V4L2学习（四）VIVI分析

　　vivi 相对于后面要分析的 usb 摄像头驱动程序，它没有真正的硬件相关层的操作，也就是说抛开了复杂的 usb 层的相关知识，便于理解 V4L2 驱动框架，侧重于驱动和应用的交互。

  前面我们提到，V4L2 的核心是 v4l2-dev.c 它向上提供统一的文件操作接口 v4l2_fops ，向下提供 video_device 注册接口 register_video_device ，作为一个具体的驱动，需要做的工作就是分配、设置、注册一个 video_device.框架很简单，复杂的是视频设备相关众多的 ioctl。

一、vivi 框架分析

static int __init vivi_init(void)
{
    ret = vivi_create_instance(i);
    ...
    return ret;
}
module_init(vivi_init);

 　　 vivi 分配了一个 video_device 指针，没有去设置而是直接让它指向了一个现成的 video_device 结构 vivi_template ，那么全部的工作都将围绕 vivi_template 展开。

static int __init vivi_create_instance(int inst)
{
    struct vivi_dev *dev;
    struct video_device *vfd;
    struct v4l2_ctrl_handler *hdl;
    struct vb2_queue *q;
 
    // 分配一个 vivi_dev 结构体
    dev = kzalloc(sizeof(*dev), GFP_KERNEL);
    
    // v4l2_dev 初始化,并没有什么作用
    ret = v4l2_device_register(NULL, &dev->v4l2_dev);
    
    // 设置 dev 的一些参数，比如图像格式、大小
    dev->fmt = &formats[0];
    dev->width = 640;
    dev->height = 480;
    dev->pixelsize = dev->fmt->depth / 8;
    ...
    
    // vivi_dev->vb_vidq(vb2_queue) 初始化
    q = &dev->vb_vidq;
    memset(q, 0, sizeof(dev->vb_vidq));
    q->type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    q->io_modes = VB2_MMAP | VB2_USERPTR | VB2_DMABUF | VB2_READ;
    q->drv_priv = dev;
    q->buf_struct_size = sizeof(struct vivi_buffer);
    
    // vivi_dev->vb_vidq(vb2_queue)->ops
    q->ops     = &vivi_video_qops;
    
    // vivi_dev->vb_vidq(vb2_queue)->mem_ops
    q->mem_ops = &vb2_vmalloc_memops;
    
    // 初始化一些锁之类的东西
    vb2_queue_init(q);
 
    /* init video dma queues */
    INIT_LIST_HEAD(&dev->vidq.active);
    init_waitqueue_head(&dev->vidq.wq);
    
    // 分配一个 video_device ,这才是重点
    vfd = video_device_alloc();
 
    *vfd = vivi_template;
    vfd->debug = debug;
    vfd->v4l2_dev = &dev->v4l2_dev;
    set_bit(V4L2_FL_USE_FH_PRIO, &vfd->flags);
 
    vfd->lock = &dev->mutex;
    
    // 注册 video_device ！！！
    ret = video_register_device(vfd, VFL_TYPE_GRABBER, video_nr);
    // 把 vivi_dev 放入 video_device->dev->p->driver_data ,这个后边经常用到
    video_set_drvdata(vfd, dev);
 
    /* Now that everything is fine, let's add it to device list */
    list_add_tail(&dev->vivi_devlist, &vivi_devlist);
 
    if (video_nr != -1)
        video_nr++;
    // vivi_dev->vfd(video_device) =  vfd
    dev->vfd = vfd;
    v4l2_info(&dev->v4l2_dev, "V4L2 device registered as %s
",
          video_device_node_name(vfd));
    return 0;
}

　　用户空间调用的是 v4l2_fops ，但是最终会调用到 vivi_fops ，vivi_fops 中的 ioctl 调用 video_ioctl2

static struct video_device vivi_template = {
    .name        = "vivi",
    .fops           = &vivi_fops,
    .ioctl_ops     = &vivi_ioctl_ops,
    .minor        = -1,
    .release    = video_device_release,
 
    .tvnorms              = V4L2_STD_525_60,
    .current_norm         = V4L2_STD_NTSC_M,
};

　　video_register_device 过程就不详细分析了，前面的文章中分析过，大概就是向核心层注册 video_device 结构体，核心层注册字符设备并提供一个统一的 fops ，当用户空间 read write ioctl 等，最终还是会跳转到 video_device->fops ,还有一点就是核心层会把我们注册进来的 video_device 结构放入一个全局的 video_device数组。

static const struct v4l2_file_operations vivi_fops = {
    .owner        = THIS_MODULE,
    .open           = v4l2_fh_open,
    .release        = vivi_close,
    .read           = vivi_read,
    .poll        = vivi_poll,
    .unlocked_ioctl = video_ioctl2, /* V4L2 ioctl handler */
    .mmap           = vivi_mmap,
};

  这里，先看一下 v4l2_fh_open 函数

int v4l2_fh_open(struct file *filp)
{
  // 前面注册时，我们将 video_device 结构体放入了全局数组 video_device ，现在通过     video_devdata 函数取出来，后面经常用到这种做法
    struct video_device *vdev = video_devdata(filp);
    // 分配一个 v4l2_fh 结构，放入file->private_data 中
    struct v4l2_fh *fh = kzalloc(sizeof(*fh), GFP_KERNEL);
    filp->private_data = fh;
    if (fh == NULL)
        return -ENOMEM;
    v4l2_fh_init(fh, vdev);
    v4l2_fh_add(fh);
    return 0;
}

  1、我们随时可以通过 video_devdata 取出我们注册的 video_device 结构进行操作
  2、我们随时可以通过 file->private_data 取出 v4l2_fh 结构，虽然现在还不知道它有啥用

  下面来分析 ioctl ...首先来看一下调用过程 

long video_ioctl2(struct file *file,
           unsigned int cmd, unsigned long arg)
{
    return video_usercopy(file, cmd, arg, __video_do_ioctl);
}

static long __video_do_ioctl(struct file *file,
        unsigned int cmd, void *arg)
{
    struct video_device *vfd = video_devdata(file);
    const struct v4l2_ioctl_ops *ops = vfd->ioctl_ops;
    void *fh = file->private_data;
    struct v4l2_fh *vfh = NULL;
    int use_fh_prio = 0;
    long ret = -ENOTTY;
 
    if (ops == NULL) {
        printk(KERN_WARNING "videodev: "%s" has no ioctl_ops.
",
                vfd->name);
        return ret;
    }
 
    if (test_bit(V4L2_FL_USES_V4L2_FH, &vfd->flags)) {
        vfh = file->private_data;
        use_fh_prio = test_bit(V4L2_FL_USE_FH_PRIO, &vfd->flags);
    }
 
    if (v4l2_is_known_ioctl(cmd)) {
        struct v4l2_ioctl_info *info = &v4l2_ioctls[_IOC_NR(cmd)];
 
            if (!test_bit(_IOC_NR(cmd), vfd->valid_ioctls) &&
            !((info->flags & INFO_FL_CTRL) && vfh && vfh->ctrl_handler))
            return -ENOTTY;
 
        if (use_fh_prio && (info->flags & INFO_FL_PRIO)) {
            ret = v4l2_prio_check(vfd->prio, vfh->prio);
            if (ret)
                return ret;
        }
    }
 
    if ((vfd->debug & V4L2_DEBUG_IOCTL) &&
                !(vfd->debug & V4L2_DEBUG_IOCTL_ARG)) {
        v4l_print_ioctl(vfd->name, cmd);
        printk(KERN_CONT "
");
    }
 
    switch (cmd) {
 
    /* --- capabilities ------------------------------------------ */
    case VIDIOC_QUERYCAP:
    {
        struct v4l2_capability *cap = (struct v4l2_capability *)arg;
 
        cap->version = LINUX_VERSION_CODE;
        ret = ops->vidioc_querycap(file, fh, cap);
        if (!ret)
            dbgarg(cmd, "driver=%s, card=%s, bus=%s, "
                    "version=0x%08x, "
                    "capabilities=0x%08x, "
                    "device_caps=0x%08x
",
                    cap->driver, cap->card, cap->bus_info,
                    cap->version,
                    cap->capabilities,
                    cap->device_caps);
        break;
    }

  vivi 驱动就复杂在这些 ioctl 上，下面按照应用层与驱动的交互顺序来具体的分析这些 ioctl。

二、ioctl 深入分析

  应用空间的一个视频 app 与驱动的交互流程大致如下图所示：

下面就根据流程，分析每一个 ioctl 在 vivi 中的具体实现。把以上的过程吃透，自己写一个虚拟摄像头程序应该就不成问题了。

　　2.1 VIDIOC_QUERYCAP 查询设备能力

应用层：

struct v4l2_capability {
    __u8    driver[16];    /* i.e. "bttv" */
    __u8    card[32];    /* i.e. "Hauppauge WinTV" */
    __u8    bus_info[32];    /* "PCI:" + pci_name(pci_dev) */
    __u32   version;        <span style="white-space:pre">    </span>/* should use KERNEL_VERSION() */
    __u32    capabilities;    /* Device capabilities */
    __u32    reserved[4];
};
 
struct v4l2_capability cap;
ret = ioctl(fd,VIDIOC_QUERYCAP,&cap);
if (ret < 0) {
    LOG("VIDIOC_QUERYCAP failed (%d)
", ret);
    return ret;
}

驱动层：

void *fh = file->private_data;
ops->vidioc_querycap(file, fh, cap);
static int vidioc_querycap(struct file *file, void  *priv, struct v4l2_capability *cap)
{
    struct vivi_fh  *fh  = priv;
    struct vivi_dev *dev = fh->dev;
 
 
    // 这里只是将一些信息写回用户空间而已，非常简单
    strcpy(cap->driver, "vivi");  
    strcpy(cap->card, "vivi");
    strlcpy(cap->bus_info, dev->v4l2_dev.name, sizeof(cap->bus_info));
    cap->version =     VIVI_VERSION; cap->capabilities =V4L2_CAP_VIDEO_CAPTURE |V4L2_CAP_STREAMING     | V4L2_CAP_READWRITE;return 0;}
}

一般我们只关心 capabilities 成员，比如V4L2_CAP_VIDEO_CAPTURE 具有视频捕获能力，其它定义如下：

/* Values for 'capabilities' field */
#define V4L2_CAP_VIDEO_CAPTURE        0x00000001  /* Is a video capture device */
#define V4L2_CAP_VIDEO_OUTPUT        0x00000002  /* Is a video output device */
#define V4L2_CAP_VIDEO_OVERLAY        0x00000004  /* Can do video overlay */
#define V4L2_CAP_VBI_CAPTURE        0x00000010  /* Is a raw VBI capture device */
#define V4L2_CAP_VBI_OUTPUT        0x00000020  /* Is a raw VBI output device */
#define V4L2_CAP_SLICED_VBI_CAPTURE    0x00000040  /* Is a sliced VBI capture device */
#define V4L2_CAP_SLICED_VBI_OUTPUT    0x00000080  /* Is a sliced VBI output device */
#define V4L2_CAP_RDS_CAPTURE        0x00000100  /* RDS data capture */
#define V4L2_CAP_VIDEO_OUTPUT_OVERLAY    0x00000200  /* Can do video output overlay */
#define V4L2_CAP_HW_FREQ_SEEK        0x00000400  /* Can do hardware frequency seek  */
#define V4L2_CAP_RDS_OUTPUT        0x00000800  /* Is an RDS encoder */

　　2.2 VIDIOC_ENUM_FMT 枚举(查询)设备支持的视频格式

应用层：

struct v4l2_fmtdesc {
    __u32            index;             /* Format number      */
    enum v4l2_buf_type  type;              /* buffer type        */
    __u32               flags;
    __u8            description[32];   /* Description string */
    __u32            pixelformat;       /* Format fourcc      */
    __u32            reserved[4];
};
 
struct v4l2_fmtdesc fmtdesc;
fmtdesc.index=0;
fmtdesc.type=V4L2_BUF_TYPE_VIDEO_CAPTURE;
while(ioctl(fd,VIDIOC_ENUM_FMT,&fmtdesc)!=-1)
{
    printf("SUPPORT	%d.%s
",fmtdesc.index+1,fmtdesc.description);
    fmtdesc.index++;
}

驱动层：

static struct vivi_fmt formats[] = {
    {
        .name     = "4:2:2, packed, YUYV",
        .fourcc   = V4L2_PIX_FMT_YUYV,
        .depth    = 16,
    },
    ...
}
static int vidioc_enum_fmt_vid_cap(struct file *file, void  *priv,
                    struct v4l2_fmtdesc *f)
{
    struct vivi_fmt *fmt;
 
    if (f->index >= ARRAY_SIZE(formats))
        return -EINVAL;
 
    fmt = &formats[f->index];
 
    strlcpy(f->description, fmt->name, sizeof(f->description));
    f->pixelformat = fmt->fourcc;
    return 0;
}

　　一般一个设备支持多种视频格式，比如 vivi 它所支持的格式存放在 formats 数组中，由于应用层并不知道设备支持多少种格式，也不知道某种格式具体存放在哪个数组项中，因此通过index从0开始尝试，对于驱动层来说就是遍历所有的数组项，返回每一个index对应的视频格式，比如 V4L2_PIX_FMT_YUYV .

　　2.3 VIDIOC_S_FMT 设置视频格式

应用层：

struct v4l2_format {
    enum v4l2_buf_type type;
    union {
        struct v4l2_pix_format        pix;     /* V4L2_BUF_TYPE_VIDEO_CAPTURE */
        struct v4l2_window        win;     /* V4L2_BUF_TYPE_VIDEO_OVERLAY */
        struct v4l2_vbi_format        vbi;     /* V4L2_BUF_TYPE_VBI_CAPTURE */
        struct v4l2_sliced_vbi_format    sliced;  /* V4L2_BUF_TYPE_SLICED_VBI_CAPTURE */
        __u8    raw_data[200];                   /* user-defined */
    } fmt;
};
struct v4l2_pix_format {
    __u32                 width;
    __u32            height;
    __u32            pixelformat;
    enum v4l2_field      field;
    __u32                bytesperline;    /* for padding, zero if unused */
    __u32                  sizeimage;
    enum v4l2_colorspace    colorspace;
    __u32            priv;        /* private data, depends on pixelformat */
};
struct v4l2_format fmt;
memset(&fmt, 0, sizeof(fmt));
fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;//格式类型
fmt.fmt.pix.width //宽度
fmt.fmt.pix.height //高度
fmt.fmt.pix.pixelformat = VIDEO_FORMAT;//这一项必须是前面查询出来的某种格式，对应 vivi formats数组
fmt.fmt.pix.field       = V4L2_FIELD_INTERLACED;//好像是隔行扫描的意思
ret = ioctl(fd, VIDIOC_S_FMT, &fmt);
if (ret < 0) {
    LOG("VIDIOC_S_FMT failed (%d)
", ret);
    return ret;
}

驱动层：

static int vidioc_s_fmt_vid_cap(struct file *file, void *priv,
                    struct v4l2_format *f)
{
    struct vivi_dev *dev = video_drvdata(file);
    struct vb2_queue *q = &dev->vb_vidq;
 
    int ret = vidioc_try_fmt_vid_cap(file, priv, f);
    //if (fmt->fourcc == f->fmt.pix.pixelformat)返回formats[k]
    dev->fmt = get_format(f);
    dev->pixelsize     = dev->fmt->depth / 8;
    dev->width         = f->fmt.pix.width;
    dev->height     = f->fmt.pix.height;
    dev->field         = f->fmt.pix.field;
 
    return 0;
}
static int vidioc_try_fmt_vid_cap(struct file *file, void *priv,
            struct v4l2_format *f)
{
    struct vivi_dev *dev = video_drvdata(file);
    struct vivi_fmt *fmt;
    enum v4l2_field field;
 
    fmt = get_format(f);
 
    field = f->fmt.pix.field;
 
    if (field == V4L2_FIELD_ANY) {
        field = V4L2_FIELD_INTERLACED;
    } 
 
    f->fmt.pix.field = field;
    v4l_bound_align_image(&f->fmt.pix.width, 48, MAX_WIDTH, 2,
                  &f->fmt.pix.height, 32, MAX_HEIGHT, 0, 0);
    f->fmt.pix.bytesperline =
        (f->fmt.pix.width * fmt->depth) >> 3;
    f->fmt.pix.sizeimage =
        f->fmt.pix.height * f->fmt.pix.bytesperline;
    if (fmt->fourcc == V4L2_PIX_FMT_YUYV ||
        fmt->fourcc == V4L2_PIX_FMT_UYVY)
        f->fmt.pix.colorspace = V4L2_COLORSPACE_SMPTE170M;
    else
        f->fmt.pix.colorspace = V4L2_COLORSPACE_SRGB;
    return 0;
}

　　这里将应用层传进来的视频格式简单处理后存放进了一个 vivi_dev 结构，vivi_dev 哪里来的呢？，在一开始的时候 vivi_create_instance ,我们创建了一个 video_device 结构代表我们的设备，并设置了一个 vivi_dev 作为 video_device->dev->privatedata ，之后 register_video_device ，内核会自动将我们的 video_device 放入全局数组 video_device[] 中。
　　2.4 VIDIOC_G_FMT 获得设置好的视频格式

应用层：

ret = ioctl(fd, VIDIOC_G_FMT, &fmt);
if (ret < 0) {
    LOG("VIDIOC_G_FMT failed (%d)
", ret);
    return ret;
}
// Print Stream Format
LOG("Stream Format Informations:
");
LOG(" type: %d
", fmt.type);
LOG("  %d
", fmt.fmt.pix.width);
LOG(" height: %d
", fmt.fmt.pix.height);
char fmtstr[8];
memset(fmtstr, 0, 8);
memcpy(fmtstr, &fmt.fmt.pix.pixelformat, 4);
LOG(" pixelformat: %s
", fmtstr);
LOG(" field: %d
", fmt.fmt.pix.field);
LOG(" bytesperline: %d
", fmt.fmt.pix.bytesperline);
LOG(" sizeimage: %d
", fmt.fmt.pix.sizeimage);
LOG(" colorspace: %d
", fmt.fmt.pix.colorspace);
LOG(" priv: %d
", fmt.fmt.pix.priv);
LOG(" raw_date: %s
", fmt.fmt.raw_data);

驱动层：

static int vidioc_g_fmt_vid_cap(struct file *file, void *priv,
                    struct v4l2_format *f)
{
    struct vivi_dev *dev = video_drvdata(file);
<span style="white-space:pre">    </span>// 把记录在 vivi_dev 中的参数写回用户空间
    f->fmt.pix.width        = dev->width;
    f->fmt.pix.height       = dev->height;
    f->fmt.pix.field        = dev->field;
    f->fmt.pix.pixelformat  = dev->fmt->fourcc;
    f->fmt.pix.bytesperline =
        (f->fmt.pix.width * dev->fmt->depth) >> 3;
    f->fmt.pix.sizeimage =
        f->fmt.pix.height * f->fmt.pix.bytesperline;
    if (dev->fmt->fourcc == V4L2_PIX_FMT_YUYV ||
        dev->fmt->fourcc == V4L2_PIX_FMT_UYVY)
        f->fmt.pix.colorspace = V4L2_COLORSPACE_SMPTE170M;
    else
        f->fmt.pix.colorspace = V4L2_COLORSPACE_SRGB;
    return 0;
}

　　将我们之前设置的格式返回而已。

　　2.5 VIDIOC_REQBUFS 请求在内核空间分配视频缓冲区

    分配的内存位于内核空间,应用程序无法直接访问,需要通过调用mmap内存映射函数,把内核空间的内存映射到用户空间,应用才可以用用户空间地址来访问内核空间。
应用层：

struct v4l2_requestbuffers {
    __u32            count;
    __u32            type;        /* enum v4l2_buf_type */
    __u32            memory;        /* enum v4l2_memory */
    __u32            reserved[2];
};
struct v4l2_requestbuffers reqbuf;
reqbuf.type     = V4L2_BUF_TYPE_VIDEO_CAPTURE;
reqbuf.memory     = V4L2_MEMORY_MMAP;
reqbuf.count     = BUFFER_COUNT;
ret = ioctl(fd , VIDIOC_REQBUFS, &reqbuf);
if(ret < 0) {
    LOG("VIDIOC_REQBUFS failed (%d)
", ret);
    return ret;
}

驱动层：

static int vidioc_reqbufs(struct file *file, void *priv,
              struct v4l2_requestbuffers *p)
{
    struct vivi_dev *dev = video_drvdata(file);
    return vb2_reqbufs(&dev->vb_vidq, p);    //核心层提供的标准函数
}

vb_vidq 是 vivi_dev 的一个成员，前面我们提到它有两个 ops ，一个是 ops 另一个是 mem_ops

static struct vb2_ops vivi_video_qops = {
    .queue_setup    = queue_setup,
    .buf_init        = buffer_init,
    .buf_prepare    = buffer_prepare,
    .buf_finish            = buffer_finish,
    .buf_cleanup        = buffer_cleanup,
    .buf_queue        = buffer_queue,
    .start_streaming= start_streaming,
    .stop_streaming        = stop_streaming,
    .wait_prepare        = vivi_unlock,
    .wait_finish        = vivi_lock,
};

static int vidioc_reqbufs(struct file *file, void *priv,
              struct v4l2_requestbuffers *p)
{
    struct vivi_dev *dev = video_drvdata(file);
    return vb2_reqbufs(&dev->vb_vidq, p);    //核心层提供的标准函数
}

int vb2_reqbufs(struct vb2_queue *q, struct v4l2_requestbuffers *req)
{
    unsigned int num_buffers, allocated_buffers, num_planes = 0;
    int ret = 0;
    // 判断 re->count 是否小于 VIDEO_MAX_FRAME
    num_buffers = min_t(unsigned int, req->count, VIDEO_MAX_FRAME);
    memset(q->plane_sizes, 0, sizeof(q->plane_sizes));
    memset(q->alloc_ctx, 0, sizeof(q->alloc_ctx));
    q->memory = req->memory;
 
    //(q)->ops->queue_setup(q,NULL,...)
    ret = call_qop(q, queue_setup, q, NULL, &num_buffers, &num_planes,
               q->plane_sizes, q->alloc_ctx);
    /* Finally, allocate buffers and video memory */
    ret = __vb2_queue_alloc(q, req->memory, num_buffers, num_planes);
 
    allocated_buffers = ret;
 
    q->num_buffers = allocated_buffers;
    req->count = allocated_buffers;
    return 0;
}

static int queue_setup(struct vb2_queue *vq, const struct v4l2_format *fmt,
                unsigned int *nbuffers, unsigned int *nplanes,
                unsigned int sizes[], void *alloc_ctxs[])
{
    struct vivi_dev *dev = vb2_get_drv_priv(vq);
    unsigned long size;
    // 每一个buffer 的大小
    size = dev->width * dev->height * dev->pixelsize;
    if (0 == *nbuffers)
        *nbuffers = 32;
    // 如果申请的buffer过多，导致空间不够减少buffer
    while (size * *nbuffers > vid_limit * 1024 * 1024)
        (*nbuffers)--;
    *nplanes = 1;
    // 把总大小放入 vivi_dev->vb_vidq->plane_size[0]
    sizes[0] = size;
    return 0;
}

static int __vb2_queue_alloc(struct vb2_queue *q, enum v4l2_memory memory,
                 unsigned int num_buffers, unsigned int num_planes)
{
    unsigned int buffer;
    struct vb2_buffer *vb;
    int ret;
    // 分配多个 vb2_buffer 填充并放入 vivi_dev->vb_vidq->bufs[]
    for (buffer = 0; buffer < num_buffers; ++buffer) {
        /* Allocate videobuf buffer structures */
        vb = kzalloc(q->buf_struct_size, GFP_KERNEL);
        
        /* Length stores number of planes for multiplanar buffers */
        if (V4L2_TYPE_IS_MULTIPLANAR(q->type))
            vb->v4l2_buf.length = num_planes;
 
        vb->state = VB2_BUF_STATE_DEQUEUED;
        vb->vb2_queue = q;
        vb->num_planes = num_planes;
        vb->v4l2_buf.index = q->num_buffers + buffer;
        vb->v4l2_buf.type = q->type;
        vb->v4l2_buf.memory = memory;
 
        /* Allocate video buffer memory for the MMAP type */
        if (memory == V4L2_MEMORY_MMAP) {
            ret = __vb2_buf_mem_alloc(vb);//核心提供的标准函数
            ret = call_qop(q, buf_init, vb);//q->ops->buf_init
        }
 
        q->bufs[q->num_buffers + buffer] = vb;
    }
    __setup_offsets(q, buffer);
    return buffer;
}

static int __vb2_buf_mem_alloc(struct vb2_buffer *vb)
{
    struct vb2_queue *q = vb->vb2_queue;
    void *mem_priv;
    int plane;
 
    /* num_planes == 1 */
    for (plane = 0; plane < vb->num_planes; ++plane) {
        mem_priv = call_memop(q, alloc, q->alloc_ctx[plane],
                      q->plane_sizes[plane]);
 
        /* Associate allocator private data with this plane */
        vb->planes[plane].mem_priv = mem_priv;
        vb->v4l2_planes[plane].length = q->[plane];
    }
 
    return 0;
}

static void *vb2_vmalloc_alloc(void *alloc_ctx, unsigned long size)
{
    struct vb2_vmalloc_buf *buf;
 
    buf = kzalloc(sizeof(*buf), GFP_KERNEL);
 
    buf->size = size;
    // 分配空间
    buf->vaddr = vmalloc_user(buf->size);
    buf->handler.refcount = &buf->refcount;
    buf->handler.put = vb2_vmalloc_put;
    buf->handler.arg = buf;
 
    atomic_inc(&buf->refcount);
    return buf;
}

　　2.6 VIDIOC_QUERYBUF 查询分配好的 buffer 信息
    查询已经分配好的V4L2视频缓冲区的相关信息,包括缓冲区的使用状态、在内核空间的偏移地址、缓冲区长度等,然后应用程序根据这些信息使用mmap把内核空间地址映射到用户空间。
应用层：

struct v4l2_buffer {
    __u32                    index;
    enum v4l2_buf_type      type;
    __u32                    bytesused;
    __u32                    flags;
    enum v4l2_field            field;
    struct timeval            timestamp;
    struct v4l2_timecode    timecode;
    __u32                    sequence;
 
    /* memory location */
    enum v4l2_memory        memory;
    union {
        __u32               offset;
        unsigned long       userptr;
    } m;
    __u32                    length;
    __u32                    input;
    __u32                    reserved;
};
v4l2_buffer buf;
buf.index = i;
buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_MMAP;
ret = ioctl(fd , VIDIOC_QUERYBUF, &buf);
if(ret < 0) {
    LOG("VIDIOC_QUERYBUF (%d) failed (%d)
", i, ret);
    return ret;
}

驱动层：

ops->vidioc_querybuf(file, fh, p);
static int vidioc_querybuf(struct file *file, void *priv, struct v4l2_buffer *p)
{
    struct vivi_dev *dev = video_drvdata(file);
    return vb2_querybuf(&dev->vb_vidq, p);
}

int vb2_querybuf(struct vb2_queue *q, struct v4l2_buffer *b)
{
    struct vb2_buffer *vb;
    // 取出 buf
    vb = q->bufs[b->index];
    // 将 buf 信息写回用户空间传递的 b
    return __fill_v4l2_buffer(vb, b);
}

static int __fill_v4l2_buffer(struct vb2_buffer *vb, struct v4l2_buffer *b)
{
    struct vb2_queue *q = vb->vb2_queue;
    int ret;
 
    /* Copy back data such as timestamp, flags, input, etc. */
    memcpy(b, &vb->v4l2_buf, offsetof(struct v4l2_buffer, m));
    b->input = vb->v4l2_buf.input;
    b->reserved = vb->v4l2_buf.reserved;
 
    if (V4L2_TYPE_IS_MULTIPLANAR(q->type)) {
        ret = __verify_planes_array(vb, b);
        if (ret)
            return ret;
 
        /*
         * Fill in plane-related data if userspace provided an array
         * for it. The memory and size is verified above.
         */
        memcpy(b->m.planes, vb->v4l2_planes,
            b->length * sizeof(struct v4l2_plane));
 
        if (q->memory == V4L2_MEMORY_DMABUF) {
            unsigned int plane;
            for (plane = 0; plane < vb->num_planes; ++plane)
                b->m.planes[plane].m.fd = 0;
        }
    } else {
        /*
         * We use length and offset in v4l2_planes array even for
         * single-planar buffers, but userspace does not.
         */
        b->length = vb->v4l2_planes[0].length;
        b->bytesused = vb->v4l2_planes[0].bytesused;
        if (q->memory == V4L2_MEMORY_MMAP)
            b->m.offset = vb->v4l2_planes[0].m.mem_offset;
        else if (q->memory == V4L2_MEMORY_USERPTR)
            b->m.userptr = vb->v4l2_planes[0].m.userptr;
        else if (q->memory == V4L2_MEMORY_DMABUF)
            b->m.fd = 0;
    }
 
    /*
     * Clear any buffer state related flags.
     */
    b->flags &= ~V4L2_BUFFER_STATE_FLAGS;
 
    switch (vb->state) {
    case VB2_BUF_STATE_QUEUED:
    case VB2_BUF_STATE_ACTIVE:
        b->flags |= V4L2_BUF_FLAG_QUEUED;
        break;
    case VB2_BUF_STATE_ERROR:
        b->flags |= V4L2_BUF_FLAG_ERROR;
        /* fall through */
    case VB2_BUF_STATE_DONE:
        b->flags |= V4L2_BUF_FLAG_DONE;
        break;
    case VB2_BUF_STATE_PREPARED:
        b->flags |= V4L2_BUF_FLAG_PREPARED;
        break;
    case VB2_BUF_STATE_DEQUEUED:
        /* nothing */
        break;
    }
 
    if (__buffer_in_use(q, vb))
        b->flags |= V4L2_BUF_FLAG_MAPPED;
 
    return 0;
}

2.7 mmap

 应用层：

v4l2_buffer framebuf[]
framebuf[i].length = buf.length;
framebuf[i].start = (char *) mmap(
    NULL,         // 欲指向内存的起始地址，一般为NULL,表示系统自动分配
    buf.length,    //映射长度
    PROT_READ|PROT_WRITE,     //可读可写
    MAP_SHARED,     //对映射区的读写会写回内核空间，而且允许其它映射该内核空间地址的进程共享
    fd, 
    buf.m.offset
);
if (framebuf[i].start == MAP_FAILED) {
    LOG("mmap (%d) failed: %s
", i, strerror(errno));
    return -1;
}

驱动层：

static int vivi_mmap(struct file *file, struct vm_area_struct *vma)
{
    struct vivi_dev *dev = video_drvdata(file);
    int ret;
    ret = vb2_mmap(&dev->vb_vidq, vma);//核心层提供的函数
    return ret;
}

　　2.8 VIDIOC_QBUF 
  投放一个空的视频缓冲区到视频缓冲区输入队列，执行成功后，在启动视频设备拍摄图像时，相应的视频数据被保存到视频输入队列相应的视频缓冲区中。
应用层：

ret = ioctl(fd , VIDIOC_QBUF, &buf);
if (ret < 0) {
    LOG("VIDIOC_QBUF (%d) failed (%d)
", i, ret);
    return -1;

驱动层：

static int vidioc_qbuf(struct file *file, void *priv, struct v4l2_buffer *p)
{
    struct vivi_dev *dev = video_drvdata(file);
    return vb2_qbuf(&dev->vb_vidq, p);
}

int vb2_qbuf(struct vb2_queue *q, struct v4l2_buffer *b)
{
    struct rw_semaphore *mmap_sem = NULL;
    struct vb2_buffer *vb;
    int ret = 0;
 
    vb = q->bufs[b->index];
 
    switch (vb->state) {
    case VB2_BUF_STATE_DEQUEUED:
        ret = __buf_prepare(vb, b);
    }
    // 将这个 buffer 挂入 q->queued_list
    list_add_tail(&vb->queued_entry, &q->queued_list);
    vb->state = VB2_BUF_STATE_QUEUED;
 
    if (q->streaming)
        __enqueue_in_driver(vb);
 
    /* Fill buffer information for the userspace */
    __fill_v4l2_buffer(vb, b);
 
unlock:
    if (mmap_sem)
        up_read(mmap_sem);
    return ret;
}

  实质上就是取出一个 vb2_buffer 挂入 vivi_dev->vb_vidq->queued_list
　　2.9 VIDIOC_STREAMON
应用层：

enum v4l2_buf_type type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
ret = ioctl(fd, VIDIOC_STREAMON, &type);
if (ret < 0) {
    LOG("VIDIOC_STREAMON failed (%d)
", ret);
    return ret;
}

驱动层：

static int vidioc_streamon(struct file *file, void *priv, enum v4l2_buf_type i)
{
    struct vivi_dev *dev = video_drvdata(file);
    return vb2_streamon(&dev->vb_vidq, i);
}

int vb2_streamon(struct vb2_queue *q, enum v4l2_buf_type type)
{
    struct vb2_buffer *vb;
    int ret;
    vb->state = VB2_BUF_STATE_ACTIVE;
    // 在 queued_list 链表中取出每一个 buffer 调用buffer queue,对于vivi来说就是放入 vidq->active 链表
    list_for_each_entry(vb, &q->queued_list, queued_entry)
            __enqueue_in_driver(vb);
    ret = call_qop(q, start_streaming, q, atomic_read(&q->queued_count));
 
    q->streaming = 1;
    return 0;
}

static void __enqueue_in_driver(struct vb2_buffer *vb)
{
    struct vb2_queue *q = vb->vb2_queue;
    vb->state = VB2_BUF_STATE_ACTIVE;
 
    /* sync buffers */
    for (plane = 0; plane < vb->num_planes; ++plane)
    call_memop(q, prepare, vb->planes[plane].mem_priv);
 
    q->ops->buf_queue(vb);//    list_add_tail(&buf->list, &vidq->active);    
}

static int start_streaming(struct vb2_queue *vq, unsigned int count)
{
    struct vivi_dev *dev = vb2_get_drv_priv(vq);
    dprintk(dev, 1, "%s
", __func__);
    return vivi_start_generating(dev);
}

static int vivi_start_generating(struct vivi_dev *dev)
{
    struct vivi_dmaqueue *dma_q = &dev->vidq;
 
    /* Resets frame counters */
    dev->ms = 0;
    dev->mv_count = 0;
    dev->jiffies = jiffies;
 
    dma_q->frame = 0;
    dma_q->ini_jiffies = jiffies;
    // 创建一个内核线程，入口函数 vivi_thread
    dma_q->kthread = kthread_run(vivi_thread, dev, dev->v4l2_dev.name);
    
    /* Wakes thread */
    wake_up_interruptible(&dma_q->wq);
 
    return 0;
}

static int vivi_thread(void *data)
{
    struct vivi_dev *dev = data;
 
    dprintk(dev, 1, "thread started
");
 
    set_freezable();
 
    for (;;) {
        vivi_sleep(dev);
 
        if (kthread_should_stop())
            break;
    }
    dprintk(dev, 1, "thread: exit
");
    return 0;
}

static void vivi_sleep(struct vivi_dev *dev)
{
    struct vivi_dmaqueue *dma_q = &dev->vidq;
    int timeout;
    DECLARE_WAITQUEUE(wait, current);
 
    add_wait_queue(&dma_q->wq, &wait);
    if (kthread_should_stop())
        goto stop_task;
 
    /* Calculate time to wake up */
    timeout = msecs_to_jiffies(frames_to_ms(1));
 
    vivi_thread_tick(dev);
 
    schedule_timeout_interruptible(timeout);
 
stop_task:
    remove_wait_queue(&dma_q->wq, &wait);
    try_to_freeze();
}

  每次调用 vivi_sleep 这个线程都被挂入等待队列，调用 vivi_thread_tick 填充数据，然后休眠指定的时间自动唤醒，一直循环下去。这样就生成了一帧一帧的视频数据。

static void vivi_thread_tick(struct vivi_dev *dev)
{
    struct vivi_dmaqueue *dma_q = &dev->vidq;
    struct vivi_buffer *buf;
    unsigned long flags = 0;
 
    spin_lock_irqsave(&dev->slock, flags);
 
    buf = list_entry(dma_q->active.next, struct vivi_buffer, list);
    list_del(&buf->list);
    spin_unlock_irqrestore(&dev->slock, flags);
 
    do_gettimeofday(&buf->vb.v4l2_buf.timestamp);
 
    /* 填充Buffer */
    vivi_fillbuff(dev, buf);
 
    vb2_buffer_done(&buf->vb, VB2_BUF_STATE_DONE);
}

void vb2_buffer_done(struct vb2_buffer *vb, enum vb2_buffer_state state)
{
    struct vb2_queue *q = vb->vb2_queue;
    unsigned long flags;
    unsigned int plane;
 
    /* sync buffers */
    for (plane = 0; plane < vb->num_planes; ++plane)
        call_memop(q, finish, vb->planes[plane].mem_priv);
 
    /* Add the buffer to the done buffers list */
    spin_lock_irqsave(&q->done_lock, flags);
    vb->state = state;
    list_add_tail(&vb->done_entry, &q->done_list);
    atomic_dec(&q->queued_count);
#ifdef CONFIG_SYNC
    sw_sync_timeline_inc(q->timeline, 1);
#endif
    spin_unlock_irqrestore(&q->done_lock, flags);
 
    /* 应用程序select 时 poll_wait 里休眠，现在有数据了唤醒 */
    wake_up(&q->done_wq);
}

  开始的时候我们将以一个 vb_buffer 挂入 vb_vidq->queued_list ，当启动视频传输之后，它被取出挂入 vb_vidq->vidq->active 队列，然后在内核线程中每一个 tick ，又将它取出填充视频数据之后，再挂入 vb_vidq->done_list ，唤醒正在休眠等待视频数据的应用程序。
　　2.10 select
驱动层：

vivi_poll(struct file *file, struct poll_table_struct *wait)
{
    struct vivi_dev *dev = video_drvdata(file);
    struct vb2_queue *q = &dev->vb_vidq;
 
    return vb2_poll(q, file, wait);
}

unsigned int vb2_poll(struct vb2_queue *q, struct file *file, poll_table *wait)
{
    // 挂入休眠队列，是否休眠还要看返回值，大概没有数据就休眠，有数据就不休眠
    poll_wait(file, &q->done_wq, wait);
 
    if (!list_empty(&q->done_list))
        vb = list_first_entry(&q->done_list, struct vb2_buffer,
                    done_entry);
    spin_unlock_irqrestore(&q->done_lock, flags);
 
    if (vb && (vb->state == VB2_BUF_STATE_DONE
            || vb->state == VB2_BUF_STATE_ERROR)) {
        return (V4L2_TYPE_IS_OUTPUT(q->type)) ?
                res | POLLOUT | POLLWRNORM :
                res | POLLIN | POLLRDNORM;
    }
    return res;
}

  唤醒之后，我们就可以去从视频输出队列中取出buffer，然后根据映射关系，在应用空间取出视频数据了
　　2.11 VIDIOC_DQBUF
应用层：

ret = ioctl(fd, VIDIOC_DQBUF, &buf);
if (ret < 0) {
    LOG("VIDIOC_DQBUF failed (%d)
", ret);
    return ret;
}

static int vidioc_dqbuf(struct file *file, void *priv, struct v4l2_buffer *p)
{
    struct vivi_dev *dev = video_drvdata(file);
    return vb2_dqbuf(&dev->vb_vidq, p, file->f_flags & O_NONBLOCK);
}

int vb2_dqbuf(struct vb2_queue *q, struct v4l2_buffer *b, bool nonblocking)
{
    struct vb2_buffer *vb = NULL;
    int ret;
    // 等待在 q->done_list 取出第一个可用的 buffer
    ret = __vb2_get_done_vb(q, &vb, nonblocking);
 
    ret = call_qop(q, buf_finish, vb);
 
    /* 写回buffer的信息到用户空间，应用程序找个这个buffer的mmap之后的地址读数据 */
    __fill_v4l2_buffer(vb, b);
    /* Remove from videobuf queue */
    list_del(&vb->queued_entry);
 
    vb->state = VB2_BUF_STATE_DEQUEUED;
    return 0;
}

static int __vb2_get_done_vb(struct vb2_queue *q, struct vb2_buffer **vb,int nonblocking)
{
    unsigned long flags;
    int ret;
    /*
    * Wait for at least one buffer to become available on the done_list.
    */
    ret = __vb2_wait_for_done_vb(q, nonblocking);
 
 
    spin_lock_irqsave(&q->done_lock, flags);
    *vb = list_first_entry(&q->done_list, struct vb2_buffer, done_entry);
    list_del(&(*vb)->done_entry);
    spin_unlock_irqrestore(&q->done_lock, flags);
    return 0;
}

static int buffer_finish(struct vb2_buffer *vb)
{
    struct vivi_dev *dev = vb2_get_drv_priv(vb->vb2_queue);
    dprintk(dev, 1, "%s
", __func__);
    return 0;
}