Android项目总结复盘4

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UVC协议文档网址:https://www.usb.org/documents?search=&type%5B0%5D=55&items_per_page=50

主要下载USB Video Class 1_5,关注下载zip包中的UVC 1.5 Class specification.pdf文件,里面有接口相关的解释。

Android盒子控制摄像头项目已经差不多4年了,很多知识点已经忘记,现在重新温固一遍,下面两篇文章可以帮助回顾。

https://my.oschina.net/u/2007478/blog/968470

https://blog.csdn.net/go_str/article/details/80844175

下边两个网址中可以找到代码中各种结构体的解释:

https://www.kernel.org/doc/html/v4.13/media/v4l-drivers/uvcvideo.html

https://www.linuxtv.org/downloads/legacy/video4linux/API/V4L2_API/spec-single/v4l2.html

1、Linux kernel下的UVC

我们先从Android官网git clone一下kernel的源码:

https://android.googlesource.com/kernel/goldfish/

git clone https://android.googlesource.com/kernel/goldfish

clone到本地之后就可以通过Source Insight查看源码了。查看源码之前先project -> rebuild project,这样代码中各对象之间可以点击跳转。

1.1 初始化

goldfish\drivers\media\usb\uvc\uvc_driver.c

static int __init uvc_init(void)
{
	int ret;
	ret = usb_register(&uvc_driver.driver);
	return 0;
}

struct uvc_driver uvc_driver = {
	.driver = {
		.name		= "uvcvideo",
		.probe		= uvc_probe,
		.disconnect	= uvc_disconnect,
		.suspend	= uvc_suspend,
		.resume		= uvc_resume,
		.reset_resume	= uvc_reset_resume,
		.id_table	= uvc_ids,
		.supports_autosuspend = 1,
	},
};

在入口函数uvc_init中,核心的一行是usb_register,也就是注册USB设备,在注册完成之后会调用uvc_probe函数。

goldfish\include\linux\usb.h

struct usb_driver {
	const char *name;

	int (*probe) (struct usb_interface *intf,
		      const struct usb_device_id *id);

看看这个uvc_probe函数:

goldfish\drivers\media\usb\uvc\uvc_driver.c

static int uvc_probe(struct usb_interface *intf,
		     const struct usb_device_id *id)
{
	struct usb_device *udev = interface_to_usbdev(intf);
	struct uvc_device *dev;
	int ret;

	if (id->idVendor && id->idProduct)
		uvc_trace(UVC_TRACE_PROBE, "Probing known UVC device %s "
				"(%04x:%04x)\n", udev->devpath, id->idVendor,
				id->idProduct);
	else
		uvc_trace(UVC_TRACE_PROBE, "Probing generic UVC device %s\n",
				udev->devpath);
    
    uvc_parse_control(dev);//1
    v4l2_device_register(&intf->dev, &dev->vdev);//2
    uvc_ctrl_init_device(dev);//3
    uvc_scan_device(dev);
    uvc_register_chains(dev);
    usb_set_intfdata(intf, dev);
    ret = uvc_status_init(dev);
    usb_enable_autosuspend(udev);
}
  • 每一个摄像头设备在底层初始化完成之后,都会有一个vendorId和productId。
  • uvc_parse_control会根据设备的vendorId和productId去对特定厂商的摄像头做一些适配。
  • v4l2_device_register,该方法将设备注册到v4l2,v4l2是Video for linux2的简称,为linux中关于视频设备的内核驱动。该方法在goldfish\drivers\media\v4l2-core\v4l2-device.c中。
  • uvc_ctrl_init_device,初始化设备控制。

1.2 初始化设备控制

这里是我们需要重点关注的。可以先跟踪一下这个调用栈。

goldfish\drivers\media\usb\uvc\uvc_ctrl.c

int uvc_ctrl_init_device(struct uvc_device *dev)
{
    list_for_each_entry(entity, &dev->entities, list) {
		struct uvc_control *ctrl;
		unsigned int bControlSize = 0, ncontrols;
		__u8 *bmControls = NULL;
		//第一部分
		if (UVC_ENTITY_TYPE(entity) == UVC_VC_EXTENSION_UNIT) {
			bmControls = entity->extension.bmControls;
			bControlSize = entity->extension.bControlSize;
		} else if (UVC_ENTITY_TYPE(entity) == UVC_VC_PROCESSING_UNIT) {
			bmControls = entity->processing.bmControls;
			bControlSize = entity->processing.bControlSize;
		} else if (UVC_ENTITY_TYPE(entity) == UVC_ITT_CAMERA) {
			bmControls = entity->camera.bmControls;
			bControlSize = entity->camera.bControlSize;
		}
		//第二部分
		/* Initialize all supported controls */
		ctrl = entity->controls;
		for (i = 0; i < bControlSize * 8; ++i) {
			if (uvc_test_bit(bmControls, i) == 0)
				continue;

			ctrl->entity = entity;
			ctrl->index = i;

			uvc_ctrl_init_ctrl(dev, ctrl);
			ctrl++;
		}
	}
}

1.2.1 ENTITY类型过滤

用于区分终端类型,重点关注UVC_ITT_CAMERA类型,看看这个类型在UVC协议文档里面的定义:

摄像头传感器,仅用于描述摄像头终端。那么代码里面的描述是:

goldfish\include\uapi\linux\usb\video.h

/* B.2. Input Terminal Types */
#define UVC_ITT_VENDOR_SPECIFIC				0x0200
#define UVC_ITT_CAMERA					0x0201
#define UVC_ITT_MEDIA_TRANSPORT_INPUT			0x0202

1.2.2 初始化uvc_control

从第一部分中取出camera.bmControlscamera.bControlSize,这两个变量是干嘛的呢,还是看协议文档:

bControlSize是对应着bmControls的位大小; bmControls对应着摄像头支持的控制参数,如果控制参数位置为1,表示支持该控制。

1.2.2.1 方法uvc_ctrl_init_ctrl

接下来调用uvc_ctrl_init_ctrl方法:

goldfish\drivers\media\usb\uvc\uvc_ctrl.c

static void uvc_ctrl_init_ctrl(struct uvc_device *dev, struct uvc_control *ctrl)
{
	const struct uvc_control_info *info = uvc_ctrls;
	const struct uvc_control_info *iend = info + ARRAY_SIZE(uvc_ctrls);
	const struct uvc_control_mapping *mapping = uvc_ctrl_mappings;
	const struct uvc_control_mapping *mend = mapping + ARRAY_SIZE(uvc_ctrl_mappings);
	
	for (; info < iend; ++info) {
		if (uvc_entity_match_guid(ctrl->entity, info->entity) &&
		    ctrl->index == info->index) {
			uvc_ctrl_add_info(dev, ctrl, info);
			break;
		 }
	}

	if (!ctrl->initialized)
		return;

	for (; mapping < mend; ++mapping) {
		if (uvc_entity_match_guid(ctrl->entity, mapping->entity) &&
		    ctrl->info.selector == mapping->selector)
			__uvc_ctrl_add_mapping(dev, ctrl, mapping);
	}
}
1.2.2.2 结构体uvc_control_info
  • uvc_ctrls,这个结构体的类型是uvc_control_info,是一个静态数组。可以理解为一个实体类别下对应着多个控制功能,每个功能有对应着不同的操作方式,以当前项目需要用到的功能举例:
static struct uvc_control_info uvc_ctrls[] = {
	{
		.entity		= UVC_GUID_UVC_CAMERA,
		.selector	= UVC_CT_PANTILT_ABSOLUTE_CONTROL,
		.index		= 11,
		.size		= 8,
		.flags		= UVC_CTRL_FLAG_SET_CUR
				| UVC_CTRL_FLAG_GET_RANGE
				| UVC_CTRL_FLAG_RESTORE
				| UVC_CTRL_FLAG_AUTO_UPDATE,
	},
	{
		.entity		= UVC_GUID_UVC_CAMERA,
		.selector	= UVC_CT_PANTILT_RELATIVE_CONTROL,
		.index		= 12,
		.size		= 4,
		.flags		= UVC_CTRL_FLAG_SET_CUR | UVC_CTRL_FLAG_GET_MIN
				| UVC_CTRL_FLAG_GET_MAX | UVC_CTRL_FLAG_GET_RES
				| UVC_CTRL_FLAG_GET_DEF
				| UVC_CTRL_FLAG_AUTO_UPDATE,
	},
}
  • entity 实体是一种类型,UVC_GUID_UVC_CAMERA只是其中一种,还有UVC_GUID_UVC_PROCESSING等。

  • selector 对应的是实体下的一种功能,比如相对绝对转动。

  • index 对应着在uvc_ctrls中的序号。

    /* Bit index in bmControls */

  • size 对应着具体操作位的长度,比如上边列举出来的UVC_CT_PANTILT_ABSOLUTE_CONTROLUVC_CT_PANTILT_RELATIVE_CONTROL控制,看看在协议文档中的定义:

对于PanTilt Absolute来说,高四位代表着左右的角度,低四位代表着上下的角度,都是有符号整数,总共八位,所以size为8。

对于PanTilt Relative来说,总共四位,每一位代表不同的控制属性,第一位表示左右相对;第二位表示左右控制的速度;第三位表示上下相对;第四位表示上下的速度。所以size为4。

  • flags 表示对于这些selector支持的功能操作: 详细的解释如下:
1.2.2.3 结构体uvc_control_mapping

uvc_ctrl_mappingsuvc_control_mapping类型的结构体变量,也是一个静态的结构体,简略看下里面定义了啥:

static struct uvc_control_mapping uvc_ctrl_mappings[] = {
    {
		.id		= V4L2_CID_PAN_ABSOLUTE,
		.name		= "Pan (Absolute)",
		.entity		= UVC_GUID_UVC_CAMERA,
		.selector	= UVC_CT_PANTILT_ABSOLUTE_CONTROL,
		.size		= 32,
		.offset		= 0,
		.v4l2_type	= V4L2_CTRL_TYPE_INTEGER,
		.data_type	= UVC_CTRL_DATA_TYPE_UNSIGNED,
	},
	{
		.id		= V4L2_CID_TILT_ABSOLUTE,
		.name		= "Tilt (Absolute)",
		.entity		= UVC_GUID_UVC_CAMERA,
		.selector	= UVC_CT_PANTILT_ABSOLUTE_CONTROL,
		.size		= 32,
		.offset		= 32,
		.v4l2_type	= V4L2_CTRL_TYPE_INTEGER,
		.data_type	= UVC_CTRL_DATA_TYPE_UNSIGNED,
	},
}

可以看到这里的id开头都是V4L2,而entityselector都对应着uvc_control_info uvc_ctrls中定义的entityselector

另外v4l2_type对应着设置的数据类型,data_type则定义了数据为有符号还是无符号。

这个结构体从我的理解来看,就是将UVC定义的控制,映射到v4l2,并建立两者之间的关系。

1.2.2.4 方法uvc_ctrl_add_info

这个方法核心就一行代码:

goldfish\drivers\media\usb\uvc\uvc_ctrl.c

static int uvc_ctrl_add_info(struct uvc_device *dev, struct uvc_control *ctrl,
	const struct uvc_control_info *info)
{
    ctrl->info = *info;
}

将上一步uvc_control_info变量给到uvc_control的info。uvc_control的定义如下:

goldfish\drivers\media\usb\uvc\uvcvideo.h

struct uvc_control {
	struct uvc_entity *entity;
	struct uvc_control_info info;

	__u8 index;	/* Used to match the uvc_control entry with a
			   uvc_control_info. */
	__u8 dirty:1,
	     loaded:1,
	     modified:1,
	     cached:1,
	     initialized:1;

	__u8 *uvc_data;
};
1.2.2.5 方法__uvc_ctrl_add_mapping

同样的,将uvc_control_mapping数据赋值到uvc_control对象中:

goldfish\drivers\media\usb\uvc\uvc_ctrl.c

static int __uvc_ctrl_add_mapping(struct uvc_device *dev,
	struct uvc_control *ctrl, const struct uvc_control_mapping *mapping)
{
	struct uvc_control_mapping *map;
	map = kmemdup(mapping, sizeof(*mapping), GFP_KERNEL);
	map->menu_info = kmemdup(mapping->menu_info, size, GFP_KERNEL);
	list_add_tail(&map->list, &ctrl->info.mappings);
}

uvc_control_infomappings作为链表头,将map->list添加到后面。

1.3 总结

在初始化的过程中可以将UVC协议的文档跟代码建立联系,以帮助理解代码的逻辑。待理解了各种数据类型定义的原理及流程之后,发现其实现了UVC与V4L2的连接,这样下一步的工作就比较好开展了。

UVC初始化的部分到这里告一段落,接下来要根据具体需求做一些定制的工作。