https://blog.csdn.net/ljsbuct/article/details/7094670
https://wenku.baidu.com/view/8bcfa6b2011ca300a7c390dd.html
https://blog.csdn.net/xiewenhao12/article/details/53305540
https://blog.csdn.net/hesunwen/article/details/54377811
https://blog.csdn.net/jmq_0000/article/details/7545540
https://blog.csdn.net/htjacky/article/details/54969842
https://blog.csdn.net/sq336/article/details/39204169
Camera2-------------------------------------------
https://blog.csdn.net/hejjunlin/article/details/53138945
CameraMetadata------------------------------------
https://blog.csdn.net/armwind/article/details/52027010
camera HAL(HIDL)----------------------------------
https://blog.csdn.net/u011279649/article/details/80392386

显示IMX6  ------->   https://blog.csdn.net/qq_29729577/article/details/51134315                                                    http://www.cnblogs.com/emouse/archive/2013/03/04/2943243.html

Camera的架构与Android系统的整体架构保持一致，如下图所示，本文主要从以下四个方面对其进行说明。

1. Framework：Camera.java
2. Android Runtime：android_hardware_Camera.cpp
3. Library：Camera Client和Camera Service
4. HAL：CameraHardwareInterface

Camera系统架构

一、Framework：Camera.java

Camera是应用层软件直接使用的类，涵盖了启动、预览、拍摄及关闭等操作摄像头的全部接口。Camera.java在Android源码中的路径为：framework/base/core/java/android/hardware。为了说明整个Camera系统的架构，这里暂不横向分析Camera.java的功能，下面从open()方法着手：

public static Camera open() {int numberOfCameras = getNumberOfCameras();CameraInfo cameraInfo = new CameraInfo();for (int i = 0; i < numberOfCameras; i++) {getCameraInfo(i, cameraInfo);if (cameraInfo.facing == CameraInfo.CAMERA_FACING_BACK) {return new Camera(i);}}return null;
}

open()方法需要注意以下几点：

• getNumberOfCameras为native方法，实现在android_hardware_Camera.cpp中；
• CameraInfo是Camera定义的静态内部类，包含facing、orientation、canDisableShutterSound；
• getCameraInfo内部调用native方法_getCameraInfo获取摄像头信息；
• open()默认启动的是后置摄像头（CAMERA_FACING_BACK）。

/** used by Camera#open, Camera#open(int) */
Camera(int cameraId) {int err = cameraInitNormal(cameraId);if (checkInitErrors(err)) {switch(err) {case EACCESS:throw new RuntimeException("Fail to connect to camera service");case ENODEV:throw new RuntimeException("Camera initialization failed");default:// Should never hit this.throw new RuntimeException("Unknown camera error");}}
}

Camera构造器的核心实现在cameraInitNormal中，cameraInitNormal调用cameraInitVersion，并传入参数cameraId和CAMERA_HAL_API_VERSION_NORMAL_CONNECT，后者代表HAL的版本。

private int cameraInitVersion(int cameraId, int halVersion) {……String packageName = ActivityThread.currentPackageName();return native_setup(new WeakReference<Camera>(this), cameraId, halVersion, packageName);
}

cameraInitNormal调用本地方法native_setup()，由此进入到android_hardware_Camera.cpp中，native_setup()的签名如下：

private native final int native_setup(Object camera_this, int cameraId, int halVersion, String packageName);

二、Android Runtime：android_hardware_Camera.cpp

native_setup()被动态注册到JNI，通过JNI调用android_hardware_Camera_native_setup()方法。

static JNINativeMethod camMethods[] = {……{ "native_setup",    "(Ljava/lang/Object;ILjava/lang/String;)V",(void*)android_hardware_Camera_native_setup }……
};

JNI的重点是android_hardware_Camera_native_setup()方法的实现：

// connect to camera service
static jint android_hardware_Camera_native_setup(JNIEnv *env, jobject thiz,jobject weak_this, jint cameraId, jint halVersion, jstring clientPackageName)
{// Convert jstring to String16const char16_t *rawClientName = env->GetStringChars(clientPackageName, NULL);jsize rawClientNameLen = env->GetStringLength(clientPackageName);String16 clientName(rawClientName, rawClientNameLen);env->ReleaseStringChars(clientPackageName, rawClientName);sp<Camera> camera;if (halVersion == CAMERA_HAL_API_VERSION_NORMAL_CONNECT) {// Default path: hal version is don't care, do normal camera connect.camera = Camera::connect(cameraId, clientName,Camera::USE_CALLING_UID);} else {jint status = Camera::connectLegacy(cameraId, halVersion, clientName,Camera::USE_CALLING_UID, camera);if (status != NO_ERROR) {return status;}}if (camera == NULL) {return -EACCES;}// make sure camera hardware is aliveif (camera->getStatus() != NO_ERROR) {return NO_INIT;}jclass clazz = env->GetObjectClass(thiz);if (clazz == NULL) {// This should never happenjniThrowRuntimeException(env, "Can't find android/hardware/Camera");return INVALID_OPERATION;}// We use a weak reference so the Camera object can be garbage collected.// The reference is only used as a proxy for callbacks.sp<JNICameraContext> context = new JNICameraContext(env, weak_this, clazz, camera);context->incStrong((void*)android_hardware_Camera_native_setup);camera->setListener(context);// save context in opaque fieldenv->SetLongField(thiz, fields.context, (jlong)context.get());return NO_ERROR;
}

android_hardware_Camera_native_setup()方法通过调用Camera::connect()方法请求连接CameraService服务。入参中：

• clientName是通过将clientPackageName从jstring转换为String16格式得到；
• Camera::USE_CALLING_UID是定义在Camera.h中的枚举类型，其值为ICameraService::USE_CALLING_UID（同样为枚举类型，值为-1）。

Camera::connect()位于Camera.cpp中，由此进入到Library层。

三、Library：Camera Client和Camera Service

如上述架构图中所示，ICameraService.h、ICameraClient.h和ICamera.h三个类定义了Camera的接口和架构，ICameraService.cpp和Camera.cpp两个文件用于Camera架构的实现，Camera的具体功能在下层调用硬件相关的接口来实现。Camera.h是Camera系统对上层的接口。

具体的，Camera类继承模板类CameraBase，Camera::connect()调用了CameraBase.cpp中的connect()方法。

sp<Camera> Camera::connect(int cameraId, const String16& clientPackageName,int clientUid)
{return CameraBaseT::connect(cameraId, clientPackageName, clientUid);
}

CameraBase实际上又继承了IBinder的DeathRecipient内部类，DeathRecipient虚拟继承自RefBase。RefBase是Android中的引用计数基础类，其中定义了incStrong、decStrong、incWeak和decWeak等涉及sp/wp的指针操作函数，当然这扯远了。

template <typename TCam>
struct CameraTraits {
};template <typename TCam, typename TCamTraits = CameraTraits<TCam> >
class CameraBase : public IBinder::DeathRecipient
{
public:static sp<TCam>      connect(int cameraId,const String16& clientPackageName,int clientUid);……
}

class DeathRecipient : public virtual RefBase
{
public:virtual void binderDied(const wp<IBinder>& who) = 0;
};

回到Camera::connect()的实现上，其中，new TCam(cameraId)生成BnCameraClient对象，BnCameraClient定义在ICameraClient.h文件中，继承自模板类BnInterface。getCameraService()方法返回CameraService的服务代理BpCameraService，BpCameraService同样继承自模板类BnInterface。然后通过Binder通信发送CONNECT命令，当BnCameraService收到CONNECT命令后调用CameraService的connect()成员函数来做相应的处理。

template <typename TCam, typename TCamTraits>
sp<TCam> CameraBase<TCam, TCamTraits>::connect(int cameraId,const String16& clientPackageName,int clientUid)
{ALOGV("%s: connect", __FUNCTION__);sp<TCam> c = new TCam(cameraId); // BnCameraClient sp<TCamCallbacks> cl = c;status_t status = NO_ERROR;const sp<ICameraService>& cs = getCameraService(); // BpCameraServiceif (cs != 0) {TCamConnectService fnConnectService = TCamTraits::fnConnectService;status = (cs.get()->*fnConnectService)(cl, cameraId, clientPackageName, clientUid,/*out*/ c->mCamera);}if (status == OK && c->mCamera != 0) {c->mCamera->asBinder()->linkToDeath(c);c->mStatus = NO_ERROR;} else {ALOGW("An error occurred while connecting to camera: %d", cameraId);c.clear();}return c;
}

class BnCameraClient: public BnInterface<ICameraClient>
{
public:virtual status_t    onTransact( uint32_t code,const Parcel& data,Parcel* reply,uint32_t flags = 0);
};

class BpCameraService: public BpInterface<ICameraService>
{
public:BpCameraService(const sp<IBinder>& impl): BpInterface<ICameraService>(impl){}……
}

注：connect()函数在BpCameraService和BnCameraService的父类ICameraService中声明为纯虚函数，在BpCameraService和CameraService中分别给出了实现，BpCameraService作为代理类，提供接口给客户端，真正实现在BnCameraService的子类CameraService中。

在BpCameraService中，connect()函数实现如下：

// connect to camera service (android.hardware.Camera)virtual status_t connect(const sp<ICameraClient>& cameraClient, int cameraId,const String16 &clientPackageName, int clientUid,/*out*/sp<ICamera>& device){Parcel data, reply;data.writeInterfaceToken(ICameraService::getInterfaceDescriptor());data.writeStrongBinder(cameraClient->asBinder());data.writeInt32(cameraId);data.writeString16(clientPackageName);data.writeInt32(clientUid);// BpBinder的transact()函数向IPCThreadState实例发送消息，通知其有消息要发送给binder driverremote()->transact(BnCameraService::CONNECT, data, &reply); if (readExceptionCode(reply)) return -EPROTO;status_t status = reply.readInt32();if (reply.readInt32() != 0) {device = interface_cast<ICamera>(reply.readStrongBinder()); // client端读出server返回的bind}return status;}

首先将传递过来的Camera对象cameraClient转换成IBinder类型，将调用的参数写到Parcel（可理解为Binder通信的管道）中，通过BpBinder的transact()函数发送消息，然后由BnCameraService去响应该连接，最后就是等待服务端返回，如果成功则生成一个BpCamera实例。

真正的服务端响应实现在BnCameraService的onTransact()函数中，其负责解包收到的Parcel并执行client端的请求的方法。

status_t BnCameraService::onTransact(uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{switch(code) {……case CONNECT: {CHECK_INTERFACE(ICameraService, data, reply);sp<ICameraClient> cameraClient =interface_cast<ICameraClient>(data.readStrongBinder()); // 使用Camera的Binder对象生成Camera客户代理BpCameraClient实例int32_t cameraId = data.readInt32();const String16 clientName = data.readString16();int32_t clientUid = data.readInt32();sp<ICamera> camera;status_t status = connect(cameraClient, cameraId,clientName, clientUid, /*out*/camera); // 将生成的BpCameraClient对象作为参数传递到CameraService的connect()函数中reply->writeNoException();reply->writeInt32(status); // 将BpCamera对象以IBinder的形式打包到Parcel中返回if (camera != NULL) {reply->writeInt32(1);reply->writeStrongBinder(camera->asBinder());} else {reply->writeInt32(0);}return NO_ERROR;} break;……}
}

主要的处理包括：

1. 通过data中Camera的Binder对象生成Camera客户代理BpCameraClient实例；
2. 将生成的BpCameraClient对象作为参数传递到CameraService（/frameworks/av/services/camera /libcameraservice/CameraService.cpp）的connect()函数中，该函数会返回一个BpCamera实例；
3. 将在上述实例对象以IBinder的形式打包到Parcel中返回。

最后，BpCamera实例是通过CameraService::connect()函数返回的。CameraService::connect()实现的核心是调用connectHelperLocked()函数根据HAL不同API的版本创建不同的client实例（早期版本中好像没有connectHelperLocked()这个函数，但功能基本相似）。

status_t CameraService::connectHelperLocked(/*out*/sp<Client>& client,/*in*/const sp<ICameraClient>& cameraClient,int cameraId,const String16& clientPackageName,int clientUid,int callingPid,int halVersion,bool legacyMode) {int facing = -1;int deviceVersion = getDeviceVersion(cameraId, &facing);if (halVersion < 0 || halVersion == deviceVersion) {// Default path: HAL version is unspecified by caller, create CameraClient// based on device version reported by the HAL.switch(deviceVersion) {case CAMERA_DEVICE_API_VERSION_1_0:client = new CameraClient(this, cameraClient,clientPackageName, cameraId,facing, callingPid, clientUid, getpid(), legacyMode);break;case CAMERA_DEVICE_API_VERSION_2_0:case CAMERA_DEVICE_API_VERSION_2_1:case CAMERA_DEVICE_API_VERSION_3_0:case CAMERA_DEVICE_API_VERSION_3_1:case CAMERA_DEVICE_API_VERSION_3_2:client = new Camera2Client(this, cameraClient,clientPackageName, cameraId,facing, callingPid, clientUid, getpid(), legacyMode);break;case -1:ALOGE("Invalid camera id %d", cameraId);return BAD_VALUE;default:ALOGE("Unknown camera device HAL version: %d", deviceVersion);return INVALID_OPERATION;}} else {// A particular HAL version is requested by caller. Create CameraClient// based on the requested HAL version.if (deviceVersion > CAMERA_DEVICE_API_VERSION_1_0 &&halVersion == CAMERA_DEVICE_API_VERSION_1_0) {// Only support higher HAL version device opened as HAL1.0 device.client = new CameraClient(this, cameraClient,clientPackageName, cameraId,facing, callingPid, clientUid, getpid(), legacyMode);} else {// Other combinations (e.g. HAL3.x open as HAL2.x) are not supported yet.ALOGE("Invalid camera HAL version %x: HAL %x device can only be"" opened as HAL %x device", halVersion, deviceVersion,CAMERA_DEVICE_API_VERSION_1_0);return INVALID_OPERATION;}}status_t status = connectFinishUnsafe(client, client->getRemote());if (status != OK) {// this is probably not recoverable.. maybe the client can try againreturn status;}mClient[cameraId] = client;LOG1("CameraService::connect X (id %d, this pid is %d)", cameraId,getpid());return OK;
}

可见，在CAMERA_DEVICE_API_VERSION_2_0之前使用CameraClient进行实例化，之后则采用Camera2Client进行实例化。以CameraClient为例，其initialize()函数如下：

status_t CameraClient::initialize(camera_module_t *module) {int callingPid = getCallingPid();status_t res;LOG1("CameraClient::initialize E (pid %d, id %d)", callingPid, mCameraId);// Verify ops permissionsres = startCameraOps();if (res != OK) {return res;}char camera_device_name[10];snprintf(camera_device_name, sizeof(camera_device_name), "%d", mCameraId);mHardware = new CameraHardwareInterface(camera_device_name);res = mHardware->initialize(&module->common);if (res != OK) {ALOGE("%s: Camera %d: unable to initialize device: %s (%d)",__FUNCTION__, mCameraId, strerror(-res), res);mHardware.clear();return res;}mHardware->setCallbacks(notifyCallback,dataCallback,dataCallbackTimestamp,(void *)(uintptr_t)mCameraId);// Enable zoom, error, focus, and metadata messages by defaultenableMsgType(CAMERA_MSG_ERROR | CAMERA_MSG_ZOOM | CAMERA_MSG_FOCUS |CAMERA_MSG_PREVIEW_METADATA | CAMERA_MSG_FOCUS_MOVE);LOG1("CameraClient::initialize X (pid %d, id %d)", callingPid, mCameraId);return OK;
}

上述函数中，主要注意以下流程：

加粗的代码CameraHardwareInterface新建了了一个Camera硬件接口，当然，camera_device_name为摄像头设备名；

1. mHardware->initialize(&module->common)调用底层硬件的初始化方法；
2. mHardware->setCallbacks将CamerService处的回调函数注册到HAL处。
3. CameraHardwareInterface定义了Camera的硬件抽象特征，由此进入到HAL。

四、HAL：CameraHardwareInterface

CameraHardwareInterface的作用在于链接Camera Server和V4L2，通过实现CameraHardwareInterface可以屏蔽不同的driver对Camera Server的影响。CameraHardwareInterface同样虚拟继承自RefBase。

class CameraHardwareInterface : public virtual RefBase {
public:CameraHardwareInterface(const char *name){mDevice = 0;mName = name;}……
}

CameraHardwareInterface中包含了控制通道和数据通道，控制通道用于处理预览和视频获取的开始/停止、拍摄照片、自动对焦等功能，数据通道通过回调函数来获得预览、视频录制、自动对焦等数据。当需要支持新的硬件时就需要继承于CameraHardwareInterface ，来实现对应的功能。CameraHardwareInterface提供的public方法如下：

CameraHardwareInterface

在前一节中，initialize()函数调用了mHardware->initialize和mHardware->setCallbacks，下面来看下CameraHardwareInterface.h对其的实现。

status_t initialize(hw_module_t *module){ALOGI("Opening camera %s", mName.string());camera_module_t *cameraModule = reinterpret_cast<camera_module_t *>(module);camera_info info;status_t res = cameraModule->get_camera_info(atoi(mName.string()), &info);if (res != OK) return res;int rc = OK;if (module->module_api_version >= CAMERA_MODULE_API_VERSION_2_3 &&info.device_version > CAMERA_DEVICE_API_VERSION_1_0) {// Open higher version camera device as HAL1.0 device.rc = cameraModule->open_legacy(module, mName.string(),CAMERA_DEVICE_API_VERSION_1_0,(hw_device_t **)&mDevice);} else {rc = CameraService::filterOpenErrorCode(module->methods->open(module, mName.string(), (hw_device_t **)&mDevice));}if (rc != OK) {ALOGE("Could not open camera %s: %d", mName.string(), rc);return rc;}initHalPreviewWindow();return rc;}

在initialize()方法中，通过cameraModule->open_legacy打开摄像头模组，initHalPreviewWindow()用于初始化Preview的相关流opspreview_stream_ops，初始化hal的预览窗口。

void initHalPreviewWindow(){mHalPreviewWindow.nw.cancel_buffer = __cancel_buffer;mHalPreviewWindow.nw.lock_buffer = __lock_buffer;mHalPreviewWindow.nw.dequeue_buffer = __dequeue_buffer;mHalPreviewWindow.nw.enqueue_buffer = __enqueue_buffer;mHalPreviewWindow.nw.set_buffer_count = __set_buffer_count;mHalPreviewWindow.nw.set_buffers_geometry = __set_buffers_geometry;mHalPreviewWindow.nw.set_crop = __set_crop;mHalPreviewWindow.nw.set_timestamp = __set_timestamp;mHalPreviewWindow.nw.set_usage = __set_usage;mHalPreviewWindow.nw.set_swap_interval = __set_swap_interval;mHalPreviewWindow.nw.get_min_undequeued_buffer_count =__get_min_undequeued_buffer_count;}

/** Set the notification and data callbacks */void setCallbacks(notify_callback notify_cb,data_callback data_cb,data_callback_timestamp data_cb_timestamp,void* user){mNotifyCb = notify_cb;mDataCb = data_cb;mDataCbTimestamp = data_cb_timestamp;mCbUser = user;ALOGV("%s(%s)", __FUNCTION__, mName.string());if (mDevice->ops->set_callbacks) {mDevice->ops->set_callbacks(mDevice,__notify_cb,__data_cb,__data_cb_timestamp,__get_memory,this);}}

set_callbacks中，__notify_cb、__data_cb、__data_cb_timestamp和__get_memory分别消息回调，数据回调，时间戳回调，以及内存相关操作的回调。

以上通过简略分析应用层调用Camera.open()之后在Framework、ART、Library以及HAL层的响应，来说明Android中Camera系统的整体架构，希望对读者能有一定的帮助。

作者：yhthu
链接：https://www.jianshu.com/p/592b08e2e00e
來源：简书
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