UVC 摄像头驱动（三）配置摄像头，实时数据采集

　　前面分析了 UVC 摄像头的硬件模型和描述符，对于一个 usb 摄像头来说，内部大概分为一个 VC 接口和一个 VS 接口，VC 接口内部有许多 unit 和 terminal 用来“控制”摄像头，比如我们可以通过 Process unit 设置白平衡、曝光等等。对于 VS 接口来说，标准 VS 接口往往含有许多个设置，每一个设置都包含一个实时传输端点，虽然它们的端点地址可能相同，但是它们的最大传输包大小不同，在 Class specific VS 接口中，包含多个 Format ，每一个 Format 包含多个 Frame ,Format 指的 YUYV MJPG 等等，Frame 就是各种分辨率 480*320 640 * 480 等等。以上这些信息，都是通过分析描述符来获得。

VideoStreaming Requests

　　参考 UVC 1.5 Class specification 4.3 节

我们需要使用控制传输来和VS通信，Probe and commit 设置，请求格式参考上图。

• bmRequestType 请求类型，参考标准USB协议
• bRequest 子类，定义在 Table A-8
• CS ，Control Selector ，定义在 Table A-16 ，例如是probe 还是 commit
• wIndex 高字节为0，低字节为接口号
• wLength 和 Data 和标准USB协议一样，为数据长度和数据

参数设置的过程需要主机和USB设备进行协商， 协商的过程大致如下图所示：

• Host 先将期望的设置发送给USB设备(PROBE)
• 设备将Host期望设置在自身能力范围之内进行修改，返回给Host(PROBE)
• Host 认为设置可行的话，Commit 提交(COMMIT)
• 设置接口的当前设置为某一个设置
  　　
  那么协商哪些数据？这些数据在哪里定义？参考Table 4-75 ,里面包含了使用哪一个Frame 哪一个 Frame 帧频率，一次传输包大小等等的信息。
  参考代码：

static int myuvc_try_streaming_params(struct myuvc_streaming_control *ctrl){    __u8 *data;    __u16 size;    int ret;    __u8 type = USB_TYPE_CLASS | USB_RECIP_INTERFACE;    unsigned int pipe;    memset(ctrl, 0, sizeof *ctrl);    ctrl->bmHint = 1;   /* dwFrameInterval */    ctrl->bFormatIndex = 1;    ctrl->bFrameIndex  = 1;    ctrl->dwFrameInterval = 333333;    ctrl->dwClockFrequency = 48000000;    ctrl->wCompQuality = 61;    size = 34;    data = kzalloc(size, GFP_KERNEL);    if (data == NULL)        return -ENOMEM;    *(__le16 *)&data[0] = cpu_to_le16(ctrl->bmHint);    data[2] = ctrl->bFormatIndex;    data[3] = ctrl->bFrameIndex;    *(__le32 *)&data[4] = cpu_to_le32(ctrl->dwFrameInterval);    *(__le16 *)&data[8] = cpu_to_le16(ctrl->wKeyFrameRate);    *(__le16 *)&data[10] = cpu_to_le16(ctrl->wPFrameRate);    *(__le16 *)&data[12] = cpu_to_le16(ctrl->wCompQuality);    *(__le16 *)&data[14] = cpu_to_le16(ctrl->wCompWindowSize);    *(__le16 *)&data[16] = cpu_to_le16(ctrl->wDelay);    put_unaligned_le32(ctrl->dwMaxVideoFrameSize, &data[18]);    put_unaligned_le32(ctrl->dwMaxPayloadTransferSize, &data[22]);    if (size == 34) {        put_unaligned_le32(ctrl->dwClockFrequency, &data[26]);        data[30] = ctrl->bmFramingInfo;        data[31] = ctrl->bPreferedVersion;        data[32] = ctrl->bMinVersion;        data[33] = ctrl->bMaxVersion;    }    pipe = usb_sndctrlpipe(myuvc_udev, 0);    type |= USB_DIR_OUT;    ret = usb_control_msg(myuvc_udev, pipe, 0x01, type, 0x01 << 8,            0 << 8 | myuvc_streaming_intf, data, size, 5000);    kfree(data);    return (ret < 0) ? ret : 0;}static int myuvc_get_streaming_params(struct myuvc_streaming_control *ctrl){    __u8 *data;    __u16 size;    int ret;    __u8 type = USB_TYPE_CLASS | USB_RECIP_INTERFACE;    unsigned int pipe;    size = 34;    data = kmalloc(size, GFP_KERNEL);    if (data == NULL)        return -ENOMEM;    pipe = usb_rcvctrlpipe(myuvc_udev, 0);    type |= USB_DIR_IN;    ret = usb_control_msg(myuvc_udev, pipe, 0x81, type, 0x01 << 8,            0 << 8 | myuvc_streaming_intf, data, size, 5000);    if (ret < 0)        goto done;    ctrl->bmHint = le16_to_cpup((__le16 *)&data[0]);    ctrl->bFormatIndex = data[2];    ctrl->bFrameIndex = data[3];    ctrl->dwFrameInterval = le32_to_cpup((__le32 *)&data[4]);    ctrl->wKeyFrameRate = le16_to_cpup((__le16 *)&data[8]);    ctrl->wPFrameRate = le16_to_cpup((__le16 *)&data[10]);    ctrl->wCompQuality = le16_to_cpup((__le16 *)&data[12]);    ctrl->wCompWindowSize = le16_to_cpup((__le16 *)&data[14]);    ctrl->wDelay = le16_to_cpup((__le16 *)&data[16]);    ctrl->dwMaxVideoFrameSize = get_unaligned_le32(&data[18]);    ctrl->dwMaxPayloadTransferSize = get_unaligned_le32(&data[22]);    if (size == 34) {        ctrl->dwClockFrequency = get_unaligned_le32(&data[26]);        ctrl->bmFramingInfo = data[30];        ctrl->bPreferedVersion = data[31];        ctrl->bMinVersion = data[32];        ctrl->bMaxVersion = data[33];    } else {        //ctrl->dwClockFrequency = video->dev->clock_frequency;        ctrl->bmFramingInfo = 0;        ctrl->bPreferedVersion = 0;        ctrl->bMinVersion = 0;        ctrl->bMaxVersion = 0;    }done:    kfree(data);    return (ret < 0) ? ret : 0;}static int myuvc_set_streaming_params(struct myuvc_streaming_control *ctrl){    __u8 *data;    __u16 size;    int ret;    __u8 type = USB_TYPE_CLASS | USB_RECIP_INTERFACE;    unsigned int pipe;    size = 34;    data = kzalloc(size, GFP_KERNEL);    if (data == NULL)        return -ENOMEM;    *(__le16 *)&data[0] = cpu_to_le16(ctrl->bmHint);    data[2] = ctrl->bFormatIndex;    data[3] = ctrl->bFrameIndex;    *(__le32 *)&data[4] = cpu_to_le32(ctrl->dwFrameInterval);    *(__le16 *)&data[8] = cpu_to_le16(ctrl->wKeyFrameRate);    *(__le16 *)&data[10] = cpu_to_le16(ctrl->wPFrameRate);    *(__le16 *)&data[12] = cpu_to_le16(ctrl->wCompQuality);    *(__le16 *)&data[14] = cpu_to_le16(ctrl->wCompWindowSize);    *(__le16 *)&data[16] = cpu_to_le16(ctrl->wDelay);    put_unaligned_le32(ctrl->dwMaxVideoFrameSize, &data[18]);    put_unaligned_le32(ctrl->dwMaxPayloadTransferSize, &data[22]);    if (size == 34) {        put_unaligned_le32(ctrl->dwClockFrequency, &data[26]);        data[30] = ctrl->bmFramingInfo;        data[31] = ctrl->bPreferedVersion;        data[32] = ctrl->bMinVersion;        data[33] = ctrl->bMaxVersion;    }    pipe = usb_sndctrlpipe(myuvc_udev, 0);    type |= USB_DIR_OUT;    ret = usb_control_msg(myuvc_udev, pipe, 0x01, type, 0x02 << 8,            0 << 8 | myuvc_streaming_intf, data, size, 5000);    kfree(data);    return (ret < 0) ? ret : 0;}

VideoControl Requests

　　这里我们主要分析 ＶＣ 接口里的 Processing Unit Control Requests 以亮度为例：

static void myuvc_set_le_value(__s32 value, __u8 *data){    int bits = 16;    int offset = 0;    __u8 mask;    data += offset / 8;    offset &= 7;    for (; bits > 0; data++) {        mask = ((1LL << bits) - 1) << offset;        *data = (*data & ~mask) | ((value << offset) & mask);        value >>= offset ? offset : 8;        bits -= 8 - offset;        offset = 0;    }}static __s32 myuvc_get_le_value(const __u8 *data){    int bits = 16;    int offset = 0;    __s32 value = 0;    __u8 mask;    data += offset / 8;    offset &= 7;    mask = ((1LL << bits) - 1) << offset;    for (; bits > 0; data++) {        __u8 byte = *data & mask;        value |= offset > 0 ? (byte >> offset) : (byte << (-offset));        bits -= 8 - (offset > 0 ? offset : 0);        offset -= 8;        mask = (1 << bits) - 1;    }    /* Sign-extend the value if needed. */    value |= -(value & (1 << (16 - 1)));    return value;}/* 参考:uvc_query_v4l2_ctrl */    int myuvc_vidioc_queryctrl (struct file *file, void *fh,                struct v4l2_queryctrl *ctrl){    __u8 type = USB_TYPE_CLASS | USB_RECIP_INTERFACE;    unsigned int pipe;    int ret;    u8 data[2];    if (ctrl->id != V4L2_CID_BRIGHTNESS)        return -EINVAL;    memset(ctrl, 0, sizeof *ctrl);    ctrl->id   = V4L2_CID_BRIGHTNESS;    ctrl->type = V4L2_CTRL_TYPE_INTEGER;    strcpy(ctrl->name, "MyUVC_BRIGHTNESS");    ctrl->flags = 0;    pipe = usb_rcvctrlpipe(udev, 0);    type |= USB_DIR_IN;    /* 发起USB传输确定这些值 */    ret = usb_control_msg(udev, pipe, GET_MIN, type, PU_BRIGHTNESS_CONTROL << 8,            ProcessingUnitID << 8 | myuvc_control_intf, data, 2, 5000);    if (ret != 2)        return -EIO;    ctrl->minimum = myuvc_get_le_value(data);   /* Note signedness */    ret = usb_control_msg(udev, pipe, GET_MAX, type,  PU_BRIGHTNESS_CONTROL << 8,            ProcessingUnitID << 8 | myuvc_control_intf, data, 2, 5000);    if (ret != 2)        return -EIO;    ctrl->maximum = myuvc_get_le_value(data);   /* Note signedness */    ret = usb_control_msg(udev, pipe, GET_RES, type, PU_BRIGHTNESS_CONTROL << 8,             ProcessingUnitID << 8 | myuvc_control_intf, data, 2, 5000);    if (ret != 2)        return -EIO;    ctrl->step = myuvc_get_le_value(data);  /* Note signedness */    ret = usb_control_msg(udev, pipe, GET_DEF, type, PU_BRIGHTNESS_CONTROL << 8,            ProcessingUnitID << 8 | myuvc_control_intf, data, 2, 5000);    if (ret != 2)        return -EIO;    ctrl->default_value = myuvc_get_le_value(data); /* Note signedness */    printk("Brightness: min =%d, max = %d, step = %d, default = %d\n", ctrl->minimum, ctrl->maximum, ctrl->step, ctrl->default_value);    return 0;}/* 参考 : uvc_ctrl_get */int myuvc_vidioc_g_ctrl (struct file *file, void *fh,                struct v4l2_control *ctrl){    __u8 type = USB_TYPE_CLASS | USB_RECIP_INTERFACE;    unsigned int pipe;    int ret;    u8 data[2];    if (ctrl->id != V4L2_CID_BRIGHTNESS)        return -EINVAL;    pipe = usb_rcvctrlpipe(udev, 0);    type |= USB_DIR_IN;    ret = usb_control_msg(udev, pipe, GET_CUR, type, PU_BRIGHTNESS_CONTROL << 8,            ProcessingUnitID << 8 | myuvc_control_intf, data, 2, 5000);    if (ret != 2)        return -EIO;    ctrl->value = myuvc_get_le_value(data); /* Note signedness */    return 0;}/* 参考: uvc_ctrl_set/uvc_ctrl_commit */int myuvc_vidioc_s_ctrl (struct file *file, void *fh,                struct v4l2_control *ctrl){    __u8 type = USB_TYPE_CLASS | USB_RECIP_INTERFACE;    unsigned int pipe;    int ret;    u8 data[2];    if (ctrl->id != V4L2_CID_BRIGHTNESS)        return -EINVAL;    myuvc_set_le_value(ctrl->value, data);    pipe = usb_sndctrlpipe(udev, 0);    type |= USB_DIR_OUT;    ret = usb_control_msg(udev, pipe, SET_CUR, type, PU_BRIGHTNESS_CONTROL << 8,            ProcessingUnitID  << 8 | myuvc_control_intf, data, 2, 5000);    if (ret != 2)        return -EIO;    return 0;}

数据采集

　　数据采集时，我们需要和标准VS接口设置里的实时端点通信获取数据，分配、设置、提交URB。
　　以 640*320 分辨率的图像为例，每一个像素16bit，因此一帧图像占用空间 640*320*2字节 ，对于我的摄像头一次传输，3060字节。需要注意的是，标准UVC摄像头驱动中限制了一个URB的Buffer数量最大为32个，因此一个URB能承载的数据为 3060 * 32 ,经过计算，一帧图像数据需要多个 URB 来传输。因此在将图像数据拷贝到用户空间Buffer时候，可能在某一个URB包含两帧图像的数据，需要不要处理完前一帧就把第二帧图像数据丢弃了，那样会造成图像数据丢失。

static int myuvc_alloc_init_urbs(void){    u16 psize;    u32 size;    int npackets;    int i,j;    struct urb *urb;    //struct urb *urb;    psize = 3060; /* 实时传输端点一次能传输的最大字节数 */    size  = 614400;  /* 一帧数据的最大长度 */    npackets = DIV_ROUND_UP(size, psize);    if (npackets > 32)        npackets = 32;    myprintk("psize %d npackets %d\n",psize,npackets);    size =  psize * npackets;    for (i = 0; i < 5; ++i) {         /* 1. 分配usb_buffers */        urb_buffer[i] = usb_alloc_coherent(            myuvc_udev, size,            GFP_KERNEL | __GFP_NOWARN, &urb_dma[i]);        /* 2. 分配urb */        myurb[i] = usb_alloc_urb(npackets, GFP_KERNEL);        if (!urb_buffer[i] || !myurb[i])        {            //myuvc_uninit_urbs();            myprintk("alloc buffer or urb failed\n");            return -ENOMEM;        }    }    /* 3. 设置urb */     for (i = 0; i < 5; ++i) {        urb = myurb[i];        urb->dev = myuvc_udev;        urb->context = NULL;        urb->pipe = usb_rcvisocpipe(myuvc_udev, 0x81);        urb->transfer_flags = URB_ISO_ASAP | URB_NO_TRANSFER_DMA_MAP;        urb->interval = 1;        urb->transfer_buffer = urb_buffer[i];        urb->transfer_dma = urb_dma[i];        urb->complete = myuvc_video_complete;        urb->number_of_packets = npackets;        urb->transfer_buffer_length = size;        for (j = 0; j < npackets; ++j) {            urb->iso_frame_desc[j].offset = j * psize;            urb->iso_frame_desc[j].length = psize;        }    }      return 0;}

static void myuvc_video_complete(struct urb *urb){    u8 *src;    //u8 *dest;    int ret, i;    int len;    int maxlen;//    int nbytes;//    struct myuvc_buffer *buf;    myprintk("video complete\n");    switch (urb->status) {    case 0:        break;    default:        myprintk("Non-zero status (%d) in video "            "completion handler.\n", urb->status);        return;    }    for (i = 0; i < urb->number_of_packets; ++i) {        if (urb->iso_frame_desc[i].status < 0) {            myprintk("USB isochronous frame "                "lost (%d).\n", urb->iso_frame_desc[i].status);            continue;        }        src  = urb->transfer_buffer + urb->iso_frame_desc[i].offset;        //dest = myuvc_queue.mem + buf->buf.m.offset + buf->buf.bytesused;        len = urb->iso_frame_desc[i].actual_length;        myprintk("len %d\n",urb->iso_frame_desc[i].actual_length);        /* 判断数据是否有效 */        /* URB数据含义:         * data[0] : 头部长度         * data[1] : 错误状态         */        if (len < 2 || src[0] < 2 || src[0] > len)            continue;        /* Skip payloads marked with the error bit ("error frames"). */        if (src[1] & UVC_STREAM_ERR) {            myprintk("Dropping payload (error bit set).\n");            continue;        }        /* 除去头部后的数据长度 */        len -= src[0];        /* 缓冲区最多还能存多少数据 */        //maxlen = buf->buf.length - buf->buf.bytesused;        //nbytes = min(len, maxlen);        /* 复制数据 */        //memcpy(dest, src + src[0], nbytes);        //buf->buf.bytesused += nbytes;        /* 判断一帧数据是否已经全部接收到 */        if (len > maxlen) {            //buf->state = VIDEOBUF_DONE;        }        /* Mark the buffer as done if the EOF marker is set. */        if (src[1] & UVC_STREAM_EOF) {            myprintk("Frame complete (EOF found).\n");            if (len == 0)                myprintk("EOF in empty payload.\n");            //buf->state = VIDEOBUF_DONE;        }    }    /* 再次提交URB */    if ((ret = usb_submit_urb(urb, GFP_ATOMIC)) < 0) {        myprintk("Failed to resubmit video URB (%d).\n", ret);    }}