295 lines
9.6 KiB
ReStructuredText
295 lines
9.6 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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======================================
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CoreSight System Configuration Manager
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======================================
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:Author: Mike Leach <mike.leach@linaro.org>
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:Date: October 2020
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Introduction
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============
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The CoreSight System Configuration manager is an API that allows the
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programming of the CoreSight system with pre-defined configurations that
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can then be easily enabled from sysfs or perf.
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Many CoreSight components can be programmed in complex ways - especially ETMs.
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In addition, components can interact across the CoreSight system, often via
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the cross trigger components such as CTI and CTM. These system settings can
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be defined and enabled as named configurations.
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Basic Concepts
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==============
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This section introduces the basic concepts of a CoreSight system configuration.
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Features
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--------
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A feature is a named set of programming for a CoreSight device. The programming
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is device dependent, and can be defined in terms of absolute register values,
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resource usage and parameter values.
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The feature is defined using a descriptor. This descriptor is used to load onto
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a matching device, either when the feature is loaded into the system, or when the
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CoreSight device is registered with the configuration manager.
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The load process involves interpreting the descriptor into a set of register
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accesses in the driver - the resource usage and parameter descriptions
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translated into appropriate register accesses. This interpretation makes it easy
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and efficient for the feature to be programmed onto the device when required.
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The feature will not be active on the device until the feature is enabled, and
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the device itself is enabled. When the device is enabled then enabled features
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will be programmed into the device hardware.
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A feature is enabled as part of a configuration being enabled on the system.
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Parameter Value
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~~~~~~~~~~~~~~~
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A parameter value is a named value that may be set by the user prior to the
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feature being enabled that can adjust the behaviour of the operation programmed
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by the feature.
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For example, this could be a count value in a programmed operation that repeats
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at a given rate. When the feature is enabled then the current value of the
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parameter is used in programming the device.
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The feature descriptor defines a default value for a parameter, which is used
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if the user does not supply a new value.
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Users can update parameter values using the configfs API for the CoreSight
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system - which is described below.
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The current value of the parameter is loaded into the device when the feature
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is enabled on that device.
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Configurations
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--------------
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A configuration defines a set of features that are to be used in a trace
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session where the configuration is selected. For any trace session only one
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configuration may be selected.
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The features defined may be on any type of device that is registered
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to support system configuration. A configuration may select features to be
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enabled on a class of devices - i.e. any ETMv4, or specific devices, e.g. a
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specific CTI on the system.
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As with the feature, a descriptor is used to define the configuration.
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This will define the features that must be enabled as part of the configuration
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as well as any preset values that can be used to override default parameter
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values.
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Preset Values
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~~~~~~~~~~~~~
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Preset values are easily selectable sets of parameter values for the features
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that the configuration uses. The number of values in a single preset set, equals
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the sum of parameter values in the features used by the configuration.
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e.g. a configuration consists of 3 features, one has 2 parameters, one has
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a single parameter, and another has no parameters. A single preset set will
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therefore have 3 values.
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Presets are optionally defined by the configuration, up to 15 can be defined.
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If no preset is selected, then the parameter values defined in the feature
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are used as normal.
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Operation
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~~~~~~~~~
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The following steps take place in the operation of a configuration.
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1) In this example, the configuration is 'autofdo', which has an
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associated feature 'strobing' that works on ETMv4 CoreSight Devices.
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2) The configuration is enabled. For example 'perf' may select the
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configuration as part of its command line::
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perf record -e cs_etm/autofdo/ myapp
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which will enable the 'autofdo' configuration.
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3) perf starts tracing on the system. As each ETMv4 that perf uses for
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trace is enabled, the configuration manager will check if the ETMv4
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has a feature that relates to the currently active configuration.
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In this case 'strobing' is enabled & programmed into the ETMv4.
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4) When the ETMv4 is disabled, any registers marked as needing to be
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saved will be read back.
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5) At the end of the perf session, the configuration will be disabled.
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Viewing Configurations and Features
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===================================
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The set of configurations and features that are currently loaded into the
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system can be viewed using the configfs API.
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Mount configfs as normal and the 'cs-syscfg' subsystem will appear::
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$ ls /config
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cs-syscfg stp-policy
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This has two sub-directories::
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$ cd cs-syscfg/
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$ ls
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configurations features
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The system has the configuration 'autofdo' built in. It may be examined as
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follows::
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$ cd configurations/
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$ ls
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autofdo
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$ cd autofdo/
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$ ls
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description feature_refs preset1 preset3 preset5 preset7 preset9
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enable preset preset2 preset4 preset6 preset8
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$ cat description
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Setup ETMs with strobing for autofdo
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$ cat feature_refs
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strobing
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Each preset declared has a 'preset<n>' subdirectory declared. The values for
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the preset can be examined::
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$ cat preset1/values
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strobing.window = 0x1388 strobing.period = 0x2
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$ cat preset2/values
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strobing.window = 0x1388 strobing.period = 0x4
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The 'enable' and 'preset' files allow the control of a configuration when
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using CoreSight with sysfs.
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The features referenced by the configuration can be examined in the features
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directory::
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$ cd ../../features/strobing/
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$ ls
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description matches nr_params params
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$ cat description
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Generate periodic trace capture windows.
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parameter 'window': a number of CPU cycles (W)
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parameter 'period': trace enabled for W cycles every period x W cycles
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$ cat matches
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SRC_ETMV4
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$ cat nr_params
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2
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Move to the params directory to examine and adjust parameters::
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cd params
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$ ls
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period window
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$ cd period
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$ ls
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value
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$ cat value
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0x2710
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# echo 15000 > value
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# cat value
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0x3a98
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Parameters adjusted in this way are reflected in all device instances that have
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loaded the feature.
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Using Configurations in perf
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============================
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The configurations loaded into the CoreSight configuration management are
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also declared in the perf 'cs_etm' event infrastructure so that they can
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be selected when running trace under perf::
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$ ls /sys/devices/cs_etm
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cpu0 cpu2 events nr_addr_filters power subsystem uevent
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cpu1 cpu3 format perf_event_mux_interval_ms sinks type
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The key directory here is 'events' - a generic perf directory which allows
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selection on the perf command line. As with the sinks entries, this provides
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a hash of the configuration name.
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The entry in the 'events' directory uses perfs built in syntax generator
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to substitute the syntax for the name when evaluating the command::
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$ ls events/
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autofdo
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$ cat events/autofdo
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configid=0xa7c3dddd
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The 'autofdo' configuration may be selected on the perf command line::
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$ perf record -e cs_etm/autofdo/u --per-thread <application>
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A preset to override the current parameter values can also be selected::
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$ perf record -e cs_etm/autofdo,preset=1/u --per-thread <application>
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When configurations are selected in this way, then the trace sink used is
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automatically selected.
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Using Configurations in sysfs
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=============================
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Coresight can be controlled using sysfs. When this is in use then a configuration
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can be made active for the devices that are used in the sysfs session.
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In a configuration there are 'enable' and 'preset' files.
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To enable a configuration for use with sysfs::
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$ cd configurations/autofdo
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$ echo 1 > enable
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This will then use any default parameter values in the features - which can be
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adjusted as described above.
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To use a preset<n> set of parameter values::
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$ echo 3 > preset
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This will select preset3 for the configuration.
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The valid values for preset are 0 - to deselect presets, and any value of
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<n> where a preset<n> sub-directory is present.
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Note that the active sysfs configuration is a global parameter, therefore
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only a single configuration can be active for sysfs at any one time.
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Attempting to enable a second configuration will result in an error.
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Additionally, attempting to disable the configuration while in use will
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also result in an error.
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The use of the active configuration by sysfs is independent of the configuration
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used in perf.
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Creating and Loading Custom Configurations
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==========================================
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Custom configurations and / or features can be dynamically loaded into the
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system by using a loadable module.
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An example of a custom configuration is found in ./samples/coresight.
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This creates a new configuration that uses the existing built in
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strobing feature, but provides a different set of presets.
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When the module is loaded, then the configuration appears in the configfs
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file system and is selectable in the same way as the built in configuration
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described above.
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Configurations can use previously loaded features. The system will ensure
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that it is not possible to unload a feature that is currently in use, by
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enforcing the unload order as the strict reverse of the load order.
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