版权声明：本文为博主原创文章，未经博主允许不得转载。目录(?)[-]    前言    底层配置        1 打开配置        2 添加权限        3 Debug        4 几个比较有用的调试命令    上层应用        1 操作流程        2 具体代码实现    解码mjpeg格式        1 jni层 - 插入huffman表        2 jave层 - 解码并显示    总结1. 前言前段时间调试了一个uvc摄像头，这里做下记录。硬件平台为mt6735，软件平台为Android 5.02. 底层配置UVC全称是usb video class，一种usb视频规范。所有遵循uvc协议的摄像头都不需要安装额外的驱动，只需要一个通用驱动即可。Linux内核已经集成了uvc驱动，代码路径是kernel-3.10/drivers/media/usb/uvc/2.1 打开配置linux内核需要打开以下配置来支持uvc设备CONFIG_MEDIA_SUPPORT=yCONFIG_MEDIA_CAMERA_SUPPORT=yCONFIG_VIDEO_DEV=yCONFIG_VIDEO_V4L2=yCONFIG_VIDEOBUF2_CORE=yCONFIG_VIDEOBUF2_MEMOPS=yCONFIG_VIDEOBUF2_VMALLOC=yCONFIG_MEDIA_USB_SUPPORT=yCONFIG_USB_VIDEO_CLASS=yMTK平台还需要额外打开otg配置CONFIG_USB_MTK_OTG=y CONFIG_USB_MTK_HDRC=y CONFIG_USB_MTK_HDRC_HCD=y插入摄像头，如果生成了/dev/video0设备节点，则证明uvc摄像头已经加载成功了。成功生成驱动节点后还需要为它添加权限2.2 添加权限在uevent.rc中加入/dev/video0               0666   root       root在system_app.te中加入allow system_app video_device:chr_file { read write open getattr };2.3 Debug如果没有出现/dev/video0节点，需要先判断是否枚举成功。在shell终端cat相关的节点查询cat /sys/kernel/debug/usb/devices如果该摄像头枚举成功，则能找到对应的设备信息T:  Bus=01 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#=1 Spd=480 MxCh=1D:  Ver=2.00 Cls=00(>ifc) Sub=00 Prot=00 MxPS=64 #Cfgs=1P:  Vendor=18EC ProdID=3399 Rev=0.00S:  Manufacturer=ARKMICROS:  Product=USB PC CAMERA如果枚举成功则需要判断当前的usb摄像头是不是遵循uvc协议的摄像头。将usb摄像头插到PC上(ubuntu操作系统)，通过”lsusb”命令查找是否有视频类接口信息lsusb -d 18ec:3399 -v | grep "14 Video"如果该摄像头遵循UVC协议，则会输出以下类似信息bFunctionClass 14 VideobInterfaceClass 14 VideobInterfaceClass 14 VideobInterfaceClass 14 Video其中18ec:3399是摄像头的vid和pid，而14 video代表uvc规范2.4 几个比较有用的调试命令打开/关闭linux uvc driver logecho 0xffff > /sys/module/uvcvideo/parameters/trace //打开echo 0 > /sys/module/uvcvideo/parameters/trace //关闭获取详细的usb设备描述符lsusb -d 18ec:3399 –v3. 上层应用v4l2 - Video for Linux 2，是Linux内核中关于视频设备的内核驱动框架，为上层的访问底层的视频设备提供了统一的接口。同时是针对uvc免驱usb设备的编程框架，主要用于采集usb摄像头等。MTK标准的Camera并没有采用v4l2框架，所以需要在jni层实现基本的v4l2视频采集流程。3.1 操作流程在v4l2编程中，一般使用ioctl函数来对设备进行操作：extern int ioctl (int __fd, unsigned long int __request, …) __THROW;    1    1__fd：设备的ID，例如用open函数打开/dev/video0后返回的cameraFd；__request：具体的命令标志符。在进行V4L2开发中，一般会用到以下的命令标志符：VIDIOC_REQBUFS：分配内存VIDIOC_QUERYBUF：把VIDIOC_REQBUFS中分配的数据缓存转换成物理地址VIDIOC_QUERYCAP：查询驱动功能VIDIOC_ENUM_FMT：获取当前驱动支持的视频格式VIDIOC_S_FMT：设置当前驱动的视频格式VIDIOC_G_FMT：读取当前驱动的视频格式VIDIOC_TRY_FMT：验证当前驱动的视频格式VIDIOC_CROPCAP：查询驱动的修剪能力VIDIOC_S_CROP：设置视频信号的边框VIDIOC_G_CROP：读取视频信号的边框VIDIOC_QBUF：把数据放回缓存队列VIDIOC_DQBUF：把数据从缓存中读取出来VIDIOC_STREAMON：开始视频采集VIDIOC_STREAMOFF：结束视频采集VIDIOC_QUERYSTD：检查当前视频设备支持的标准，例如PAL或NTSC。这些IO调用，有些是必须的，有些是可选择的。在网上有开源的应用simplewebcam，它已经实现了基本的v4l2视频采集流程。大概看下它是怎么做的操作流程v4l23.2 具体代码实现(1) 打开设备驱动节点int opendevice(int i){    struct stat st;    sprintf(dev_name,"/dev/video%d",i);    if (-1 == stat (dev_name, &st)) {        LOGE("Cannot identify '%s': %d, %s", dev_name, errno, strerror (errno));        return ERROR_LOCAL;    }    if (!S_ISCHR (st.st_mode)) {        LOGE("%s is no device", dev_name);        return ERROR_LOCAL;    }    fd = open (dev_name, O_RDWR);    if (-1 == fd) {        LOGE("Cannot open '%s': %d, %s", dev_name, errno, strerror (errno));        return ERROR_LOCAL;    }    return SUCCESS_LOCAL;}(2) 查询驱动功能int initdevice(void) {    struct v4l2_capability cap;    struct v4l2_format fmt;    unsigned int min;    if (-1 == xioctl (fd, VIDIOC_QUERYCAP, &cap)) {        if (EINVAL == errno) {            LOGE("%s is no V4L2 device", dev_name);            return ERROR_LOCAL;        } else {            return errnoexit ("VIDIOC_QUERYCAP");        }    }    if (!(cap.capabilities & V4L2_CAP_VIDEO_CAPTURE)) {        LOGE("%s is no video capture device", dev_name);        return ERROR_LOCAL;    }    if (!(cap.capabilities & V4L2_CAP_STREAMING)) {        LOGE("%s does not support streaming i/o", dev_name);        return ERROR_LOCAL;    }    ......}(3) 设置视频格式int initdevice(void) {    struct v4l2_capability cap;    struct v4l2_format fmt;    ......    CLEAR (fmt);    fmt.type                = V4L2_BUF_TYPE_VIDEO_CAPTURE;    fmt.fmt.pix.width       = IMG_WIDTH;     fmt.fmt.pix.height      = IMG_HEIGHT;    fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_MJPEG;    if (-1 == xioctl (fd, VIDIOC_S_FMT, &fmt))        return errnoexit ("VIDIOC_S_FMT");    ......}(4) 申请帧缓存并映射到用户空间int initmmap(void){    struct v4l2_requestbuffers req;    CLEAR (req);    req.count               = 4;    req.type                = V4L2_BUF_TYPE_VIDEO_CAPTURE;    req.memory              = V4L2_MEMORY_MMAP;    if (-1 == xioctl (fd, VIDIOC_REQBUFS, &req)) {        if (EINVAL == errno) {            LOGE("%s does not support memory mapping", dev_name);            return ERROR_LOCAL;        } else {            return errnoexit ("VIDIOC_REQBUFS");        }    }    if (req.count < 2) {        LOGE("Insufficient buffer memory on %s", dev_name);        return ERROR_LOCAL;    }    buffers = calloc (req.count, sizeof (*buffers));    if (!buffers) {        LOGE("Out of memory");        return ERROR_LOCAL;    }    for (n_buffers = 0; n_buffers < req.count; ++n_buffers) {        struct v4l2_buffer buf;        CLEAR (buf);        buf.type        = V4L2_BUF_TYPE_VIDEO_CAPTURE;        buf.memory      = V4L2_MEMORY_MMAP;        buf.index       = n_buffers;        if (-1 == xioctl (fd, VIDIOC_QUERYBUF, &buf))            return errnoexit ("VIDIOC_QUERYBUF");        buffers[n_buffers].length = buf.length;        buffers[n_buffers].start =        mmap (NULL ,            buf.length,            PROT_READ | PROT_WRITE,            MAP_SHARED,            fd, buf.m.offset);        if (MAP_FAILED == buffers[n_buffers].start)            return errnoexit ("mmap");    }    return SUCCESS_LOCAL;}(5) 将帧缓存加入缓存队列并启动视频采集int startcapturing(void){    unsigned int i;    struct v4l2_buffer buf;    enum v4l2_buf_type type;    for (i = 0; i < n_buffers; ++i) {        CLEAR (buf);        buf.type        = V4L2_BUF_TYPE_VIDEO_CAPTURE;        buf.memory      = V4L2_MEMORY_MMAP;        buf.index       = i;        if (-1 == xioctl (fd, VIDIOC_QBUF, &buf))            return errnoexit ("VIDIOC_QBUF");    }    type = V4L2_BUF_TYPE_VIDEO_CAPTURE;    if (-1 == xioctl (fd, VIDIOC_STREAMON, &type))        return errnoexit ("VIDIOC_STREAMON");    return SUCCESS_LOCAL;}(6) 从缓存队列中取出一帧int readframeonce(void){    for (;;) {        fd_set fds;        struct timeval tv;        int r;        FD_ZERO (&fds);        FD_SET (fd, &fds);        tv.tv_sec = 2;        tv.tv_usec = 0;        r = select (fd + 1, &fds, NULL, NULL, &tv);        if (-1 == r) {            if (EINTR == errno)                continue;            return errnoexit ("select");        }        if (0 == r) {            LOGE("select timeout");            return ERROR_LOCAL;        }        if (readframe ()==1)            break;    }    return realImageSize;}int readframe(void){    struct v4l2_buffer buf;    unsigned int i;    CLEAR (buf);    buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;    buf.memory = V4L2_MEMORY_MMAP;    if (-1 == xioctl (fd, VIDIOC_DQBUF, &buf)) {        switch (errno) {            case EAGAIN:                return 0;            case EIO:            default:                return errnoexit ("VIDIOC_DQBUF");        }    }    assert (buf.index < n_buffers);    convert2JPEG(buffers[buf.index].start, buf.bytesused);    if (-1 == xioctl (fd, VIDIOC_QBUF, &buf))        return errnoexit ("VIDIOC_QBUF");    return 1;}4. 解码mjpeg格式我所使用的usb摄像头是mjpeg格式，而从网上下载的simplewebcam应用只支持yuyv格式，所以需要重写解码模块。4.1 jni层 - 插入huffman表安卓自带的libjpeg解码库只能解码jpeg格式。而mjpeg格式需要在v4l2读出的帧中找到SOF0（Start Of Frame 0），插入huffman表后就可以用libjpeg库解码成rgb。static int convert2JPEG(const void *p, int size){    char *mjpgBuf = NULL;    if (pImageBuf == NULL) {        return errnoexit("pImageBuf isn't initialized in JNI");    }    /* Clear pImageBuf and realImageSize */    memset(pImageBuf, 0, (IMG_WIDTH*IMG_HEIGHT)*2);    realImageSize = 0;    /* insert dht data to p, and then save them to pImageBuf */    realImageSize = insert_huffman(p, size, pImageBuf);    return SUCCESS_LOCAL;}static int insert_huffman(const void *in_buf, int buf_size, void *out_buf) {    int pos = 0;    int size_start = 0;       char *pcur = (char *)in_buf;        char *pdeb = (char *)in_buf;       char *plimit = (char *)in_buf + buf_size;        char *jpeg_buf = (char *)out_buf;        /* find the SOF0(Start Of Frame 0) of JPEG */        while ( (((pcur[0] << 8) | pcur[1]) != 0xffc0) && (pcur < plimit) ){         pcur++;    }    LOGD("pcur: 0x%x, plimit: 0x%x", pcur, plimit);    /* SOF0 of JPEG exist */    if (pcur < plimit){        if (jpeg_buf != NULL)        {            /* insert huffman table after SOF0 */            size_start = pcur - pdeb;            memcpy(jpeg_buf, in_buf, size_start);            pos += size_start;            memcpy(jpeg_buf + pos, dht_data, sizeof(dht_data));            pos += sizeof(dht_data);            memcpy(jpeg_buf + pos, pcur, buf_size - size_start);            pos += buf_size - size_start;            return pos;        }    } else{        LOGE("SOF0 does not exist");    }    return 0;}const static unsigned char dht_data[] = {    0xff, 0xc4, 0x01, 0xa2, 0x00, 0x00, 0x01, 0x05, 0x01, 0x01, 0x01, 0x01,    0x01, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x02,    0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x01, 0x00, 0x03,    0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x00, 0x00, 0x00,    0x00, 0x00, 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09,    0x0a, 0x0b, 0x10, 0x00, 0x02, 0x01, 0x03, 0x03, 0x02, 0x04, 0x03, 0x05,    0x05, 0x04, 0x04, 0x00, 0x00, 0x01, 0x7d, 0x01, 0x02, 0x03, 0x00, 0x04,    0x11, 0x05, 0x12, 0x21, 0x31, 0x41, 0x06, 0x13, 0x51, 0x61, 0x07, 0x22,    0x71, 0x14, 0x32, 0x81, 0x91, 0xa1, 0x08, 0x23, 0x42, 0xb1, 0xc1, 0x15,    0x52, 0xd1, 0xf0, 0x24, 0x33, 0x62, 0x72, 0x82, 0x09, 0x0a, 0x16, 0x17,    0x18, 0x19, 0x1a, 0x25, 0x26, 0x27, 0x28, 0x29, 0x2a, 0x34, 0x35, 0x36,    0x37, 0x38, 0x39, 0x3a, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49, 0x4a,    0x53, 0x54, 0x55, 0x56, 0x57, 0x58, 0x59, 0x5a, 0x63, 0x64, 0x65, 0x66,    0x67, 0x68, 0x69, 0x6a, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78, 0x79, 0x7a,    0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89, 0x8a, 0x92, 0x93, 0x94, 0x95,    0x96, 0x97, 0x98, 0x99, 0x9a, 0xa2, 0xa3, 0xa4, 0xa5, 0xa6, 0xa7, 0xa8,    0xa9, 0xaa, 0xb2, 0xb3, 0xb4, 0xb5, 0xb6, 0xb7, 0xb8, 0xb9, 0xba, 0xc2,    0xc3, 0xc4, 0xc5, 0xc6, 0xc7, 0xc8, 0xc9, 0xca, 0xd2, 0xd3, 0xd4, 0xd5,    0xd6, 0xd7, 0xd8, 0xd9, 0xda, 0xe1, 0xe2, 0xe3, 0xe4, 0xe5, 0xe6, 0xe7,    0xe8, 0xe9, 0xea, 0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8, 0xf9,    0xfa, 0x11, 0x00, 0x02, 0x01, 0x02, 0x04, 0x04, 0x03, 0x04, 0x07, 0x05,    0x04, 0x04, 0x00, 0x01, 0x02, 0x77, 0x00, 0x01, 0x02, 0x03, 0x11, 0x04,    0x05, 0x21, 0x31, 0x06, 0x12, 0x41, 0x51, 0x07, 0x61, 0x71, 0x13, 0x22,    0x32, 0x81, 0x08, 0x14, 0x42, 0x91, 0xa1, 0xb1, 0xc1, 0x09, 0x23, 0x33,    0x52, 0xf0, 0x15, 0x62, 0x72, 0xd1, 0x0a, 0x16, 0x24, 0x34, 0xe1, 0x25,    0xf1, 0x17, 0x18, 0x19, 0x1a, 0x26, 0x27, 0x28, 0x29, 0x2a, 0x35, 0x36,    0x37, 0x38, 0x39, 0x3a, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49, 0x4a,    0x53, 0x54, 0x55, 0x56, 0x57, 0x58, 0x59, 0x5a, 0x63, 0x64, 0x65, 0x66,    0x67, 0x68, 0x69, 0x6a, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78, 0x79, 0x7a,    0x82, 0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89, 0x8a, 0x92, 0x93, 0x94,    0x95, 0x96, 0x97, 0x98, 0x99, 0x9a, 0xa2, 0xa3, 0xa4, 0xa5, 0xa6, 0xa7,    0xa8, 0xa9, 0xaa, 0xb2, 0xb3, 0xb4, 0xb5, 0xb6, 0xb7, 0xb8, 0xb9, 0xba,    0xc2, 0xc3, 0xc4, 0xc5, 0xc6, 0xc7, 0xc8, 0xc9, 0xca, 0xd2, 0xd3, 0xd4,    0xd5, 0xd6, 0xd7, 0xd8, 0xd9, 0xda, 0xe2, 0xe3, 0xe4, 0xe5, 0xe6, 0xe7,    0xe8, 0xe9, 0xea, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8, 0xf9, 0xfa};第28-31行，找到SOF0所在的位置，并让pcur指向它第39-47行，在SOF0所在的位置之后插入huffman表，也就是dht_data数组。可被libjpeg解码的图像最终保存在pImageBuf中4.2 jave层 - 解码并显示jni层把图像保存在pImageBuf，这个buffer对应Java层的mImageBuffer。Jave层获取到图像之后调用BitmapFactory.decodeByteArray进行解码，并通过Canvas显示图像@Overridepublic void run() {    while (true && cameraExists) {        ......        imageSize = processCamera();        if(imageSize == -1 || imageSize == 0)            continue;        bmp = BitmapFactory.decodeByteArray(mImageBuffer.array(), mImageBuffer.arrayOffset(), imageSize);        if(bmp == null)            continue;        Canvas canvas = getHolder().lockCanvas();        if (canvas != null)        {            // draw camera bmp on canvas            canvas.drawBitmap(bmp,null,rect,null);            getHolder().unlockCanvasAndPost(canvas);        }    }}5. 总结底层配置，只需要使能otg功能并把uvc相关的配置宏打开，插入设备后生成了/dev/videoX设备节点则说明usb摄像头枚举并初始化成功了上层应用，采用网上的开源应用simplewebcam，这个应用只支持yuyv格式，所以需要重写解码模块。需要在数据帧中手动插入huffman表之后，才能用android的libjpeg库来解码mjpeg格式另外，在调试过程中出现了”uvcvideo: Non-zero status (-71) in video completion handler”这样的log，那是因为mt6735平台的usb host controller对iso端点的支持不太好，经常出现丢包现象，这个问题需要打上mtk提供的patch才能解决问题