v4l2接收流程

内核media驱动目录结构

目录media/driver,子目录说明如下，主要列举本文中使用到的目录

目录	功能
I2C	摄像头，解串器（max9296/9295等）
platform	控制器的驱动，例如mipi控制等
v4l2_core	ioctl 入口等
media\common\videobuf2\	通用buffer分配及管理等

通过此目录结构，我们可以得知： 摄像头的控制路径很多是通过I2C通道。

用户态编程的几个关键IOCTL

VIDIOC_REQBUFS

功能： 向控制器驱动申请接收视频流的buffer个数。如下代码总共申请5个buffer，也就是最多缓存5帧数据。

req.count = 5;
	req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE;
	req.memory = V4L2_MEMORY_MMAP;
	if (ioctl(fd, VIDIOC_REQBUFS, &req) < 0) {
		printf("Reqbufs fail\n");
		goto err1;
	}

VIDIOC_QUERYBUF

功能：获取驱动分配的buffer信息，以便用户态进行mmap映射

	for(i = 0; i < req.count; i++) {
		memset(&buf, 0, sizeof(buf));
		planes_buffer = calloc(num_planes, sizeof(*planes_buffer));
		plane_start = calloc(num_planes, sizeof(*plane_start));
		memset(planes_buffer, 0, sizeof(*planes_buffer));
		buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE;
		buf.memory = V4L2_MEMORY_MMAP;
		buf.m.planes = planes_buffer;
		buf.length = num_planes;
		buf.index = i;
		if (-1 == ioctl (fd, VIDIOC_QUERYBUF, &buf)) {
			printf("Querybuf fail\n");
			req.count = i;
			goto err2;
		}

VIDIOC_DQBUF

功能：等待有视频数据的buf。可以是阻塞与非阻塞，后续在分析驱动时，着重介绍。

	memset(&buf, 0, sizeof(buf));
		buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE;
		buf.memory = V4L2_MEMORY_MMAP;
		buf.m.planes = tmp_plane;
		buf.length = num_planes;
		if (ioctl (fd, VIDIOC_DQBUF, &buf) < 0)
			printf("dqbuf fail\n");

VIDIOC_QBUF

功能：虽然如下两个功能最终操作一样，但应用的时间点不一样。

1）在接收完数据后，用户态应用将buf还给驱动，进而驱动可以继续往buffer里面填数据

if (ioctl (fd, VIDIOC_QBUF, &buf) < 0)
			printf("failture VIDIOC_QBUF\n");

2）在初始化时，将申请到的buffer挂载到驱动的队列上

for (i = 0; i < req.count; ++i) {
		memset(&buf, 0, sizeof(buf));

		buf.type        = V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE;
		buf.memory      = V4L2_MEMORY_MMAP;
		buf.length   	= num_planes;
		buf.index       = i;
		buf.m.planes 	= (buffers + i)->planes_buffer;

		if (ioctl (fd, VIDIOC_QBUF, &buf) < 0)
			printf ("VIDIOC_QBUF failed\n");
	}

对应代码入口

\kernel\drivers\media\v4l2-core\v4l2-ioctl.c

static const struct v4l2_ioctl_info v4l2_ioctls[] = {
	IOCTL_INFO(VIDIOC_QUERYCAP, v4l_querycap, v4l_print_querycap, 0),
	IOCTL_INFO(VIDIOC_ENUM_FMT, v4l_enum_fmt, v4l_print_fmtdesc, 0),
	IOCTL_INFO(VIDIOC_G_FMT, v4l_g_fmt, v4l_print_format, 0),
	IOCTL_INFO(VIDIOC_S_FMT, v4l_s_fmt, v4l_print_format, INFO_FL_PRIO),
	IOCTL_INFO(VIDIOC_REQBUFS, v4l_reqbufs, v4l_print_requestbuffers, INFO_FL_PRIO | INFO_FL_QUEUE),
	IOCTL_INFO(VIDIOC_QUERYBUF, v4l_querybuf, v4l_print_buffer, INFO_FL_QUEUE | INFO_FL_CLEAR(v4l2_buffer, length)),
	IOCTL_INFO(VIDIOC_G_FBUF, v4l_stub_g_fbuf, v4l_print_framebuffer, 0),
	IOCTL_INFO(VIDIOC_S_FBUF, v4l_stub_s_fbuf, v4l_print_framebuffer, INFO_FL_PRIO),
	IOCTL_INFO(VIDIOC_OVERLAY, v4l_overlay, v4l_print_u32, INFO_FL_PRIO),
	IOCTL_INFO(VIDIOC_QBUF, v4l_qbuf, v4l_print_buffer, INFO_FL_QUEUE),

总体而言：由于dma操作对物理地址的要求，buffer的分配由内核驱动完成。进而映射到用户空间，用户空间仅仅负责将数据取出，buffer的管理都有内核统一完成。

因而在内核中抽象出对buffer的统一管理。

buffer的管理

首先，从用户态入手，了解几个关键的数据结构。

struct v4l2_requestbuffers

req.count = 5;
	req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE;
	req.memory = V4L2_MEMORY_MMAP;

count：即申请5个buffer

type： MPLANE，V4L2_BUF_TYPE_VIDEO_CAPTURE 等，影响buffer的分布。

struct v4l2_buffer

	for(i = 0; i < req.count; i++) {
		memset(&buf, 0, sizeof(buf));
		planes_buffer = calloc(num_planes, sizeof(*planes_buffer));
		plane_start = calloc(num_planes, sizeof(*plane_start));
		memset(planes_buffer, 0, sizeof(*planes_buffer));
		buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE;
		buf.memory = V4L2_MEMORY_MMAP;
		buf.m.planes = planes_buffer;
		buf.length = num_planes;
		buf.index = i;
		if (-1 == ioctl (fd, VIDIOC_QUERYBUF, &buf)) {
			printf("Querybuf fail\n");
			req.count = i;
			goto err2;
		}

mplane之宏定义

在让驱动分配内存，已经应用向驱动获取内存信息时，都用到一个字段，即type：

此字段的取值包括：

\kernel\include\uapi\linux\videodev2.h

V4L2_BUF_TYPE_VIDEO_CAPTURE
V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE

const char *v4l2_type_names[] = {
	[0]				   = "0",
	[V4L2_BUF_TYPE_VIDEO_CAPTURE]      = "vid-cap",
	[V4L2_BUF_TYPE_VIDEO_OVERLAY]      = "vid-overlay",
	[V4L2_BUF_TYPE_VIDEO_OUTPUT]       = "vid-out",
	[V4L2_BUF_TYPE_VBI_CAPTURE]        = "vbi-cap",
	[V4L2_BUF_TYPE_VBI_OUTPUT]         = "vbi-out",
	[V4L2_BUF_TYPE_SLICED_VBI_CAPTURE] = "sliced-vbi-cap",
	[V4L2_BUF_TYPE_SLICED_VBI_OUTPUT]  = "sliced-vbi-out",
	[V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY] = "vid-out-overlay",
	[V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE] = "vid-cap-mplane",
	[V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE] = "vid-out-mplane",
	[V4L2_BUF_TYPE_SDR_CAPTURE]        = "sdr-cap",
	[V4L2_BUF_TYPE_SDR_OUTPUT]         = "sdr-out",
	[V4L2_BUF_TYPE_META_CAPTURE]       = "meta-cap",
	[V4L2_BUF_TYPE_META_OUTPUT]	   = "meta-out",
};

此处我们关注两个video_capture

mplane之定义

1）何为mplane？

所谓plane，即为一个分量所占的内存空间。mplane即多个plane表示一帧图像。如下图Y U V分布占用一个plane。

如下，Y与UV分别占用一个plane

常见的NV12 NV16等都属于此范畴。

2）与之相对应的 Interleaved，即一块内存即为完整的一帧图像

例如： YUYV ;YVYU等采用此方式。

了解了上述结构后，不同的存储形式对内存分配的要求不同。如果采用plane方式，则可能需要分配2-3块内存。而如果采用interlaced 方式，则只分配一块内存即可。

内存分配

内存分配入口

\kernel\drivers\media\v4l2-core\v4l2-ioctl.c

static int v4l_reqbufs(const struct v4l2_ioctl_ops *ops,
				struct file *file, void *fh, void *arg)
{
	struct v4l2_requestbuffers *p = arg;
	int ret = check_fmt(file, p->type);

	if (ret)
		return ret;

	CLEAR_AFTER_FIELD(p, capabilities);

	return ops->vidioc_reqbufs(file, fh, p);
}

驱动层接口

kernel\drivers\media\platform\rockchip\cif\capture.c

驱动层最终调用了buff的通用管理接口

static const struct v4l2_ioctl_ops rkcif_v4l2_ioctl_ops = {
	.vidioc_reqbufs = vb2_ioctl_reqbufs,
	.vidioc_querybuf = vb2_ioctl_querybuf,
	.vidioc_create_bufs = vb2_ioctl_create_bufs,
	.vidioc_qbuf = vb2_ioctl_qbuf,
	.vidioc_expbuf = vb2_ioctl_expbuf,
	.vidioc_dqbuf = vb2_ioctl_dqbuf,
	.vidioc_prepare_buf = vb2_ioctl_prepare_buf,
	.vidioc_streamon = vb2_ioctl_streamon,
	.vidioc_streamoff = vb2_ioctl_streamoff,
	.vidioc_enum_input = rkcif_enum_input,
	.vidioc_try_fmt_vid_cap_mplane = rkcif_try_fmt_vid_cap_mplane,
	.vidioc_enum_fmt_vid_cap = rkcif_enum_fmt_vid_cap_mplane,
	.vidioc_s_fmt_vid_cap_mplane = rkcif_s_fmt_vid_cap_mplane,
	.vidioc_g_fmt_vid_cap_mplane = rkcif_g_fmt_vid_cap_mplane,
	.vidioc_querycap = rkcif_querycap,
	.vidioc_s_selection = rkcif_s_selection,
	.vidioc_g_selection = rkcif_g_selection,
	.vidioc_enum_frameintervals = rkcif_enum_frameintervals,
	.vidioc_enum_framesizes = rkcif_enum_framesizes,
	.vidioc_default = rkcif_ioctl_default,
};

通用分配接口

两套入口

kernel\drivers\media\v4l2-core\videobuf-core.c

int videobuf_reqbufs(struct videobuf_queue *q,
		 struct v4l2_requestbuffers *req)
{
  q->ops->buf_setup(q, &count, &size);
  .......
  ......
  retval = __videobuf_mmap_setup(q, count, size, req->memory);

}

第二套为我们所关心

\kernel\drivers\media\common\videobuf2\videobuf2-v4l2.c

首先查询，然后分配

int vb2_ioctl_reqbufs(struct file *file, void *priv,
			  struct v4l2_requestbuffers *p)
{

int vb2_core_reqbufs(struct vb2_queue *q, enum vb2_memory memory,
		     unsigned int *count)
{

   /*
	 * Ask the driver how many buffers and planes per buffer it requires.
	 * Driver also sets the size and allocator context for each plane.
     *  获取驱动buffer的数目以及每个bufffer里面plane的数目
	 */
	ret = call_qop(q, queue_setup, q, &num_buffers, &num_planes,
		       plane_sizes, q->alloc_devs);


}

驱动层提供plane信息

在通用buffer代码分配内存前，需要了解分配几个内存区域。而这由控制器驱动具体提供。

以 3588 为例：

static struct vb2_ops rkcif_vb2_ops = {
	.queue_setup = rkcif_queue_setup,
	.buf_queue = rkcif_buf_queue,
	.wait_prepare = vb2_ops_wait_prepare,
	.wait_finish = vb2_ops_wait_finish,
	.stop_streaming = rkcif_stop_streaming,
	.start_streaming = rkcif_start_streaming,
};

q->ops = &rkcif_vb2_ops;

//输出mplanes数目，而mplanes数目
static int rkcif_queue_setup(struct vb2_queue *queue,
			     unsigned int *num_buffers,
			     unsigned int *num_planes,
			     unsigned int sizes[],
			     struct device *alloc_ctxs[])
{
	struct rkcif_stream *stream = queue->drv_priv;
	struct rkcif_extend_info *extend_line = &stream->extend_line;
	struct rkcif_device *dev = stream->cifdev;
	const struct v4l2_pix_format_mplane *pixm = NULL;
	const struct cif_output_fmt *cif_fmt;
	const struct cif_input_fmt *in_fmt;
	bool is_extended = false;
	u32 i, height;

	pixm = &stream->pixm;
	cif_fmt = stream->cif_fmt_out;
	in_fmt = stream->cif_fmt_in;

	*num_planes = cif_fmt->mplanes;

//具体说明根据类型不同而定，例如
static const struct cif_output_fmt out_fmts[] = {
	{
		.fourcc = V4L2_PIX_FMT_NV16,
		.cplanes = 2,
		.mplanes = 1,
		.fmt_val = YUV_OUTPUT_422 | UV_STORAGE_ORDER_UVUV,
		.bpp = { 8, 16 },
		.csi_fmt_val = CSI_WRDDR_TYPE_YUV422,
		.fmt_type = CIF_FMT_TYPE_YUV,
	},

核心分配buffer接口

nel\drivers\media\common\videobuf2\videobuf2-core.c


/*
 * __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);

	for (buffer = 0; buffer < num_buffers; ++buffer) {
		/* Allocate videobuf buffer structures */
		vb = kzalloc(q->buf_struct_size, GFP_KERNEL);
		if (!vb) {
			dprintk(q, 1, "memory alloc for buffer struct failed\n");
			break;
		}

		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;
		/*
		 * We need to set these flags here so that the videobuf2 core
		 * will call ->prepare()/->finish() cache sync/flush on vb2
		 * buffers when appropriate. However, we can avoid explicit
		 * ->prepare() and ->finish() cache sync for DMABUF buffers,
		 * because DMA exporter takes care of it.
		 */
		if (q->memory != VB2_MEMORY_DMABUF) {
			vb->need_cache_sync_on_prepare = 1;
			vb->need_cache_sync_on_finish = 1;
		}
		for (plane = 0; plane < num_planes; ++plane) {
			vb->planes[plane].length = plane_sizes[plane];
			vb->planes[plane].min_length = plane_sizes[plane];
		}
		call_void_bufop(q, init_buffer, vb);

		q->bufs[vb->index] = vb;

		/* Allocate video buffer memory for the MMAP type */
        //  这里即采用我们的分配接口
		if (memory == VB2_MEMORY_MMAP) {
			ret = __vb2_buf_mem_alloc(vb);
			if (ret) {
				dprintk(q, 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(q, 1, "buffer %d %p initialization failed\n",
					buffer, vb);
				__vb2_buf_mem_free(vb);
				q->bufs[vb->index] = NULL;
				kfree(vb);
				break;
			}
		}
	}

	dprintk(q, 3, "allocated %d buffers, %d plane(s) each\n",
		buffer, num_planes);

	return buffer;
}


/*
 * __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) {
		/* Memops alloc requires size to be page aligned. */
		unsigned long size = PAGE_ALIGN(vb->planes[plane].length);

		/* Did it wrap around? */
		if (size < vb->planes[plane].length)
			goto free;
        // 此处调用实际的内存分配接口
		mem_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;
}

驱动提供的内存分配接口

kernel\drivers\media\platform\rockchip\cif\capture.c

\kernel\drivers\media\platform\rockchip\cif\hw.c

static int rkcif_init_vb2_queue(struct vb2_queue *q,
				struct rkcif_stream *stream,
				enum v4l2_buf_type buf_type)
{
	struct rkcif_hw *hw_dev = stream->cifdev->hw_dev;

	q->type = buf_type;
	q->io_modes = VB2_MMAP | VB2_DMABUF;
	q->drv_priv = stream;
	q->ops = &rkcif_vb2_ops;
	q->mem_ops = hw_dev->mem_ops;

//驱动探测时，赋值mem_ops
static int rkcif_plat_hw_probe(struct platform_device *pdev)
{
	
	struct rkcif_hw *cif_hw;
	
	cif_hw->mem_ops = &vb2_cma_sg_memops;
    ............................
}

真正内存分配接口

\kernel\drivers\media\common\videobuf2\videobuf2-cma-sg.c

const struct vb2_mem_ops vb2_cma_sg_memops = {
	.alloc		= vb2_cma_sg_alloc,
	.put		= vb2_cma_sg_put,
	.get_userptr	= vb2_cma_sg_get_userptr,
	.put_userptr	= vb2_cma_sg_put_userptr,
	.prepare	= vb2_cma_sg_prepare,
	.finish		= vb2_cma_sg_finish,
	.vaddr		= vb2_cma_sg_vaddr,
	.mmap		= vb2_cma_sg_mmap,
	.num_users	= vb2_cma_sg_num_users,
	.get_dmabuf	= vb2_cma_sg_get_dmabuf,
	.map_dmabuf	= vb2_cma_sg_map_dmabuf,
	.unmap_dmabuf	= vb2_cma_sg_unmap_dmabuf,
	.attach_dmabuf	= vb2_cma_sg_attach_dmabuf,
	.detach_dmabuf	= vb2_cma_sg_detach_dmabuf,
	.cookie		= vb2_cma_sg_cookie,
};

接口流程综述

经过了core与具体设备的交替，其中驱动向core层注册的接口包括：

1）采用的buffer分配入口注册

2） buffer信息的注册。例如控制器的plane信息等

3) 实际的分配内存接口注册。

mmap映射

针对RK的芯片，其存在mplane与cplane，两者之间的关系是怎么样的？这里查询到的plane_num是多少？

	for(i = 0; i < req.count; i++) {
			memset(&buf, 0, sizeof(buf));
			planes_buffer = calloc(num_planes, sizeof(*planes_buffer));
			plane_start = calloc(num_planes, sizeof(*plane_start));
			memset(planes_buffer, 0, sizeof(*planes_buffer));
			buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE;
			buf.memory = V4L2_MEMORY_MMAP;
			buf.m.planes = planes_buffer;  //记录每个plane的长度，偏移量信息
			buf.length = num_planes;
			buf.index = i;
			if (-1 == ioctl (fd, VIDIOC_QUERYBUF, &buf)) {
				printf("Querybuf fail\n");
				req.count = i;
				goto err2;
			}
	
			(buffers + i)->planes_buffer = planes_buffer;
			(buffers + i)->plane_start = plane_start;
			for(j = 0; j < num_planes; j++) { //针对每个plane映射
				printf("plane[%d]: length = %d\n", j, (planes_buffer + j)->length);
				printf("plane[%d]: offset = %d\n", j, (planes_buffer + j)->m.mem_offset);
				(plane_start + j)->start = mmap (NULL /* start anywhere */,
										(planes_buffer + j)->length,
										PROT_READ | PROT_WRITE /* required */,
										MAP_SHARED /* recommended */,
										fd,
										(planes_buffer + j)->m.mem_offset);
				if (MAP_FAILED == (plane_start +j)->start) {
					printf ("mmap failed\n");
					req.count = i;
					goto unmmap;
				}
			}

sun4i-csi驱动

在明了内存分配后，通过一个驱动示例，了解其接收数据的流程。

问题：数据到来后，通过中断通知，用户通过ioctl dqbuf接口调用时，是否一直阻塞中？

该驱动结构及流程相对简单，但采用了V4L2的主要接口，利于分析。涉及到的文件

media\platform\sunxi\sun4i-csi

中断入口

请思考，此处csi->qlock保护的是什么？

static irqreturn_t sun4i_csi_irq(int irq, void *data)
{
	struct sun4i_csi *csi = data;
	u32 reg;

	reg = readl(csi->regs + CSI_INT_STA_REG);

	/* Acknowledge the interrupts */
	writel(reg, csi->regs + CSI_INT_STA_REG);

	if (!(reg & CSI_INT_FRM_DONE))
		return IRQ_HANDLED;

	spin_lock(&csi->qlock); //这里加锁的目的是什么？保护什么？
	if (sun4i_csi_buffer_flip(csi, csi->sequence++)) {
		dev_warn(csi->dev, "%s: Flip failed\n", __func__);
		sun4i_csi_capture_stop(csi);
	}
	spin_unlock(&csi->qlock);

	return IRQ_HANDLED;
}

static int sun4i_csi_buffer_flip(struct sun4i_csi *csi, unsigned int sequence)
{
	u32 reg = readl(csi->regs + CSI_BUF_CTRL_REG);
	unsigned int next;

	/* Our next buffer is not the current buffer */
	next = !(reg & CSI_BUF_CTRL_DBS);

	/* Report the previous buffer as done */
    //此处调用V4L2的接口，将带有视频帧的buffer通知用户
	sun4i_csi_buffer_mark_done(csi, next, sequence);
    //然后将新的buffer的地址 填写到CSI控制器中，这样控制器就可以采用新的内存
	/* Put a new buffer in there */
	return sun4i_csi_buffer_fill_slot(csi, next);
}

buffer done

此接口主要完成两个事情：

1）将buffer添加到 done_list 队列

2）唤醒等候在此工作队列的进程

void vb2_buffer_done(struct vb2_buffer *vb, enum vb2_buffer_state state)
{
    struct vb2_queue *q = vb->vb2_queue;
    /* Add the buffer to the done buffers list */
	spin_lock_irqsave(&q->done_lock, flags);	//锁控制对done链表
    list_add_tail(&vb->done_entry, &q->done_list);
    spin_unlock_irqrestore(&q->done_lock, flags);
    wake_up(&q->done_wq);
}

buffer 的申请

在一次buffer提交用户后，需要从驱动里面再申请一个buffer填写到控制器的相关寄存器。


static int sun4i_csi_buffer_fill_slot(struct sun4i_csi *csi, unsigned int slot)
{
	struct sun4i_csi_buffer *c_buf;
	struct vb2_v4l2_buffer *v_buf;
	unsigned int plane;

	/*
	 * We should never end up in a situation where we overwrite an
	 * already filled slot.
	 */
	if (WARN_ON(csi->current_buf[slot]))
		return -EINVAL;
    //如下buffer  list即为中断的锁所保护的对象。也就是锁加在此处即可。
    
	if (list_empty(&csi->buf_list))
		return sun4i_csi_setup_scratch_buffer(csi, slot);

	c_buf = list_first_entry(&csi->buf_list, struct sun4i_csi_buffer, list);
	list_del_init(&c_buf->list);

	v_buf = &c_buf->vb;
	csi->current_buf[slot] = v_buf;

	for (plane = 0; plane < csi->fmt.num_planes; plane++) {
		dma_addr_t buf_addr;

		buf_addr = vb2_dma_contig_plane_dma_addr(&v_buf->vb2_buf,
							 plane);
		writel(buf_addr, csi->regs + CSI_BUF_ADDR_REG(plane, slot));
	}

	return 0;
}

dqbuf的实现

流程走到此处已经需要查看被阻塞进程对收到的buffer的处理了。

根据上一章《内存分配》的代码流程，很容易得知dqbuf的代码实现。

\kernel\drivers\media\common\videobuf2\videobuf2-v4l2.c

int vb2_core_dqbuf(struct vb2_queue *q, unsigned int *pindex, void *pb,
		   bool nonblocking)
{
	struct vb2_buffer *vb = NULL;
	int ret;
    
	ret = __vb2_get_done_vb(q, &vb, pb, nonblocking);

    。。。。。。。。。。。。。。。。。
    。。。。。。。。。。。。。。。。。。
    call_void_vb_qop(vb, buf_finish, vb);

}



/*
 * __vb2_get_done_vb() - get a buffer ready for dequeuing
 *
 * Will sleep if required for nonblocking == false.
 */
static int __vb2_get_done_vb(struct vb2_queue *q, struct vb2_buffer **vb,
			     void *pb, int nonblocking)
{
	unsigned long flags;
	int ret = 0;

	/*
	 * Wait for at least one buffer to become available on the done_list.
	 */
     // 这个接口中的函数与中断处理中的wake up工作队列相对应，注意有nonblocking参数，
     // 也就是说支持nonblocking的操作
	ret = __vb2_wait_for_done_vb(q, nonblocking);
	if (ret)
		return ret;

	/*
	 * Driver's lock has been held since we last verified that done_list
	 * is not empty, so no need for another list_empty(done_list) check.
	 */
	spin_lock_irqsave(&q->done_lock, flags); //这里再次对done_lock进行了操作，此处即
    //和 中断处理中，buffer的添加对应了。这里为删除。
	*vb = list_first_entry(&q->done_list, struct vb2_buffer, done_entry);
	/*
	 * Only remove the buffer from done_list if all planes can be
	 * handled. Some cases such as V4L2 file I/O and DVB have pb
	 * == NULL; skip the check then as there's nothing to verify.
	 */
	if (pb)
		ret = call_bufop(q, verify_planes_array, *vb, pb);
	if (!ret)
		list_del(&(*vb)->done_entry);
	spin_unlock_irqrestore(&q->done_lock, flags);

	return ret;
}

//等待done_list  非空

/*
 * __vb2_wait_for_done_vb() - wait for a buffer to become available
 * for dequeuing
 *
 * Will sleep if required for nonblocking == false.
 */
static int __vb2_wait_for_done_vb(struct vb2_queue *q, int nonblocking)
{
	/*
	 * All operations on vb_done_list are performed under done_lock
	 * spinlock protection. However, buffers may be removed from
	 * it and returned to userspace only while holding both driver's
	 * lock and the done_lock spinlock. Thus we can be sure that as
	 * long as we hold the driver's lock, the list will remain not
	 * empty if list_empty() check succeeds.
	 */

	for (;;) { //死循环
        
		int ret;

		if (q->waiting_in_dqbuf) {
			dprintk(q, 1, "another dup()ped fd is waiting for a buffer\n");
			return -EBUSY;
		}

		if (!q->streaming) {
			dprintk(q, 1, "streaming off, will not wait for buffers\n");
			return -EINVAL;
		}

		if (q->error) {
			dprintk(q, 1, "Queue in error state, will not wait for buffers\n");
			return -EIO;
		}

		if (q->last_buffer_dequeued) {
			dprintk(q, 3, "last buffer dequeued already, will not wait for buffers\n");
			return -EPIPE;
		}

		if (!list_empty(&q->done_list)) { 
			/*
			 * Found a buffer that we were waiting for.
			 */
			break;
		}

		if (nonblocking) {
			dprintk(q, 3, "nonblocking and no buffers to dequeue, will not wait\n");
			return -EAGAIN;
		}

		q->waiting_in_dqbuf = 1;
		/*
		 * We are streaming and blocking, wait for another buffer to
		 * become ready or for streamoff. Driver's lock is released to
		 * allow streamoff or qbuf to be called while waiting.
		 */
		call_void_qop(q, wait_prepare, q); //休眠前释放锁vb2_queue->lock
          //为了保证qbuf接口和stream off接口被调用

		/*
		 * All locks have been released, it is safe to sleep now.
		 */
		dprintk(q, 3, "will sleep waiting for buffers\n");
        //将休眠在done_wq的队列上，和中断相对应
		ret = wait_event_interruptible(q->done_wq,
				!list_empty(&q->done_list) || !q->streaming ||
				q->error);

		/*
		 * We need to reevaluate both conditions again after reacquiring
		 * the locks or return an error if one occurred.
		 */
		call_void_qop(q, wait_finish, q);
		q->waiting_in_dqbuf = 0;
		if (ret) {
			dprintk(q, 1, "sleep was interrupted\n");
			return ret;
		}
	}
	return 0;
}

至此，带有一帧图像的buffer 被传递到用户APP中。

qbuf的实现

int vb2_core_qbuf(struct vb2_queue *q, unsigned int index, void *pb,
		  struct media_request *req)
{

     /*
	 * Add to the queued buffers list, a buffer will stay on it until
	 * dequeued in dqbuf.
	 */
     注意： 这里对queued_list的操作没有锁。那么buffer从list里面删除的时候和这个add的过程不冲突
     ，这个过程又在哪里呢？
	orig_state = vb->state;
	list_add_tail(&vb->queued_entry, &q->queued_list);
	q->queued_count++;
	q->waiting_for_buffers = false;
	vb->state = VB2_BUF_STATE_QUEUED;

    /*
	 * If already streaming, give the buffer to driver for processing.
	 * If not, the buffer will be given to driver on next streamon.
	 */
    这里将buffer 由具体的驱动管理。为何？
	if (q->start_streaming_called)
		__enqueue_in_driver(vb);


}

控制器驱动回收buffer

static void sun4i_csi_buffer_queue(struct vb2_buffer *vb)
{
	struct sun4i_csi *csi = vb2_get_drv_priv(vb->vb2_queue);
	struct sun4i_csi_buffer *buf = vb2_to_csi_buffer(vb);
	unsigned long flags;
    
    //此处由用户空间操作，进行buffer归还驱动，相关链表加锁
	spin_lock_irqsave(&csi->qlock, flags);  
	list_add_tail(&buf->list, &csi->buf_list);
	spin_unlock_irqrestore(&csi->qlock, flags);
}

至此，完成了视频数据接收过程。