1. 概述

VIDIOC_REQBUFS 请求在内核空间分配视频缓冲区
功能：
请求V4L2驱动分配视频缓冲区（申请V4L2视频驱动分配内存）
V4L2是视频设备的驱动层，位于内核空间，所以通过 VIDIOC_REQBUFS控制命令申请的内存位于内核空间，应用程序不能直接访问，需要通过调用mmap内存映射函数，把内核空间内存映射到用户空间后，应用程序通过访问用户空间地址来访问内核空间。
参数说明：
参数类型为 V4L2的申请缓冲区数据结构体类型 struct v4l2_requestbuffers；v4l2_requestbuffers 结构中定义了缓存的数量，驱动会据此申请对应数量的视频缓存。多个缓存可以用于建立FIFO，来提高视频采集的效率。
一般不超过5个，CAP_BUF_NUM = 4
返回值说明：
执行成功时，函数返回值为 0；V4L2驱动层分配好了视频缓冲区；

2. 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 req;
/* 申请设备的缓存区 */
memset(&req, 0, sizeof(req));
req.count = CAP_BUF_NUM;  //申请一个拥有四个缓冲帧的缓冲区
req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
req.memory = V4L2_MEMORY_MMAP;if (ioctl(dev->fd, VIDIOC_REQBUFS, &req) < 0)
{if (EINVAL == errno){printf(stderr, "%s does not support ""memory mapping\n", dev->dev);return TFAIL;}else{printf(stderr, "%s does not support ""memory mapping, unknow error\n", dev->dev);return TFAIL;}
}
if (req.count < 2)
{printf(stderr, "Insufficient buffer memory on %s\n",dev->dev);return TFAIL;
}

3. 内核驱动

3.1 vb2_ioctl_reqbufs函数

/* vb2 ioctl helpers */int vb2_ioctl_reqbufs(struct file *file, void *priv, struct v4l2_requestbuffers *p)
{// 获取devstruct video_device *vdev = video_devdata(file);// 检查队列memory类型是否一致，不一致则直接退出int res = vb2_verify_memory_type(vdev->queue, p->memory, p->type);if (res)return res;if (vb2_queue_is_busy(vdev, file))return -EBUSY;res = vb2_core_reqbufs(vdev->queue, p->memory, &p->count);/** 注意*     1. 当count=0的时候，释放所有buffer*//* If count == 0, then the owner has released all buffers and heis no longer owner of the queue. Otherwise we have a new owner. */if (res == 0)vdev->queue->owner = p->count ? file->private_data : NULL;return res;
}
EXPORT_SYMBOL_GPL(vb2_ioctl_reqbufs);

3.2 vb2_core_reqbufs函数

int vb2_core_reqbufs(struct vb2_queue *q, enum vb2_memory memory, unsigned int *count)
{unsigned int num_buffers, allocated_buffers, num_planes = 0;// #define VB2_MAX_PLANES  (8)unsigned plane_sizes[VB2_MAX_PLANES] = { };int ret;// 正在取流中if (q->streaming) {dprintk(1, "streaming active\n");return -EBUSY;}/** 注意* 1。 这里进行count=0判断，memory类型判断，删除所有buffer，所以为什么应用层会进行0 buffer的申请操作。*/if (*count == 0 || q->num_buffers != 0 || q->memory != memory) {/** We already have buffers allocated, so first check if they* are not in use and can be freed.*/mutex_lock(&q->mmap_lock);if (q->memory == VB2_MEMORY_MMAP && __buffers_in_use(q)) {mutex_unlock(&q->mmap_lock);dprintk(1, "memory in use, cannot free\n");return -EBUSY;}/** Call queue_cancel to clean up any buffers in the PREPARED or* QUEUED state which is possible if buffers were prepared or* queued without ever calling STREAMON.*/__vb2_queue_cancel(q);ret = __vb2_queue_free(q, q->num_buffers);mutex_unlock(&q->mmap_lock);if (ret)return ret;/** In case of REQBUFS(0) return immediately without calling* driver's queue_setup() callback and allocating resources.*/if (*count == 0)return 0;}/** Make sure the requested values and current defaults are sane.*/// #define VB2_MAX_FRAME   (32)num_buffers = min_t(unsigned int, *count, VB2_MAX_FRAME);// min_buffers_needed = 2一般为2num_buffers = max_t(unsigned int, num_buffers, q->min_buffers_needed);memset(q->alloc_devs, 0, sizeof(q->alloc_devs));q->memory = memory;// 从驱动层获取到可以支持多少buffers和planes per buffer/** Ask the driver how many buffers and planes per buffer it requires.* Driver also sets the size and allocator context for each plane.*/ret = call_qop(q, queue_setup, q, &num_buffers, &num_planes, plane_sizes, q->alloc_devs);if (ret)return ret;// 最后分配buffer/* Finally, allocate buffers and video memory */allocated_buffers =__vb2_queue_alloc(q, memory, num_buffers, num_planes, plane_sizes);if (allocated_buffers == 0) {dprintk(1, "memory allocation failed\n");return -ENOMEM;}// 检查是否大于最小需求buffer/** There is no point in continuing if we can't allocate the minimum* number of buffers needed by this vb2_queue.*/if (allocated_buffers < q->min_buffers_needed)ret = -ENOMEM;/** Check if driver can handle the allocated number of buffers.*/if (!ret && allocated_buffers < num_buffers) {num_buffers = allocated_buffers;/** num_planes is set by the previous queue_setup(), but since it* signals to queue_setup() whether it is called from create_bufs()* vs reqbufs() we zero it here to signal that queue_setup() is* called for the reqbufs() case.*/num_planes = 0;ret = call_qop(q, queue_setup, q, &num_buffers,&num_planes, plane_sizes, q->alloc_devs);if (!ret && allocated_buffers < num_buffers)ret = -ENOMEM;/** Either the driver has accepted a smaller number of buffers,* or .queue_setup() returned an error*/}mutex_lock(&q->mmap_lock);q->num_buffers = allocated_buffers;if (ret < 0) {/** Note: __vb2_queue_free() will subtract 'allocated_buffers'* from q->num_buffers.*/__vb2_queue_free(q, allocated_buffers);mutex_unlock(&q->mmap_lock);return ret;}mutex_unlock(&q->mmap_lock);/** 注意*   1.这里对应用层传递过来的count进行了，已经分配buffer数量的更新操作。*//** Return the number of successfully allocated buffers* to the userspace.*/*count = allocated_buffers;q->waiting_for_buffers = !q->is_output;return 0;
}
EXPORT_SYMBOL_GPL(vb2_core_reqbufs);

3.3 __vb2_queue_alloc函数

/*** __vb2_queue_alloc() - allocate videobuf buffer structures and (for MMAP type)* video buffer memory for all buffers/planes on the queue and initializes the* queue** Returns the number of buffers successfully allocated.*/
static int __vb2_queue_alloc(struct vb2_queue *q, enum vb2_memory memory,unsigned int num_buffers, unsigned int num_planes,const unsigned plane_sizes[VB2_MAX_PLANES])
{unsigned int buffer, plane;struct vb2_buffer *vb;int ret;/* Ensure that q->num_buffers+num_buffers is below VB2_MAX_FRAME */num_buffers = min_t(unsigned int, num_buffers,VB2_MAX_FRAME - q->num_buffers);// 分配 vidoe buffer了for (buffer = 0; buffer < num_buffers; ++buffer) {/* Allocate videobuf buffer structures */vb = kzalloc(q->buf_struct_size, GFP_KERNEL);if (!vb) {dprintk(1, "memory alloc for buffer struct failed\n");break;}// 设置video buffer的数据成员vb->state = VB2_BUF_STATE_DEQUEUED;vb->vb2_queue = q;vb->num_planes = num_planes;vb->index = q->num_buffers + buffer;vb->type = q->type;vb->memory = memory;for (plane = 0; plane < num_planes; ++plane) {vb->planes[plane].length = plane_sizes[plane];vb->planes[plane].min_length = plane_sizes[plane];}/** 注意*  1. 这里使用的idx索引*/q->bufs[vb->index] = vb;/* Allocate video buffer memory for the MMAP type */if (memory == VB2_MEMORY_MMAP){ret = __vb2_buf_mem_alloc(vb);if (ret) {dprintk(1, "failed allocating memory for buffer %d\n",buffer);q->bufs[vb->index] = NULL;kfree(vb);break;}__setup_offsets(vb);/** Call the driver-provided buffer initialization* callback, if given. An error in initialization* results in queue setup failure.*/ret = call_vb_qop(vb, buf_init, vb);if (ret) {dprintk(1, "buffer %d %p initialization failed\n",buffer, vb);__vb2_buf_mem_free(vb);q->bufs[vb->index] = NULL;kfree(vb);break;}}}dprintk(1, "allocated %d buffers, %d plane(s) each\n",buffer, num_planes);return buffer;
}

3.4 __setup_offsets函数

// 其实主要就是内存模块的对齐 page
/*** __setup_offsets() - setup unique offsets ("cookies") for every plane in* the buffer.*/
static void __setup_offsets(struct vb2_buffer *vb)
{struct vb2_queue *q = vb->vb2_queue;unsigned int plane;unsigned long off = 0;if (vb->index) {struct vb2_buffer *prev = q->bufs[vb->index - 1];struct vb2_plane *p = &prev->planes[prev->num_planes - 1];off = PAGE_ALIGN(p->m.offset + p->length);}for (plane = 0; plane < vb->num_planes; ++plane) {vb->planes[plane].m.offset = off;dprintk(3, "buffer %d, plane %d offset 0x%08lx\n",vb->index, plane, off);off += vb->planes[plane].length;off = PAGE_ALIGN(off);}
}

3.5 __vb2_buf_mem_alloc函数

/*** __vb2_buf_mem_alloc() - allocate video memory for the given buffer*/
static int __vb2_buf_mem_alloc(struct vb2_buffer *vb)
{struct vb2_queue *q = vb->vb2_queue;void *mem_priv;int plane;int ret = -ENOMEM;/** Allocate memory for all planes in this buffer* NOTE: mmapped areas should be page aligned*/for (plane = 0; plane < vb->num_planes; ++plane) {// page对齐unsigned long size = PAGE_ALIGN(vb->planes[plane].length);// 内核mmapmem_priv = call_ptr_memop(vb, alloc,q->alloc_devs[plane] ? : q->dev,q->dma_attrs, size, q->dma_dir, q->gfp_flags);if (IS_ERR_OR_NULL(mem_priv)) {if (mem_priv)ret = PTR_ERR(mem_priv);goto free;}/* Associate allocator private data with this plane */vb->planes[plane].mem_priv = mem_priv;}return 0;
free:/* Free already allocated memory if one of the allocations failed */for (; plane > 0; --plane) {call_void_memop(vb, put, vb->planes[plane - 1].mem_priv);vb->planes[plane - 1].mem_priv = NULL;}return ret;
}

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VIDIOC_REQBUFS向驱动申请帧缓冲代码详解

1. 概述

2. 应用层

3. 内核驱动

3.1 vb2_ioctl_reqbufs函数

3.2 vb2_core_reqbufs函数

3.3 __vb2_queue_alloc函数

3.4 __setup_offsets函数

3.5 __vb2_buf_mem_alloc函数

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