[PATCH v4 22/22] Documentation: amd-pstate: add amd-pstate driver introduction

[Huang Rui]

Introduce the amd-pstate driver design and implementation.

Signed-off-by: Huang Rui <ray.huang@xxxxxxx>
---
 Documentation/admin-guide/pm/amd-pstate.rst   | 373 ++++++++++++++++++
 .../admin-guide/pm/working-state.rst          |   1 +
 2 files changed, 374 insertions(+)
 create mode 100644 Documentation/admin-guide/pm/amd-pstate.rst

diff --git a/Documentation/admin-guide/pm/amd-pstate.rst b/Documentation/admin-guide/pm/amd-pstate.rst
new file mode 100644
index 000000000000..24a88476fc69
--- /dev/null
+++ b/Documentation/admin-guide/pm/amd-pstate.rst
@@ -0,0 +1,373 @@
+.. SPDX-License-Identifier: GPL-2.0
+.. include:: <isonum.txt>
+
+===============================================
+``amd-pstate`` CPU Performance Scaling Driver
+===============================================
+
+:Copyright: |copy| 2021 Advanced Micro Devices, Inc.
+
+:Author: Huang Rui <ray.huang@xxxxxxx>
+
+
+Introduction
+===================
+
+``amd-pstate`` is the AMD CPU performance scaling driver that introduces a
+new CPU frequency control mechanism on modern AMD APU and CPU series in
+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
+flexible, low-latency interface for the Linux kernel to directly
+communicate the performance hints to hardware.
+
+``amd-pstate`` leverages the Linux kernel governors such as ``schedutil``,
+``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
+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.
+
+
+AMD CPPC Overview
+=======================
+
+Collaborative Processor Performance Control (CPPC) interface enumerates a
+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
+interpreter for performance adjustments. ``amd-pstate`` will initialize a
+``struct cpufreq_driver`` instance ``amd_pstate_driver`` with the callbacks
+to manage each performance update behavior. ::
+
+ Highest Perf ------>+-----------------------+                         +-----------------------+
+                     |                       |                         |                       |
+                     |                       |                         |                       |
+                     |                       |          Max Perf  ---->|                       |
+                     |                       |                         |                       |
+                     |                       |                         |                       |
+ Nominal Perf ------>+-----------------------+                         +-----------------------+
+                     |                       |                         |                       |
+                     |                       |                         |                       |
+                     |                       |                         |                       |
+                     |                       |                         |                       |
+                     |                       |                         |                       |
+                     |                       |                         |                       |
+                     |                       |      Desired Perf  ---->|                       |
+                     |                       |                         |                       |
+                     |                       |                         |                       |
+                     |                       |                         |                       |
+                     |                       |                         |                       |
+                     |                       |                         |                       |
+                     |                       |                         |                       |
+                     |                       |                         |                       |
+                     |                       |                         |                       |
+                     |                       |                         |                       |
+  Lowest non-        |                       |                         |                       |
+  linear perf ------>+-----------------------+                         +-----------------------+
+                     |                       |                         |                       |
+                     |                       |       Lowest perf  ---->|                       |
+                     |                       |                         |                       |
+  Lowest perf ------>+-----------------------+                         +-----------------------+
+                     |                       |                         |                       |
+                     |                       |                         |                       |
+                     |                       |                         |                       |
+          0   ------>+-----------------------+                         +-----------------------+
+
+                                     AMD P-States Performance Scale
+
+
+.. _perf_cap:
+
+AMD CPPC Performance Capability
+--------------------------------
+
+Highest Performance (RO)
+.........................
+
+It 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
+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
+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
+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
+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
+performance level lower than the lowest nonlinear performance level may
+cause an efficiency penalty but should reduce the instantaneous power
+consumption of the processor.
+
+AMD CPPC Performance Control
+------------------------------
+
+``amd-pstate`` passes performance goals through these registers. The
+register drives the behavior of the desired performance target.
+
+Minimum requested performance (RW)
+...................................
+
+``amd-pstate`` specifies the minimum allowed performance level.
+
+Maximum requested performance (RW)
+...................................
+
+``amd-pstate`` specifies a limit the maximum performance that is expected
+to be supplied by the hardware.
+
+Desired performance target (RW)
+...................................
+
+``amd-pstate`` specifies a desired target in the CPPC performance scale as
+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
+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).
+
+
+Key Governors Support
+=======================
+
+``amd-pstate`` can be used with all the (generic) scaling governors listed
+by the ``scaling_available_governors`` policy attribute in ``sysfs``. Then,
+it is responsible for the configuration of policy objects corresponding to
+CPUs and provides the ``CPUFreq`` core (and the scaling governors attached
+to the policy objects) with accurate information on the maximum and minimum
+operating frequencies supported by the hardware. Users can check the
+``scaling_cur_freq`` information comes from the ``CPUFreq`` core.
+
+``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
+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.
+
+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 21h, Revision B0 Processors [3]_) to indicate the different
+types. ``amd-pstate`` is to register different ``amd_pstate_perf_funcs``
+instances for different hardware implementations.
+
+Currently, some of Zen2 and Zen3 processors support ``amd-pstate``. In the
+future, it will be supported on more and more AMD processors.
+
+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.
+
+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``
+uses the ``cppc_acpi`` helper methods to implement the callback functions
+of ``amd_pstate_perf_funcs``.
+
+
+AMD P-States and ACPI hardware P-States always can be supported in one
+processor. But AMD P-States has the higher priority and if it is enabled
+with :c:macro:`MSR_AMD_CPPC_ENABLE` or ``cppc_set_enable``, it will respond
+to the request from AMD P-States.
+
+
+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
+``/sys/devices/system/cpu/cpufreq/policyX/`` directory and affect all CPUs. ::
+
+ root@hr-test1:/home/ray# ls /sys/devices/system/cpu/cpufreq/policy0/*amd*
+ /sys/devices/system/cpu/cpufreq/policy0/amd_pstate_highest_perf
+ /sys/devices/system/cpu/cpufreq/policy0/amd_pstate_lowest_nonlinear_freq
+ /sys/devices/system/cpu/cpufreq/policy0/amd_pstate_lowest_nonlinear_perf
+ /sys/devices/system/cpu/cpufreq/policy0/amd_pstate_lowest_perf
+ /sys/devices/system/cpu/cpufreq/policy0/amd_pstate_max_freq
+ /sys/devices/system/cpu/cpufreq/policy0/amd_pstate_min_freq
+ /sys/devices/system/cpu/cpufreq/policy0/amd_pstate_nominal_freq
+ /sys/devices/system/cpu/cpufreq/policy0/amd_pstate_nominal_perf
+
+
+``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
+performance supported in `AMD CPPC Performance Capability <perf_cap_>`_).
+This attribute is read-only.
+
+``amd_pstate_nominal_perf / amd_pstate_nominal_freq``
+
+Nominal CPPC performance and CPU frequency that the driver is allowed to
+set in percent of the maximum supported CPPC performance level (Please see
+nominal performance in `AMD CPPC Performance Capability <perf_cap_>`_).
+This attribute is read-only.
+
+``amd_pstate_lowest_nonlinear_perf / amd_pstate_lowest_nonlinear_freq``
+
+The lowest non-linear CPPC performance and 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_>`_).
+This attribute is read-only.
+
+``amd_pstate_lowest_perf``
+
+The lowest physical CPPC performance. The minimum CPU frequency can be read
+back from ``cpuinfo`` member of ``cpufreq_policy``, so we won't expose it
+here.
+This attribute is read-only.
+
+
+``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
+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
+efficiency frequency management method on AMD processors.
+
+``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. ::
+
+ root@hr-test1:/home/ray# cpupower frequency-info
+ analyzing CPU 0:
+   driver: amd-pstate
+   CPUs which run at the same hardware frequency: 0
+   CPUs which need to have their frequency coordinated by software: 0
+   maximum transition latency: 131 us
+   hardware limits: 400 MHz - 4.68 GHz
+   available cpufreq governors: ondemand conservative powersave userspace performance schedutil
+   current policy: frequency should be within 400 MHz and 4.68 GHz.
+                   The governor "schedutil" may decide which speed to use
+                   within this range.
+   current CPU frequency: Unable to call hardware
+   current CPU frequency: 4.02 GHz (asserted by call to kernel)
+   boost state support:
+     Supported: yes
+     Active: yes
+     AMD PSTATE Highest Performance: 166. Maximum Frequency: 4.68 GHz.
+     AMD PSTATE Nominal Performance: 117. Nominal Frequency: 3.30 GHz.
+     AMD PSTATE Lowest Non-linear Performance: 39. Lowest Non-linear Frequency: 1.10 GHz.
+     AMD PSTATE Lowest Performance: 15. Lowest Frequency: 400 MHz.
+
+
+Diagnostics and Tuning
+=======================
+
+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
+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
+configured to support event tracing). ::
+
+ root@hr-test1:/home/ray# cd /sys/kernel/tracing/
+ root@hr-test1:/sys/kernel/tracing# echo 1 > events/amd_cpu/enable
+ root@hr-test1:/sys/kernel/tracing# cat trace
+ # tracer: nop
+ #
+ # entries-in-buffer/entries-written: 47827/42233061   #P:2
+ #
+ #                                _-----=> irqs-off
+ #                               / _----=> need-resched
+ #                              | / _---=> hardirq/softirq
+ #                              || / _--=> preempt-depth
+ #                              ||| /     delay
+ #           TASK-PID     CPU#  ||||   TIMESTAMP  FUNCTION
+ #              | |         |   ||||      |         |
+          <idle>-0       [015] dN...  4995.979886: amd_pstate_perf: amd_min_perf=85 amd_des_perf=85 amd_max_perf=166 cpu_id=15 changed=false fast_switch=true
+          <idle>-0       [007] d.h..  4995.979893: amd_pstate_perf: amd_min_perf=85 amd_des_perf=85 amd_max_perf=166 cpu_id=7 changed=false fast_switch=true
+             cat-2161    [000] d....  4995.980841: amd_pstate_perf: amd_min_perf=85 amd_des_perf=85 amd_max_perf=166 cpu_id=0 changed=false fast_switch=true
+            sshd-2125    [004] d.s..  4995.980968: amd_pstate_perf: amd_min_perf=85 amd_des_perf=85 amd_max_perf=166 cpu_id=4 changed=false fast_switch=true
+          <idle>-0       [007] d.s..  4995.980968: amd_pstate_perf: amd_min_perf=85 amd_des_perf=85 amd_max_perf=166 cpu_id=7 changed=false fast_switch=true
+          <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
+governor (for the policies it is attached to), or by the ``CPUFreq`` core (for the
+policies with other scaling governors).
+
+
+Reference
+===========
+
+.. [1] AMD64 Architecture Programmer's Manual Volume 2: System Programming,
+       https://www.amd.com/system/files/TechDocs/24593.pdf
+
+.. [2] Advanced Configuration and Power Interface Specification,
+       https://uefi.org/sites/default/files/resources/ACPI_Spec_6_4_Jan22.pdf
+
+.. [3] Processor Programming Reference (PPR) for AMD Family 19h Model 21h, Revision B0 Processors
+       https://www.amd.com/system/files/TechDocs/55898_B1_pub_0.50.zip
+
diff --git a/Documentation/admin-guide/pm/working-state.rst b/Documentation/admin-guide/pm/working-state.rst
index f40994c422dc..5d2757e2de65 100644
--- a/Documentation/admin-guide/pm/working-state.rst
+++ b/Documentation/admin-guide/pm/working-state.rst
@@ -11,6 +11,7 @@ Working-State Power Management
    intel_idle
    cpufreq
    intel_pstate
+   amd-pstate
    cpufreq_drivers
    intel_epb
    intel-speed-select
-- 
2.25.1