[PATCH v7 10/10] Documentation: coresight: Add documentation for CoreSight config
Mike Leach
mike.leach at linaro.org
Wed Apr 14 20:19:03 BST 2021
Adds documentation for the CoreSight System configuration manager.
Cc: Jonathan Corbet <corbet at lwn.net>
Cc: linux-doc at vger.kernel.org
Signed-off-by: Mike Leach <mike.leach at linaro.org>
Reviewed-by: Mathieu Poirier <mathieu.poirier at linaro.org>
---
.../trace/coresight/coresight-config.rst | 244 ++++++++++++++++++
Documentation/trace/coresight/coresight.rst | 16 ++
2 files changed, 260 insertions(+)
create mode 100644 Documentation/trace/coresight/coresight-config.rst
diff --git a/Documentation/trace/coresight/coresight-config.rst b/Documentation/trace/coresight/coresight-config.rst
new file mode 100644
index 000000000000..a4e3ef295240
--- /dev/null
+++ b/Documentation/trace/coresight/coresight-config.rst
@@ -0,0 +1,244 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+======================================
+CoreSight System Configuration Manager
+======================================
+
+ :Author: Mike Leach <mike.leach at linaro.org>
+ :Date: October 2020
+
+Introduction
+============
+
+The CoreSight System Configuration manager is an API that allows the
+programming of the CoreSight system with pre-defined configurations that
+can then be easily enabled from sysfs or perf.
+
+Many CoreSight components can be programmed in complex ways - especially ETMs.
+In addition, components can interact across the CoreSight system, often via
+the cross trigger components such as CTI and CTM. These system settings can
+be defined and enabled as named configurations.
+
+
+Basic Concepts
+==============
+
+This section introduces the basic concepts of a CoreSight system configuration.
+
+
+Features
+--------
+
+A feature is a named set of programming for a CoreSight device. The programming
+is device dependent, and can be defined in terms of absolute register values,
+resource usage and parameter values.
+
+The feature is defined using a descriptor. This descriptor is used to load onto
+a matching device, either when the feature is loaded into the system, or when the
+CoreSight device is registered with the configuration manager.
+
+The load process involves interpreting the descriptor into a set of register
+accesses in the driver - the resource usage and parameter descriptions
+translated into appropriate register accesses. This interpretation makes it easy
+and efficient for the feature to be programmed onto the device when required.
+
+The feature will not be active on the device until the feature is enabled, and
+the device itself is enabled. When the device is enabled then enabled features
+will be programmed into the device hardware.
+
+A feature is enabled as part of a configuration being enabled on the system.
+
+
+Parameter Value
+~~~~~~~~~~~~~~~
+
+A parameter value is a named value that may be set by the user prior to the
+feature being enabled that can adjust the behaviour of the operation programmed
+by the feature.
+
+For example, this could be a count value in a programmed operation that repeats
+at a given rate. When the feature is enabled then the current value of the
+parameter is used in programming the device.
+
+The feature descriptor defines a default value for a parameter, which is used
+if the user does not supply a new value.
+
+Users can update parameter values using the configfs API for the CoreSight
+system - which is described below.
+
+The current value of the parameter is loaded into the device when the feature
+is enabled on that device.
+
+
+Configurations
+--------------
+
+A configuration defines a set of features that are to be used in a trace
+session where the configuration is selected. For any trace session only one
+configuration may be selected.
+
+The features defined may be on any type of device that is registered
+to support system configuration. A configuration may select features to be
+enabled on a class of devices - i.e. any ETMv4, or specific devices, e.g. a
+specific CTI on the system.
+
+As with the feature, a descriptor is used to define the configuration.
+This will define the features that must be enabled as part of the configuration
+as well as any preset values that can be used to override default parameter
+values.
+
+
+Preset Values
+~~~~~~~~~~~~~
+
+Preset values are easily selectable sets of parameter values for the features
+that the configuration uses. The number of values in a single preset set, equals
+the sum of parameter values in the features used by the configuration.
+
+e.g. a configuration consists of 3 features, one has 2 parameters, one has
+a single parameter, and another has no parameters. A single preset set will
+therefore have 3 values.
+
+Presets are optionally defined by the configuration, up to 15 can be defined.
+If no preset is selected, then the parameter values defined in the feature
+are used as normal.
+
+
+Operation
+~~~~~~~~~
+
+The following steps take place in the operation of a configuration.
+
+1) In this example, the configuration is 'autofdo', which has an
+ associated feature 'strobing' that works on ETMv4 CoreSight Devices.
+
+2) The configuration is enabled. For example 'perf' may select the
+ configuration as part of its command line::
+
+ perf record -e cs_etm/autofdo/ myapp
+
+ which will enable the 'autofdo' configuration.
+
+3) perf starts tracing on the system. As each ETMv4 that perf uses for
+ trace is enabled, the configuration manager will check if the ETMv4
+ has a feature that relates to the currently active configuration.
+ In this case 'strobing' is enabled & programmed into the ETMv4.
+
+4) When the ETMv4 is disabled, any registers marked as needing to be
+ saved will be read back.
+
+5) At the end of the perf session, the configuration will be disabled.
+
+
+Viewing Configurations and Features
+===================================
+
+The set of configurations and features that are currently loaded into the
+system can be viewed using the configfs API.
+
+Mount configfs as normal and the 'cs-syscfg' subsystem will appear::
+
+ $ ls /config
+ cs-syscfg stp-policy
+
+This has two sub-directories::
+
+ $ cd cs-syscfg/
+ $ ls
+ configurations features
+
+The system has the configuration 'autofdo' built in. It may be examined as
+follows::
+
+ $ cd configurations/
+ $ ls
+ autofdo
+ $ cd autofdo/
+ $ ls
+ description preset1 preset3 preset5 preset7 preset9
+ feature_refs preset2 preset4 preset6 preset8
+ $ cat description
+ Setup ETMs with strobing for autofdo
+ $ cat feature_refs
+ strobing
+
+Each preset declared has a preset<n> subdirectory declared. The values for
+the preset can be examined::
+
+ $ cat preset1/values
+ strobing.window = 0x1388 strobing.period = 0x2
+ $ cat preset2/values
+ strobing.window = 0x1388 strobing.period = 0x4
+
+The features referenced by the configuration can be examined in the features
+directory::
+
+ $ cd ../../features/strobing/
+ $ ls
+ description matches nr_params params
+ $ cat description
+ Generate periodic trace capture windows.
+ parameter 'window': a number of CPU cycles (W)
+ parameter 'period': trace enabled for W cycles every period x W cycles
+ $ cat matches
+ SRC_ETMV4
+ $ cat nr_params
+ 2
+
+Move to the params directory to examine and adjust parameters::
+
+ cd params
+ $ ls
+ period window
+ $ cd period
+ $ ls
+ value
+ $ cat value
+ 0x2710
+ # echo 15000 > value
+ # cat value
+ 0x3a98
+
+Parameters adjusted in this way are reflected in all device instances that have
+loaded the feature.
+
+
+Using Configurations in perf
+============================
+
+The configurations loaded into the CoreSight configuration management are
+also declared in the perf 'cs_etm' event infrastructure so that they can
+be selected when running trace under perf::
+
+ $ ls /sys/devices/cs_etm
+ configurations format perf_event_mux_interval_ms sinks type
+ events nr_addr_filters power
+
+Key directories here are 'configurations' - which lists the loaded
+configurations, and 'events' - a generic perf directory which allows
+selection on the perf command line.::
+
+ $ ls configurations/
+ autofdo
+ $ cat configurations/autofdo
+ 0xa7c3dddd
+
+As with the sinks entries, this provides a hash of the configuration name.
+The entry in the 'events' directory uses perfs built in syntax generator
+to substitute the syntax for the name when evaluating the command::
+
+ $ ls events/
+ autofdo
+ $ cat events/autofdo
+ configid=0xa7c3dddd
+
+The 'autofdo' configuration may be selected on the perf command line::
+
+ $ perf record -e cs_etm/autofdo/u --per-thread <application>
+
+A preset to override the current parameter values can also be selected::
+
+ $ perf record -e cs_etm/autofdo,preset=1/u --per-thread <application>
+
+When configurations are selected in this way, then the trace sink used is
+automatically selected.
diff --git a/Documentation/trace/coresight/coresight.rst b/Documentation/trace/coresight/coresight.rst
index 169749efd8d1..7ec656c9f0dc 100644
--- a/Documentation/trace/coresight/coresight.rst
+++ b/Documentation/trace/coresight/coresight.rst
@@ -619,6 +619,20 @@ A separate documentation file is provided to explain the use of these devices.
(:doc:`coresight-ect`) [#fourth]_.
+CoreSight System Configuration
+------------------------------
+
+CoreSight components can be complex devices with many programming options.
+Furthermore, components can be programmed to interact with each other across the
+complete system.
+
+A CoreSight System Configuration manager is provided to allow these complex programming
+configurations to be selected and used easily from perf and sysfs.
+
+See the separate document for further information.
+(:doc:`coresight-config`) [#fifth]_.
+
+
.. [#first] Documentation/ABI/testing/sysfs-bus-coresight-devices-stm
.. [#second] Documentation/trace/stm.rst
@@ -626,3 +640,5 @@ A separate documentation file is provided to explain the use of these devices.
.. [#third] https://github.com/Linaro/perf-opencsd
.. [#fourth] Documentation/trace/coresight/coresight-ect.rst
+
+.. [#fifth] Documentation/trace/coresight/coresight-config.rst
--
2.17.1
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