【AudioPolicy To AudioHAL笔记（三）】安卓S上audio_policy

【AudioPolicy To AudioHAL笔记（三）】安卓S上audio_policy_configuration.xml 加载过程分析

安卓S上audio_policy_configuration.xml 加载过程分析

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上一篇文章启动过程提到:

/frameworks/av/services/audiopolicy/managerdefault/AudioPolicyManager.cpp
void AudioPolicyManager::loadConfig() {ALOGE("Mylog_AP:AudioPolicyManager::loadConfig() !");if (deserializeAudioPolicyXmlConfig(getConfig()) != NO_ERROR) {ALOGE("could not load audio policy configuration file, setting defaults");getConfig().setDefault();}
}

本文详细讲一下这个解析配置文件的过程。

deserializeAudioPolicyXmlConfig()的定义在：

/frameworks/av/services/audiopolicy/managerdefault/AudioPolicyManager.cpp
static status_t deserializeAudioPolicyXmlConfig(AudioPolicyConfig &config) {if (std::string audioPolicyXmlConfigFile = audio_get_audio_policy_config_file();!audioPolicyXmlConfigFile.empty()) {status_t ret = deserializeAudioPolicyFile(audioPolicyXmlConfigFile.c_str(), &config);if (ret == NO_ERROR) {config.setSource(audioPolicyXmlConfigFile);}return ret;}return BAD_VALUE;
}

这个函数主要做了两个事情：

调用函数audio_get_audio_policy_config_file获取配置文件的信息
调用函数deserializeAudioPolicyFile处理配置文件

其中第一步骤的函数在如下的定义(有可能这个函数会被客制化修改）：

//https://cs.android.com/android/platform/superproject/+/android-12.0.0_r8:system/media/audio/include/system/audio_config.h?hl=zh-cn
/system/media/audio/include/system/audio_config.h
static inline std::string audio_get_audio_policy_config_file()

函数内容不是很难，感兴趣可以自己阅读源码，我这里总结一下：
这个函数主要是根据系统的属性选择不同的配置文件，简单的画个系统属性和配置文件的关系图：
系统属性与配置文件的关系

这里我们主要分析第二个函数deserializeAudioPolicyFile.但在分析这个函数前需要了解一下知识：

也就是audio_policy_config_file中child的概念，我画一张简图表示一下这个关系（以audio_policy_configuration.xml为例）：

看图应该不难分析，就是套娃又套娃。一个modules对应一个so文件,例如，这个就是对于一个primary HAL；其中一个module允许有多个设备列表。

有上面的了解后，再看一下源码：

/frameworks/av/services/audiopolicy/common/managerdefinitions/src/Serializer.cpp
status_t deserializeAudioPolicyFile(const char *fileName, AudioPolicyConfig *config)
{PolicySerializer serializer;status_t status = serializer.deserialize(fileName, config);   --> 重构+函数模板if (status != OK) config->clear();return status;
}

结合C++的语法，上面函数调用到：

/frameworks/av/services/audiopolicy/common/managerdefinitions/src/Serializer.cppstatus_t PolicySerializer::deserialize(const char *configFile, AudioPolicyConfig *config,bool ignoreVendorExtensions)
{mIgnoreVendorExtensions = ignoreVendorExtensions;auto doc = make_xmlUnique(xmlParseFile(configFile));    --> 解析XML配置文件if (doc == nullptr) {ALOGE("%s: Could not parse %s document.", __func__, configFile);return BAD_VALUE;}xmlNodePtr root = xmlDocGetRootElement(doc.get());     --> 获取XML文档的根节点if (root == NULL) {ALOGE("%s: Could not parse %s document: empty.", __func__, configFile);return BAD_VALUE;}if (xmlXIncludeProcess(doc.get()) < 0) {               --> 处理XML文档中的XInclude指令ALOGE("%s: libxml failed to resolve XIncludes on %s document.", __func__, configFile);}if (xmlStrcmp(root->name, reinterpret_cast<const xmlChar*>(rootName)))  {   -->// 检查根节点的名称是否与"rootName = "audioPolicyConfiguration""的一致ALOGE("%s: No %s root element found in xml data %s.", __func__, rootName,reinterpret_cast<const char*>(root->name));return BAD_VALUE;}std::string version = getXmlAttribute(root, versionAttribute);     --> 获取根节点的版本属性if (version.empty()) {ALOGE("%s: No version found in root node %s", __func__, rootName);return BAD_VALUE;}if (version == "7.0") {mChannelMasksSeparator = mSamplingRatesSeparator = mFlagsSeparator = " ";} else if (version != "1.0") {ALOGE("%s: Version does not match; expected \"1.0\" or \"7.0\" got \"%s\"",__func__, version.c_str());return BAD_VALUE;}// Let's deserialize children// ModulesModuleTraits::Collection modules;status_t status = deserializeCollection<ModuleTraits>(root, &modules, config);if (status != NO_ERROR) {return status;}config->setHwModules(modules);// Global Configurationdeserialize<GlobalConfigTraits>(root, config);  --> 利用C++的函数模板语法调用到不同的deserialize// Surround configurationdeserialize<SurroundSoundTraits>(root, config);return android::OK;
}

上面这个函数重点关注一下下面的语句，其是在这个函数中解析文件的：

status_t status = deserializeCollection<ModuleTraits>(root, &modules, config);
其调用的函数是：
/frameworks/av/services/audiopolicy/common/managerdefinitions/src/Serializer.cpp
template <class Trait>
status_t PolicySerializer::deserializeCollection(const xmlNode *cur,typename Trait::Collection *collection,typename Trait::PtrSerializingCtx serializingContext)
{--> 用两个for循环，遍历当前xml节点的所有子节点for (cur = cur->xmlChildrenNode; cur != NULL; cur = cur->next) {const xmlNode *child = NULL;if (!xmlStrcmp(cur->name, reinterpret_cast<const xmlChar*>(Trait::collectionTag))) {child = cur->xmlChildrenNode;} else if (!xmlStrcmp(cur->name, reinterpret_cast<const xmlChar*>(Trait::tag))) {child = cur;}for (; child != NULL; child = child->next) {if (!xmlStrcmp(child->name, reinterpret_cast<const xmlChar*>(Trait::tag))) {auto maybeElement = deserialize<Trait>(child, serializingContext);if (maybeElement.index() == 1) {status_t status = Trait::addElementToCollection(std::get<1>(maybeElement), collection);if (status != NO_ERROR) {ALOGE("%s: could not add element to %s collection", __func__,Trait::collectionTag);return status;}} else if (mIgnoreVendorExtensions && std::get<status_t>(maybeElement) == NO_INIT) {// Skip a vendor extension element.} else {return BAD_VALUE;}}}if (!xmlStrcmp(cur->name, reinterpret_cast<const xmlChar*>(Trait::tag))) {return NO_ERROR;}}return NO_ERROR;
}

单从函数分析并不难，找到对应模板的名字，然后再调用

auto maybeElement = deserialize<Trait>(child, serializingContext);

最后的结果就是：

/frameworks/av/services/audiopolicy/common/managerdefinitions/include/AudioPolicyConfig.h
class AudioPolicyConfig
{
public:....private:std::string mSource;std::string mEngineLibraryNameSuffix;HwModuleCollection &mHwModules; /**< Collection of Module, with Profiles, i.e. Mix Ports. */DeviceVector &mOutputDevices;   // xm中所以output devices模块集合DeviceVector &mInputDevices;    //xml中所以input devices模块集合sp<DeviceDescriptor> &mDefaultOutputDevice;   //默认的output device...
};