学步园

CameraService.cpp (frameworks\base\services\camera\libcameraservice)

中调用hw_get_module

void CameraService::onFirstRef()
{
    BnCameraService::onFirstRef();

    if (hw_get_module(CAMERA_HARDWARE_MODULE_ID,
                (const hw_module_t **)&mModule) < 0) {
        LOGE("Could not load camera HAL module");
        mNumberOfCameras = 0;
    }
    else {
        mNumberOfCameras = mModule->get_number_of_cameras();
        if (mNumberOfCameras > MAX_CAMERAS) {
            LOGE("Number of cameras(%d) > MAX_CAMERAS(%d).",
                    mNumberOfCameras, MAX_CAMERAS);
            mNumberOfCameras = MAX_CAMERAS;
        }
        for (int i = 0; i < mNumberOfCameras; i++) {
            setCameraFree(i);
        }
    }
}

看一下hw_get_module是怎么回事

int hw_get_module(const char *id, const struct hw_module_t **module)
{
    return hw_get_module_by_class(id, NULL, module);
}

他只是一个封装实际调用了hw_get_module_by_class@Hardware.c (hardware\libhardware）

好在不长，看看吧

int hw_get_module_by_class(const char *class_id, const char *inst,
                           const struct hw_module_t **module)
{
    int status;
    int i;
    const struct hw_module_t *hmi = NULL;
    char prop[PATH_MAX];//几个关键的数组
    char path[PATH_MAX];//在下面起了重要
    char name[PATH_MAX];//作用

    if (inst)
        snprintf(name, PATH_MAX, "%s.%s", class_id, inst);
    else
        strlcpy(name, class_id, PATH_MAX);//走这里

    /*
     * 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.
     */

    /* Loop through the configuration variants looking for a module */
    for (i=0 ; i<HAL_VARIANT_KEYS_COUNT+1 ; i++) {
        if (i < HAL_VARIANT_KEYS_COUNT) {
            if (property_get(variant_keys[i], prop, NULL) == 0)
   //在这里将prop的路径得到，分别从
    "ro.hardware[qcom]"
    "ro.product.board"[7x27],
    "ro.board.platform"[msm7627a],
    "ro.arch",
    "ro.hw_platform"[QRD_SKU3-1100]
    这几个属性文件中获得硬件的信息
    有些硬件信息的字符串会出现在编译后生成的.so名字中
           {
		      continue;
            }
            snprintf(path, sizeof(path), "%s/%s.%s.so",
                     HAL_LIBRARY_PATH2, name, prop);
            if (access(path, R_OK) == 0) break;

            snprintf(path, sizeof(path), "%s/%s.%s.so",
                     HAL_LIBRARY_PATH1, name, prop);//走这里,在这里得到/system/lib/hw/camera.msm7627a.so
                                              这样一个路径，这个库里有QualcommCamera.cpp，这是
                                              camera模块HAL代码开始的地方
            if (access(path, R_OK) == 0) break;
        } else {
            snprintf(path, sizeof(path), "%s/%s.default.so",
                     HAL_LIBRARY_PATH1, name);
            if (access(path, R_OK) == 0) break;
        }
    }

    status = -ENOENT;
    if (i < HAL_VARIANT_KEYS_COUNT+1) {
        /* load the module, if this fails, we're doomed, and we should not try
         * to load a different variant. */
        status = load(class_id, path, module);//这里关键,函数的三个参数可以串联成一句话：
        到path（/system/lib/hw/camera.msm7627a.so）这个路径下找到一个id（camera）匹配的module
    }

    return status;
}

再来看看load这个函数@hardware.c (hardware\libhardware)

static int load(const char *id,
        const char *path,
        const struct hw_module_t **pHmi)
{
    int status;
    void *handle;
    struct hw_module_t *hmi;

    /*
     * 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();
        LOGE("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) {
        LOGE("load: couldn't find symbol %s", sym);
        status = -EINVAL;
        goto done;
    }

    /* Check that the id matches */
    if (strcmp(id, hmi->id) != 0) {
        LOGE("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 {
        LOGV("loaded HAL id=%s path=%s hmi=%p handle=%p",
                id, path, *pHmi, handle);
    }

    *pHmi = hmi;

    return status;
}

在打开的.so（camera.msm7627a.so）中查找HMI符号的地址,并保存在hmi中。至此,.so中的hw_module_t已经被成功获取,从而可以根
据它获取别的相关接口。
1)HAL通过hw_get_module函数获取hw_module_t
2)HAL通过hw_module_t->methods->open获取hw_device_t指针,并在此open函数中初始化hw_device_t的包装结构中的
函数及hw_device_t中的close函数,如gralloc_device_open。
3)三个重要的数据结构:
    a) struct hw_device_t: 表示硬件设备,存储了各种硬件设备的公共属性和方法       
    b)struct hw_module_t: 可用hw_get_module进行加载的module
    c)struct hw_module_methods_t: 用于定义操作设备的方法,其中只定义了一个打开设备的方法open.