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前言

上一篇关于分析了 hw_get_module() 的调用逻辑，本篇通过追踪 Camera.startPreview() 方法，以加深对控制流的理解。

1 Frameworks

1.1 Camera.java

• 路径：frameworks/base/core/java/android/hardware/Camera.java
• startPreview()：
  • 给上层 application 提供一个接口。
  • 进入 Runtime 层。

/**
* Starts capturing and drawing preview frames to the screen.
* Preview will not actually start until a surface is supplied
* with {@link #setPreviewDisplay(SurfaceHolder)} or
* {@link #setPreviewTexture(SurfaceTexture)}.
*
* <p>If {@link #setPreviewCallback(Camera.PreviewCallback)},
* {@link #setOneShotPreviewCallback(Camera.PreviewCallback)}, or
* {@link #setPreviewCallbackWithBuffer(Camera.PreviewCallback)} were
* called, {@link Camera.PreviewCallback#onPreviewFrame(byte[], Camera)}
* will be called when preview data becomes available.
*/
    public native final void startPreview();

2 Android Runtime

2.1 android_hardware_Camera.cpp

• 路径：frameworks/base/core/jni/android_hardware_Camera.cpp
• android_hardware_Camera_startPreview()：
  • 调用 get_native_camera() 函数获取一个 Camera 实例。
  • 调用 Camera::startPreview()。

static void android_hardware_Camera_startPreview(JNIEnv *env, jobject thiz)
{
    ALOGV("startPreview");
    sp<Camera> camera = get_native_camera(env, thiz, NULL);
    if (camera == 0) return;
    
    if (camera->startPreview() != NO_ERROR) {
        jniThrowRuntimeException(env, "startPreview failed");
        return;
    }
}

• get_native_camera()：
  • 从 DVM 中获取关于 Camera 的上下文。
  • 从上下文信息中获取 Camera 实例。

sp<Camera> get_native_camera(JNIEnv *env, jobject thiz, JNICameraContext** pContext)
{
    sp<Camera> camera;
    Mutex::Autolock _l(sLock);
    JNICameraContext* context = reinterpret_cast<JNICameraContext*>(env->GetLongField(thiz, fields.context));
    if (context != NULL) {
        camera = context->getCamera();
    }
    ALOGV("get_native_camera: context=%p, camera=%p", context, camera.get());
    if (camera == 0) {
        jniThrowRuntimeException(env,
                "Camera is being used after Camera.release() was called");
    }

    if (pContext != NULL) *pContext = context;
    return camera;
}

3 Libraries

3.1 Camera.cpp

• 路径：frameworks/av/camera/Camera.cpp
• startPreview()：
  • mCamera 即是在 connect 过程中返回的 CameraClient，它具体实现了 startPreview() 接口。
  • 调用 CameraClient::startPreview()。

// start preview mode
status_t Camera::startPreview()
{
    ALOGV("startPreview");
    sp <::android::hardware::ICamera> c = mCamera;
    if (c == 0) return NO_INIT;
    return c->startPreview();
}

3.2 CameraClient.cpp

• 路径：frameworks/av/services/camera/libcameraservice/api1/CameraClient.cpp
• startPreview()：
  • 通过 startCameraMode 函数进入具体的实现逻辑。

// start preview mode
status_t CameraClient::startPreview() {
    LOG1("startPreview (pid %d)", getCallingPid());
    return startCameraMode(CAMERA_PREVIEW_MODE);
}

• startCameraMode()：
  • 根据传入的参数 CAMERA_PREVIEW_MODE 确定进入的分支。
  • 调用 startPreviewMode() 。

// start preview or recording
status_t CameraClient::startCameraMode(camera_mode mode) {
    LOG1("startCameraMode(%d)", mode);
    Mutex::Autolock lock(mLock);
    status_t result = checkPidAndHardware();
    if (result != NO_ERROR) return result;

    switch(mode) {
        case CAMERA_PREVIEW_MODE:
            if (mSurface == 0 && mPreviewWindow == 0) {
                LOG1("mSurface is not set yet.");
                // still able to start preview in this case.
            }
            return startPreviewMode();
        case CAMERA_RECORDING_MODE:
            if (mSurface == 0 && mPreviewWindow == 0) {
                ALOGE("mSurface or mPreviewWindow must be set before startRecordingMode.");
                return INVALID_OPERATION;
            }
            return startRecordingMode();
        default:
            return UNKNOWN_ERROR;
    }
}

• startPreviewMode()：
  • 如果预览已经存在，则直接返回成功信息。
  • 如果未存在，则继续往下走。
  • mHardware 是 CameraHardwareInterface 的实例，在 connect 过程的最后被初始化。
  • 通过 mHardware 调用 setPreviewWindow() 和 startPreview() 接口。
  • 进入 HAL 层。

status_t CameraClient::startPreviewMode() {
    LOG1("startPreviewMode");
    status_t result = NO_ERROR;

    // if preview has been enabled, nothing needs to be done
    if (mHardware->previewEnabled()) {
        return NO_ERROR;
    }

    if (mPreviewWindow != 0) {
        mHardware->setPreviewScalingMode(
            NATIVE_WINDOW_SCALING_MODE_SCALE_TO_WINDOW);
        mHardware->setPreviewTransform(mOrientation);
    }
    mHardware->setPreviewWindow(mPreviewWindow);
    result = mHardware->startPreview();
    if (result == NO_ERROR) {
        mCameraService->updateProxyDeviceState(
            ICameraServiceProxy::CAMERA_STATE_ACTIVE,
            String8::format("%d", mCameraId));
    }
    return result;
}

4 HAL

4.1 CameraHardwareInterface.h

• 路径：frameworks/av/services/camera/libcameraservice/device1/CameraHardwareInterface.h
• previewEnable()：
  • 通过 mDevice->ops 继续向下调用（不是我们主要追踪的）。
  • mDevice 即是通过 hw_get_module() 相关流程进行初始化的设备实例，它的类型是 camera_device_t 。
  • 如果 preview 存在，则返回 true 。

    /**
     * Returns true if preview is enabled.
     */
    int previewEnabled()
    {
        ALOGV("%s(%s)", __FUNCTION__, mName.string());
        if (mDevice->ops->preview_enabled)
            return mDevice->ops->preview_enabled(mDevice);
        return false;
    }

• setPreviewWindow()：
  • 如果 set_preview_window 函数指针为空，则返回失败信息。
  • 若否，通过 mDevice->ops 继续向下调用（不是我们主要追踪的）。

    /** Set the ANativeWindow to which preview frames are sent */
    status_t setPreviewWindow(const sp<ANativeWindow>& buf)
    {
        ALOGV("%s(%s) buf %p", __FUNCTION__, mName.string(), buf.get());
        if (mDevice->ops->set_preview_window) {
            mPreviewWindow = buf;
            if (buf != nullptr) {
                if (mPreviewScalingMode != NOT_SET) {
                    setPreviewScalingMode(mPreviewScalingMode);
                }
                if (mPreviewTransform != NOT_SET) {
                    setPreviewTransform(mPreviewTransform);
                }
            }
            mHalPreviewWindow.user = this;
            ALOGV("%s &mHalPreviewWindow %p mHalPreviewWindow.user %p", __FUNCTION__,
                    &mHalPreviewWindow, mHalPreviewWindow.user);
            return mDevice->ops->set_preview_window(mDevice,
                    buf.get() ? &mHalPreviewWindow.nw : 0);
        }
        return INVALID_OPERATION;
    }

• startPreview()：
  • 若 start_preview() 函数指针为空，则返回失败信息。
  • 若否，则通过 mDevice 进行下一步操作。
  • 关于 mDevice，我们结合 Camera.open() 流程与 hw_get_module() 相关逻辑，可以知道它的逻辑是这样的：
    • 在 CameraService 启动时，会调用 onFirstRef() 对 module 进行初始化，获取 module 实例。
    • 在 open 过程中，CameraClient 连接 CameraServer 成功时，会实例化 CameraHardwareInterface，并传入 module 实例对其初始化。
    • 在初始化过程中，通过 module 实例对应的 open 方法，我们获得一个 device 实例，即 mDevice，这对应了具体的摄像头设备。
    • 通过 mDevice，我们就可以将对应的指令传达到硬件设备。
  • 通过对 camera_device_t 类型进行追踪，可以找到函数指针的一个具体指向。

    /**
     * Start preview mode.
     */
    status_t startPreview()
    {
        ALOGV("%s(%s)", __FUNCTION__, mName.string());
        if (mDevice->ops->start_preview)
            return mDevice->ops->start_preview(mDevice);
        return INVALID_OPERATION;
    }

4.2 camera.h

• 路径：hardware/libhardware/include/hardware/camera.h
• struct camera_device：
  • 这里就声明了我们想要追踪的 camera_device_t 。
  • ops 对应的类型是 camera_device_ops_t ，这个结构中声明了函数指针。

typedef struct camera_device {
    /**
     * camera_device.common.version must be in the range
     * HARDWARE_DEVICE_API_VERSION(0,0)-(1,FF). CAMERA_DEVICE_API_VERSION_1_0 is
     * recommended.
     */
    hw_device_t common;
    camera_device_ops_t *ops;
    void *priv;
} camera_device_t;

• struct camera_device_ops：
  • 可以看到，所有关于 Camera 设备的操作，对应的函数指针都在这里声明了。但是看不出函数指针具体指向哪里。
  • 在 Linux 下用 find . -name "*.cpp" | xargs grep "start_preview =" 可以找到一些对应的文件，这些文件所处的路径与具体的设备商有关。
  • 在这些文件中，就确定了函数指针的指向。

typedef struct camera_device_ops {
    int (*set_preview_window)(struct camera_device *,
            struct preview_stream_ops *window);

    void (*set_callbacks)(struct camera_device *,
            camera_notify_callback notify_cb,
            camera_data_callback data_cb,
            camera_data_timestamp_callback data_cb_timestamp,
            camera_request_memory get_memory,
            void *user);

    void (*enable_msg_type)(struct camera_device *, int32_t msg_type);

    void (*disable_msg_type)(struct camera_device *, int32_t msg_type);

    int (*msg_type_enabled)(struct camera_device *, int32_t msg_type);

    /**
     * Start preview mode.
     */
    int (*start_preview)(struct camera_device *);

    void (*stop_preview)(struct camera_device *);

    int (*preview_enabled)(struct camera_device *);

    int (*store_meta_data_in_buffers)(struct camera_device *, int enable);

    int (*start_recording)(struct camera_device *);

    void (*stop_recording)(struct camera_device *);

    int (*recording_enabled)(struct camera_device *);

    void (*release_recording_frame)(struct camera_device *,
                    const void *opaque);

    int (*auto_focus)(struct camera_device *);

    int (*cancel_auto_focus)(struct camera_device *);

    int (*take_picture)(struct camera_device *);

    int (*cancel_picture)(struct camera_device *);

    int (*set_parameters)(struct camera_device *, const char *parms);

    char *(*get_parameters)(struct camera_device *);

    void (*put_parameters)(struct camera_device *, char *);

    int (*send_command)(struct camera_device *,
                int32_t cmd, int32_t arg1, int32_t arg2);

    void (*release)(struct camera_device *);

    int (*dump)(struct camera_device *, int fd);
} camera_device_ops_t;

4.3 hardware/ti/omap4-aah/camera

通过 find 指令，找到了一些与函数指针指向有关的文件。从这些文件的路径上看，它们与不同的设备供应商有关。下面去 ti/omap4-aah 子文件夹去一探究竟。

4.3.1 CameraHal_Module.cpp

• 路径：hardware/ti/omap4-aah/camera/CameraHal_Module.cpp
• camera_device_open()：
  • 在 open 流程中，就指定了 ops 中指针的对应关系。

        memset(camera_device, 0, sizeof(*camera_device));
        memset(camera_ops, 0, sizeof(*camera_ops));

        camera_device->base.common.tag = HARDWARE_DEVICE_TAG;
        camera_device->base.common.version = 0;
        camera_device->base.common.module = (hw_module_t *)(module);
        camera_device->base.common.close = camera_device_close;
        camera_device->base.ops = camera_ops;

        camera_ops->set_preview_window = camera_set_preview_window;
        camera_ops->set_callbacks = camera_set_callbacks;
        camera_ops->enable_msg_type = camera_enable_msg_type;
        camera_ops->disable_msg_type = camera_disable_msg_type;
        camera_ops->msg_type_enabled = camera_msg_type_enabled;
        camera_ops->start_preview = camera_start_preview;
        camera_ops->stop_preview = camera_stop_preview;
        camera_ops->preview_enabled = camera_preview_enabled;
        camera_ops->store_meta_data_in_buffers = camera_store_meta_data_in_buffers;
        camera_ops->start_recording = camera_start_recording;
        camera_ops->stop_recording = camera_stop_recording;
        camera_ops->recording_enabled = camera_recording_enabled;
        camera_ops->release_recording_frame = camera_release_recording_frame;
        camera_ops->auto_focus = camera_auto_focus;
        camera_ops->cancel_auto_focus = camera_cancel_auto_focus;
        camera_ops->take_picture = camera_take_picture;
        camera_ops->cancel_picture = camera_cancel_picture;
        camera_ops->set_parameters = camera_set_parameters;
        camera_ops->get_parameters = camera_get_parameters;
        camera_ops->put_parameters = camera_put_parameters;
        camera_ops->send_command = camera_send_command;
        camera_ops->release = camera_release;
        camera_ops->dump = camera_dump;

        *device = &camera_device->base.common;

        // -------- TI specific stuff --------

        camera_device->cameraid = cameraid;

• camera_start_preview()：
  • 注意 gCameraHals 是 CameraHal * 。
  • 通过调用 CameraHal::startPreview() 完成业务逻辑。

int camera_start_preview(struct camera_device * device)
{
    CAMHAL_LOG_MODULE_FUNCTION_NAME;

    int rv = -EINVAL;
    ti_camera_device_t* ti_dev = NULL;

    if(!device)
        return rv;

    ti_dev = (ti_camera_device_t*) device;

    rv = gCameraHals[ti_dev->cameraid]->startPreview();

    return rv;
}

4.3.2 CameraHal.cpp

• 路径：hardware/ti/omap4-aah/camera/CameraHal.cpp
• 这个文件对应的功能是，将 Camera Hardware Interface 映射到 V4L2。
• 注意两个声明：
  • extern "C" CameraAdapter* OMXCameraAdapter_Factory(size_t);
  • extern "C" CameraAdapter* V4LCameraAdapter_Factory(size_t);

• 分别对应 OMX 与 V4L 的适配器工厂，这里可能是将 Adapter 模式与 Factory 模式结合使用。

• startPreview()：
  • 源代码中带有大量注释，在这里我将其去掉，只关注调用逻辑。
  • 首先调用了 cameraPreviewInitialization() 函数进行初始化。
  • 通过 CameraAdapter 发送 CAMERA_START_PREVIEW 指令，若成功执行，则完成流程。

status_t CameraHal::startPreview() {
    LOG_FUNCTION_NAME;

    status_t ret = cameraPreviewInitialization();

    if (!mPreviewInitializationDone) return ret;

    mPreviewInitializationDone = false;

    if(mDisplayAdapter.get() != NULL) {
        CAMHAL_LOGDA("Enabling display");
        int width, height;
        mParameters.getPreviewSize(&width, &height);

#if PPM_INSTRUMENTATION || PPM_INSTRUMENTATION_ABS
        ret = mDisplayAdapter->enableDisplay(width, height, &mStartPreview);
#else
        ret = mDisplayAdapter->enableDisplay(width, height, NULL);
#endif

        if ( ret != NO_ERROR ) {
            CAMHAL_LOGEA("Couldn't enable display");
            CAMHAL_ASSERT_X(false,
                "At this stage mCameraAdapter->mStateSwitchLock is still locked, "
                "deadlock is guaranteed");

            goto error;
        }
    }

    CAMHAL_LOGDA("Starting CameraAdapter preview mode");

    ret = mCameraAdapter->sendCommand(CameraAdapter::CAMERA_START_PREVIEW);

    if(ret!=NO_ERROR) {
        CAMHAL_LOGEA("Couldn't start preview w/ CameraAdapter");
        goto error;
    }
    CAMHAL_LOGDA("Started preview");

    mPreviewEnabled = true;
    mPreviewStartInProgress = false;
    return ret;

    error:

        CAMHAL_LOGEA("Performing cleanup after error");

        //Do all the cleanup
        freePreviewBufs();
        mCameraAdapter->sendCommand(CameraAdapter::CAMERA_STOP_PREVIEW);
        if(mDisplayAdapter.get() != NULL) {
            mDisplayAdapter->disableDisplay(false);
        }
        mAppCallbackNotifier->stop();
        mPreviewStartInProgress = false;
        mPreviewEnabled = false;
        LOG_FUNCTION_NAME_EXIT;

        return ret;
}

• cameraPreviewInitialization()：
  • 这段代码比较长，但从注释看，它主要做三件事：
    • 通过 Adapter 设置相关参数；
    • 申请 Buffers 空间；
    • 对 Buffers 进行相应设置以进行预览。
  • 了解了大体思路，再看看具体代码：
    • mCameraAdapter->setParameters()：设置参数。
    • allocPreviewBufs() ：申请 Buffers。
    • desc：注意到这个变量对应着 CameraAdapter::BuffersDescriptor，在申请 Buffers 空间成功后，便对其进行相应的成员设置。
    • mAppCallbackNotifier->start()：开启回调通知。
    • mAppCallbackNotifier->startPreviewCallbacks()：将 Buffers 对应到相应的回调函数中，以供上层 APP 获取预览所需的数据。
  • NOTE：
    • 代码中不断使用 mCameraAdapter->sendCommand() 来发送指令，并获取一些数据。
    • 指令发送到对应的 Adapter （如 V4L Adapter），就会调用相应的函数进行处理。

/**
   @brief Set preview mode related initialization
          -> Camera Adapter set params
          -> Allocate buffers
          -> Set use buffers for preview
   @param none
   @return NO_ERROR
   @todo Update function header with the different errors that are possible
*/
status_t CameraHal::cameraPreviewInitialization()
{
    status_t ret = NO_ERROR;
    CameraAdapter::BuffersDescriptor desc;
    CameraFrame frame;
    unsigned int required_buffer_count;
    unsigned int max_queueble_buffers;

#if PPM_INSTRUMENTATION || PPM_INSTRUMENTATION_ABS
        gettimeofday(&mStartPreview, NULL);
#endif

    LOG_FUNCTION_NAME;

    if (mPreviewInitializationDone) {
        return NO_ERROR;
    }

    if ( mPreviewEnabled ){
      CAMHAL_LOGDA("Preview already running");
      LOG_FUNCTION_NAME_EXIT;
      return ALREADY_EXISTS;
    }

    if ( NULL != mCameraAdapter ) {
      ret = mCameraAdapter->setParameters(mParameters);
    }

    if ((mPreviewStartInProgress == false) && (mDisplayPaused == false)){
      ret = mCameraAdapter->sendCommand(CameraAdapter::CAMERA_QUERY_RESOLUTION_PREVIEW,( int ) &frame);
      if ( NO_ERROR != ret ){
        CAMHAL_LOGEB("Error: CAMERA_QUERY_RESOLUTION_PREVIEW %d", ret);
        return ret;
      }

      ///Update the current preview width and height
      mPreviewWidth = frame.mWidth;
      mPreviewHeight = frame.mHeight;
    }

    ///If we don't have the preview callback enabled and display adapter,
    if(!mSetPreviewWindowCalled || (mDisplayAdapter.get() == NULL)){
      CAMHAL_LOGD("Preview not started. Preview in progress flag set");
      mPreviewStartInProgress = true;
      ret = mCameraAdapter->sendCommand(CameraAdapter::CAMERA_SWITCH_TO_EXECUTING);
      if ( NO_ERROR != ret ){
        CAMHAL_LOGEB("Error: CAMERA_SWITCH_TO_EXECUTING %d", ret);
        return ret;
      }
      return NO_ERROR;
    }

    if( (mDisplayAdapter.get() != NULL) && ( !mPreviewEnabled ) && ( mDisplayPaused ) )
        {
        CAMHAL_LOGDA("Preview is in paused state");

        mDisplayPaused = false;
        mPreviewEnabled = true;
        if ( NO_ERROR == ret )
            {
            ret = mDisplayAdapter->pauseDisplay(mDisplayPaused);

            if ( NO_ERROR != ret )
                {
                CAMHAL_LOGEB("Display adapter resume failed %x", ret);
                }
            }
        //restart preview callbacks
        if(mMsgEnabled & CAMERA_MSG_PREVIEW_FRAME)
        {
            mAppCallbackNotifier->enableMsgType (CAMERA_MSG_PREVIEW_FRAME);
        }

        signalEndImageCapture();
        return ret;
        }

    required_buffer_count = atoi(mCameraProperties->get(CameraProperties::REQUIRED_PREVIEW_BUFS));

    ///Allocate the preview buffers
    ret = allocPreviewBufs(mPreviewWidth, mPreviewHeight, mParameters.getPreviewFormat(), required_buffer_count, max_queueble_buffers);

    if ( NO_ERROR != ret )
        {
        CAMHAL_LOGEA("Couldn't allocate buffers for Preview");
        goto error;
        }

    if ( mMeasurementEnabled )
        {

        ret = mCameraAdapter->sendCommand(CameraAdapter::CAMERA_QUERY_BUFFER_SIZE_PREVIEW_DATA,
                                          ( int ) &frame,
                                          required_buffer_count);
        if ( NO_ERROR != ret )
            {
            return ret;
            }

         ///Allocate the preview data buffers
        ret = allocPreviewDataBufs(frame.mLength, required_buffer_count);
        if ( NO_ERROR != ret ) {
            CAMHAL_LOGEA("Couldn't allocate preview data buffers");
            goto error;
           }

        if ( NO_ERROR == ret )
            {
            desc.mBuffers = mPreviewDataBuffers;
            desc.mOffsets = mPreviewDataOffsets;
            desc.mFd = mPreviewDataFd;
            desc.mLength = mPreviewDataLength;
            desc.mCount = ( size_t ) required_buffer_count;
            desc.mMaxQueueable = (size_t) required_buffer_count;

            mCameraAdapter->sendCommand(CameraAdapter::CAMERA_USE_BUFFERS_PREVIEW_DATA,
                                        ( int ) &desc);
            }

        }

    ///Pass the buffers to Camera Adapter
    desc.mBuffers = mPreviewBuffers;
    desc.mOffsets = mPreviewOffsets;
    desc.mFd = mPreviewFd;
    desc.mLength = mPreviewLength;
    desc.mCount = ( size_t ) required_buffer_count;
    desc.mMaxQueueable = (size_t) max_queueble_buffers;

    ret = mCameraAdapter->sendCommand(CameraAdapter::CAMERA_USE_BUFFERS_PREVIEW,
                                      ( int ) &desc);

    if ( NO_ERROR != ret )
        {
        CAMHAL_LOGEB("Failed to register preview buffers: 0x%x", ret);
        freePreviewBufs();
        return ret;
        }

    ///Start the callback notifier
    ret = mAppCallbackNotifier->start();

    if( ALREADY_EXISTS == ret )
        {
        //Already running, do nothing
        CAMHAL_LOGDA("AppCallbackNotifier already running");
        ret = NO_ERROR;
        }
    else if ( NO_ERROR == ret ) {
        CAMHAL_LOGDA("Started AppCallbackNotifier..");
        mAppCallbackNotifier->setMeasurements(mMeasurementEnabled);
        }
    else
        {
        CAMHAL_LOGDA("Couldn't start AppCallbackNotifier");
        goto error;
        }

    if (ret == NO_ERROR) mPreviewInitializationDone = true;

    mAppCallbackNotifier->startPreviewCallbacks(mParameters, mPreviewBuffers, mPreviewOffsets, mPreviewFd, mPreviewLength, required_buffer_count);

    return ret;

    error:

        CAMHAL_LOGEA("Performing cleanup after error");

        //Do all the cleanup
        freePreviewBufs();
        mCameraAdapter->sendCommand(CameraAdapter::CAMERA_STOP_PREVIEW);
        if(mDisplayAdapter.get() != NULL)
            {
            mDisplayAdapter->disableDisplay(false);
            }
        mAppCallbackNotifier->stop();
        mPreviewStartInProgress = false;
        mPreviewEnabled = false;
        LOG_FUNCTION_NAME_EXIT;

        return ret;
}

4.3.3 BaseCameraAdapter.cpp

• 路径：hardware/ti/omap4-aah/camera/BaseCameraAdapter.cpp
• 注意到文件中定义的常量：
  • 分别对应不同的命令。

const LUT cameraCommandsUserToHAL[] = {
    { "CAMERA_START_PREVIEW",                   CameraAdapter::CAMERA_START_PREVIEW },
    { "CAMERA_STOP_PREVIEW",                    CameraAdapter::CAMERA_STOP_PREVIEW },
    { "CAMERA_START_VIDEO",                     CameraAdapter::CAMERA_START_VIDEO },
    { "CAMERA_STOP_VIDEO",                      CameraAdapter::CAMERA_STOP_VIDEO },
    { "CAMERA_START_IMAGE_CAPTURE",             CameraAdapter::CAMERA_START_IMAGE_CAPTURE },
    { "CAMERA_STOP_IMAGE_CAPTURE",              CameraAdapter::CAMERA_STOP_IMAGE_CAPTURE },
    { "CAMERA_PERFORM_AUTOFOCUS",               CameraAdapter::CAMERA_PERFORM_AUTOFOCUS },
    { "CAMERA_CANCEL_AUTOFOCUS",                CameraAdapter::CAMERA_CANCEL_AUTOFOCUS },
    { "CAMERA_PREVIEW_FLUSH_BUFFERS",           CameraAdapter::CAMERA_PREVIEW_FLUSH_BUFFERS },
    { "CAMERA_START_SMOOTH_ZOOM",               CameraAdapter::CAMERA_START_SMOOTH_ZOOM },
    { "CAMERA_STOP_SMOOTH_ZOOM",                CameraAdapter::CAMERA_STOP_SMOOTH_ZOOM },
    { "CAMERA_USE_BUFFERS_PREVIEW",             CameraAdapter::CAMERA_USE_BUFFERS_PREVIEW },
    { "CAMERA_SET_TIMEOUT",                     CameraAdapter::CAMERA_SET_TIMEOUT },
    { "CAMERA_CANCEL_TIMEOUT",                  CameraAdapter::CAMERA_CANCEL_TIMEOUT },
    { "CAMERA_START_BRACKET_CAPTURE",           CameraAdapter::CAMERA_START_BRACKET_CAPTURE },
    { "CAMERA_STOP_BRACKET_CAPTURE",            CameraAdapter::CAMERA_STOP_BRACKET_CAPTURE },
    { "CAMERA_QUERY_RESOLUTION_PREVIEW",        CameraAdapter::CAMERA_QUERY_RESOLUTION_PREVIEW },
    { "CAMERA_QUERY_BUFFER_SIZE_IMAGE_CAPTURE", CameraAdapter::CAMERA_QUERY_BUFFER_SIZE_IMAGE_CAPTURE },
    { "CAMERA_QUERY_BUFFER_SIZE_PREVIEW_DATA",  CameraAdapter::CAMERA_QUERY_BUFFER_SIZE_PREVIEW_DATA },
    { "CAMERA_USE_BUFFERS_IMAGE_CAPTURE",       CameraAdapter::CAMERA_USE_BUFFERS_IMAGE_CAPTURE },
    { "CAMERA_USE_BUFFERS_PREVIEW_DATA",        CameraAdapter::CAMERA_USE_BUFFERS_PREVIEW_DATA },
    { "CAMERA_TIMEOUT_EXPIRED",                 CameraAdapter::CAMERA_TIMEOUT_EXPIRED },
    { "CAMERA_START_FD",                        CameraAdapter::CAMERA_START_FD },
    { "CAMERA_STOP_FD",                         CameraAdapter::CAMERA_STOP_FD },
    { "CAMERA_SWITCH_TO_EXECUTING",             CameraAdapter::CAMERA_SWITCH_TO_EXECUTING },
    { "CAMERA_USE_BUFFERS_VIDEO_CAPTURE",       CameraAdapter::CAMERA_USE_BUFFERS_VIDEO_CAPTURE },
#ifdef OMAP_ENHANCEMENT_CPCAM
    { "CAMERA_USE_BUFFERS_REPROCESS",           CameraAdapter::CAMERA_USE_BUFFERS_REPROCESS },
    { "CAMERA_START_REPROCESS",                 CameraAdapter::CAMERA_START_REPROCESS },
#endif
};

• BaseCameraAdapter::sendCommand()：
  • 利用 switch 将不同的命令对应到各自的逻辑中。
  • 此处的 BaseCameraAdapter::startPreview() 实际上做什么操作，具体的实现是在其子类中，接下来选取一个子类 V4LCameraAdapter 继续深入。

    case CameraAdapter::CAMERA_START_PREVIEW:
        {

            CAMHAL_LOGDA("Start Preview");

        if ( ret == NO_ERROR )
            {
            ret = setState(operation);
            }

        if ( ret == NO_ERROR )
            {
            ret = startPreview();
            }

        if ( ret == NO_ERROR )
            {
            ret = commitState();
            }
        else
            {
            ret |= rollbackState();
            }

        break;

        }

4.3.4 V4LCameraAdapter.h

• 路径：hardware/ti/omap4-aah/camera/inc/V4LCameraAdapter/V4LCameraAdapter.h
• 类 V4LCameraAdapter 继承了 BaseCameraAdapter。
• 注意它有一个内部私有类：
  • threadLoop() 应该是与线程的循环执行有关。
  • 这个线程不断执行 Adapter 中的 previewThread() 函数。

private:

    class PreviewThread : public android::Thread {
            V4LCameraAdapter* mAdapter;
        public:
            PreviewThread(V4LCameraAdapter* hw) :
                    Thread(false), mAdapter(hw) { }
            virtual void onFirstRef() {
                run("CameraPreviewThread", android::PRIORITY_URGENT_DISPLAY);
            }
            virtual bool threadLoop() {
                mAdapter->previewThread();
                // loop until we need to quit
                return true;
            }
        };

    //Used for calculation of the average frame rate during preview
    status_t recalculateFPS();

    char * GetFrame(int &index);

    int previewThread();

4.3.5 V4LCameraAdapter.cpp

• 路径：hardware/ti/omap4-aah/camera/V4LCameraAdapter/V4LCameraAdapter.cpp
• startPreview()：
  • 通过 v4lIoctl() 函数从硬件获取需要的数据，并存入 Buffers。
  • 启动一个 PreviewThread，用于接收从 V4L 摄像头设备传回的数据。
  • 最后设置一些 flag 表明预览功能已开启，startPreview 的控制流程就结束了。

status_t V4LCameraAdapter::startPreview()
{
    status_t ret = NO_ERROR;

    LOG_FUNCTION_NAME;
    android::AutoMutex lock(mPreviewBufsLock);

    if(mPreviewing) {
        ret = BAD_VALUE;
        goto EXIT;
    }

    for (int i = 0; i < mPreviewBufferCountQueueable; i++) {

        mVideoInfo->buf.index = i;
        mVideoInfo->buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        mVideoInfo->buf.memory = V4L2_MEMORY_MMAP;

        ret = v4lIoctl(mCameraHandle, VIDIOC_QBUF, &mVideoInfo->buf);
        if (ret < 0) {
            CAMHAL_LOGEA("VIDIOC_QBUF Failed");
            goto EXIT;
        }
        nQueued++;
    }

    ret = v4lStartStreaming();

    // Create and start preview thread for receiving buffers from V4L Camera
    if(!mCapturing) {
        mPreviewThread = new PreviewThread(this);
        CAMHAL_LOGDA("Created preview thread");
    }

    //Update the flag to indicate we are previewing
    mPreviewing = true;
    mCapturing = false;

EXIT:
    LOG_FUNCTION_NAME_EXIT;
    return ret;
}

• previewThread()：
  • 在 PreviewThread 线程中不断被调用。
  • 获取设备传回的数据，并进行一些格式转换操作：
    • convertYUV422ToNV12Tiler()
  • 给帧数据进行一些必要的参数设置，如帧大小、时间戳等。
  • 将帧数据发送给用户：
    • sendFrameToSubscribers(&frame)

int V4LCameraAdapter::previewThread()
{
    status_t ret = NO_ERROR;
    int width, height;
    CameraFrame frame;
    void *y_uv[2];
    int index = 0;
    int stride = 4096;
    char *fp = NULL;

    mParams.getPreviewSize(&width, &height);

    if (mPreviewing) {

        fp = this->GetFrame(index);
        if(!fp) {
            ret = BAD_VALUE;
            goto EXIT;
        }
        CameraBuffer *buffer = mPreviewBufs.keyAt(index);
        CameraFrame *lframe = (CameraFrame *)mFrameQueue.valueFor(buffer);
        if (!lframe) {
            ret = BAD_VALUE;
            goto EXIT;
        }

        debugShowFPS();

        if ( mFrameSubscribers.size() == 0 ) {
            ret = BAD_VALUE;
            goto EXIT;
        }
        y_uv[0] = (void*) lframe->mYuv[0];
        //y_uv[1] = (void*) lframe->mYuv[1];
        //y_uv[1] = (void*) (lframe->mYuv[0] + height*stride);
        convertYUV422ToNV12Tiler ( (unsigned char*)fp, (unsigned char*)y_uv[0], width, height);
        CAMHAL_LOGVB("##...index= %d.;camera buffer= 0x%x; y= 0x%x; UV= 0x%x.",index, buffer, y_uv[0], y_uv[1] );

#ifdef SAVE_RAW_FRAMES
        unsigned char* nv12_buff = (unsigned char*) malloc(width*height*3/2);
        //Convert yuv422i to yuv420sp(NV12) & dump the frame to a file
        convertYUV422ToNV12 ( (unsigned char*)fp, nv12_buff, width, height);
        saveFile( nv12_buff, ((width*height)*3/2) );
        free (nv12_buff);
#endif

        frame.mFrameType = CameraFrame::PREVIEW_FRAME_SYNC;
        frame.mBuffer = buffer;
        frame.mLength = width*height*3/2;
        frame.mAlignment = stride;
        frame.mOffset = 0;
        frame.mTimestamp = systemTime(SYSTEM_TIME_MONOTONIC);
        frame.mFrameMask = (unsigned int)CameraFrame::PREVIEW_FRAME_SYNC;

        if (mRecording)
        {
            frame.mFrameMask |= (unsigned int)CameraFrame::VIDEO_FRAME_SYNC;
            mFramesWithEncoder++;
        }

        ret = setInitFrameRefCount(frame.mBuffer, frame.mFrameMask);
        if (ret != NO_ERROR) {
            CAMHAL_LOGDB("Error in setInitFrameRefCount %d", ret);
        } else {
            ret = sendFrameToSubscribers(&frame);
        }
    }
EXIT:

    return ret;
}

5 流程简图

总结

本篇我们从 Camera.startPreview() 方法作为切入点，结合 hw_get_module() 相关内容，对其整个流程进行追踪分析，从而对之前已有初步了解的 Camera 控制流逻辑有一个更全面的理解。在 HAL 层中，涉及到了不同平台的实现，而且这些具体实现还是有比较大的区别的。下一篇，我们将分析 Camera 数据流的逻辑。