【genius_platform软件平台开发】第六十一讲：Linux系统之V4L2视频驱动-VIDIOC_QBUF入队列

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1. 概述

• 操作系统一般把系统使用的内存划分成用户空间和内核空间，分别由应用程序管理和操作系统管理。
  应用程序可以直接访问内存的地址，而内核空间存放的是 供内核访问的代码和数据，用户不能直接访问。
  v4l2捕获的数据，最初是存放在内核空间的，这意味着用户不能直接访问该段内存，必须通过某些手段来转换地址；
  具体流程可以查看【genius_platform软件平台开发】第五十三讲：Linux系统之V4L2视频驱动-应用层代码如何编写篇章
  

2. 应用层

struct v4l2_buffer info;
MEMSET(info);
 info.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
 info.memory = V4L2_MEMORY_MMAP;
 info.index = nIdx;

 // 将申请的内核缓冲区放入视频采集输入队列中排队
 if (-1 == this->ioCtrl(m_nFd, VIDIOC_QBUF, &info))
 {
     LOGERROR("CV4l2CaptureIr::initMmap is error... VIDIOC_QBUF");
     return ReturnCode_Error;
 }

3. 驱动层

3.1 vb2_qbuf函数

int vb2_qbuf(struct vb2_queue *q, struct v4l2_buffer *b)
{
	int ret;

	if (vb2_fileio_is_active(q)) {
		dprintk(1, "file io in progress\n");
		return -EBUSY;
	}

	ret = vb2_queue_or_prepare_buf(q, b, "qbuf");
	return ret ? ret : vb2_core_qbuf(q, b->index, b);
}
EXPORT_SYMBOL_GPL(vb2_qbuf);

3.2 vb2_queue_or_prepare_buf函数

// 一些前期的准备检查
static int vb2_queue_or_prepare_buf(struct vb2_queue *q, struct v4l2_buffer *b,
				    const char *opname)
{
	// 主要做了一些检查
	if (b->type != q->type) {
		dprintk(1, "%s: invalid buffer type\n", opname);
		return -EINVAL;
	}
	
	// 索引检查
	if (b->index >= q->num_buffers) {
		dprintk(1, "%s: buffer index out of range\n", opname);
		return -EINVAL;
	}

	// 是否为空检查
	if (q->bufs[b->index] == NULL) {
		/* Should never happen */
		dprintk(1, "%s: buffer is NULL\n", opname);
		return -EINVAL;
	}
	
	// memory类型检查
	if (b->memory != q->memory) {
		dprintk(1, "%s: invalid memory type\n", opname);
		return -EINVAL;
	}

	return __verify_planes_array(q->bufs[b->index], b);
}

3.3 __verify_planes_array函数

/**
 * __verify_planes_array() - verify that the planes array passed in struct
 * v4l2_buffer from userspace can be safely used
 */
static int __verify_planes_array(struct vb2_buffer *vb, const struct v4l2_buffer *b)
{
	if (!V4L2_TYPE_IS_MULTIPLANAR(b->type))
		return 0;

	/* Is memory for copying plane information present? */
	if (b->m.planes == NULL) 
	{
		dprintk(1, "multi-planar buffer passed but planes array not provided\n");
		return -EINVAL;
	}

	if (b->length < vb->num_planes || b->length > VB2_MAX_PLANES) 
	{
		dprintk(1, "incorrect planes array length, expected %d, got %d\n",
			vb->num_planes, b->length);
		return -EINVAL;
	}

	return 0;
}

3.4 vb2_core_qbuf函数

// 核心处理函数
int vb2_core_qbuf(struct vb2_queue *q, unsigned int index, void *pb)
{
	struct vb2_buffer *vb;
	int ret;

	if (q->error) {
		dprintk(1, "fatal error occurred on queue\n");
		return -EIO;
	}

	// 根据索引取出vb
	vb = q->bufs[index];

	switch (vb->state) 
	{
	/*
	前面的所有分析中只有reqbufs的时候
    * 状态设置成了VB2_BUF_STATE_DEQUEUED
    * 表示在用户空间控制
    * 所以要分析__buf_prepare这个函数
    * */
	case VB2_BUF_STATE_DEQUEUED:
		ret = __buf_prepare(vb, pb);
		if (ret)
			return ret;
		break;
	case VB2_BUF_STATE_PREPARED:
		break;
	case VB2_BUF_STATE_PREPARING:
		dprintk(1, "buffer still being prepared\n");
		return -EINVAL;
	default:
		dprintk(1, "invalid buffer state %d\n", vb->state);
		return -EINVAL;
	}

	// 添加到队列缓冲区buffer list中，数据帧将一直保留在该list队列中，直到在dqbuf中退出队列
	// 将这个 buffer 挂入 q->queued_list
	/*
	 * Add to the queued buffers list, a buffer will stay on it until
	 * dequeued in dqbuf.
	 */
	list_add_tail(&vb->queued_entry, &q->queued_list);
	q->queued_count++;
	q->waiting_for_buffers = false;
	vb->state = VB2_BUF_STATE_QUEUED;

	if (pb)
		call_void_bufop(q, copy_timestamp, vb, pb);

	trace_vb2_qbuf(q, vb);

	/*
	 * If already streaming, give the buffer to driver for processing.
	 * If not, the buffer will be given to driver on next streamon.
	 */
	if (q->start_streaming_called)
		__enqueue_in_driver(vb);

	/* Fill buffer information for the userspace */
	if (pb)
		call_void_bufop(q, fill_user_buffer, vb, pb);

	// 如果之前是在start_streaming_called 或者streaming状态，则只有当buffer足够了才会执行start streaming
	/*
	 * If streamon has been called, and we haven't yet called
	 * start_streaming() since not enough buffers were queued, and
	 * we now have reached the minimum number of queued buffers,
	 * then we can finally call start_streaming().
	 */
	if (q->streaming && !q->start_streaming_called &&
	    q->queued_count >= q->min_buffers_needed) {
		ret = vb2_start_streaming(q);
		if (ret)
			return ret;
	}

	dprintk(2, "qbuf of buffer %d succeeded\n", vb->index);
	return 0;
}
EXPORT_SYMBOL_GPL(vb2_core_qbuf);

3.5 __buf_prepare函数

static int __buf_prepare(struct vb2_buffer *vb, const void *pb)
{
	struct vb2_queue *q = vb->vb2_queue;
	unsigned int plane;
	int ret;
 
	if (q->error) {
		dprintk(1, "fatal error occurred on queue\n");
		return -EIO;
	}
 
 	// 设置state=VB2_BUF_STATE_PREPARING
	vb->state = VB2_BUF_STATE_PREPARING;
 
	switch (q->memory) {
        /*
         * 驱动会执行MMAP
         */
	case VB2_MEMORY_MMAP:
		ret = __prepare_mmap(vb, pb);
		break;
	case VB2_MEMORY_USERPTR:
		ret = __prepare_userptr(vb, pb);
		break;
	case VB2_MEMORY_DMABUF:
		ret = __prepare_dmabuf(vb, pb);
		break;
	default:
		WARN(1, "Invalid queue type\n");
		ret = -EINVAL;
	}
...

## 3.6 __prepare_mmap函数

```c
static int __prepare_mmap(struct vb2_buffer *vb, const void *pb)
{
	int ret = 0;
	
	/*
	* pb是用户空间传入的，所以存在
	*/
	if (pb)
	/*
	* 变换之后 vb->vb2_queue->buf_ops->fill_vb2_buffer
	* call_bufop 对应__fill_vb2_buffer
	*/
	ret = call_bufop(vb->vb2_queue, fill_vb2_buffer,
			 vb, pb, vb->planes);
    /*
       * vb->vb2_queue->ops->buf_prepare
       * 就是buf_prepare
       */
	return ret ? ret : call_vb_qop(vb, buf_prepare, vb);
}

3.6 __fill_vb2_buffer函数

/**
 * __fill_vb2_buffer() - fill a vb2_buffer with information provided in a
 * v4l2_buffer by the userspace. It also verifies that struct
 * v4l2_buffer has a valid number of planes.
 */
static int __fill_vb2_buffer(struct vb2_buffer *vb,
		const void *pb, struct vb2_plane *planes)
{
	struct vb2_queue *q = vb->vb2_queue;
	const struct v4l2_buffer *b = pb;
	struct vb2_v4l2_buffer *vbuf = to_vb2_v4l2_buffer(vb);
	unsigned int plane;
	int ret;

	// 对于capature length=0
	ret = __verify_length(vb, b);
	if (ret < 0) {
		dprintk(1, "plane parameters verification failed: %d\n", ret);
		return ret;
	}
	
	if (b->field == V4L2_FIELD_ALTERNATE && q->is_output) {
		/*
		 * If the format's field is ALTERNATE, then the buffer's field
		 * should be either TOP or BOTTOM, not ALTERNATE since that
		 * makes no sense. The driver has to know whether the
		 * buffer represents a top or a bottom field in order to
		 * program any DMA correctly. Using ALTERNATE is wrong, since
		 * that just says that it is either a top or a bottom field,
		 * but not which of the two it is.
		 */
		dprintk(1, "the field is incorrectly set to ALTERNATE for an output buffer\n");
		return -EINVAL;
	}
	vb->timestamp = 0;
	vbuf->sequence = 0;

	// 多平面适配格式
	if (V4L2_TYPE_IS_MULTIPLANAR(b->type)) {
		if (b->memory == VB2_MEMORY_USERPTR) {
			for (plane = 0; plane < vb->num_planes; ++plane) {
				planes[plane].m.userptr =
					b->m.planes[plane].m.userptr;
				planes[plane].length =
					b->m.planes[plane].length;
			}
		}
		if (b->memory == VB2_MEMORY_DMABUF) {
			for (plane = 0; plane < vb->num_planes; ++plane) {
				planes[plane].m.fd =
					b->m.planes[plane].m.fd;
				planes[plane].length =
					b->m.planes[plane].length;
			}
		}

		/* Fill in driver-provided information for OUTPUT types */
		if (V4L2_TYPE_IS_OUTPUT(b->type)) {
			/*
			 * Will have to go up to b->length when API starts
			 * accepting variable number of planes.
			 *
			 * If bytesused == 0 for the output buffer, then fall
			 * back to the full buffer size. In that case
			 * userspace clearly never bothered to set it and
			 * it's a safe assumption that they really meant to
			 * use the full plane sizes.
			 *
			 * Some drivers, e.g. old codec drivers, use bytesused == 0
			 * as a way to indicate that streaming is finished.
			 * In that case, the driver should use the
			 * allow_zero_bytesused flag to keep old userspace
			 * applications working.
			 */
			for (plane = 0; plane < vb->num_planes; ++plane) {
				struct vb2_plane *pdst = &planes[plane];
				struct v4l2_plane *psrc = &b->m.planes[plane];

				if (psrc->bytesused == 0)
					vb2_warn_zero_bytesused(vb);

				if (vb->vb2_queue->allow_zero_bytesused)
					pdst->bytesused = psrc->bytesused;
				else
					pdst->bytesused = psrc->bytesused ?
						psrc->bytesused : pdst->length;
				pdst->data_offset = psrc->data_offset;
			}
		}
	} else {
		// 单平面视频格式
		/*
		 * Single-planar buffers do not use planes array,
		 * so fill in relevant v4l2_buffer struct fields instead.
		 * In videobuf we use our internal V4l2_planes struct for
		 * single-planar buffers as well, for simplicity.
		 *
		 * If bytesused == 0 for the output buffer, then fall back
		 * to the full buffer size as that's a sensible default.
		 *
		 * Some drivers, e.g. old codec drivers, use bytesused == 0 as
		 * a way to indicate that streaming is finished. In that case,
		 * the driver should use the allow_zero_bytesused flag to keep
		 * old userspace applications working.
		 */
		 // 用户空间指针，直接赋值
		if (b->memory == VB2_MEMORY_USERPTR) {
			planes[0].m.userptr = b->m.userptr;
			planes[0].length = b->length;
		}
		// dma方式
		if (b->memory == VB2_MEMORY_DMABUF) {
			planes[0].m.fd = b->m.fd;
			planes[0].length = b->length;
		}
		/*
		*enum v4l2_buf_type {
        V4L2_BUF_TYPE_VIDEO_CAPTURE        = 1,
        V4L2_BUF_TYPE_VIDEO_OUTPUT         = 2,
        V4L2_BUF_TYPE_VIDEO_OVERLAY        = 3,
        V4L2_BUF_TYPE_VBI_CAPTURE          = 4,
        V4L2_BUF_TYPE_VBI_OUTPUT           = 5,
        V4L2_BUF_TYPE_SLICED_VBI_CAPTURE   = 6,
        V4L2_BUF_TYPE_SLICED_VBI_OUTPUT    = 7,
        V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY = 8,
        V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE = 9,
        V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE  = 10,
        V4L2_BUF_TYPE_SDR_CAPTURE          = 11,
        V4L2_BUF_TYPE_SDR_OUTPUT           = 12,
        V4L2_BUF_TYPE_META_CAPTURE         = 13,
        V4L2_BUF_TYPE_PRIVATE              = 0x80,
};
#define V4L2_TYPE_IS_MULTIPLANAR(type)                  \
        ((type) == V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE   \
         || (type) == V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE)

#define V4L2_TYPE_IS_OUTPUT(type)                               \
        ((type) == V4L2_BUF_TYPE_VIDEO_OUTPUT                   \
         || (type) == V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE         \
         || (type) == V4L2_BUF_TYPE_VIDEO_OVERLAY               \
         || (type) == V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY        \
         || (type) == V4L2_BUF_TYPE_VBI_OUTPUT                  \
         || (type) == V4L2_BUF_TYPE_SLICED_VBI_OUTPUT           \
         || (type) == V4L2_BUF_TYPE_SDR_OUTPUT)
		*/
		if (V4L2_TYPE_IS_OUTPUT(b->type)) 
		{
			if (b->bytesused == 0)
				vb2_warn_zero_bytesused(vb);

			if (vb->vb2_queue->allow_zero_bytesused)
				planes[0].bytesused = b->bytesused;
			else
				planes[0].bytesused = b->bytesused ?
					b->bytesused : planes[0].length;
		} else
		{
			// 所以V4L2_BUF_TYPE_VIDEO_CAPTURE会走到这里
			planes[0].bytesused = 0;
		}
	}
	
	// 下面都不用的
	/* Zero flags that the vb2 core handles */
	vbuf->flags = b->flags & ~V4L2_BUFFER_MASK_FLAGS;
	if (!vb->vb2_queue->copy_timestamp || !V4L2_TYPE_IS_OUTPUT(b->type)) {
		/*
		 * Non-COPY timestamps and non-OUTPUT queues will get
		 * their timestamp and timestamp source flags from the
		 * queue.
		 */
		vbuf->flags &= ~V4L2_BUF_FLAG_TSTAMP_SRC_MASK;
	}

	if (V4L2_TYPE_IS_OUTPUT(b->type)) {
		/*
		 * For output buffers mask out the timecode flag:
		 * this will be handled later in vb2_qbuf().
		 * The 'field' is valid metadata for this output buffer
		 * and so that needs to be copied here.
		 */
		vbuf->flags &= ~V4L2_BUF_FLAG_TIMECODE;
		vbuf->field = b->field;
	} else {
		/* Zero any output buffer flags as this is a capture buffer */
		vbuf->flags &= ~V4L2_BUFFER_OUT_FLAGS;
	}

	return 0;
}

3.7 buffer_prepare函数

static int buffer_prepare(struct vb2_buffer *vb)
{
	struct vivi_dev *dev = vb2_get_drv_priv(vb->vb2_queue);
	struct vivi_buffer *buf = container_of(vb, struct vivi_buffer, vb);
	unsigned long size;
 
	BUG_ON(NULL == dev->fmt);
 
 	// 检查wh等数值
	/*
	 * Theses properties only change when queue is idle, see s_fmt.
	 * The below checks should not be performed here, on each
	 * buffer_prepare (i.e. on each qbuf). Most of the code in this function
	 * should thus be moved to buffer_init and s_fmt.
	 */
	if (dev->width  < 48 || dev->width  > MAX_WIDTH ||
	    dev->height < 32 || dev->height > MAX_HEIGHT)
		return -EINVAL;
 
	size = dev->width * dev->height * 2;
    /*
    *vb->planes[i].length == size
    * 这是之前reqbufs时候设置的
    */
	if (vb2_plane_size(vb, 0) < size) {
		dprintk(dev, 1, "%s data will not fit into plane (%lu < %lu)\n",
				__func__, vb2_plane_size(vb, 0), size);
		return -EINVAL;
	}
	   
    /*
     * vb2_set_plane_payload-> vb->planes[0].bytesused = size;
     * 终于找到bytesused设置的地方了
     * 这里只设置planes[0]，而不考虑planes[1]，是单平面的视频格式
     */
	vb2_set_plane_payload(&buf->vb, 0, size);
 
	buf->fmt = dev->fmt;  
	precalculate_bars(dev);
	precalculate_line(dev);
 
	return 0;
}

3.8 vb2_set_plane_payload函数

/**
 * vb2_set_plane_payload() - set bytesused for the plane plane_no
 * @vb:		buffer for which plane payload should be set
 * @plane_no:	plane number for which payload should be set
 * @size:	payload in bytes
 */
static inline void vb2_set_plane_payload(struct vb2_buffer *vb,
				 unsigned int plane_no, unsigned long size)
{
	if (plane_no < vb->num_planes)
		vb->planes[plane_no].bytesused = size;
}