PFW（Parameter-Framework）

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Overview

Media Policy uses PFW to construct various states such as routing and audio policy. PFW is a general framework for constructing state machines based on configuration files and callback functions.

• In the PFW configuration file, we will define which states are in the state machine, the conditions for each state to be established, and the variables used for judgment.
• The callback function constitutes the action of the state machine.

Project Directory

.
├── context.c
├── criterion.c
├── dump.c
├── include
│   └── pfw.h
├── internal.h
├── Kconfig
├── Make.defs
├── Makefile
├── parser.c
├── README.md
├── README_zh-cn.md
├── sanitizer.c
├── system.c
├── test
│   ├── criteria.txt
│   ├── Makefile
│   ├── settings.pfw
│   └── test.c
└── vector.c

Function Introduction

The PFW module mainly includes functions such as creating a system and modifying variables.

• Create a system: Provide a configuration file path and plugin to create a pfw system. By implementing the on_load/on_save method, the PFW system can have the functions of reading and instant saving.
• Modify variables: pfw_setint and other methods provide an interface for modifying the value of a single variable.
• Query variables: Query the value of a single variable, or print the status of the entire system through dump.
• Apply changes: Apply the current variable value to the state machine. If a change occurs, the corresponding plugin will be called according to the logic in the configuration file.
• Subscribe to plugin: Subscribe to the specified plugin by name, register a callback to the plugin, so that when the corresponding plugin is called, the previously registered callback will also be called to notify the subscriber.

Write PFW configuration file

Configuration file composition

The PFW configuration file consists of two files: criteria.txt and settings.pfw:

• criteria.txt: defines all variables in the system and the dependencies between variables.
• settings.pfw: defines the rules in the system and the transition conditions between states.

Criteria Syntax

In the criteria.txt file, each line defines a variable, and the right side of each line of := and |= represents a possible syntax expansion, in the format of:

<TYPE> <NAMES>:<RANGES>

Type variables are essentially int32_t, but the values ​​can be interpreted as three types:

• NumericalCriterion: Numeric type, that is, the most primitive int type, with a value range of INT32_MIN to INT32_MAX.
• ExclusiveCriterion: Enumeration type, similar to enum in C language, several values ​​starting from 0 are interpreted as meaningful strings.
• InclusiveCriterion: Mask type, several bits from low to high are interpreted as meaningful strings.

NAMES: Each variable can have one or more names, and these names are bound to the same variable entity when reading and writing variables.

RANGES:

• The value range of NumericalCriterion is represented by one or more open intervals.
• The value ranges of ExclusiveCriterion and InclusiveCriterion are represented by one or more strings.

Example of writing a Criteria file

Take Audio policy as an example:

NumericalCriterion MediaVolume MusicVolume : [0,10] = 5
ExclusiveCriterion AudioMode               : normal phone
InclusiveCriterion AvailableDevices        : mic sco a2dp = mic

• Media Volume: numeric, value range is 0 to 10; initial value is 5.
• Audio mode: enumerated, value 0 corresponds to normal mode, value 1 corresponds to phone mode; initial value is 0 by default.
• Available devices: mask type, value 0 is the empty set, value 1 means mic, value 2 means sco, value 3 means mic|sco, ......, value 7 means mic|sco|a2dp; initially turns on mic.

Settings.pfw Syntax

The settings file consists of several domain, each domain represents a state machine. This configuration file uses indentation to distinguish the hierarchy. Each tab or 4 spaces represents a level of indentation, and tab and spaces cannot be mixed (the same as python).

• Each state machine has several confs, each representing a state.

• Each state has two parts, condition and action. When we apply a criterion, we traverse all the states in order, and we enter the state that satisfies the condition first. We will enter the state that satisfies the condition first, and if we enter a new state, we will execute its action.

  domain: string
      conf: string
          <RULES>
          <ACTS>

• The RULES part is the constraints on the criterion values, which can be recursively combined through AND and OR; different types of criterion constraints use different predicates.

• ACTS is the execution of one or more plugin callbacks. In ACTS, if you want to refer to the current value of the criterion variable instead of a fixed value, we support the following syntax:

  param%criterion%param

Example of writing a Settings file

Taking the Audio sco node control as an example, when sco is available and the user needs it, the sampling rate will be updated through the FFmpegCommand plug-in, and the sco input and output nodes will be opened:

domain: SCO
    conf: enable
        ALL
            UsingDevices Includes sco
            AvailableDevices Includes sco
        FFmpegCommand = sco,sample_rate,%HFPSampleRate%;sco,play,;
    conf: disable
        ALL
        FFmpegCommand = sco,pause,;

Check Tool

In the pfw/test directory, we have configured a Makefile for use on the host, which can compile and generate the test command line tool, in which you can enter commands to debug the two .conf configuration files in the same folder. At the same time, when developing new pfw features, you can also use this tool to quickly check the code.

pfw/test$ make cc -o test ../system.c ../parser.c ../context.c ../vector.c ../sanitizer.c ../criterion.c test.c -Wall -Werror -O0 -g -I ../include -D CONFIG_LIB_PFW_DEBUG -fsanitize=address -fsanitize=leak
pfw/test$ ./test

pfw> dump         //Print dump information.
pfw> setint persist.media.MediaVolume 7     //Set the MediaVolume value to 7.