+thermal, etc.), this is the performance level the processor is expected to
be able to maintain continuously. All cores/processors are expected to be
able to sustain their nominal performance state simultaneously.
Lowest non-linear Performance (RO)
...................................
-It is the lowest performance level at which nonlinear power savings are
+This is the lowest performance level at which nonlinear power savings are
achieved, for example, due to the combined effects of voltage and frequency
scaling. Above this threshold, lower performance levels should be generally
more energy efficient than higher performance levels. This register
@@ -119,7 +119,7 @@ effectively conveys the most efficient performance level to ``amd-pstate``.
Lowest Performance (RO)
........................
-It is the absolute lowest performance level of the processor. Selecting a
+This is the absolute lowest performance level of the processor. Selecting a
performance level lower than the lowest nonlinear performance level may
cause an efficiency penalty but should reduce the instantaneous power
consumption of the processor.
@@ -149,14 +149,14 @@ a relative number. This can be expressed as percentage of nominal
performance (infrastructure max). Below the nominal sustained performance
level, desired performance expresses the average performance level of the
processor subject to hardware. Above the nominal performance level,
-processor must provide at least nominal performance requested and go higher
+the processor must provide at least nominal performance requested and go higher
if current operating conditions allow.
Energy Performance Preference (EPP) (RW)
.........................................
-Provides a hint to the hardware if software wants to bias toward performance
-(0x0) or energy efficiency (0xff).
+This attribute provides a hint to the hardware if software wants to bias
+toward performance (0x0) or energy efficiency (0xff).
Key Governors Support
@@ -173,35 +173,34 @@ operating frequencies supported by the hardware. Users can check the
``amd-pstate`` mainly supports ``schedutil`` and ``ondemand`` for dynamic
frequency control. It is to fine tune the processor configuration on
``amd-pstate`` to the ``schedutil`` with CPU CFS scheduler. ``amd-pstate``
-registers adjust_perf callback to implement the CPPC similar performance
-update behavior. It is initialized by ``sugov_start`` and then populate the
-CPU's update_util_data pointer to assign ``sugov_update_single_perf`` as
-the utilization update callback function in CPU scheduler. CPU scheduler
-will call ``cpufreq_update_util`` and assign the target performance
-according to the ``struct sugov_cpu`` that utilization update belongs to.
-Then ``amd-pstate`` updates the desired performance according to the CPU
+registers the adjust_perf callback to implement performance update behavior
+similar to CPPC. It is initialized by ``sugov_start`` and then populates the
+CPU's update_util_data pointer to assign ``sugov_update_single_perf`` as the
+utilization update callback function in the CPU scheduler. The CPU scheduler
+will call ``cpufreq_update_util`` and assigns the target performance according
+to the ``struct sugov_cpu`` that the utilization update belongs to.
+Then, ``amd-pstate`` updates the desired performance according to the CPU
scheduler assigned.
Processor Support
=======================
-The ``amd-pstate`` initialization will fail if the _CPC in ACPI SBIOS is
-not existed at the detected processor, and it uses ``acpi_cpc_valid`` to
-check the _CPC existence. All Zen based processors support legacy ACPI
-hardware P-States function, so while the ``amd-pstate`` fails to be
-initialized, the kernel will fall back to initialize ``acpi-cpufreq``
-driver.
+The ``amd-pstate`` initialization will fail if the ``_CPC`` entry in the ACPI
+SBIOS does not exist in the detected processor. It uses ``acpi_cpc_valid``
+to check the existence of ``_CPC``. All Zen based processors support the legacy
+ACPI hardware P-States function, so when ``amd-pstate`` fails initialization,
+the kernel will fall back to initialize the ``acpi-cpufreq`` driver.
There are two types of hardware implementations for ``amd-pstate``: one is
`Full MSR Support <perf_cap_>`_ and another is `Shared Memory Support
-<perf_cap_>`_. It can use :c:macro:`X86_FEATURE_CPPC` feature flag (for
-details refer to Processor Programming Reference (PPR) for AMD Family
-19h Model 51h, Revision A1 Processors [3]_) to indicate the different
-types. ``amd-pstate`` is to register different ``static_call`` instances
-for different hardware implementations.
+<perf_cap_>`_. It can use the :c:macro:`X86_FEATURE_CPPC` feature flag to
+indicate the different types. (For details, refer to the Processor Programming
+Reference (PPR) for AMD Family 19h Model 51h, Revision A1 Processors [3]_.)
+``amd-pstate`` is to register different ``static_call`` instances for different
+hardware implementations.
-Currently, some of Zen2 and Zen3 processors support ``amd-pstate``. In the
+Currently, some of the Zen2 and Zen3 processors support ``amd-pstate``. In the
future, it will be supported on more and more AMD processors.
Full MSR Support
@@ -210,18 +209,18 @@ Full MSR Support
Some new Zen3 processors such as Cezanne provide the MSR registers directly
while the :c:macro:`X86_FEATURE_CPPC` CPU feature flag is set.
``amd-pstate`` can handle the MSR register to implement the fast switch
-function in ``CPUFreq`` that can shrink latency of frequency control on the
-interrupt context. The functions with ``pstate_xxx`` prefix represent the
-operations of MSR registers.
+function in ``CPUFreq`` that can reduce the latency of frequency control in
+interrupt context. The functions with a ``pstate_xxx`` prefix represent the
+operations on MSR registers.
Shared Memory Support
----------------------
-If :c:macro:`X86_FEATURE_CPPC` CPU feature flag is not set, that means the
-processor supports shared memory solution. In this case, ``amd-pstate``
+If the :c:macro:`X86_FEATURE_CPPC` CPU feature flag is not set, the
+processor supports the shared memory solution. In this case, ``amd-pstate``
uses the ``cppc_acpi`` helper methods to implement the callback functions
-that defined on ``static_call``. The functions with ``cppc_xxx`` prefix
-represent the operations of acpi cppc helpers for shared memory solution.
+that are defined on ``static_call``. The functions with the ``cppc_xxx`` prefix
+represent the operations of ACPI CPPC helpers for the shared memory solution.
AMD P-States and ACPI hardware P-States always can be supported in one
@@ -234,7 +233,7 @@ User Space Interface in ``sysfs``
==================================
``amd-pstate`` exposes several global attributes (files) in ``sysfs`` to
-control its functionality at the system level. They located in the
+control its functionality at the system level. They are located in the
``/sys/devices/system/cpu/cpufreq/policyX/`` directory and affect all CPUs. ::
root at hr-test1:/home/ray# ls /sys/devices/system/cpu/cpufreq/policy0/*amd*
@@ -246,38 +245,38 @@ control its functionality at the system level. They located in the
``amd_pstate_highest_perf / amd_pstate_max_freq``
Maximum CPPC performance and CPU frequency that the driver is allowed to
-set in percent of the maximum supported CPPC performance level (the highest
+set, in percent of the maximum supported CPPC performance level (the highest
performance supported in `AMD CPPC Performance Capability <perf_cap_>`_).
-In some of ASICs, the highest CPPC performance is not the one in the _CPC
-table, so we need to expose it to sysfs. If boost is not active but
-supported, this maximum frequency will be larger than the one in
+In some ASICs, the highest CPPC performance is not the one in the ``_CPC``
+table, so we need to expose it to sysfs. If boost is not active, but
+still supported, this maximum frequency will be larger than the one in
``cpuinfo``.
This attribute is read-only.
``amd_pstate_lowest_nonlinear_freq``
-The lowest non-linear CPPC CPU frequency that the driver is allowed to set
-in percent of the maximum supported CPPC performance level (Please see the
+The lowest non-linear CPPC CPU frequency that the driver is allowed to set,
+in percent of the maximum supported CPPC performance level. (Please see the
lowest non-linear performance in `AMD CPPC Performance Capability
-<perf_cap_>`_).
+<perf_cap_>`_.)
This attribute is read-only.
-For other performance and frequency values, we can read them back from
+Other performance and frequency values can be read back from
``/sys/devices/system/cpu/cpuX/acpi_cppc/``, see :ref:`cppc_sysfs`.
``amd-pstate`` vs ``acpi-cpufreq``
======================================
-On majority of AMD platforms supported by ``acpi-cpufreq``, the ACPI tables
-provided by the platform firmware used for CPU performance scaling, but
-only provides 3 P-states on AMD processors.
-However, on modern AMD APU and CPU series, it provides the collaborative
-processor performance control according to ACPI protocol and customize this
-for AMD platforms. That is fine-grain and continuous frequency range
+On the majority of AMD platforms supported by ``acpi-cpufreq``, the ACPI tables
+provided by the platform firmware are used for CPU performance scaling, but
+only provide 3 P-states on AMD processors.
+However, on modern AMD APU and CPU series, hardware provides the Collaborative
+Processor Performance Control according to the ACPI protocol and customizes this
+for AMD platforms. That is, fine-grained and continuous frequency ranges
instead of the legacy hardware P-states. ``amd-pstate`` is the kernel
-module which supports the new AMD P-States mechanism on most of future AMD
-platforms. The AMD P-States mechanism will be the more performance and energy
+module which supports the new AMD P-States mechanism on most of the future AMD
+platforms. The AMD P-States mechanism is the more performance and energy
efficiency frequency management method on AMD processors.
Kernel Module Options for ``amd-pstate``
@@ -287,25 +286,25 @@ Kernel Module Options for ``amd-pstate``
Use a module param (shared_mem) to enable related processors manually with
**amd_pstate.shared_mem=1**.
Due to the performance issue on the processors with `Shared Memory Support
-<perf_cap_>`_, so we disable it for the moment and will enable this by default
-once we address performance issue on this solution.
+<perf_cap_>`_, we disable it presently and will re-enable this by default
+once we address performance issue with this solution.
-The way to check whether current processor is `Full MSR Support <perf_cap_>`_
+To check whether the current processor is using `Full MSR Support <perf_cap_>`_
or `Shared Memory Support <perf_cap_>`_ : ::
ray at hr-test1:~$ lscpu | grep cppc
Flags: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdtscp lm constant_tsc rep_good nopl nonstop_tsc cpuid extd_apicid aperfmperf rapl pni pclmulqdq monitor ssse3 fma cx16 sse4_1 sse4_2 x2apic movbe popcnt aes xsave avx f16c rdrand lahf_lm cmp_legacy svm extapic cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw ibs skinit wdt tce topoext perfctr_core perfctr_nb bpext perfctr_llc mwaitx cpb cat_l3 cdp_l3 hw_pstate ssbd mba ibrs ibpb stibp vmmcall fsgsbase bmi1 avx2 smep bmi2 erms invpcid cqm rdt_a rdseed adx smap clflushopt clwb sha_ni xsaveopt xsavec xgetbv1 xsaves cqm_llc cqm_occup_llc cqm_mbm_total cqm_mbm_local clzero irperf xsaveerptr rdpru wbnoinvd cppc arat npt lbrv svm_lock nrip_save tsc_scale vmcb_clean flushbyasid decodeassists pausefilter pfthreshold avic v_vmsave_vmload vgif v_spec_ctrl umip pku ospke vaes vpclmulqdq rdpid overflow_recov succor smca fsrm
-If CPU Flags have cppc, then this processor supports `Full MSR Support
-<perf_cap_>`_. Otherwise it supports `Shared Memory Support <perf_cap_>`_.
+If the CPU flags have ``cppc``, then this processor supports `Full MSR Support
+<perf_cap_>`_. Otherwise, it supports `Shared Memory Support <perf_cap_>`_.
``cpupower`` tool support for ``amd-pstate``
===============================================
-``amd-pstate`` is supported on ``cpupower`` tool that can be used to dump the frequency
-information. And it is in progress to support more and more operations for new
-``amd-pstate`` module with this tool. ::
+``amd-pstate`` is supported by the ``cpupower`` tool, which can be used to dump
+frequency information. Development is in progress to support more and more
+operations for the new ``amd-pstate`` module with this tool. ::
root at hr-test1:/home/ray# cpupower frequency-info
analyzing CPU 0:
@@ -336,10 +335,10 @@ Trace Events
--------------
There are two static trace events that can be used for ``amd-pstate``
-diagnostics. One of them is the cpu_frequency trace event generally used
+diagnostics. One of them is the ``cpu_frequency`` trace event generally used
by ``CPUFreq``, and the other one is the ``amd_pstate_perf`` trace event
specific to ``amd-pstate``. The following sequence of shell commands can
-be used to enable them and see their output (if the kernel is generally
+be used to enable them and see their output (if the kernel is
configured to support event tracing). ::
root at hr-test1:/home/ray# cd /sys/kernel/tracing/
@@ -364,7 +363,7 @@ configured to support event tracing). ::
<idle>-0 [003] d.s.. 4995.980971: amd_pstate_perf: amd_min_perf=85 amd_des_perf=85 amd_max_perf=166 cpu_id=3 changed=false fast_switch=true
<idle>-0 [011] d.s.. 4995.980996: amd_pstate_perf: amd_min_perf=85 amd_des_perf=85 amd_max_perf=166 cpu_id=11 changed=false fast_switch=true
-The cpu_frequency trace event will be triggered either by the ``schedutil`` scaling
+The ``cpu_frequency`` trace event will be triggered either by the ``schedutil`` scaling
governor (for the policies it is attached to), or by the ``CPUFreq`` core (for the
policies with other scaling governors).
--
2.34.1
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