Documentation/trace/coresight/coresight-config.rst - linux/kernel/git/torvalds/linux - Git at Google

 .. SPDX-License-Identifier: GPL-2.0

 ======================================
 CoreSight System Configuration Manager
 ======================================

     :Author:   Mike Leach <mike.leach@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  feature_refs  preset1  preset3  preset5  preset7  preset9
     enable       preset        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 'enable' and 'preset' files allow the control of a configuration when
 using CoreSight with sysfs.

 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
     cpu0  cpu2  events  nr_addr_filters		power  subsystem  uevent
     cpu1  cpu3  format  perf_event_mux_interval_ms	sinks  type

 The key directory here is 'events' - a generic perf directory which allows
 selection on the perf command line. 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.

 Using Configurations in sysfs
 =============================

 Coresight can be controlled using sysfs. When this is in use then a configuration
 can be made active for the devices that are used in the sysfs session.

 In a configuration there are 'enable' and 'preset' files.

 To enable a configuration for use with sysfs::

     $ cd configurations/autofdo
     $ echo 1 > enable

 This will then use any default parameter values in the features - which can be
 adjusted as described above.

 To use a preset<n> set of parameter values::

     $ echo 3 > preset

 This will select preset3 for the configuration.
 The valid values for preset are 0 - to deselect presets, and any value of
 <n> where a preset<n> sub-directory is present.

 Note that the active sysfs configuration is a global parameter, therefore
 only a single configuration can be active for sysfs at any one time.
 Attempting to enable a second configuration will result in an error.
 Additionally, attempting to disable the configuration while in use will
 also result in an error.

 The use of the active configuration by sysfs is independent of the configuration
 used in perf.


 Creating and Loading Custom Configurations
 ==========================================

 Custom configurations and / or features can be dynamically loaded into the
 system by using a loadable module.

 An example of a custom configuration is found in ./samples/coresight.

 This creates a new configuration that uses the existing built in
 strobing feature, but provides a different set of presets.

 When the module is loaded, then the configuration appears in the configfs
 file system and is selectable in the same way as the built in configuration
 described above.

 Configurations can use previously loaded features. The system will ensure
 that it is not possible to unload a feature that is currently in use, by
 enforcing the unload order as the strict reverse of the load order.
	.. SPDX-License-Identifier: GPL-2.0

	======================================
	CoreSight System Configuration Manager
	======================================

	:Author: Mike Leach <mike.leach@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 feature_refs preset1 preset3 preset5 preset7 preset9
	enable preset 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 'enable' and 'preset' files allow the control of a configuration when
	using CoreSight with sysfs.

	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
	cpu0 cpu2 events nr_addr_filters power subsystem uevent
	cpu1 cpu3 format perf_event_mux_interval_ms sinks type

	The key directory here is 'events' - a generic perf directory which allows
	selection on the perf command line. 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.

	Using Configurations in sysfs
	=============================

	Coresight can be controlled using sysfs. When this is in use then a configuration
	can be made active for the devices that are used in the sysfs session.

	In a configuration there are 'enable' and 'preset' files.

	To enable a configuration for use with sysfs::

	$ cd configurations/autofdo
	$ echo 1 > enable

	This will then use any default parameter values in the features - which can be
	adjusted as described above.

	To use a preset<n> set of parameter values::

	$ echo 3 > preset

	This will select preset3 for the configuration.
	The valid values for preset are 0 - to deselect presets, and any value of
	<n> where a preset<n> sub-directory is present.

	Note that the active sysfs configuration is a global parameter, therefore
	only a single configuration can be active for sysfs at any one time.
	Attempting to enable a second configuration will result in an error.
	Additionally, attempting to disable the configuration while in use will
	also result in an error.

	The use of the active configuration by sysfs is independent of the configuration
	used in perf.


	Creating and Loading Custom Configurations
	==========================================

	Custom configurations and / or features can be dynamically loaded into the
	system by using a loadable module.

	An example of a custom configuration is found in ./samples/coresight.

	This creates a new configuration that uses the existing built in
	strobing feature, but provides a different set of presets.

	When the module is loaded, then the configuration appears in the configfs
	file system and is selectable in the same way as the built in configuration
	described above.

	Configurations can use previously loaded features. The system will ensure
	that it is not possible to unload a feature that is currently in use, by
	enforcing the unload order as the strict reverse of the load order.