[RFC PATCH v3 9/9] coresight: docs: Add documentation for CoreSight config.

Thu Nov 26 12:52:14 EST 2020

On Fri, Oct 30, 2020 at 05:56:55PM +0000, Mike Leach wrote:
> Adds documentation for the CoreSight System configuration manager.
> 
> Signed-off-by: Mike Leach <mike.leach at linaro.org>
> ---
>  .../trace/coresight/coresight-config.rst      | 230 ++++++++++++++++++
>  Documentation/trace/coresight/coresight.rst   |  16 ++
>  2 files changed, 246 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..8afcf18c090c
> --- /dev/null
> +++ b/Documentation/trace/coresight/coresight-config.rst
> @@ -0,0 +1,230 @@
> +.. 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 complex 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
> +device is registered with the configuration manager.

In the above paragraph I would explicitly write "coresight device" when
referring to a device.  That way there is no doubt as to what is happening.

> +
> +The load process involves interpreting the descriptor into a the set of register

s/into a the set/into a set/

> +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.

If I understand correctly the above paragraph is for the sysfs and configfs
interfaces.  From perf enabling feature and devices doesn't apply.

> +
> +A feature can be enabled either as part of a configuration being enabled on the
> +system, or individually on a device instance using the configfs API.
> +
> +
> +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.
> +
> +
> +Virtual Device
> +--------------
> +
> +The CoreSight System Configuration manager also includes a virtual CoreSight
> +device. This provides the system with access to kernel objects and device
> +management to own elements such as the sysfs entries used to expose the
> +configurations into the cs_etm event in perf.
> +
> +This device 'system_cfg'  appears on a custom bus - 'coresight-config' -
> +rather than the normal 'coresight' bus to avoid complications when the
> +coresight system is searching for real devices on the bus.
> +
> +
> +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 'coresight-syscfg' subsystem will appear::
> +
> +    $ ls /config
> +    coresight-syscfg  stp-policy
> +
> +This has two sub-directories::
> +
> +    $ cd coresight-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  nr_presets  preset_values  refs
> +    $ cat description
> +    Setup ETMs with strobing for autofdo
> +    $ cat refs
> +    references 1 features:-
> +    strobing
> +    $ cat nr_presets
> +    2
> +    $ cat preset_values
> +    2 presets, 2 parameters per preset
> +    preset[1]: strobing.window = 0x2710 strobing.period = 0x1388
> +    preset[2]: strobing.window = 0x1388 strobing.period = 0x1388
> +
> +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 0b73acb44efa..0122d9bebf35 100644
> --- a/Documentation/trace/coresight/coresight.rst
> +++ b/Documentation/trace/coresight/coresight.rst
> @@ -587,6 +587,20 @@ 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
> @@ -594,3 +608,5 @@ 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
> 