Linux多媒体基础 - v4l2 vb2_queue的用法

V4L2 Framework - vb2_queue Initialization and Usage

Introduction

In the Linux kernel, the Video4Linux2 (V4L2) framework facilitates the exchange of video data between device drivers and applications. The vb2_queue structure is central to the Video Buffer 2 (vb2) API, which manages buffer operations.

Initialization of vb2_queue

1. Define vb2_queue Structure:
   Typically, define a vb2_queue variable within your device or driver structure.

2. Initialize vb2_queue:
   Use the vb2_queue_init function to initialize this structure. Provide a filled vb2_queue_init structure that includes callback functions and settings.

   struct vb2_queue q;
   struct vb2_ops ops = {
       .queue_setup            = my_queue_setup,
       .buf_prepare            = my_buf_prepare,
       .buf_queue              = my_buf_queue,
       .start_streaming        = my_start_streaming,
       .stop_streaming         = my_stop_streaming,
       .buf_finish             = my_buf_finish,
       .buf_cleanup            = my_buf_cleanup,
       .wait_prepare           = my_wait_prepare,
       .wait_finish            = my_wait_finish,
   };
   
   memset(&q, 0, sizeof(q));
   q.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
   q.io_modes = VB2_MMAP | VB2_USERPTR;
   q.ops = &ops;
   q.mem_ops = &vb2_vmalloc_memops;
   q.drv_priv = my_private_data;
   q.buf_struct_size = sizeof(my_buffer_struct);
   q.timestamp_flags = V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC;
   q.min_buffers_needed = 2;
   
   int ret = vb2_queue_init(&q);
   if (ret) {
       // Handle error
   }
   

Usage of vb2_queue

After initialization, the vb2_queue can be used for streaming operations:

• Buffer Enqueue and Dequeue:
  Applications request the kernel to enqueue and dequeue buffers via ioctl calls like VIDIOC_QBUF and VIDIOC_DQBUF.

• Stream Control:
  Streaming is typically started with VIDIOC_STREAMON and stopped with VIDIOC_STREAMOFF.

• Buffer Handling Callbacks:
  Callback functions specified in the vb2_ops structure are invoked at appropriate times, such as after buffer preparation, before enqueue, and after dequeue.

Considerations

• Memory Allocation:
  Choose the appropriate memory operations set, such as vb2_dma_contig_memops for contiguous physical memory or vb2_vmalloc_memops for virtual memory.

• Error Handling:
  Perform thorough error checking and handling during initialization and usage.

• Thread Safety in Multithreaded Environments:
  If your driver supports multithreading, ensure that driver locks are appropriately released and reacquired in the wait_prepare and wait_finish callbacks.