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\n",
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.\n",
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 "\n");
}
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\n",
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)\n", 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\t%d.%s\n",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)\n", 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)\n", ret);
return ret;
}
// Print Stream Format
LOG("Stream Format Informations:\n");
LOG(" type: %d\n", fmt.type);
LOG(" width: %d\n", fmt.fmt.pix.width);
LOG(" height: %d\n", fmt.fmt.pix.height);
char fmtstr[8];
memset(fmtstr, 0, 8);
memcpy(fmtstr, &fmt.fmt.pix.pixelformat, 4);
LOG(" pixelformat: %s\n", fmtstr);
LOG(" field: %d\n", fmt.fmt.pix.field);
LOG(" bytesperline: %d\n", fmt.fmt.pix.bytesperline);
LOG(" sizeimage: %d\n", fmt.fmt.pix.sizeimage);
LOG(" colorspace: %d\n", fmt.fmt.pix.colorspace);
LOG(" priv: %d\n", fmt.fmt.pix.priv);
LOG(" raw_date: %s\n", 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)\n", 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)\n", 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\n", 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)\n", 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)\n", 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\n", __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\n");
set_freezable();
for (;;) {
vivi_sleep(dev);
if (kthread_should_stop())
break;
}
dprintk(dev, 1, "thread: exit\n");
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)\n", 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\n", __func__);
return 0;
}
原文地址：https://www.cnblogs.com/Lxk0825/p/10407913.html
时间： 2024-11-06 07:23:49