android中camera的hal模塊怎麼被調用

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

中調用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是怎麼回事

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

他只是一個封裝實際調用了[email protected] (hardware\libhardware）

好在不長，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];//幾個關鍵的數組
    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);//走這裏

    /*
     * 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)
   //在這裏將prop的路徑得到，分別從
    "ro.hardware[qcom]"
    "ro.product.board"[7x27],
    "ro.board.platform"[msm7627a],
    "ro.arch",
    "ro.hw_platform"[QRD_SKU3-1100]
    這幾個屬性文件中獲得硬件的信息
    有些硬件信息的字符串會出現在編譯後生成的.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);//走這裏,在這裏得到/system/lib/hw/camera.msm7627a.so
                                              這樣一個路徑，這個庫裏有QualcommCamera.cpp，這是
                                              camera模塊HAL代碼開始的地方
            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);//這裏關鍵,函數的三個參數可以串聯成一句話：
        到path（/system/lib/hw/camera.msm7627a.so）這個路徑下找到一個id（camera）匹配的module
    }

    return status;
}

再來看看load這個函數@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;
}

在打開的.so（camera.msm7627a.so）中查找HMI符號的地址,並保存在hmi中。至此,.so中的hw_module_t已經被成功獲取,從而可以根
據它獲取別的相關接口。
1)HAL通過hw_get_module函數獲取hw_module_t
2)HAL通過hw_module_t->methods->open獲取hw_device_t指針,並在此open函數中初始化hw_device_t的包裝結構中的
函數及hw_device_t中的close函數,如gralloc_device_open。
3)三個重要的數據結構:
    a) struct hw_device_t: 表示硬件設備,存儲了各種硬件設備的公共屬性和方法       
    b)struct hw_module_t: 可用hw_get_module進行加載的module
    c)struct hw_module_methods_t: 用於定義操作設備的方法,其中只定義了一個打開設備的方法open.