android 怎么选择audio hal

最近遇到一个问题，就是不知道audioFlinger 在选择hal 的时候是怎么选择的，为什么选择的是vendor 的audio hal 而不是default 的？

本文急于5.1 的代码。

audioflinger 什么时候会去打开audio hal 的so， 5.1版本的不是初始化audioflinger 时就去选择。会在openoutput 时找hw 时去选择，如果已有就不需要找，第一次就要找，看下面代码

status_t AudioFlinger::openOutput(audio_module_handle_t module,audio_io_handle_t *output,audio_config_t *config,audio_devices_t *devices,const String8& address,uint32_t *latencyMs,audio_output_flags_t flags)
{ALOGV("openOutput(), module %d Device %x, SamplingRate %d, Format %#08x, Channels %x, flags %x",module,(devices != NULL) ? *devices : 0,config->sample_rate,config->format,config->channel_mask,flags);if (*devices == AUDIO_DEVICE_NONE) {return BAD_VALUE;}Mutex::Autolock _l(mLock);sp<PlaybackThread> thread = openOutput_l(module, output, config, *devices, address, flags);if (thread != 0) {

sp<AudioFlinger::PlaybackThread> AudioFlinger::openOutput_l(audio_module_handle_t module,audio_io_handle_t *output,audio_config_t *config,audio_devices_t devices,const String8& address,audio_output_flags_t flags)
{AudioHwDevice *outHwDev = findSuitableHwDev_l(module, devices);if (outHwDev == NULL) {return 0;}

AudioFlinger::AudioHwDevice* AudioFlinger::findSuitableHwDev_l(audio_module_handle_t module,audio_devices_t devices)
{// if module is 0, the request comes from an old policy manager and we should load// well known modulesif (module == 0) {ALOGW("findSuitableHwDev_l() loading well know audio hw modules");for (size_t i = 0; i < ARRAY_SIZE(audio_interfaces); i++) {loadHwModule_l(audio_interfaces[i]);}// then try to find a module supporting the requested device.for (size_t i = 0; i < mAudioHwDevs.size(); i++) {AudioHwDevice *audioHwDevice = mAudioHwDevs.valueAt(i);audio_hw_device_t *dev = audioHwDevice->hwDevice();if ((dev->get_supported_devices != NULL) &&(dev->get_supported_devices(dev) & devices) == devices)return audioHwDevice;

audio_module_handle_t AudioFlinger::loadHwModule_l(const char *name)
{for (size_t i = 0; i < mAudioHwDevs.size(); i++) {if (strncmp(mAudioHwDevs.valueAt(i)->moduleName(), name, strlen(name)) == 0) {ALOGW("loadHwModule() module %s already loaded", name);return mAudioHwDevs.keyAt(i);}}audio_hw_device_t *dev;int rc = load_audio_interface(name, &dev);if (rc) {ALOGI("loadHwModule() error %d loading module %s ", rc, name);return 0;}

static int load_audio_interface(const char *if_name, audio_hw_device_t **dev)
{const hw_module_t *mod;int rc;rc = hw_get_module_by_class(AUDIO_HARDWARE_MODULE_ID, if_name, &mod);ALOGE_IF(rc, "%s couldn't load audio hw module %s.%s (%s)", __func__,AUDIO_HARDWARE_MODULE_ID, if_name, strerror(-rc));if (rc) {goto out;}

int hw_get_module_by_class(const char *class_id, const char *inst,const struct hw_module_t **module)
{int i;char prop[PATH_MAX];char path[PATH_MAX];char name[PATH_MAX];char prop_name[PATH_MAX];if (inst)snprintf(name, PATH_MAX, "%s.%s", class_id, inst);elsestrlcpy(name, class_id, PATH_MAX);/** Here we rely on the fact that calling dlopen multiple times on* the same .so will simply increment a refcount (and not load* a new copy of the library).* We also assume that dlopen() is thread-safe.*//* First try a property specific to the class and possibly instance */snprintf(prop_name, sizeof(prop_name), "ro.hardware.%s", name);if (property_get(prop_name, prop, NULL) > 0) {                  //这里会找出cpu 厂商对cpu型号的命名if (hw_module_exists(path, sizeof(path), name, prop) == 0) {// 合理会去找对应的so 是否存在， 如果存在就跳到foundgoto found;}}/* Loop through the configuration variants looking for a module */for (i=0 ; i<HAL_VARIANT_KEYS_COUNT; i++) {if (property_get(variant_keys[i], prop, NULL) == 0) {continue;}if (hw_module_exists(path, sizeof(path), name, prop) == 0) {goto found;}}/* Nothing found, try the default */if (hw_module_exists(path, sizeof(path), name, "default") == 0) { //找不到厂商的， 就会去找default 的sogoto found;}return -ENOENT;found:/* load the module, if this fails, we're doomed, and we should not try* to load a different variant. */return load(class_id, path, module);
}

static int hw_module_exists(char *path, size_t path_len, const char *name,const char *subname)
{snprintf(path, path_len, "%s/%s.%s.so",HAL_LIBRARY_PATH2, name, subname);if (access(path, R_OK) == 0)return 0;snprintf(path, path_len, "%s/%s.%s.so",HAL_LIBRARY_PATH1, name, subname);if (access(path, R_OK) == 0)return 0;return -ENOENT;
}

所有的玄机都在函数 hw_get_module_by_class，在这个函数，会先去找目录/vendor/lib, 64bit 是/vendor/64lib/ 目录下的so，然后再去找/system/lib，同样64bit 找/system/64lib 下的so，先找vendor 的so，找不到就会找default的so，一般是system/lib/audio.primary.default.so

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