Parse the bootargs from the device tree (dtb format data)

1、函数调用关系

start_kernel
	setup_arch
		setup_machine_fdt
			early_init_dt_scan_nodes	//Traverse device tree node,Analyze the important information for the kernel boot
				of_scan_flat_dt		//Analytical device tree node
					early_init_dt_scan_chosen	//用于解析chosen节点的函数
					
	//函数调用
	of_scan_flat_dt(early_init_dt_scan_chosen, boot_command_line);

(1)调用of_scan_flat_dt()函数遍历dtb的所有节点;
(2)Each node will callearly_init_dt_scan_chosen()函数进行解析,判断是不是chosen节点;
(3)如果是chosen节点,Just resolutionbootargsProperties and save toboot_command_line变量中;

2、dtb中bootargs的来源

(1)内核启动参数bootargsStored in the device treechosen节点的bootargs属性;
(2)bootargs数据可以是在dts源文件中定义,也可以是ubootStart the kernel passed to the kernel;
(3)优先级：uboot传递的bootargsParameter priority is higher thandts中定义的bootargs;
(4)如果是uboot传递的bootargs,In the kernel decompression stage is calledatags_to_fdt()函数将tag中的bootargs参数转换成dtb的格式,写进dtb数据中;

3、of_scan_flat_dt()函数

int __init of_scan_flat_dt(int (*it)(unsigned long node,const char *uname, int depth,
				     void *data), void *data)
{
    
	//dtb数据的地址,Is the root node address
	const void *blob = initial_boot_params;
	
	const char *pathp;
	int offset, rc = 0, depth = -1;
	if (!blob)
		return 0;

	//从根节点遍历dtb中每个节点,返回的offsetIs each node address
	for (offset = fdt_next_node(blob, -1, &depth);
	     offset >= 0 && depth >= 0 && !rc;
	     offset = fdt_next_node(blob, offset, &depth)) {
    

		//Resolve the node name
		pathp = fdt_get_name(blob, offset, NULL);
		if (*pathp == '/')
			pathp = kbasename(pathp);

		//回调函数：解析节点
		rc = it(offset, pathp, depth, data);
	}
	return rc;
}

(1)The device tree ordtb格式的二进制数据,从dtbParsing nodes and attributes in a format offer proprietary function,Specific how to parse out the don't care;
(2)blob：dtbThe data start address,Is also the address of the root node;
(3)offset：Said the address of the node relative to the root node of the offset,Lies the node data address;
(4)depth:On behalf of the depth of the node relative to the root node,Such as the depth of the root node is0,/chosen节点是1;
(5)it：Is passed in the device tree node of analytic function,What need to parse the message is passed to the corresponding node analytical function;

early_init_dt_scan_chosen()函数

int __init early_init_dt_scan_chosen(unsigned long node, const char *uname,
				     int depth, void *data)
{
    
	int l;
	const char *p;

	pr_debug("search \"chosen\", depth: %d, uname: %s\n", depth, uname);

	//The depth of the node for1,Data does not makeNULL,At the same time, the node name is"chosen"或者 "[email protected]"
	if (depth != 1 || !data ||
	    (strcmp(uname, "chosen") != 0 && strcmp(uname, "[email protected]") != 0))
		return 0;

	//解析initrd相关
	early_init_dt_check_for_initrd(node);

	/* 从chosenNode of parsing outbootargs属性 */
	p = of_get_flat_dt_prop(node, "bootargs", &l);
	if (p != NULL && l > 0)
		strlcpy(data, p, min((int)l, COMMAND_LINE_SIZE));	//将bootargs拷贝到data中

	//The following code is fromchosenParsing of nodes are less thanbootargs,采用默认的bootargs
#ifdef CONFIG_CMDLINE
#if defined(CONFIG_CMDLINE_EXTEND)
	strlcat(data, " ", COMMAND_LINE_SIZE);
	strlcat(data, CONFIG_CMDLINE, COMMAND_LINE_SIZE);
#elif defined(CONFIG_CMDLINE_FORCE)
	strlcpy(data, CONFIG_CMDLINE, COMMAND_LINE_SIZE);
#else
	/* No arguments from boot loader, use kernel's cmdl*/
	if (!((char *)data)[0])
		strlcpy(data, CONFIG_CMDLINE, COMMAND_LINE_SIZE);
#endif
#endif /* CONFIG_CMDLINE */

	pr_debug("Command line is: %s\n", (char*)data);

	/* break now */
	return 1;
}

(1)Figure out whether incoming nodechosen节点,If is the parse outbootargs,并保存到data中(也就是boot_command_line);
(2)如果找不到chosen节点,或者chosen节点的bootargs属性是空,则使用默认的CONFIG_CMDLINE;