android平台多路摄像头实现方式的简析

采用Camera API2 HAL3架构，android Ｐ和Ｑ的初始化预览流程无异。

流程大概为CameraManager.openCamera打开摄像头设备回调onOpened方法得到CameraDevice，通过CameraDevice创建CaptureRequest预览请求，设置Surface容器同时创建CameraCaptureSession会话，从而实现视频预览。

多路摄像头应用MIPI CSI2架构，通过MIPI CSI2接口结合N4芯片达到多路视频显示的目的。
实现多路摄像头需要对Camera框架进行了适配，通过MIPI虚拟通道，每个通道对应一个虚拟cameraID，从cameraID-8开始+1，前后左右分别对应8，9，10，11分别对应虚拟通道1，2，3，4，cameraID-12则为打开四路通道的对应Id（后续看需求，可用于四路视频的录制）。camlitehal层处理camera相关的驱动操作，camera配置文件在device/…/camera_config.xml（区别于device/…/manifest.xml），并不是安装传统的打开设备节点legecy/0，legecy/1的方式打开）

最终会调用到mtkcam的hal层和驱动层来操控摄像头sensor和isp。
N4摄像头sensor处理源码路径：

n4_yuv kernel-4.9/drivers/misc/mediatek/imgsensor/src/common/v1

mtkcam部分目录为vendor/mediatek/proprietary/hardware/mtkcam/，按照mtkcam进行扩展实现对camera多路以及单路摄像头的预览。

客户端预览多路摄像头流程

可开启线程池，打开不同cameraId的摄像头，多任务实现多摄像头的同时预览。

    public static final String FRONT_CAMERA_ID = "8";
    public static final String BACK_CAMERA_ID = "9";
    public static final String LEFT_CAMERA_ID = "10";
    public static final String RIGHT_CAMERA_ID = "11";
    
/** 打开摄像头 openCamera**/
   CameraManager manager = (CameraManager) mContext.getSystemService(Context.CAMERA_SERVICE);
   manager.openCamera(mCameraId, mDeviceStateCallback, mBgHandler);
   
private final CameraDevice.StateCallback mDeviceStateCallback = new CameraDevice.StateCallback() {
        @Override
        public void onOpened(CameraDevice cameraDevice) {
            mCameraDevice = cameraDevice;
            startPreview();
        }

        @Override
        public void onDisconnected(CameraDevice cameraDevice) {
            cameraDevice.close();
        }

        @Override
        public void onError(CameraDevice cameraDevice, int error) {
            cameraDevice.close();
        }
    };

/** startPreview 视频预览**/
        mPreviewBuilder = mCameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);
        
        Surface surface = mSurfaceView.getHolder().getSurface();
                        mPreviewBuilder.addTarget(surface);


        mCameraDevice.createCaptureSession(Collections.singletonList(surface), mSessionStateCallback, null);
private CameraCaptureSession.StateCallback mSessionStateCallback = new CameraCaptureSession.StateCallback() {
                        @Override
                        public void onConfigured(CameraCaptureSession cameraCaptureSession) {
                            mPreviewSession = cameraCaptureSession;
                            updatePreview();
                        }

                        @Override
                        public void onConfigureFailed(CameraCaptureSession cameraCaptureSession) {
       cameraCaptureSession.close();
                            mPreviewSession = null;
                        }
                    }

/** updatePreview **/
        mPreviewRequest = mPreviewBuilder.build();
            mPreviewBuilder.set(CaptureRequest.CONTROL_MODE, CameraMetadata.CONTROL_MODE_AUTO);   mPreviewSession.setRepeatingRequest(mPreviewRequest, mCaptureCallback, mBgHandler);

private CameraCaptureSession.CaptureCallback mCaptureCallback = new CameraCaptureSession.CaptureCallback() {
        @Override
        public void onCaptureCompleted(CameraCaptureSession session, CaptureRequest request, TotalCaptureResult result) {
            //result为预览视频的结果，以此获取摄像头预览画面信息
        }
    };

openSession流程（MTK）

CameraDeviceClient frameworks/av/services/camera/libcameraservice/common

Camera3Device frameworks/av/services/camera/libcameraservice/device3

CameraProviderManager openSession frameworksavservicescameralibcameraservicecommon

openSession方法调用findDeviceInfoLocked从mDevices中获取匹配cameraId的Device
android 从camhallite 的hal层中加载获取cameraId,

hardware::Return CameraProviderManager::ProviderInfo::cameraDeviceStatusChange

cameraDeviceStatusChange方法将CameraDevice添加到mDevices中,接着通过
deviceInfo3->mInterface->open的流程打开摄像头设备

之后在源码中CameraDevice实现类：

vendor/mediatek/proprietary/hardware/mtkcam/main/hal/device/3.x/device/CameraDevice3Impl.cpp
vendor/mediatek/proprietary/hardware/mtkcam3/main/hal/device/3.x/device/CameraDevice3SessionImpl.cpp
vendor/mediatek/proprietary/hardware/mtkcam3/main/hal/devicemgr/CameraDeviceManagerBase.cpp

详细见博客介绍：Camera Hal3 Open/Close

之后根据操作打开n4则去切换mipi，打开cvbs单路摄像头则去打开/dev/video10节点。

传统方式openSession（google原生方式）

CameraDevice
源码地址：hardwareinterfacescameradevice3.2defaultCameraDevice.cpp

CameraModule
源码地址：hardwareinterfacescameracommon1.0defaultCameraModule.cpp

QCamera2Factory
源码地址：hardwareqcomcameraqcamera2QCamera2Factory.cpp

QCamera3HardwareInterface hardwareqcomcameraqcamera2hal3QCamera3HWI.cpp

mm_camera_interface.c hardwareqcomcameraqcamera2stackmm-camera-interfacesrc

mm_camera.c hardwareqcomcameraqcamera2stackmm-camera-interfacesrcmm_camera.c

mtkcam mtk Camera Half分析

CameraId获取流程

CameraProviderManager mInterface->getCameraIdList

ICameraProvider获取 hardware/interfaces/camera/provider/2.4/
再从hal层 camhal3lite获取

camhal3lite通过mipivcWrapper.cpp 对mipi端口进行操控vendor/mediatek/proprietary/hardware/mtkcam/drv/src/mipivc/mipivcWrapper.cpp

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