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dts节点展开为platform_device结构过程分析
1.概述
本文主要是记录学习Linux解析dts的代码分析，以便进行后续回顾。
平台：ARM Vexpress
内核版本：linux-4.9
2.dts节点展开为platform_device结构过程分析
自从ARM引入的dts之后，bsp驱动代码产生了非常之大的变化，像在linux-2.6.32这些版本的platform驱动中，会存在大量类似一下的代码：
static struct resource xxx_resources[] = {
[0] = {
.start = …,
.end = …,
.flags = IORESOURCE_MEM,
},
[1] = {
.start = …,
.end = …,
.flags = IORESOURCE_IRQ,
},
};
static struct platform_device xxx_device = {
.name = "xxx",
.id = -1,
.dev = {
.platform_data = &xxx_data,
},
.resource = xxx_resources,
.num_resources = ARRAY_SIZE(xxx_resources),
};
但是通过dts，像上述的代码不再需要我们程序员进行手动配置，只需在dts相应的节点通过reg、interrupt等属性的配置，就可以通过内核提供的解析dts的接口把dts中的节点信息展开为platform_device，然后把reg、interrupt等属性的信息填充到struct resource资源结构体中，有效的减少了我们驱动代码的冗余，做到配置简单，易读。以下就是通过分析代码，了解linux是如何把dts节点信息展开为struct platform_device结构体的过程。
将dts节点展开为struct platform_device结构体的过程主要是交给of_platform_populate()函数完成，通过对该函数使用dump_stack()回溯其调用过程可以得到以下log:
futex hash table entries: 1024 (order: 4, 65536 bytes)
NET: Registered protocol family 16
DMA: preallocated 256 KiB pool for atomic coherent allocations
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.9.56+ #7
Hardware name: ARM-Versatile Express
[<80111158>] (unwind_backtrace) from [<8010ca80>] (show_stack+0x20/0x24)
[<8010ca80>] (show_stack) from [<803dd1b4>] (dump_stack+0xac/0xd8)
[<803dd1b4>] (dump_stack) from [<80586318>] (of_platform_populate+0x30/0xbc)
[<80586318>] (of_platform_populate) from [<80928410>] (vexpress_config_init+0xb8/0xec)
[<80928410>] (vexpress_config_init) from [<80101c9c>] (do_one_initcall+0x54/0x184)
[<80101c9c>] (do_one_initcall) from [<80900f48>] (kernel_init_freeable+0x214/0x2a8)
[<80900f48>] (kernel_init_freeable) from [<806a4a94>] (kernel_init+0x18/0x124)
[<806a4a94>] (kernel_init) from [<80108190>] (ret_from_fork+0x14/0x24)
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.9.56+ #7
Hardware name: ARM-Versatile Express
[<80111158>] (unwind_backtrace) from [<8010ca80>] (show_stack+0x20/0x24)
[<8010ca80>] (show_stack) from [<803dd1b4>] (dump_stack+0xac/0xd8)
[<803dd1b4>] (dump_stack) from [<80586318>] (of_platform_populate+0x30/0xbc)
[<80586318>] (of_platform_populate) from [<80928410>] (vexpress_config_init+0xb8/0xec)
[<80928410>] (vexpress_config_init) from [<80101c9c>] (do_one_initcall+0x54/0x184)
[<80101c9c>] (do_one_initcall) from [<80900f48>] (kernel_init_freeable+0x214/0x2a8)
[<80900f48>] (kernel_init_freeable) from [<806a4a94>] (kernel_init+0x18/0x124)
[<806a4a94>] (kernel_init) from [<80108190>] (ret_from_fork+0x14/0x24)
cpuidle: using governor ladder
通过栈回溯信息，我们可以知道of_platform_populate()函数的最早调用入口是vexpress_config_init()函数,除此之外，在of_platform_default_populate()函数中也会调用该函数，分别如下所示
vexpress_config_init()函数是定义在drivers/bus/vexpress-config.c文件中，该函数实现如下：
static int __init vexpress_config_init(void)
{
int err = 0;
struct device_node *node;
/* Need the config devices early, before the "normal" devices... */
for_each_compatible_node(node, NULL, "arm,vexpress,config-bus") {
err = vexpress_config_populate(node);
if (err) {
of_node_put(node);
break;
}
}
return err;
}
postcore_initcall(vexpress_config_init);
//另外一个函数入口
int of_platform_default_populate(struct device_node *root,
const struct of_dev_auxdata *lookup,
struct device *parent)
{
return of_platform_populate(root, of_default_bus_match_table, lookup,
parent);
}
EXPORT_SYMBOL_GPL(of_platform_default_populate);
#ifndef CONFIG_PPC
static int __init of_platform_default_populate_init(void)
{
struct device_node *node;
if (!of_have_populated_dt())
return -ENODEV;
/* * Handle ramoops explicitly, since it is inside /reserved-memory, * which lacks a "compatible" property. */
node = of_find_node_by_path("/reserved-memory");
if (node) {
node = of_find_compatible_node(node, NULL, "ramoops");
if (node)
of_platform_device_create(node, NULL, NULL);
}
/* Populate everything else. */
of_platform_default_populate(NULL, NULL, NULL);
return 0;
}
arch_initcall_sync(of_platform_default_populate_init);
#endif
首先，通过postcore_initcall和arch_initcall_sync这些宏声明该platform driver的调用等级，当内核执行到以下调用时，会调用到该函数：
start_kernel(void)
->rest_init()
->kernel_init()
->kernel_init_freeable()
->do_basic_setup()
->do_initcalls()
->do_initcall_level()
->do_one_initcall()
在do_initcalls()函数中，会从0遍历到ARRAY_SIZE(initcall_levels) – 1，代码如下：
static void __init do_initcalls(void)
{
int level;
for (level = 0; level < ARRAY_SIZE(initcall_levels) - 1; level++)
do_initcall_level(level);
}
其中initcall_levels是一个指针数组，里面存放着各个level下的函数指针的首地址，该数组定义在init/main.c中，如下所示：
static initcall_t *initcall_levels[] __initdata = {
__initcall0_start,
__initcall1_start,
__initcall2_start,
__initcall3_start,
__initcall4_start,
__initcall5_start,
__initcall6_start,
__initcall7_start,
__initcall_end,
};
因此在do_initcalls()函数中，会从level=0遍历到level=8，每个driver的调用顺序可以通过以下宏来声明：
#define pure_initcall(fn) __define_initcall(fn, 0)
#define core_initcall(fn) __define_initcall(fn, 1)
#define core_initcall_sync(fn) __define_initcall(fn, 1s)
#define postcore_initcall(fn) __define_initcall(fn, 2)
#define postcore_initcall_sync(fn) __define_initcall(fn, 2s)
#define arch_initcall(fn) __define_initcall(fn, 3)
#define arch_initcall_sync(fn) __define_initcall(fn, 3s)
#define subsys_initcall(fn) __define_initcall(fn, 4)
#define subsys_initcall_sync(fn) __define_initcall(fn, 4s)
#define fs_initcall(fn) __define_initcall(fn, 5)
#define fs_initcall_sync(fn) __define_initcall(fn, 5s)
#define rootfs_initcall(fn) __define_initcall(fn, rootfs)
#define device_initcall(fn) __define_initcall(fn, 6)
#define device_initcall_sync(fn) __define_initcall(fn, 6s)
#define late_initcall(fn) __define_initcall(fn, 7)
#define late_initcall_sync(fn) __define_initcall(fn, 7s)
#define __initcall(fn) device_initcall(fn)
以上这些宏都定义在include/linux/init.h文件中。从这里可以看出postcore_initcall宏声明的driver初始化函数的调用level为2。
了解到如何调用到of_platform_default_populate()函数的过程，我们再来看看of_platform_populate()函数如何把一个dts节点展开struct platform_device结构的，分析如下：
/** * of_platform_populate() - Populate platform_devices from device tree data * @root: parent of the first level to probe or NULL for the root of the tree * @matches: match table, NULL to use the default * @lookup: auxdata table for matching id and platform_data with device nodes * @parent: parent to hook devices from, NULL for toplevel * * Similar to of_platform_bus_probe(), this function walks the device tree * and creates devices from nodes. It differs in that it follows the modern * convention of requiring all device nodes to have a 'compatible' property, * and it is suitable for creating devices which are children of the root * node (of_platform_bus_probe will only create children of the root which * are selected by the @matches argument). * * New board support should be using this function instead of * of_platform_bus_probe(). * * Returns 0 on success, < 0 on failure. */
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;
root = root ? of_node_get(root) : of_find_node_by_path("/");
if (!root)
return -EINVAL;
pr_debug("%s()\n", __func__);
pr_debug(" starting at: %s\n", root->full_name);
for_each_child_of_node(root, child) {
/* 1.遍历root节点下的所有子节点，调用of_platform_bus_create() */
rc = of_platform_bus_create(child, matches, lookup, parent, true);
if (rc) {
of_node_put(child);
break;
}
}
of_node_set_flag(root, OF_POPULATED_BUS);
of_node_put(root);
return rc;
}
EXPORT_SYMBOL_GPL(of_platform_populate);
通常root节点都是一些特定的总线节点，如platform device的simple-bus，如下所示：
const struct of_device_id of_default_bus_match_table[] = {
{
.compatible = "simple-bus", },
{
.compatible = "simple-mfd", },
{
.compatible = "isa", },
#ifdef CONFIG_ARM_AMBA
{
.compatible = "arm,amba-bus", },
#endif /* CONFIG_ARM_AMBA */
{
} /* Empty terminated list */
};
dts对应节点如下：
smb@04000000 {
compatible = "simple-bus";
#address-cells = <2>;
#size-cells = <1>;
ranges = <0 0 0x40000000 0x04000000>,
<1 0 0x44000000 0x04000000>,
<2 0 0x48000000 0x04000000>,
<3 0 0x4c000000 0x04000000>,
<7 0 0x10000000 0x00020000>;
...
}
还有arm vexpress的config-bus，dts节点如下：
dcc {
compatible = "arm,vexpress,config-bus";
arm,vexpress,config-bridge = <&v2m_sysreg>;
oscclk0: extsaxiclk {
/* ACLK clock to the AXI master port on the test chip */
compatible = "arm,vexpress-osc";
arm,vexpress-sysreg,func = <1 0>;
freq-range = <30000000 50000000>;
#clock-cells = <0>;
clock-output-names = "extsaxiclk";
};
...
}
在of_platform_bus_create()函数中，会根据节点的compatible属性进行相应的设备创建，代码分析如下：
/** * of_platform_bus_create() - Create a device for a node and its children. * @bus: device node of the bus to instantiate * @matches: match table for bus nodes * @lookup: auxdata table for matching id and platform_data with device nodes * @parent: parent for new device, or NULL for top level. * @strict: require compatible property * * Creates a platform_device for the provided device_node, and optionally * recursively create devices for all the child nodes. */
static int of_platform_bus_create(struct device_node *bus,
const struct of_device_id *matches,
const struct of_dev_auxdata *lookup,
struct device *parent, bool strict)
{
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;
/* 1.检查节点是否具备compatible属性，没有，则返回0 */
if (strict && (!of_get_property(bus, "compatible", NULL))) {
pr_debug("%s() - skipping %s, no compatible prop\n",
__func__, bus->full_name);
return 0;
}
/* 2.检查节点是否已经解析过，是，则返回0 */
if (of_node_check_flag(bus, OF_POPULATED_BUS)) {
pr_debug("%s() - skipping %s, already populated\n",
__func__, bus->full_name);
return 0;
}
auxdata = of_dev_lookup(lookup, bus);
if (auxdata) {
bus_id = auxdata->name;
platform_data = auxdata->platform_data;
}
/* 3.判断是否是amba device，如果是，则创建该类型设备 */
if (of_device_is_compatible(bus, "arm,primecell")) {
/* * Don't return an error here to keep compatibility with older * device tree files. */
of_amba_device_create(bus, bus_id, platform_data, parent);
return 0;
}
/* 4.创建platform device */
dev = of_platform_device_create_pdata(bus, bus_id, platform_data, parent);
if (!dev || !of_match_node(matches, bus))
return 0;
/* 5.如果该节点为总线节点，则遍历该节点下的所有子节点，并创建相应的设备 */
for_each_child_of_node(bus, child) {
pr_debug(" create child: %s\n", child->full_name);
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;
}
创建设备的函数of_platform_device_create_pdata()分析如下：
/** * 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;
/* 1.检查节点status属性值是否为ok或okay,不等，则返回false, * 2.status属性不存在，返回true */
if (!of_device_is_available(np) ||
of_node_test_and_set_flag(np, OF_POPULATED))
return NULL;
/* 2.分配struct platform_device结构体 * 3.解析dts mem/irq等资源信息，填充到设备结构中 */
dev = of_device_alloc(np, bus_id, parent);
if (!dev)
goto err_clear_flag;
/* 4.初始化设备bus类型，用于驱动probe */
dev->dev.bus = &platform_bus_type;
dev->dev.platform_data = platform_data;
of_dma_configure(&dev->dev, dev->dev.of_node);
of_msi_configure(&dev->dev, dev->dev.of_node);
/* 5.添加到设备链表，至此，设备创建完成 */
if (of_device_add(dev) != 0) {
of_dma_deconfigure(&dev->dev);
platform_device_put(dev);
goto err_clear_flag;
}
return dev;
err_clear_flag:
of_node_clear_flag(np, OF_POPULATED);
return NULL;
}
至此，设备创建完成，可以等待驱动的probe
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