V4L2深入理解

1. VIDIOC_REQBUFS: 请求Kernel分配Video Buffer

其申请流程如下图所示：

1.1  Kernel态相关数据结构

struct uvc_fh {struct uvc_video_chain *chain;struct uvc_streaming *stream;  //Stream--->enum uvc_handle_state state;};struct uvc_streaming {struct list_head list;struct uvc_device *dev;struct video_device *vdev;struct uvc_video_chain *chain;atomic_t active;struct usb_interface *intf;int intfnum;__u16 maxpsize;struct uvc_streaming_header header;enum v4l2_buf_type type;unsigned int nformats;struct uvc_format *format;struct uvc_streaming_control ctrl;struct uvc_format *cur_format;struct uvc_frame *cur_frame;/* Protect access to ctrl, cur_format, cur_frame and hardware video * probe control. */struct mutex mutex;unsigned int frozen : 1;struct uvc_video_queue queue; // UVC Video Queue--->void (*decode) (struct urb *urb, struct uvc_streaming *video,struct uvc_buffer *buf);/* Context data used by the bulk completion handler. */struct {__u8 header[256];unsigned int header_size;int skip_payload;__u32 payload_size;__u32 max_payload_size;} bulk;struct urb *urb[UVC_URBS];char *urb_buffer[UVC_URBS];dma_addr_t urb_dma[UVC_URBS];unsigned int urb_size;__u32 sequence;__u8 last_fid;    struct tasklet_struct *tasklet[UVC_URBS];     /* [e-mail] */};struct uvc_video_queue {enum v4l2_buf_type type;void *mem;             // 已经分配的连续虚拟内存的首地址unsigned int flags;unsigned int count;    // 已分配的buffer个数unsigned int buf_size; // 每个buffer的大小unsigned int buf_used;struct uvc_buffer buffer[UVC_MAX_VIDEO_BUFFERS];  // UVC buffer--->struct mutex mutex;/* protects buffers and mainqueue */spinlock_t irqlock;/* protects irqqueue */    wait_queue_head_t wait; /* wait if mainqueue is empty */struct list_head mainqueue;struct list_head irqqueue;    };struct uvc_buffer {unsigned long vma_use_count;struct list_head stream;/* Touched by interrupt handler. */struct v4l2_buffer buf;     // v4l2_buffer --->struct list_head queue;wait_queue_head_t wait;     // 初始化等待队列enum uvc_buffer_state state;unsigned int error;};struct v4l2_buffer {__u32index;    //buffer索引enum v4l2_buf_type      type;     //如V4L2_BUF_TYPE_VIDEO_CAPTURE__u32bytesused;__u32flags;enum v4l2_fieldfield;    // V4L2_FIELD_NONEstruct timevaltimestamp; struct v4l2_timecodetimecode;__u32sequence;/* memory location */enum v4l2_memory        memory;   // V4L2_MEMORY_MMAPunion {__u32           offset;   //在已经分配的大块内存中的偏移量,                                          //其首地址保存在uvc_video_queue->mem中unsigned long   userptr;struct v4l2_plane *planes;} m;__u32length;   //申请的内存大小__u32input;__u32reserved;};

1.2 uvc_alloc_buffers实现代码

/* * Allocate the video buffers. * * Pages are reserved to make sure they will not be swapped, as they will be * filled in the URB completion handler. * * Buffers will be individually mapped, so they must all be page aligned. */int uvc_alloc_buffers(struct uvc_video_queue *queue, unsigned int nbuffers,unsigned int buflength){unsigned int bufsize = PAGE_ALIGN(buflength);unsigned int i;void *mem = NULL;int ret;if (nbuffers > UVC_MAX_VIDEO_BUFFERS)nbuffers = UVC_MAX_VIDEO_BUFFERS;mutex_lock(&queue->mutex);if ((ret = __uvc_free_buffers(queue)) < 0)goto done;/* Bail out if no buffers should be allocated. */if (nbuffers == 0)goto done;/* Decrement the number of buffers until allocation succeeds. */for (; nbuffers > 0; --nbuffers) {mem = vmalloc_32(nbuffers * bufsize);if (mem != NULL)break;}if (mem == NULL) {ret = -ENOMEM;goto done;}for (i = 0; i < nbuffers; ++i) {memset(&queue->buffer[i], 0, sizeof queue->buffer[i]);queue->buffer[i].buf.index = i;queue->buffer[i].buf.m.offset = i * bufsize;queue->buffer[i].buf.length = buflength;queue->buffer[i].buf.type = queue->type;queue->buffer[i].buf.field = V4L2_FIELD_NONE;queue->buffer[i].buf.memory = V4L2_MEMORY_MMAP;queue->buffer[i].buf.flags = 0;init_waitqueue_head(&queue->buffer[i].wait);}queue->mem = mem;queue->count = nbuffers;queue->buf_size = bufsize;ret = nbuffers;done:mutex_unlock(&queue->mutex);return ret;}

 2. VIDIOC_QUERYBUF： 把Kernel分配的内存映射到用户空间

 

 

 3. VIDIOC_QBUF: 把uvc_buffer放入队列中

 

 

/* * Queue a video buffer. Attempting to queue a buffer that has already been * queued will return -EINVAL. */int uvc_queue_buffer(struct uvc_video_queue *queue,struct v4l2_buffer *v4l2_buf){struct uvc_buffer *buf;unsigned long flags;int ret = 0;uvc_trace(UVC_TRACE_CAPTURE, "Queuing buffer %u.\n", v4l2_buf->index);if (v4l2_buf->type != queue->type ||    v4l2_buf->memory != V4L2_MEMORY_MMAP) {uvc_trace(UVC_TRACE_CAPTURE, "[E] Invalid buffer type (%u) ""and/or memory (%u).\n", v4l2_buf->type,v4l2_buf->memory);return -EINVAL;}mutex_lock(&queue->mutex);if (v4l2_buf->index >= queue->count) {uvc_trace(UVC_TRACE_CAPTURE, "[E] Out of range index.\n");ret = -EINVAL;goto done;}buf = &queue->buffer[v4l2_buf->index];if (buf->state != UVC_BUF_STATE_IDLE) {uvc_trace(UVC_TRACE_CAPTURE, "[E] Invalid buffer state ""(%u).\n", buf->state);ret = -EINVAL;goto done;}if (v4l2_buf->type == V4L2_BUF_TYPE_VIDEO_OUTPUT &&    v4l2_buf->bytesused > buf->buf.length) {uvc_trace(UVC_TRACE_CAPTURE, "[E] Bytes used out of bounds.\n");ret = -EINVAL;goto done;}spin_lock_irqsave(&queue->irqlock, flags);if (queue->flags & UVC_QUEUE_DISCONNECTED) {spin_unlock_irqrestore(&queue->irqlock, flags);ret = -ENODEV;goto done;}buf->state = UVC_BUF_STATE_QUEUED;if (v4l2_buf->type == V4L2_BUF_TYPE_VIDEO_CAPTURE)buf->buf.bytesused = 0;elsebuf->buf.bytesused = v4l2_buf->bytesused;list_add_tail(&buf->stream, &queue->mainqueue);list_add_tail(&buf->queue, &queue->irqqueue);spin_unlock_irqrestore(&queue->irqlock, flags);done:mutex_unlock(&queue->mutex);return ret;}

 4. VIDIOC_STREAMON： 做好准备工作并提交URB请求

 

/* * Initialize bulk URBs and allocate transfer buffers. The packet size is * given by the endpoint. */static int uvc_init_video_bulk(struct uvc_streaming *stream,struct usb_host_endpoint *ep, gfp_t gfp_flags){struct urb *urb;unsigned int npackets, pipe, i;u16 psize;u32 size;            // 获取每个包的大小psize = le16_to_cpu(ep->desc.wMaxPacketSize) & 0x07ff;// 一次可传输的最大负荷传输大小size = stream->ctrl.dwMaxPayloadTransferSize;stream->bulk.max_payload_size = size;    /*    分配置urb传输buffer,返回包个数(size/psize),它指每个urb    包含多少个包; 为每个stream->urb_buffer分配DMA buffer,如下：    for (i = 0; i < UVC_URBS; ++i) {        stream->urb_size = psize * npackets; // urb_buffer的大小        //分配DMA内存，并把地址保存在urb_buffer[i]和urb_dma[i]中，        //同一块内存，不同的表示方法        //1) stream->urb_buffer[i] = offset + page->vaddr;        //2) stream->urb_dma[i] = offset + page->dma;        stream->urb_buffer[i] = usb_alloc_coherent(                     stream->dev->udev, stream->urb_size,                     gfp_flags | __GFP_NOWARN, &stream->urb_dma[i]);        if (!stream->urb_buffer[i]) {            uvc_free_urb_buffers(stream);            break;        }    }    */npackets = uvc_alloc_urb_buffers(stream, size, psize, gfp_flags);if (npackets == 0)return -ENOMEM;size = npackets * psize;if (usb_endpoint_dir_in(&ep->desc))pipe = usb_rcvbulkpipe(stream->dev->udev,       ep->desc.bEndpointAddress);elsepipe = usb_sndbulkpipe(stream->dev->udev,       ep->desc.bEndpointAddress);if (stream->type == V4L2_BUF_TYPE_VIDEO_OUTPUT)size = 0;for (i = 0; i < UVC_URBS; ++i) {        // 创建一个 urburb = usb_alloc_urb(0, gfp_flags);if (urb == NULL) {uvc_uninit_video(stream, 1);return -ENOMEM;}        /* 填充urb参数struct urb {void *transfer_buffer;// (in) associated data bufferdma_addr_t transfer_dma;// (in) dma addr for transfer_bufferusb_complete_t complete;// (in) completion routinestruct usb_iso_packet_descriptor iso_frame_desc[0];        // (in) ISO ONLY}*/usb_fill_bulk_urb(urb, stream->dev->udev, pipe,                          stream->urb_buffer[i],  //传输buffer                          size,        //传输buffer的大小                          uvc_video_complete, //URB请求完成之后的callback                          stream);urb->transfer_flags = URB_NO_TRANSFER_DMA_MAP;urb->transfer_dma = stream->urb_dma[i]; //给urb->transfer_dma赋值// 把此urb保存到stream->urb[i]中stream->urb[i] = urb;}return 0;}

5. urb数据解析 (uvc_video_complete)

       当URB请求(usb_submit_urb)完成之后，它将调用其回调函数(uvc_video_complete)，下面分析此回调函数到底做了些什么。即如何把transfer_buffer或transfer_dma中数据转换为应用程序需要的v4l2_buffer中的数据。

 5.1 uvc_video_complete

static void uvc_video_complete(struct urb *urb){    struct uvc_streaming *stream = urb->context;struct uvc_video_queue *queue = &stream->queue;struct uvc_buffer *buf = NULL;unsigned long flags;int ret;switch (urb->status) {case 0:break;default:uvc_printk(KERN_WARNING, "Non-zero status (%d) in video ""completion handler.\n", urb->status);case -ENOENT:/* usb_kill_urb() called. */if (stream->frozen)return;case -ECONNRESET:/* usb_unlink_urb() called. */case -ESHUTDOWN:/* The endpoint is being disabled. */uvc_queue_cancel(queue, urb->status == -ESHUTDOWN);return;} spin_lock_irqsave(&queue->irqlock, flags);if (!list_empty(&queue->irqqueue))buf = list_first_entry(&queue->irqqueue, struct uvc_buffer,       queue);//从queue->irqqueue中取出一个空的uvc_bufferspin_unlock_irqrestore(&queue->irqlock, flags);        //把urb中的数据转换为uvc_buffer中的数据，并设置对应的状态stream->decode(urb, stream, buf);if ((ret = usb_submit_urb(urb, GFP_ATOMIC)) < 0) {uvc_printk(KERN_ERR, "Failed to resubmit video URB (%d).\n",ret);}}

5.2  stream->decode是什么?

        现在关键是stream->decode到底做了些什么？它也是一个回调函数，首先要搞明白它是一个什么函数，其注册过程如下图所示：

5.3 uvc_video_decode_bulk

 

 

5.4 uvc_video_decode_isoc

static void uvc_video_decode_isoc(struct urb *urb, struct uvc_streaming *stream,struct uvc_buffer *buf){u8 *mem;int ret, i;for (i = 0; i < urb->number_of_packets; ++i) {if (urb->iso_frame_desc[i].status < 0) {uvc_trace(UVC_TRACE_FRAME, "USB isochronous frame ""lost (%d).\n", urb->iso_frame_desc[i].status);  /* Mark the buffer as faulty. */if (buf != NULL)buf->error = 1;continue;}/* Decode the payload header. */mem = urb->transfer_buffer + urb->iso_frame_desc[i].offset;do {ret = uvc_video_decode_start(stream, buf, mem,urb->iso_frame_desc[i].actual_length);if (ret == -EAGAIN)buf = uvc_queue_next_buffer(&stream->queue,    buf);} while (ret == -EAGAIN);if (ret < 0)continue;/* Decode the payload data. */uvc_video_decode_data(stream, buf, mem + ret,urb->iso_frame_desc[i].actual_length - ret);/* Process the header again. */uvc_video_decode_end(stream, buf, mem,urb->iso_frame_desc[i].actual_length);if (buf->state == UVC_BUF_STATE_READY) {if (buf->buf.length != buf->buf.bytesused &&    !(stream->cur_format->flags &      UVC_FMT_FLAG_COMPRESSED))buf->error = 1;buf = uvc_queue_next_buffer(&stream->queue, buf);}}}

 

6. VIDIOC_DQBUF: 获取视频数据

 

  7. CameraHAL工作流程