分析mModule的获取过程，这个变量在相机open中，会使用到这个变量。
这个函数的主要功能是根据模块ID寻找硬件模块动态链接库的地址，然后调用load去打开动态链接库并从中获取硬件模块结构体地址。
文件CameraService.cpp
入口
CameraService.cpp
{
ALOGI("CameraService process starting");
BnCameraService::onFirstRef();
// Update battery life tracking if service is restarting
BatteryNotifier& notifier(BatteryNotifier::getInstance());
notifier.noteResetCamera();
notifier.noteResetFlashlight();
camera_module_t *rawModule;
int err = hw_get_module(CAMERA_HARDWARE_MODULE_ID,
(const hw_module_t **)&rawModule);
if (err < 0) {
ALOGE("Could not load camera HAL module: %d (%s)", err, strerror(-err));
logServiceError("Could not load camera HAL module", err);
mNumberOfCameras = 0;
mNumberOfNormalCameras = 0;
return;
}
mModule = new CameraModule(rawModule);
err = mModule->init();
if (err != OK) {
ALOGE("Could not initialize camera HAL module: %d (%s)", err,
strerror(-err));
logServiceError("Could not initialize camera HAL module", err);
mNumberOfCameras = 0;
delete mModule;
mModule = nullptr;
return;
}
ALOGI("Loaded \"%s\" camera module", mModule->getModuleName());
mNumberOfCameras = mModule->getNumberOfCameras();
mNumberOfNormalCameras = mNumberOfCameras;
// Setup vendor tags before we call get_camera_info the first time
// because HAL might need to setup static vendor keys in get_camera_info
VendorTagDescriptor::clearGlobalVendorTagDescriptor();
if (mModule->getModuleApiVersion() >= CAMERA_MODULE_API_VERSION_2_2) {
setUpVendorTags();
}
mFlashlight = new CameraFlashlight(*mModule, *this);
status_t res = mFlashlight->findFlashUnits();
if (res) {
// impossible because we haven't open any camera devices.
ALOGE("Failed to find flash units.");
}
int latestStrangeCameraId = INT_MAX;
for (int i = 0; i < mNumberOfCameras; i++) {
String8 cameraId = String8::format("%d", i);
// Get camera info
struct camera_info info;
bool haveInfo = true;
status_t rc = mModule->getCameraInfo(i, &info);
if (rc != NO_ERROR) {
ALOGE("%s: Received error loading camera info for device %d, cost and"
" conflicting devices fields set to defaults for this device.",
__FUNCTION__, i);
haveInfo = false;
}
// Check for backwards-compatibility support
if (haveInfo) {
if (checkCameraCapabilities(i, info, &latestStrangeCameraId) != OK) {
delete mModule;
mModule = nullptr;
return;
}
}
// Defaults to use for cost and conflicting devices
int cost = 100;
char** conflicting_devices = nullptr;
size_t conflicting_devices_length = 0;
// If using post-2.4 module version, query the cost + conflicting devices from the HAL
if (mModule->getModuleApiVersion() >= CAMERA_MODULE_API_VERSION_2_4 && haveInfo) {
cost = info.resource_cost;
conflicting_devices = info.conflicting_devices;
conflicting_devices_length = info.conflicting_devices_length;
}
std::set<String8> conflicting;
for (size_t i = 0; i < conflicting_devices_length; i++) {
conflicting.emplace(String8(conflicting_devices[i]));
}
// Initialize state for each camera device
{
Mutex::Autolock lock(mCameraStatesLock);
mCameraStates.emplace(cameraId, std::make_shared<CameraState>(cameraId, cost,
conflicting));
}
if (mFlashlight->hasFlashUnit(cameraId)) {
mTorchStatusMap.add(cameraId,
ICameraServiceListener::TORCH_STATUS_AVAILABLE_OFF);
}
}
if (mModule->getModuleApiVersion() >= CAMERA_MODULE_API_VERSION_2_1) {
mModule->setCallbacks(this);
}
CameraDeviceFactory::registerService(this);
CameraService::pingCameraServiceProxy();
}
文件hardware.c
hw_get_module方法 很关键
int hw_get_module(const char *id, const struct hw_module_t **module)
{
return hw_get_module_by_class(id, NULL, module);
}
hw_get_module_by_class() 方法
读取库文件，尝试的顺序是：
ro.hardware
ro.product.board
ro.board.platform
ro.arch
default
通过 load 函数加载模块。
int hw_get_module_by_class(const char *class_id, const char *inst,
const struct hw_module_t **module)
{
int i = 0;
char prop[PATH_MAX] = {0};
char path[PATH_MAX] = {0};
char name[PATH_MAX] = {0};
char prop_name[PATH_MAX] = {0};
if (inst)
snprintf(name, PATH_MAX, "%s.%s", class_id, inst);
else
strlcpy(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) {
if (hw_module_exists(path, sizeof(path), name, prop) == 0) {
goto 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) {
goto 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);
}
load()：
调用 dlopen() 函数获取一个 handle。
调用 dlsym() 函数从动态链接库中获取 hw_module_t 类型的 hmi。
NOTE：
为了获取动态链接库中的结构体，我们需要用到一个字符串 sym。
sym 对应宏 HAL_MODULE_INFO_SYM_AS_STR，即 “HMI”。
我们的动态链接库 .so 文件，是一个 ELF 文件。
ELF：Executable and Linkable Format，可执行链接格式。
ELF 文件头保存了一个路线图，用于描述文件的组织结构。
通过 readelf -s 命令，我们可以查看对应的 .so 文件描述，可以看到其中有一个 Name 属性为 HMI ，其对应的位置就是我们所需要的结构体 hw_module_t。
于是我们通过 HMI 字段，就可以从动态链接库中读取出相应的结构体，从而得以在 Libraries 层中调用 HAL 层的库函数。
至此，我们就获得了最终的 rawModule，然后我们回到 onFirstRef() 中继续分析。
static int load(const char *id,
const char *path,
const struct hw_module_t **pHmi)
{
int status = -EINVAL;
void *handle = NULL;
struct hw_module_t *hmi = NULL;
/*
* load the symbols resolving undefined symbols before
* dlopen returns. Since RTLD_GLOBAL is not or'd in with
* RTLD_NOW the external symbols will not be global
*/
handle = dlopen(path, RTLD_NOW);
if (handle == NULL) {
char const *err_str = dlerror();
ALOGE("load: module=%s\n%s", path, err_str?err_str:"unknown");
status = -EINVAL;
goto done;
}
/* Get the address of the struct hal_module_info. */
const char *sym = HAL_MODULE_INFO_SYM_AS_STR;
hmi = (struct hw_module_t *)dlsym(handle, sym);
if (hmi == NULL) {
ALOGE("load: couldn't find symbol %s", sym);
status = -EINVAL;
goto done;
}
/* Check that the id matches */
if (strcmp(id, hmi->id) != 0) {
ALOGE("load: id=%s != hmi->id=%s", id, hmi->id);
status = -EINVAL;
goto done;
}
hmi->dso = handle;
/* success */
status = 0;
done:
if (status != 0) {
hmi = NULL;
if (handle != NULL) {
dlclose(handle);
handle = NULL;
}
} else {
ALOGV("loaded HAL id=%s path=%s hmi=%p handle=%p",
id, path, *pHmi, handle);
}
*pHmi = hmi;
return status;
}
在回到文件CameraModule.cpp
注意，这里的 mModule 是 camera_module_t 类型。
CameraModule::CameraModule(camera_module_t *module) {
if (module == NULL) {
ALOGE("%s: camera hardware module must not be null", __FUNCTION__);
assert(0);
}
mModule = module;
}
继续init()函数
int CameraModule::init() {
ATRACE_CALL();
int res = OK;
if (getModuleApiVersion() >= CAMERA_MODULE_API_VERSION_2_4 &&
mModule->init != NULL) {
ATRACE_BEGIN("camera_module->init");
res = mModule->init();
ATRACE_END();
}
mCameraInfoMap.setCapacity(getNumberOfCameras());
return res;
}
文件camera_common.h
声明了 camera_module_t：
结构体中声明了许多函数指针。
其中就有 init 函数指针。
这个指针指向的具体函数，是根据具体的 Camera 设备确定的，其中我查看了 QCamera2Factory.cpp ，这里就实现了对应的函数
typedef struct camera_module {
hw_module_t common;
int (*get_number_of_cameras)(void);
int (*get_camera_info)(int camera_id, struct camera_info *info);
int (*set_callbacks)(const camera_module_callbacks_t *callbacks);
void (*get_vendor_tag_ops)(vendor_tag_ops_t* ops);
int (*open_legacy)(const struct hw_module_t* module, const char* id,
uint32_t halVersion, struct hw_device_t** device);
int (*set_torch_mode)(const char* camera_id, bool enabled);
int (*init)();
/* reserved for future use */
void* reserved[5];
} camera_module_t;
小结
这篇笔记中，我们从 CameraService::onFirstRef() 入手，逐渐理顺了以 hw_get_module() 为中心的一个调用逻辑。
实际上，Android HAL 层有一个通用的入口，即宏 HAL_MODULE_INFO_SYM，通过它，我们获取 HAL 层中的模块实例，从而使得我们可以调用 HAL 层所提供的函数。
理解了 HAL 层的入口，接下来我们可以去对 Camera.startPreview() 的控制流程进行分析，从而再次加深我们对 Camera 控制流的理解。