Question

如何查看内核 device tree

Answer 1

你好，查看方法如下：
在内核初始化的时候，dtb被转换成device_node的树状结构，以便后续操作。具体的代码流程如下：
start_kernel->setup_arch->unflatten_device_tree
void __init unflatten_device_tree(void)
{
__unflatten_device_tree(initial_boot_params, &of_allnodes,
early_init_dt_alloc_memory_arch);

/* Get pointer to "/chosen" and "/aliases" nodes for use everywhere */
of_alias_scan(early_init_dt_alloc_memory_arch);
}12345678

将dtb展开，并将其组成成一个树状结构，主要功能在__unflatten_device_tree函数中实现，具体代码如下：
static void __unflatten_device_tree(struct boot_param_header *blob, //dtb在内存中的虚拟地址
struct device_node **mynodes, //一个全局的指针，后续可以通过这个指针变量dtb所有的节点
void * (*dt_alloc)(u64 size, u64 align)) //内存分配回调函数
{
unsigned long size;
void *start, *mem;
struct device_node **allnextp = mynodes;

if (!blob) {
pr_debug("No device tree pointer\n");
return;
}
...
if (be32_to_cpu(blob->magic) != OF_DT_HEADER) {
pr_err("Invalid device tree blob header\n");
return;
}

/* First pass, scan for size */
start = ((void *)blob) + be32_to_cpu(blob->off_dt_struct);
size = (unsigned long)unflatten_dt_node(blob, 0, &start, NULL, NULL, 0);
size = ALIGN(size, 4);

pr_debug(" size is %lx, allocating...\n", size);

/* Allocate memory for the expanded device tree */
mem = dt_alloc(size + 4, __alignof__(struct device_node));
memset(mem, 0, size);

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

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

/* Second pass, do actual unflattening */
start = ((void *)blob) + be32_to_cpu(blob->off_dt_struct);
unflatten_dt_node(blob, mem, &start, NULL, &allnextp, 0); //重点在这里
if (be32_to_cpup(start) != OF_DT_END)
pr_warning("Weird tag at end of tree: %08x\n", be32_to_cpup(start));
if (be32_to_cpup(mem + size) != 0xdeadbeef)
pr_warning("End of tree marker overwritten: %08x\n",
be32_to_cpup(mem + size));
*allnextp = NULL;

pr_debug(" <- unflatten_device_tree()\n");
}
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unflatten_dt_node函数真正完成了解析dtb的任务，首先找到dtb的根节点，并创建一个struct device_node ，然后把这个根节点对应的struct device_node 赋值给all_nodes全局变量。再通过递归的方法遍历根节点下面的子节点，最终创建一个树结构。
unflatten_dt_node具体代码如下：
static void * unflatten_dt_node(struct boot_param_header *blob,
void *mem,
void **p,
struct device_node *dad,
struct device_node ***allnextpp,
unsigned long fpsize)
{
struct device_node *np;
struct property *pp, **prev_pp = NULL;
char *pathp;
u32 tag;
unsigned int l, allocl;
int has_name = 0;
int new_format = 0;

tag = be32_to_cpup(*p);
if (tag != OF_DT_BEGIN_NODE) {
pr_err("Weird tag at start of node: %x\n", tag);
return mem;
}
*p += 4;
pathp = *p;
l = allocl = strlen(pathp) + 1;
*p = PTR_ALIGN(*p + l, 4);

/* version 0x10 has a more compact unit name here instead of the full
* path. we accumulate the full path size using "fpsize", we'll rebuild
* it later. We detect this because the first character of the name is
* not '/'.
*/
if ((*pathp) != '/') {
new_format = 1;
if (fpsize == 0) {
/* root node: special case. fpsize accounts for path
* plus terminating zero. root node only has '/', so
* fpsize should be 2, but we want to avoid the first
* level nodes to have two '/' so we use fpsize 1 here
*/
fpsize = 1;
allocl = 2;
l = 1;
*pathp = '\0';
} else {
/* account for '/' and path size minus terminal 0
* already in 'l'
*/
fpsize += l;
allocl = fpsize;
}
}

np = unflatten_dt_alloc(&mem, sizeof(struct device_node) + allocl,
__alignof__(struct device_node));
if (allnextpp) {
char *fn;
np->full_name = fn = ((char *)np) + sizeof(*np);
if (new_format) {
/* rebuild full path for new format */
if (dad && dad->parent) {
strcpy(fn, dad->full_name);
#ifdef DEBUG
if ((strlen(fn) + l + 1) != allocl) {
pr_debug("%s: p: %d, l: %d, a: %d\n",
pathp, (int)strlen(fn),
l, allocl);
}
#endif
fn += strlen(fn);
}
*(fn++) = '/';
}
memcpy(fn, pathp, l);

prev_pp = &np->properties;
**allnextpp = np;
*allnextpp = &np->allnext;
if (dad != NULL) {
np->parent = dad;
/* we temporarily use the next field as `last_child'*/
if (dad->next == NULL)
dad->child = np;
else
dad->next->sibling = np;
dad->next = np;
}
kref_init(&np->kref);
}
/* process properties */
while (1) {
u32 sz, noff;
char *pname;

tag = be32_to_cpup(*p);
if (tag == OF_DT_NOP) {
*p += 4;
continue;
}
if (tag != OF_DT_PROP)
break;
*p += 4;
sz = be32_to_cpup(*p);
noff = be32_to_cpup(*p + 4);
*p += 8;
if (be32_to_cpu(blob->version) < 0x10)
*p = PTR_ALIGN(*p, sz >= 8 ? 8 : 4);

pname = of_fdt_get_string(blob, noff);
if (pname == NULL) {
pr_info("Can't find property name in list !\n");
break;
}
if (strcmp(pname, "name") == 0)
has_name = 1;
l = strlen(pname) + 1;
pp = unflatten_dt_alloc(&mem, sizeof(struct property),
__alignof__(struct property));
if (allnextpp) {
/* 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") == 0) ||
(strcmp(pname, "linux,phandle") == 0)) {
if (np->phandle == 0)
np->phandle = be32_to_cpup((__be32*)*p);
}
/* And we process the "ibm,phandle" property
* used in pSeries dynamic device tree
* stuff */
if (strcmp(pname, "ibm,phandle") == 0)
np->phandle = be32_to_cpup((__be32 *)*p);
pp->name = pname;
pp->length = sz;
pp->value = *p;
*prev_pp = pp;
prev_pp = &pp->next;
}
*p = PTR_ALIGN((*p) + sz, 4);
}
/* with version 0x10 we may not have the name property, recreate
* it here from the unit name if absent
*/
if (!has_name) {
char *p1 = pathp, *ps = pathp, *pa = NULL;
int sz;

while (*p1) {
if ((*p1) == '@')
pa = p1;
if ((*p1) == '/')
ps = p1 + 1;
p1++;
}
if (pa < ps)
pa = p1;
sz = (pa - ps) + 1;
pp = unflatten_dt_alloc(&mem, sizeof(struct property) + sz,
__alignof__(struct property));
if (allnextpp) {
pp->name = "name";
pp->length = sz;
pp->value = pp + 1;
*prev_pp = pp;
prev_pp = &pp->next;
memcpy(pp->value, ps, sz - 1);
((char *)pp->value)[sz - 1] = 0;
pr_debug("fixed up name for %s -> %s\n", pathp,
(char *)pp->value);
}
}
if (allnextpp) {
*prev_pp = NULL;
np->name = of_get_property(np, "name", NULL);
np->type = of_get_property(np, "device_type", NULL);

if (!np->name)
np->name = "<NULL>";
if (!np->type)
np->type = "<NULL>";
}
while (tag == OF_DT_BEGIN_NODE || tag == OF_DT_NOP) {
if (tag == OF_DT_NOP)
*p += 4;
else
mem = unflatten_dt_node(blob, mem, p, np, allnextpp,
fpsize);
tag = be32_to_cpup(*p);
}
if (tag != OF_DT_END_NODE) {
pr_err("Weird tag at end of node: %x\n", tag);
return mem;
}
*p += 4;
return mem;
}