Linux DeviceTree学习（三）
Linux DeviceTree学习（三）
4 Linux Kernel匹配设备树
4.1 Kernel使能device tree
Boot options --->
[*] Flattened Device Tree support
(0) Compressed ROM boot loader base address
(0) Compressed ROM boot loader BSS address
[ ] Use appended device tree blob to zImage (EXPERIMENTAL)
4.2 Kernel 解析Device Tree
Kernel会根据Device Tree的struct fdt_header、struct fdt_node_header及struct fdt_property三个结构体的信息填充至Kernel能够使用的struct property结构体，该结构体描述如下：
struct property {
char *name; /* property full name */
int length; /* property value length */
void *value; /* property value */
struct property *next; /* next property under the same node */
unsigned long _flags;
unsigned int unique_id;
struct bin_attribute attr; /* 属性文件，与sysfs文件系统挂接 */
};
4.2.1 Kernel解析流程
kernel的C语言阶段的入口函数是init/main.c/stsrt_kernel()函数，在early_init_dt_scan_nodes()中会做以下三件事：
-
(1) 扫描/chosen或者/chose@0节点下面的bootargs属性值到boot_command_line，此外，还处理initrd相关的property，并保存在initrd_start和initrd_end这两个全局变量中；
-
(2) 扫描根节点下面，获取{size,address}-cells信息，并保存在dt_root_size_cells和dt_root_addr_cells全局变量中；
-
(3) 扫描具有device_type = “memory”属性的/memory或者/memory@0节点下面的reg属性值，并把相关信息保存在meminfo中，全局变量meminfo保存了系统内存相关的信息。
-
Device Tree中的每一个node节点经过kernel处理都会生成一个struct device_node的结构体，struct device_node最终一般会被挂接到具体的struct device结构体。struct device_node结构体描述如下：
struct device_node { const char *name; /* node的名称，取最后一次“/”和“@”之间子串 */ const char *type; /* device_type的属性名称，没有为*/ phandle phandle; /* phandle属性值 */ /* 指向该结构体结束的位置，存放node的路径全名，例如：/chosen */ const char *full_name; struct fwnode_handle fwnode; /* 指向该节点下的第一个属性，其他属性与该属性链表相接 */ struct property *properties; struct property *deadprops; /* removed properties */ struct device_node *parent; /* 父节点 */ struct device_node *child; /* 子节点 */ struct device_node *sibling; /* 姊妹节点，与自己同等级的node */ struct kobject kobj; /* sysfs文件系统目录体现 */ unsigned long _flags; /* 当前node状态标志位，见/include/linux/of.h line124-127 */ void *data; };
4.2.2 setup_machine_fdt( )
在Linux4.14.14内核中，首先使用setup_machine_fdt来setup machine描述符，如果返回NULL，才使用传统的方法setup_machine_tags来setup machine描述符。传统的方法需要给出__machine_arch_type（bootloader通过r1寄存器传递给kernel的）和tag list的地址（用来进行tag parse）。__machine_arch_type用来寻找machine描述符；tag list用于运行时参数的传递。
/**
* setup_machine_fdt - Machine setup when an dtb was passed to the kernel
* @dt_phys: physical address of dt blob
*
* If a dtb was passed to the kernel in r2, then use it to choose the
* correct machine_desc and to setup the system.
*/
const struct machine_desc * __init setup_machine_fdt(unsigned int dt_phys)
{
const struct machine_desc *mdesc, *mdesc_best = NULL;
#if defined(CONFIG_ARCH_MULTIPLATFORM) || defined(CONFIG_ARM_SINGLE_ARMV7M)
DT_MACHINE_START(GENERIC_DT, "Generic DT based system")
.l2c_aux_val = 0x0,
.l2c_aux_mask = ~0x0,
MACHINE_END
mdesc_best = &__mach_desc_GENERIC_DT;
#endif
if (!dt_phys || !early_init_dt_verify(phys_to_virt(dt_phys)))
return NULL;
//获取machine描述符
mdesc = of_flat_dt_match_machine(mdesc_best, arch_get_next_mach);
if (!mdesc) {
const char *prop;
int size;
unsigned long dt_root;
early_print("\nError: unrecognized/unsupported "
"device tree compatible list:\n[ ");
dt_root = of_get_flat_dt_root();
prop = of_get_flat_dt_prop(dt_root, "compatible", &size);
while (size > 0) {
early_print("'%s' ", prop);
size -= strlen(prop) + 1;
prop += strlen(prop) + 1;
}
early_print("]\n\n");
dump_machine_table(); /* does not return */
}
/* We really don't want to do this, but sometimes firmware provides buggy data */
if (mdesc->dt_fixup)
mdesc->dt_fixup();
//两个功能：为后续的DTB Scan进行准备工作，获取运行时参数bootarg传递
early_init_dt_scan_nodes();
/* Change machine number to match the mdesc we're using */
__machine_arch_type = mdesc->nr;
return mdesc;
}
4.2.3 early_init_dt_scan_nodes( )
运行时参数bootarg是在扫描DTB的chosen node时候完成的，具体的工作就是获取chosen node的bootargs、initrd等属性的value，并将其保存到全局变量（boot_command_line、initrd_start、initrd_end）中。
void __init early_init_dt_scan_nodes(void)
{
/* 扫描 /chosen node，保存运行时参数（bootargs）
到boot_command_line， 此外，还处理initrd相关的property，
并保存在initrd_start和initrd_end这两个全局变量中 */
of_scan_flat_dt(early_init_dt_scan_chosen, boot_command_line);
/* 扫描根节点，获取 {size,address}-cells信息，
并保存在dt_root_size_cells和dt_root_addr_cells全局变量中 */
of_scan_flat_dt(early_init_dt_scan_root, NULL);
/* 扫描DTB中的memory node，并把相关信息保存在meminfo中，
全局变量meminfo保存了系统内存相关的信息。*/
of_scan_flat_dt(early_init_dt_scan_memory, NULL);
}
4.2.4 unflatten_device_tree( )
Device Tree的解析首先从unflatten_device_tree()开始，代码列出如下：
/**
* unflatten_device_tree - create tree of device_nodes from flat blob
*
* unflattens the device-tree passed by the firmware, creating the
* tree of struct device_node. It also fills the "name" and "type"
* pointers of the nodes so the normal device-tree walking functions
* can be used.
*/
void __init unflatten_device_tree(void)
{
__unflatten_device_tree(initial_boot_params, &of_root,
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);
}
- 在unflatten_device_tree()中，调用函数__unflatten_device_tree()，参数initial_boot_params指向Device Tree在内存中的首地址，of_root在经过该函数处理之后，会指向根节点，early_init_dt_alloc_memory_arch是一个函数指针，为struct device_node和struct property结构体分配内存的回调函数（callback）。
4.2.5 __unflatten_device_tree( )
/**
* __unflatten_device_tree - create tree of device_nodes from flat blob
*
* unflattens a device-tree, creating the
* tree of struct device_node. It also fills the "name" and "type"
* pointers of the nodes so the normal device-tree walking functions
* can be used.
* @blob: The blob to expand
* @mynodes: The device_node tree created by the call
* @dt_alloc: An allocator that provides a virtual address to memory
* for the resulting tree
*/
static void __unflatten_device_tree(const void *blob, //需要扫描的DTB
struct device_node **mynodes, //global list指针
void * (*dt_alloc)(u64 size, u64 align)) //内存分配函数
{
unsigned long size;
int start;
void *mem;
/* 省略部分不重要部分 */
/* First pass, scan for size */
start = 0;
size = (unsigned long)unflatten_dt_node(blob, NULL, &start, NULL, NULL, 0, true);
size = ALIGN(size, 4);
/* Allocate memory for the expanded device tree */
mem = dt_alloc(size + 4, __alignof__(struct device_node));
memset(mem, 0, size);
/* Second pass, do actual unflattening */
start = 0;
unflatten_dt_node(blob, mem, &start, NULL, mynodes, 0, false);
}
- 在__unflatten_device_tree()函数中，两次调用unflatten_dt_node()函数，第一次是为了得到Device Tree转换成struct device_node和struct property结构体需要分配的内存大小，第二次调用才是具体填充每一个struct device_node和struct property结构体。
4.2.6 unflatten_dt_node( )
/**
* unflatten_dt_node - Alloc and populate a device_node from the flat tree
* @blob: The parent device tree blob
* @mem: Memory chunk to use for allocating device nodes and properties
* @poffset: pointer to node in flat tree
* @dad: Parent struct device_node
* @nodepp: The device_node tree created by the call
* @fpsize: Size of the node path up at the current depth.
* @dryrun: If true, do not allocate device nodes but still calculate needed
* memory size
*/
static void * unflatten_dt_node(const void *blob,
void *mem,
int *poffset,
struct device_node *dad,
struct device_node **nodepp,
unsigned long fpsize,
bool dryrun)
{
const __be32 *p;
struct device_node *np;
struct property *pp, **prev_pp = NULL;
const char *pathp;
unsigned int l, allocl;
static int depth;
int old_depth;
int offset;
int has_name = 0;
int new_format = 0;
/* 获取node节点的name指针到pathp中 */
pathp = fdt_get_name(blob, *poffset, &l);
if (!pathp)
return mem;
allocl = ++l;
/* 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 = "";
} else {
/* account for '/' and path size minus terminal 0
* already in 'l'
*/
fpsize += l;
allocl = fpsize;
}
}
/* 分配struct device_node内存，包括路径全称大小 */
np = unflatten_dt_alloc(&mem, sizeof(struct device_node) + allocl,
__alignof__(struct device_node));
if (!dryrun) {
char *fn;
of_node_init(np);
/* 填充full_name，full_name指向该node节点的全路径名称字符串 */
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);
fn += strlen(fn);
}
*(fn++) = '/';
}
memcpy(fn, pathp, l);
/* 节点挂接到相应的父节点、子节点和姊妹节点 */
prev_pp = &np->properties;
if (dad != NULL) {
np->parent = dad;
np->sibling = dad->child;
dad->child = np;
}
}
/* 处理该node节点下面所有的property */
for (offset = fdt_first_property_offset(blob, *poffset);
(offset >= 0);
(offset = fdt_next_property_offset(blob, offset))) {
const char *pname;
u32 sz;
if (!(p = fdt_getprop_by_offset(blob, offset, &pname, &sz))) {
offset = -FDT_ERR_INTERNAL;
break;
}
if (pname == NULL) {
pr_info("Can't find property name in list !\n");
break;
}
if (strcmp(pname, "name") == 0)
has_name = 1;
pp = unflatten_dt_alloc(&mem, sizeof(struct property),
__alignof__(struct property));
if (!dryrun) {
/* 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. */
/* 处理phandle，得到phandle值 */
if ((strcmp(pname, "phandle") == 0) ||
(strcmp(pname, "linux,phandle") == 0)) {
if (np->phandle == 0)
np->phandle = be32_to_cpup(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(p);
pp->name = (char *)pname;
pp->length = sz;
pp->value = (__be32 *)p;
*prev_pp = pp;
prev_pp = &pp->next;
}
}
/* with version 0x10 we may not have the name property, recreate
* it here from the unit name if absent
*/
/* 为每个node节点添加一个name的属性 */
if (!has_name) {
const char *p1 = pathp, *ps = pathp, *pa = NULL;
int sz;
/* 属性name的value值为node节点的名称，取“/”和“@”之间的子串 */
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 (!dryrun) {
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;
}
}
/* 填充device_node结构体中的name和type成员 */
if (!dryrun) {
*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 = "";
if (!np->type)
np->type = "";
}
old_depth = depth;
*poffset = fdt_next_node(blob, *poffset, &depth);
if (depth < 0)
depth = 0;
/* 递归调用node节点下面的子节点 */
while (*poffset > 0 && depth > old_depth)
mem = unflatten_dt_node(blob, mem, poffset, np, NULL,
fpsize, dryrun);
if (*poffset < 0 && *poffset != -FDT_ERR_NOTFOUND)
pr_err("unflatten: error %d processing FDT\n", *poffset);
/*
* Reverse the child list. Some drivers assumes node order matches .dts
* node order
*/
if (!dryrun && np->child) {
struct device_node *child = np->child;
np->child = NULL;
while (child) {
struct device_node *next = child->sibling;
child->sibling = np->child;
np->child = child;
child = next;
}
}
if (nodepp)
*nodepp = np;
return mem;
}
- 通过以上函数处理就得到了所有的struct device_node结构体，为每一个node都会自动添加一个名称为“name”的property，property.length的值为当前node的名称取最后一个“/”和“@”之间的子串（包括‘\0’）。例如：/serial@e2900800，则length = 7，property.value = device_node.name = “serial”。
4.3 platform_device和device_node绑定
machine初始化的代码可以沿着start_kernel->rest_init->kernel_init->kernel_init_freeable->do_basic_setup->do_initcalls路径寻找。在do_initcalls函数中，kernel会依次执行各个initcall函数，在这个过程中，会调用customize_machine，具体如下：
static int __init customize_machine(void)
{
/*
* customizes platform devices, or adds new ones
* On DT based machines, we fall back to populating the
* machine from the device tree, if no callback is provided,
* otherwise we would always need an init_machine callback.
*/
if (machine_desc->init_machine)
machine_desc->init_machine();
return 0;
}
- 经过以上解析，DeviceTree的数据已经全部解析出具体的struct device_node和struct property结构体，下面需要和具体的device进行绑定。首先讲解platform_device和device_node的绑定过程。在arch/arm/kernel/setup.c文件中，customize_machine()函数负责填充struct platform_device结构体。platform_device生成函数调用过程如下图所示：
onst struct of_device_id of_default_bus_match_table[ ] = {
{ .compatible = "simple-bus", },
{ .compatible = "simple-mfd", },
#ifdef CONFIG_ARM_AMBA
{ .compatible = "arm,amba-bus", },
#endif /* CONFIG_ARM_AMBA */
{} /* Empty terminated list */
};
4.3.1 of_platform_populate( )
int of_platform_populate(struct device_node *root,
const struct of_device_id *matches,
const struct of_dev_auxdata *lookup,
struct device *parent)
{
struct device_node *child;
int rc = 0;
/* 获取根节点 */
root = root ? of_node_get(root) : of_find_node_by_path("/");
if (!root)
return -EINVAL;
/* 为根节点下面的每一个节点创建platform_device结构体 */
for_each_child_of_node(root, child) {
rc = of_platform_bus_create(child, matches, lookup, parent, true);
if (rc) {
of_node_put(child);
break;
}
}
/* 更新device_node flag标志位 */
of_node_set_flag(root, OF_POPULATED_BUS);
of_node_put(root);
return rc;
}
4.3.2 of_platform_bus_create( )
static int of_platform_bus_create(struct device_node *bus, //要创建的那个device node
const struct of_device_id *matches, //要匹配的list
const struct of_dev_auxdata *lookup, //附属数据
struct device *parent, bool strict)//parent指向父节点，strict是否要求完全匹配
{
const struct of_dev_auxdata *auxdata;
struct device_node *child;
struct platform_device *dev;
const char *bus_id = NULL;
void *platform_data = NULL;
int rc = 0;
/* 只有包含"compatible"属性的node节点才会生成相应的platform_device结构体 */
/* Make sure it has a compatible property */
if (strict && (!of_get_property(bus, "compatible", NULL))) {
return 0;
}
/* 省略部分代码 */
/*
* 针对节点下面得到status = "ok" 或者status = "okay"或者不存在status属性的
* 节点分配内存并填充platform_device结构体
*/
dev = of_platform_device_create_pdata(bus, bus_id, platform_data, parent);
if (!dev || !of_match_node(matches, bus))
return 0;
/* 递归调用节点解析函数，为子节点继续生成platform_device结构体，前提是父节点
* 的”compatible”=”simple-bus”，也就是匹配of_default_bus_match_table结构体中的数据
*/
for_each_child_of_node(bus, child) {
rc = of_platform_bus_create(child, matches, lookup, &dev->dev, strict);
if (rc) {
of_node_put(child);
break;
}
}
of_node_set_flag(bus, OF_POPULATED_BUS);
return rc;
}
4.3.3 of_platform_device_create_pdata( )
/**
* of_platform_device_create_pdata - Alloc, initialize and register an of_device
* @np: pointer to node to create device for
* @bus_id: name to assign device
* @platform_data: pointer to populate platform_data pointer with
* @parent: Linux device model parent device.
*
* Returns pointer to created platform device, or NULL if a device was not
* registered. Unavailable devices will not get registered.
*/
static struct platform_device *of_platform_device_create_pdata(
struct device_node *np,
const char *bus_id,
void *platform_data,
struct device *parent)
{
struct platform_device *dev;
/*
* 针对节点下面得到status = "ok" 或者status = "okay"或者不存在status属性的
* 节点分配内存并填充platform_device结构体
*/
if (!of_device_is_available(np) ||
of_node_test_and_set_flag(np, OF_POPULATED))
return NULL;
dev = of_device_alloc(np, bus_id, parent);
if (!dev)
goto err_clear_flag;
//配置platform_device中的其他成员
dev->dev.bus = &platform_bus_type;
dev->dev.platform_data = platform_data;
of_msi_configure(&dev->dev, dev->dev.of_node);
//把platform_device加入统一的设备模型系统中
if (of_device_add(dev) != 0) {
platform_device_put(dev);
goto err_clear_flag;
}
return dev;
err_clear_flag:
of_node_clear_flag(np, OF_POPULATED);
return NULL;
}
- 总的来说，当of_platform_populate()函数执行完毕，kernel就为DTB中所有包含compatible属性名的第一级node创建platform_device结构体，并向平台设备总线注册设备信息。如果第一级node的compatible属性值等于“simple-bus”、“simple-mfd”或者"arm,amba-bus"的话，kernel会继续为当前node的第二级包含compatible属性的node创建platform_device结构体，并注册设备。Linux系统下的设备大多都是挂载在平台总线下的，因此在平台总线被注册后，会根据of_root节点的树结构，去寻找该总线的子节点，所有的子节点将被作为设备注册到该总线上。