kernel如何根据dtb文件生成device tree

device tree

dtb文件中的内容会被内核组成了device tree，整个tree上由两个数据结构组成：struct device_node和struct property。

struct device_node {
	const char *name;
	phandle phandle;
	const char *full_name;
	struct fwnode_handle fwnode;

	struct	property *properties;
	struct	property *deadprops;	/* removed properties */
	struct	device_node *parent;
	struct	device_node *child;
	struct	device_node *sibling;
#if defined(CONFIG_OF_KOBJ)
	struct	kobject kobj;
#endif
	unsigned long _flags;
	void	*data;
#if defined(CONFIG_SPARC)
	unsigned int unique_id;
	struct of_irq_controller *irq_trans;
#endif
};

struct property {
	char	*name;
	int	length;
	void	*value;
	struct property *next;
#if defined(CONFIG_OF_DYNAMIC) || defined(CONFIG_SPARC)
	unsigned long _flags;
#endif
#if defined(CONFIG_OF_PROMTREE)
	unsigned int unique_id;
#endif
#if defined(CONFIG_OF_KOBJ)
	struct bin_attribute attr;
#endif
};

以platform driver注册时为例，也需要找到同一bus下所有的device，依次对比device所对应的device_node中的property信息（一般对比compatible，type以及name信息，参照__of_device_is_compatible函数）。

dtb文件内容

那么dtb文件是如何被组织的呢？其实，主要由三部分组成：struct fdt_header、struct fdt_node_header以及struct fdt_property。

struct fdt_header {
	fdt32_t magic;			 /* magic word FDT_MAGIC */
	fdt32_t totalsize;		 /* total size of DT block */
	fdt32_t off_dt_struct;		 /* offset to structure */
	fdt32_t off_dt_strings;		 /* offset to strings */
	fdt32_t off_mem_rsvmap;		 /* offset to memory reserve map */
	fdt32_t version;		 /* format version */
	fdt32_t last_comp_version;	 /* last compatible version */

	/* version 2 fields below */
	fdt32_t boot_cpuid_phys;	 /* Which physical CPU id we're
					    booting on */
	/* version 3 fields below */
	fdt32_t size_dt_strings;	 /* size of the strings block */

	/* version 17 fields below */
	fdt32_t size_dt_struct;		 /* size of the structure block */
};

struct fdt_node_header {
	fdt32_t tag;
	char name[];
};

struct fdt_property {
	fdt32_t tag;
	fdt32_t len;
	fdt32_t nameoff;
	char data[];
};

整个dtb文件简单可以分为3大区域（不考虑reserve）：header区域、struct区域以及strings区域。

header区域即dtb开头的区域，存放struct fdt_header的数据。

struct区域存放node以及property的节点信息。

strings区域存放property的name信息。

（struct区域因为即存在node，也存在property，因此需要一些tag来区分）。

// node的开始tag
#define FDT_BEGIN_NODE	0x1		/* Start node: full name */
// node的结束tag
#define FDT_END_NODE	0x2		/* End node */

// property的开始tag，因为有len，所以不需要结束tag
#define FDT_PROP	0x3		/* Property: name off,
					   size, content */
#define FDT_NOP		0x4		/* nop */
#define FDT_END		0x9

访问dtb文件的基础API

tag通常情况下为node或property的开头，或者在读完node或者property之后（需要对齐）的位置上。

1. fdt_next_tag：返回当前offset的tag值，并修改nextoffset为下一个tag所在位置：

uint32_t fdt_next_tag(const void *fdt, int offset, int *nextoffset);

2， fdt_next_node：返回下一个node所在位置（如果没有下一个node，则*depth为-1）

int fdt_next_node(const void *fdt, int offset, int *depth)

1. nextprop_：常用于查找node的下一个property，如果下一个tag不是property，返回负值。

static int nextprop_(const void *fdt, int offset)

dtb → device tree

以riscv架构为例，dtb文件的解析发生在setup_arch中：

void __init setup_arch(char **cmdline_p)
{
	parse_dtb();
	setup_initial_init_mm(_stext, _etext, _edata, _end);

	*cmdline_p = boot_command_line;

	early_ioremap_setup();
	sbi_init();
	jump_label_init();
	parse_early_param();

	efi_init();
	paging_init();

	/* Parse the ACPI tables for possible boot-time configuration */
	acpi_boot_table_init();

#if IS_ENABLED(CONFIG_BUILTIN_DTB)
	unflatten_and_copy_device_tree();
#else
	unflatten_device_tree();
#endif
	misc_mem_init();

	init_resources();

#ifdef CONFIG_KASAN
	kasan_init();
#endif

#ifdef CONFIG_SMP
	setup_smp();
#endif

	if (!acpi_disabled)
		acpi_init_rintc_map();

	riscv_init_cbo_blocksizes();
	riscv_fill_hwcap();
	init_rt_signal_env();
	apply_boot_alternatives();
	if (IS_ENABLED(CONFIG_RISCV_ISA_ZICBOM) &&
	    riscv_isa_extension_available(NULL, ZICBOM))
		riscv_noncoherent_supported();
}

与dtb相关的是parse_dtb()函数以及unflatten_device_tree()（这里分析CONFIG_BUILTIN_DTB没有ENABLE的情况）。

parse_dtb

static void __init parse_dtb(void)
{
	/* Early scan of device tree from init memory */
	if (early_init_dt_scan(dtb_early_va)) {
		const char *name = of_flat_dt_get_machine_name();

		if (name) {
			pr_info("Machine model: %s\n", name);
			dump_stack_set_arch_desc("%s (DT)", name);
		}
	} else {
		pr_err("No DTB passed to the kernel\n");
	}

#ifdef CONFIG_CMDLINE_FORCE
	strscpy(boot_command_line, CONFIG_CMDLINE, COMMAND_LINE_SIZE);
	pr_info("Forcing kernel command line to: %s\n", boot_command_line);
#endif
}

这里重点是两个函数：early_init_dt_scan以及of_flat_dt_get_machine_name。

early_init_dt_scan函数做一些早期的检查，check_sum（校验）→{size, address}-cells→系统启动参数→将memory节点加入memblock中。

of_flat_get_machine_name去查找了根节点的model属性（调用了前面提到nextprop_接口，实际常用方式为nextprop_的上层封装函数fdt_first_property_offset以及fdt_next_property_offset）。

unflatten_device_tree

void __init unflatten_device_tree(void)
{
	__unflatten_device_tree(initial_boot_params, NULL, &of_root,
				early_init_dt_alloc_memory_arch, false);

	/* Get pointer to "/chosen" and "/aliases" nodes for use everywhere */
	of_alias_scan(early_init_dt_alloc_memory_arch);

	unittest_unflatten_overlay_base();
}

核心函数是__unflatten_device_tree，这个函数用于构建device tree。

此时：initial_boot_params为dtb首地址，of_root要生成的设备树根节点，early_init_dt_alloc_memory_arch封装了memblock_alloc（为memblock的内存分配函数）。

__unflatten_device_tree：

void *__unflatten_device_tree(const void *blob,
			      struct device_node *dad,
			      struct device_node **mynodes,
			      void *(*dt_alloc)(u64 size, u64 align),
			      bool detached)
{
	int size;
	void *mem;
	int ret;

	if (mynodes)
		*mynodes = NULL;

	pr_debug(" -> unflatten_device_tree()\n");

	if (!blob) {
		pr_debug("No device tree pointer\n");
		return NULL;
	}

	pr_debug("Unflattening device tree:\n");
	pr_debug("magic: %08x\n", fdt_magic(blob));
	pr_debug("size: %08x\n", fdt_totalsize(blob));
	pr_debug("version: %08x\n", fdt_version(blob));

	if (fdt_check_header(blob)) {
		pr_err("Invalid device tree blob header\n");
		return NULL;
	}

	/* First pass, scan for size */
	size = unflatten_dt_nodes(blob, NULL, dad, NULL);
	if (size <= 0)
		return NULL;

	size = ALIGN(size, 4);
	pr_debug("  size is %d, allocating...\n", size);

	/* Allocate memory for the expanded device tree */
	mem = dt_alloc(size + 4, __alignof__(struct device_node));
	if (!mem)
		return NULL;

	memset(mem, 0, size);

	*(__be32 *)(mem + size) = cpu_to_be32(0xdeadbeef);

	pr_debug("  unflattening %p...\n", mem);

	/* Second pass, do actual unflattening */
	ret = unflatten_dt_nodes(blob, mem, dad, mynodes);

	if (be32_to_cpup(mem + size) != 0xdeadbeef)
		pr_warn("End of tree marker overwritten: %08x\n",
			be32_to_cpup(mem + size));

	if (ret <= 0)
		return NULL;

	if (detached && mynodes && *mynodes) {
		of_node_set_flag(*mynodes, OF_DETACHED);
		pr_debug("unflattened tree is detached\n");
	}

	pr_debug(" <- unflatten_device_tree()\n");
	return mem;
}

依据pr_debug以及注释可以看到这个函数的流程很简单，核心部分在于两次unflatten_dt_nodes的调用。第一次调用计算出了需要分配的内存大小（使用局部变量base=mem，第一次调用增加mem，之后返回mem - base），分配完内存后，第二次调用填充数据（此时参数带了申请的mem）。

unflatten_dt_nodes：

static int unflatten_dt_nodes(const void *blob,
			      void *mem,
			      struct device_node *dad,
			      struct device_node **nodepp)
{
	struct device_node *root;
	int offset = 0, depth = 0, initial_depth = 0;
#define FDT_MAX_DEPTH	64
	struct device_node *nps[FDT_MAX_DEPTH];

	// 如果mem存在，则dryrun为false
	// 若mem不存在，则dryrun为true
	void *base = mem;
	bool dryrun = !base;
	int ret;

	if (nodepp)
		*nodepp = NULL;

	/*
	 * We're unflattening device sub-tree if @dad is valid. There are
	 * possibly multiple nodes in the first level of depth. We need
	 * set @depth to 1 to make fdt_next_node() happy as it bails
	 * immediately when negative @depth is found. Otherwise, the device
	 * nodes except the first one won't be unflattened successfully.
	 */
	if (dad)
		depth = initial_depth = 1;

	root = dad;
	nps[depth] = dad;

	// 遍历所有的node
	for (offset = 0;
	     offset >= 0 && depth >= initial_depth;
	     offset = fdt_next_node(blob, offset, &depth)) {
		if (WARN_ON_ONCE(depth >= FDT_MAX_DEPTH - 1))
			continue;

		if (!IS_ENABLED(CONFIG_OF_KOBJ) &&
		    !of_fdt_device_is_available(blob, offset))
			continue;

		// 核心函数populate_node
		ret = populate_node(blob, offset, &mem, nps[depth],
				   &nps[depth+1], dryrun);
		if (ret < 0)
			return ret;

		if (!dryrun && nodepp && !*nodepp)
			*nodepp = nps[depth+1];
		if (!dryrun && !root)
			root = nps[depth+1];
	}

	if (offset < 0 && offset != -FDT_ERR_NOTFOUND) {
		pr_err("Error %d processing FDT\n", offset);
		return -EINVAL;
	}

	/*
	 * Reverse the child list. Some drivers assumes node order matches .dts
	 * node order
	 */
	if (!dryrun)
		reverse_nodes(root);

	return mem - base;
}

在unflatten_dt_nodes函数中，遍历所有的node，在node的遍历过程中，调用populate_node函数。

static int populate_node(const void *blob,
			  int offset,
			  void **mem,
			  struct device_node *dad,
			  struct device_node **pnp,
			  bool dryrun)
{
	struct device_node *np;
	const char *pathp;
	int len;

	pathp = fdt_get_name(blob, offset, &len);
	if (!pathp) {
		*pnp = NULL;
		return len;
	}

	len++;

	np = unflatten_dt_alloc(mem, sizeof(struct device_node) + len,
				__alignof__(struct device_node));
	if (!dryrun) {
		char *fn;
		of_node_init(np);
		np->full_name = fn = ((char *)np) + sizeof(*np);

		memcpy(fn, pathp, len);

		if (dad != NULL) {
			np->parent = dad;
			np->sibling = dad->child;
			dad->child = np;
		}
	}

	populate_properties(blob, offset, mem, np, pathp, dryrun);
	if (!dryrun) {
		np->name = of_get_property(np, "name", NULL);
		if (!np->name)
			np->name = "<NULL>";
	}

	*pnp = np;
	return 0;
}

在使用unflatten_dt_alloc函数分配完device_node的内存之后，调用populate_properties函数分配device_node管理的所有property。

static void populate_properties(const void *blob,
				int offset,
				void **mem,
				struct device_node *np,
				const char *nodename,
				bool dryrun)
{
	struct property *pp, **pprev = NULL;
	int cur;
	bool has_name = false;

	pprev = &np->properties;
	for (cur = fdt_first_property_offset(blob, offset);
	     cur >= 0;
	     cur = fdt_next_property_offset(blob, cur)) {
		const __be32 *val;
		const char *pname;
		u32 sz;

		val = fdt_getprop_by_offset(blob, cur, &pname, &sz);
		if (!val) {
			pr_warn("Cannot locate property at 0x%x\n", cur);
			continue;
		}

		if (!pname) {
			pr_warn("Cannot find property name at 0x%x\n", cur);
			continue;
		}

		if (!strcmp(pname, "name"))
			has_name = true;

		pp = unflatten_dt_alloc(mem, sizeof(struct property),
					__alignof__(struct property));
		if (dryrun)
			continue;

		/* We accept flattened tree phandles either in
		 * ePAPR-style "phandle" properties, or the
		 * legacy "linux,phandle" properties.  If both
		 * appear and have different values, things
		 * will get weird. Don't do that.
		 */
		if (!strcmp(pname, "phandle") ||
		    !strcmp(pname, "linux,phandle")) {
			if (!np->phandle)
				np->phandle = be32_to_cpup(val);
		}

		/* And we process the "ibm,phandle" property
		 * used in pSeries dynamic device tree
		 * stuff
		 */
		if (!strcmp(pname, "ibm,phandle"))
			np->phandle = be32_to_cpup(val);

		pp->name   = (char *)pname;
		pp->length = sz;
		pp->value  = (__be32 *)val;
		*pprev     = pp;
		pprev      = &pp->next;
	}

	/* With version 0x10 we may not have the name property,
	 * recreate it here from the unit name if absent
	 */
	if (!has_name) {
		const char *p = nodename, *ps = p, *pa = NULL;
		int len;

		while (*p) {
			if ((*p) == '@')
				pa = p;
			else if ((*p) == '/')
				ps = p + 1;
			p++;
		}

		if (pa < ps)
			pa = p;
		len = (pa - ps) + 1;
		pp = unflatten_dt_alloc(mem, sizeof(struct property) + len,
					__alignof__(struct property));
		if (!dryrun) {
			pp->name   = "name";
			pp->length = len;
			pp->value  = pp + 1;
			*pprev     = pp;
			memcpy(pp->value, ps, len - 1);
			((char *)pp->value)[len - 1] = 0;
			pr_debug("fixed up name for %s -> %s\n",
				 nodename, (char *)pp->value);
		}
	}
}

这里主要是使用fdt_first_property_offset函数和fdt_next_property_offset函数对node的所有properties进行遍历并进行内容填充，注意如果没有name这个properties，则会手动创建。