虚拟摄像头之二: 配置v4l2loopback虚拟摄像头为前置或后置摄像头

前言

在android8.1系统中调用摄像头是通过 CameraManager::getCameraIdList() 方法获取系统摄像头列表，从列表中选择满足需要摄像头，用以拍照、录像或全景拍照。
上篇中以介绍如何把v4l2loopback移植到android内核，本章介绍如何配置虚拟摄像头参数，以满足android用户程序直接使用。

谁在读取camera配置信息

首先，我们看一下 class CameraBase 这个类的定义，文件路径:
@frameworks/av/camera/include/camera/CameraBase.h

class CameraBase : public IBinder::DeathRecipient
{
public:
	static status_t      getCameraInfo(int cameraId,
                                       /*out*/
                                       struct hardware::CameraInfo* cameraInfo);

    sp<TCamUser>         remote();

    // Status is set to 'UNKNOWN_ERROR' after successful (re)connection
    status_t             getStatus();
protected:
    Mutex                            mLock;
    // helper function to obtain camera service handle
    static const sp<::android::hardware::ICameraService> getCameraService();

    sp<TCamUser>                     mCamera;
    status_t                         mStatus;

    sp<TCamListener>                 mListener;

    const int                        mCameraId;

    typedef CameraBase<TCam>         CameraBaseT;    
}
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基类中 getCameraInfo() 是获取摄像头配置信息,其实现方法如下:
@frameworks/av/camera/CameraBase.cpp

// this can be in BaseCamera but it should be an instance method
template <typename TCam, typename TCamTraits>
status_t CameraBase<TCam, TCamTraits>::getCameraInfo(int cameraId,
        struct hardware::CameraInfo* cameraInfo) {
    const sp<::android::hardware::ICameraService> cs = getCameraService();
    if (cs == 0) return UNKNOWN_ERROR;
    binder::Status res = cs->getCameraInfo(cameraId, cameraInfo);
    return res.isOk() ? OK : res.serviceSpecificErrorCode();
}
template class CameraBase<Camera>;
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因为camera在android系统中是采用C/S结构,/system/bin/mediaserverr是服务端守护进程,frameworks/av/camera下实现的是client内容，
此方法是通过 getCameraService() 获取 sp<::android::hardware::ICameraService> cs 智能指针，从而调用服务端的
cs->getCameraInfo(cameraId, cameraInfo) 的方法，我们进一步跟踪。

camera 服务端守护进程

文件路径
@frameworks/av/services/camera/libcameraservice/CameraService.cpp

Status CameraService::getCameraInfo(int cameraId,
        CameraInfo* cameraInfo) {
    ATRACE_CALL();
    Mutex::Autolock l(mServiceLock);

    if (!mInitialized) {
        return STATUS_ERROR(ERROR_DISCONNECTED,
                "Camera subsystem is not available");
    }

    if (cameraId < 0 || cameraId >= mNumberOfCameras) {
        return STATUS_ERROR(ERROR_ILLEGAL_ARGUMENT,
                "CameraId is not valid");
    }

    Status ret = Status::ok();
    status_t err = mCameraProviderManager->getCameraInfo(std::to_string(cameraId), cameraInfo);
    if (err != OK) {
        ret = STATUS_ERROR_FMT(ERROR_INVALID_OPERATION,
                "Error retrieving camera info from device %d: %s (%d)", cameraId,
                strerror(-err), err);
    }
    return ret;
}
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通过跟踪源码、发现摄像属性参数是在CameraHAL.cpp文件中，构造摄像头时配置 Facing和Orientation属性值，
源码路径如下:
@vendor/nxp-opensource/imx/libcamera3/CameraHAL.cpp文件中,笔者使用的NXP的android8.1软件系统，
硬件平台IMX8QM。以此类推，摄像头属性是在摄像头HAL层构建设备时通过源码类配置该属性。

CameraHAL 配置参数分析

因此参数因硬件平台的差异，配置文件会有差异，请读者根据自有硬件平台代码追踪此过程。
@vendor/nxp-opensource/imx/libcamera3/CameraHAL.cpp

CameraHAL::CameraHAL()
  : mCameraCount(0),
    mCallbacks(NULL)
{
    // Allocate camera array and instantiate camera devices
    mCameras = new Camera*[MAX_CAMERAS];
    memset(mSets, 0, sizeof(mSets));
    memset(mCameras, 0, MAX_CAMERAS * sizeof(Camera*));

    // enumerate all camera sensors.
    enumSensorSet();                  //> 配置 camera 的参数子函数

    // check if camera exists.
    for (int32_t index=0; index<MAX_CAMERAS; index++) {
        if (!mSets[index].mExisting) {
            continue;
        }
        mCameras[index] = Camera::createCamera(index, mSets[index].mSensorName,
                                mSets[index].mFacing, mSets[index].mOrientation,
                                mSets[index].mDevPath);

        if (mCameras[index] == NULL) {
            // camera sensor is not supported now.
            // So, camera count should not change.
            ALOGW("Error: camera:%d, %s create failed", index,
                                 mSets[index].mSensorName);
        }
        else {
            mCameraCount++;
        }
    }
    ALOGI("camera number is %d", mCameraCount);

    mHotplugThread = new HotplugThread(this);
}

//> 配置 camera 的参数子函数
void CameraHAL::enumSensorSet()
{
    // get property from init.rc mSets.
    char orientStr[CAMERA_SENSOR_LENGTH];

    ALOGI("%s", __func__);
    // get back camera property.
    memset(orientStr, 0, sizeof(orientStr));
    property_get("back_camera_name", mSets[BACK_CAMERA_ID].mPropertyName, "0");
    property_get("back_camera_orient", orientStr, "0");
    mSets[BACK_CAMERA_ID].mOrientation = atoi(orientStr);
    mSets[BACK_CAMERA_ID].mFacing = CAMERA_FACING_BACK;
    mSets[BACK_CAMERA_ID].mExisting = false;

    // get front camera property.
    memset(orientStr, 0, sizeof(orientStr));
    property_get("front_camera_name", mSets[FRONT_CAMERA_ID].mPropertyName, "0");
    property_get("front_camera_orient", orientStr, "0");
    mSets[FRONT_CAMERA_ID].mOrientation = atoi(orientStr);
    mSets[FRONT_CAMERA_ID].mFacing = CAMERA_FACING_FRONT;
    mSets[FRONT_CAMERA_ID].mExisting = false;

    // make sure of back&front camera parameters.
    matchDevNodes();
}
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配置摄像头前置或后置

通过源码分析、camera的参数配置是通过property设置来管理，此参数配置是在与硬件平台相关的 init.rc 中
进行配置，IMX8QM的摄像配置参数如下:

@device/fsl/imx8qm/mek_8q/init.rc 文件

    #Define the config for dual camera
    #For landscape mode, orient is 0
    #For portrait mode, orient is 90
    #the android before honycomb are all in portrait mode
    setprop camera.disable_zsl_mode 1
    # 后置摄像头
    setprop back_camera_name imx8_ov5640_mipi,max9286_mipi,imx8_ov5640
    setprop back_camera_orient 0
    # 前置摄像头
    setprop front_camera_name imx8_ov5640,imx8_ov5640_mipi,uvc
    setprop front_camera_orient 0
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我们只需要把此摄像头名称修改为 v4l2loopback_back 和 v4l2loopback_front 名称,在构建 CameraHAL 库
时，采用此名称就可以标注出并区别开来。

NXP是如何匹配 Camera 节点属性

int32_t CameraHAL::matchDevNodes()
{
    DIR *vidDir = NULL;
    struct dirent *dirEntry;
    size_t nameLen = CAMERA_SENSOR_LENGTH - 1;
    nodeSet *nodes = NULL, *node = NULL, *last = NULL;

    ALOGI("%s", __func__);
    vidDir = opendir("/sys/class/video4linux");
    if (vidDir == NULL) {
        return -1;
    }

    while ((dirEntry = readdir(vidDir)) != NULL) {
        if (strncmp(dirEntry->d_name, "video", 5)) {
            continue;
        }

        node = (nodeSet*)malloc(sizeof(nodeSet));
        if (node == NULL) {
            ALOGE("%s malloc failed", __func__);
            break;
        }

        memset(node, 0, sizeof(nodeSet));
        if (nodes == NULL) {
            nodes = node;
        }
        else {
            last->next = node;
        }
        last = node;
        sprintf(node->devNode, "/dev/%s", dirEntry->d_name);
        getNodeName(node->devNode, node->nodeName, nameLen);
    }

    closedir(vidDir);

    for (int32_t index=0; index<MAX_CAMERAS; index++) {
        matchPropertyName(nodes, index);
    }

    node = nodes;
    while (node != NULL) {
        last = node->next;
        free(node);
        node = last;
    }

    return 0;
}
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我们看一下 sys/class/video4linux 路径下都有哪些摄像头设备

1|mek_8q:/ # ls sys/class/video4linux/                                                                                                               
video0 video1 video12 video13 video2 video3 video4 video5 
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获取摄像头名称

int32_t CameraHAL::getNodeName(const char* devNode, char name[], size_t length)
{
    int32_t ret = -1;
    int32_t fd = -1;
    size_t strLen = 0;
    struct v4l2_capability vidCap;
    struct v4l2_dbg_chip_ident vidChip;

    ALOGI("getNodeName: dev path:%s", devNode);
    if ((fd = open(devNode, O_RDWR, O_NONBLOCK)) < 0) {
        ALOGW("%s open dev path:%s failed:%s", __func__, devNode,
                strerror(errno));
        return ret;
    }

    ret = ioctl(fd, VIDIOC_QUERYCAP, &vidCap);
    if (ret < 0) {
        ALOGW("%s QUERYCAP dev path:%s failed", __func__, devNode);
        close(fd);
        fd = -1;
        return ret;
    }

    if (!(vidCap.capabilities &
          (V4L2_CAP_VIDEO_CAPTURE | V4L2_CAP_VIDEO_CAPTURE_MPLANE))) {
        ALOGW("%s dev path:%s is not capture", __func__, devNode);
        close(fd);
        fd = -1;
        ret = -1;
        return ret;
    }

    strncat(name, (const char*)vidCap.driver, length);
    strLen = strlen((const char*)vidCap.driver);
    length -= strLen;
    ALOGI("getNodeName: node name:%s", name);

    ret = ioctl(fd, VIDIOC_DBG_G_CHIP_IDENT, &vidChip);
    if (ret < 0) {
        ALOGI("%s CHIP_IDENT dev path:%s failed", __func__, devNode);
        strncat(name, ",", length);
        strLen = 1;
        length -= strLen;
        strncat(name, (const char*)vidCap.card, length);
        ALOGI("getNodeNames: node name:%s", name);
        close(fd);
        fd = -1;
        return ret;
    }

    strncat(name, ",", length);
    strLen = 1;
    length -= strLen;
    strncat(name, vidChip.match.name, length);

    ALOGI("getNodeNames: node name:%s", name);
    close(fd);
    return ret;
}
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摄像头名称属性是在驱动中标注的，需修改v4l2loopback驱动中节点名称,以此把该摄像头配置为
前置或后置摄像头。

至此位置，我们把 android8.1 中的、摄像头前置或后置的属性配置逻辑给梳理清晰了，如果感觉对你
有所启发或帮助，请点赞或关注，以资鼓励笔者，感谢喽。