Linux摄像头驱动学习之：（一）V4L2_框架分析

这段时间开始搞安卓camera底层驱动了，把以前的的Linux视频驱动回顾一下，本篇主要概述一下vfl2(video for linux 2).

一. V4L2框架: video for linux version 2
虚拟视频驱动vivi.c分析:
1．分配video_device
2．设置
3．注册：video_register_device

vivi_init
    vivi_create_instance
        v4l2_device_register   // 不是主要, 只是用于初始化一些东西，比如自旋锁、引用计数
        video_device_alloc
        // 设置
          1. vfd:
            .fops           = &vivi_fops,
            .ioctl_ops  = &vivi_ioctl_ops,
            .release = video_device_release,
          2.
            vfd->v4l2_dev = &dev->v4l2_dev;
          3. 设置"ctrl属性"(用于APP的ioctl)：
             v4l2_ctrl_handler_init(hdl, 11);
             dev->volume = v4l2_ctrl_new_std(hdl, &vivi_ctrl_ops,
               V4L2_CID_AUDIO_VOLUME, 0, 255, 1, 200);
             dev->brightness = v4l2_ctrl_new_std(hdl, &vivi_ctrl_ops,
               V4L2_CID_BRIGHTNESS, 0, 255, 1, 127);
             dev->contrast = v4l2_ctrl_new_std(hdl, &vivi_ctrl_ops,
               V4L2_CID_CONTRAST, 0, 255, 1, 16);                       
        video_register_device(video_device, type:VFL_TYPE_GRABBER, nr)
            __video_register_device
                vdev->cdev = cdev_alloc();
                vdev->cdev->ops = &v4l2_fops;
                cdev_add
               
                video_device[vdev->minor] = vdev;

if (vdev->ctrl_handler == NULL)
           vdev->ctrl_handler = vdev->v4l2_dev->ctrl_handler;

分析vivi.c的open,read,write,ioctl过程
1. open
app:     open("/dev/video0",....)
---------------------------------------------------
drv:     v4l2_fops.v4l2_open
            vdev = video_devdata(filp);  // 根据次设备号从数组中得到video_device
            ret = vdev->fops->open(filp);
                        vivi_ioctl_ops.open
                            v4l2_fh_open

2. read
app:    read ....
---------------------------------------------------
drv:    v4l2_fops.v4l2_read
            struct video_device *vdev = video_devdata(filp);
            ret = vdev->fops->read(filp, buf, sz, off);

3. ioctl
app:   ioctl
----------------------------------------------------
drv:   v4l2_fops.unlocked_ioctl
            v4l2_ioctl
                struct video_device *vdev = video_devdata(filp);
                ret = vdev->fops->unlocked_ioctl(filp, cmd, arg);
                            video_ioctl2
                                video_usercopy(file, cmd, arg, __video_do_ioctl);
                                    __video_do_ioctl
                                        struct video_device *vfd = video_devdata(file);
                                        根据APP传入的cmd来获得、设置"某些属性"v4l2_ctrl_handler的使用过程:
    __video_do_ioctl
        struct video_device *vfd = video_devdata(file);     case VIDIOC_QUERYCTRL:
     {
      struct v4l2_queryctrl *p = arg;
   
      if (vfh && vfh->ctrl_handler)
       ret = v4l2_queryctrl(vfh->ctrl_handler, p);
      else if (vfd->ctrl_handler)  // 在哪设置？在video_register_device
       ret = v4l2_queryctrl(vfd->ctrl_handler, p);
                   // 根据ID在ctrl_handler里找到v4l2_ctrl，返回它的值...}

[18721.327983] usb 2-1: USB disconnect, address 4
[18724.929301] usb 2-1: new full speed USB device using uhci_hcd and address 5
[18725.196200] usb 2-1: configuration #1 chosen from 1 choice
[18725.204999] uvcvideo: Found UVC 1.00 device <unnamed> (1b3b:2977)
[18725.222065] input: UVC Camera (1b3b:2977) as /devices/pci0000:00/0000:00:07.2/usb2/2-1/2-1:1.0/input/input7
[25252.578719] usb 2-1: USB disconnect, address 5

由以上usb摄像头插入的dmesg打印信息，搜索UVC关键字得到相关代码：uvc.driver.c ->uvc_driver结构体->video_register_device方法。

二、如何写v4l2驱动：

①构造、设置、注册： v4l2_device : v4l2_device_register, v4l2_device

②分配：video_device, video_device_alloc

③设置： a. vfd->v4l2_dev

　　　　　b. vfd: .fops = ***.open/read/write  <-上层app调用

　　　　　　　　  .ioctl_ops  <-驱动程序 v4l2_fops {open,read,write,"ioctl"}

　　　　   c.APP通过ioctl来设置，获得亮度 等信息。

驱动程序： v4l2_ctrl : 属性。

　　　　　　　v4l2_ctrl_handler : 管理 ===>（1）初始化:v4l2_ctrl_handler_init

　　　　　　　　　　　　　　　　　　　　  　　（2）设置:v4l2_ctrl_new_std , v4l2_ctrl_new_custom ,设置创建v4l2_ctrl并加入链表。

　　　　　　　　　　　　　　　　　　　　   　  （3）跟vdev关联: v4l2_dev.ctrl_handler = hdl

　　　　　　　　　　　　　　　　　　　　　　　　  video_dev -> v4l2_dev.

　　　　

具体驱动代码的编写将在下篇继续....