Linux Kernel（17.2）- Common Device Tree API

properties的value可以是empty或是以下資料型態:

Text strings (null terminated) are represented with double quotes:
string-property = "a string";
'Cells' are 32 bit unsigned integers delimited by angle brackets:
cell-property = <0xbeef 123 0xabcd1234>;
Binary data is delimited with square brackets:
binary-property = [0x01 0x23 0x45 0x67];
Data of differing representations can be concatenated together using a comma:
mixed-property = "a string", [0x01 0x23 0x45 0x67], <0x12345678>;
Commas are also used to create lists of strings:
string-list = "red fish", "blue fish";

這章節介紹幾個API，用於存取這些常見的資料型態，會以下的DTS的內容進行parse

/ {
  node1 {
    compatible = "brook,dts-test";
    a-string-property = "A string";
    a-string-list-property = "first string", "second string";
    // hex is implied in byte arrays. no '0x' prefix is required
    a-byte-data-property = [01 23 34 56];
    child-node1 {
      first-child-property;
      second-child-property = <1>;
      a-string-property = "Hello, world";
    };
    child-node2 {
    };
  };
  
  node2 {
    compatible = "brook,dts-test";
    an-empty-property;
    a-cell-property = <1 2 3 4>; /* each number (cell) is a uint32 */
    child-node@1 {
    };
    child-node@2 {
    };
  };
};

• of_match_device(): Sanity check for device that device is matching with the node
• of_property_read_string(): To read string property
• of_property_count_strings(): Find and return the number of strings from a multiple strings property.
• of_property_read_string_index(): Read a string with index from string-list property.
• of_find_property(): Find and return the property pointer by giving named
• of_property_for_each_u32(): A macro to iterate the property to get all values
• of_property_read_bool(): Returns true if the property exist false otherwise
• of_find_node_by_name(): Find a node by its "name" property
• for_each_child_of_node(): Traverse all child device node for current device node

#include <linux/module.h>
#include <linux/platform_device.h>
#include <linux/of.h>
#include <linux/of_device.h>

static struct of_device_id brook_dt_id[] = {
    {
     .compatible = "brook,dts-test",
    },
    {}
};

MODULE_DEVICE_TABLE(of, brook_dt_id);

static int brook_dts_probe(struct platform_device *pdev)
{
    const struct of_device_id *of_id;
    struct device_node *node, *child;
    struct property *pp;
    int i, ret, of_cnt, len;
    const char *str;
    const __be32 *cur;
    u32 val;

    // sanity check of_match_device() allows to get the matching entry
    of_id = of_match_device(brook_dt_id, &pdev->dev);
    if (!of_id) {
        pr_err("%s: of_id is NULL\n", __func__);
        return -1;
    }

    node = pdev->dev.of_node;
    // To read string property
    ret = of_property_read_string(node, "a-string-property", &str);
    if (!ret) {
        printk("a-string-property: %s\n", str);
    } else {
        printk("no a-string-property\n");
    }

    // Find and return the number of strings from a multiple strings property.
    of_cnt = of_property_count_strings(node, "a-string-list-property");
    if (of_cnt) {
        for (i = 0; i < of_cnt; i++) {
            ret = of_property_read_string_index(node, "a-string-list-property", i, &str);
            if (!ret) {
                printk("a-string-list-property[%d]: %s\n", i, str);
            }
        }
    }

    // to find aproperty named if arg2
    pp = of_find_property(node, "a-byte-data-property", &len);
    if (pp) {
        u8 *u8p = pp->value;
        printk("a-byte-data-property: len:%d, pp->len:%d", len, pp->length);
        for (i = 0; i < len; i++) {
            printk("a-byte-data-property:[%d] = %02x\n", i, u8p[i]);
        }
    }

    // to find aproperty named if arg2
    of_property_for_each_u32(node, "a-cell-property", pp, cur, val) {
        printk("a-cell-property: %04x/%04x\n", *cur, val);
    }

    // Returns true if the property exist false otherwise
    if (of_property_read_bool(node, "an-empty-property")) {
        printk("has an-empty-property\n");
    } else {
        printk("no an-empty-property\n");
    }

    // Traverse all child device node for current device node
    for_each_child_of_node(node, child) {
        printk("child name: %s\n", child->name);
    }

    // Find a node by its "name" property
    child = of_find_node_by_name(node, "child-node1");
    if (child) {
        printk("%s has a child %s\n", node->name, child->name);
    }

    return 0;
}

static int brook_dts_remove(struct platform_device *pdev)
{
    return 0;
}

static struct platform_driver brook_dts_platform_driver = {
    .probe = brook_dts_probe,
    .remove = brook_dts_remove,
    .driver = {
        .owner = THIS_MODULE,
        .name = "brook-dts",
        .of_match_table = brook_dt_id,
    },
};


static int __init brook_init_module(void)
{
    return platform_driver_register(&brook_dts_platform_driver);
}

static void __exit brook_exit_module(void)
{
    platform_driver_unregister(&brook_dts_platform_driver);
}

module_init(brook_init_module);
module_exit(brook_exit_module);
MODULE_LICENSE("GPL");

執行結果如下:

/ # insmod dts.ko
a-string-property: A string
a-string-list-property[0]: first string
a-string-list-property[1]: second string
a-byte-data-property: len:4, pp->len:4
a-byte-data-property:[0] = 01
a-byte-data-property:[1] = 23
a-byte-data-property:[2] = 34
a-byte-data-property:[3] = 56
no an-empty-property
child name: child-node1
child name: child-node2
node1 has a child child-node1
no a-string-property
a-cell-property: 1000000/0001
a-cell-property: 2000000/0002
a-cell-property: 3000000/0003
a-cell-property: 4000000/0004
has an-empty-property
child name: child-node
child name: child-node

以下摘錄http://www.myexception.cn/linux-unix/1910031.html, linux下devicetree中常用的of函數

linux下devicetree中常用的of函数
从device_node中获取信息：

int of_property_read_u8_array(const struct device_node *np, const char *propname,u8 *out_values, size_t sz);

int of_property_read_u16_array(const struct device_node *np, const char *propname,u16 *out_values, size_t sz);

int of_property_read_u32_array(const struct device_node *np, const char *propname,u32 *out_values, size_t sz);

从设备结点np中读取属性名为propname，类型为8、16、32、位整型数组的属性值，并放入out_values，sz指明了要读取的个数。


static inline int of_property_read_u8(const struct device_node *np,const char *propname,u8 *out_value) 

static inline int of_property_read_u16(const struct device_node *np,const char *propname,u8 *out_value) 

static inline int of_property_read_u32(const struct device_node *np,const char *propname,u8 *out_value) 

从设备结点np中读取属性名为propname，类型为8、16、32位的属性值，并放入out_values。实际上这里调用的就是sz为1的XXX_array函数。

 

int of_property_read_u32_index(const struct device_node *np,const char*propname,u32 index, u32 *out_value)

从设备结点np中读取属性名为propname的属性值中第index个u32数值给out_value

 

int of_property_read_u64(conststruct device_node *np, const char *propname,u64 *out_value)

从设备结点np中读取属性名为propname，类型为64位的属性值，并放入out_values

 

int of_property_read_string(struct device_node *np, const char *propname,const char**out_string)

从设备结点np中读取属性名为propname的字符串型属性值

 

int of_property_read_string_index(struct device_node *np, const char *propname,intindex, const char **output)

从设备结点np中读取属性名为propname的字符串型属性值数组中的第index个字符串

 

int of_property_count_strings(struct device_node *np, const char *propname)

从设备结点np中读取属性名为propname的字符串型属性值的个数

 

unsigned int irq_of_parse_and_map(struct device_node *dev, int index)

从设备节点dev中读取第index个irq号

 

int of_irq_to_resource(struct device_node *dev, int index, struct resource *r)

从设备节点dev中读取第index个irq号，并填充一个irq资源结构体

 

int of_irq_count(struct device_node *dev)

获取设备节点dev的irq个数


static inline bool of_property_read_bool(const struct device_node *np,const char *propname);

如果设备结点np含有propname属性，则返回true，否则返回false。一般用于检查空属性是否存在。

 

struct property* of_find_property(const struct device_node *np,const char *name,int *lenp)

根据name参数，在指定的设备结点np中查找匹配的property，并返回这个property

 

const void * of_get_property(const struct device_node *np, const char *name,int *lenp)

根据name参数，在指定的设备结点np中查找匹配的property，并返回这个property的属性值


struct device_node* of_get_parent(const struct device_node *node)

获得node节点的父节点的device node


int of_device_is_compatible(const struct device_node *device,const char *compat);

判断设备结点device的compatible属性是否包含compat指定的字符串


从of_allnodes中查找信息：

struct device_node* of_find_node_by_path(const char *path)
根据路径参数，在全局链表of_allnodes中，查找匹配的device_node


struct device_node* of_find_node_by_name(struct device_node *from,const char *name)
则根据name在全局链表of_allnodes中查找匹配的device_node,若from=NULL表示从头开始查找


struct device_node* of_find_node_by_type(struct device_node *from,const char *type)

根据设备类型在全局链表of_allnodes中查找匹配的device_node


struct device_node * of_find_compatible_node(struct device_node *from, const char*type, const char，*compatible);

根据compatible的属性值在全局链表of_allnodes中查找匹配的device_node，大多数情况下，from、type为NULL。

 

struct device_node* of_find_node_with_property(struct device_node *from,const char *prop_name)

根据节点属性的name在全局链表of_allnodes中查找匹配的device_node

 

struct device_node* of_find_node_by_phandle(phandle handle)

根据phandle在全局链表of_allnodes中查找匹配的device_node

 

杂：

void __iomem* of_iomap(struct device_node *node, int index);

通过设备结点直接进行设备内存区间的 ioremap()，index是内存段的索引。若设备结点的reg属性有多段，可通过index标示要ioremap的是哪一段，只有1段的情况，index为0

 

unsigned long __init of_get_flat_dt_root(void)

用来查找在dtb中的根节点，好像返回的都是0


int of_alias_get_id(struct device_node *np, const char *stem)

获取节点np对应的aliasid号

 

struct device_node* of_node_get(struct device_node *node)

void of_node_put(struct device_node *node)

device node计数增加/减少


const struct of_device_id* of_match_node(const struct of_device_id *matches,const struct device_node*node)

将matches数组中of_device_id结构的name和type与device node的compatible和type匹配，返回匹配度最高的of_device_id结构


platform_device和resource相关：

int of_address_to_resource(struct device_node *dev, int index,struct resource *r)

根据设备节点dev的reg属性值，填充资源结构体r。Index参数指明了使用reg属性中第几个属性值，一般设置为0，表示第一个。


struct platform_device* of_device_alloc(struct device_node *np,const char *bus_id,struct device *parent)

根据device node，bus_id以及父节点创建该设备的platform_device结构，同时会初始化它的resource成员。

 

int of_platform_bus_probe(struct device_node *root,const struct of_device_id *matches,struct device *parent)

遍历of_allnodes中的节点挂接到of_platform_bus_type总线上,由于此时of_platform_bus_type总线上还没有驱动,所以此时不进行匹配

 

int of_platform_populate(struct device_node *root,const struct of_device_id *matches,const struct of_dev_auxdata *lookup,struct device *parent)

遍历of_allnodes中的所有节点，生成并初始化所以节点的platform_device结构



struct platform_device* of_find_device_by_node(struct device_node *np)

根据device_node查找返回该设备对应的platform_device结构

參考資料:
• http://www.myexception.cn/linux-unix/1910031.html, linux下devicetree中常用的of函數
• https://saurabhsengarblog.wordpress.com/2015/11/28/device-tree-tutorial-arm/, Device Tree Tutorial (ARM)
• https://www.cnblogs.com/xiaojiang1025/p/6368260.html, Linux内核 设备树操作常用API

Linux Kernel（16.2）- PHY Abstraction Layer I

Purpose

多數的network devices都是透過MAC去存取PHY上的register，而network device driver就是利用這些register去決定如何配置該網路裝置， PHY的register都是遵守相同的標準，因而建立PHY Abstraction Layer的目的就是減少network device driver的loading，因為PHY的driver在此了， 三個目的如下:

• 1) Increase code-reuse
• 2) Increase overall code-maintainability
• 3) Speed development time for new network drivers, and for new systems

The MDIO bus

多數的network devices都是透過所謂的management bus與PHY溝通，不同的network devices會使用不同的bus， 這些bus要被註冊到對應的network devices上，這些bus的interface必須遵守以下規則

1. read and write function must be implemented.
   int write(struct mii_bus *bus, int mii_id, int regnum, u16 value);
   int read(struct mii_bus *bus, int mii_id, int regnum);
   mii_id是PHY在該bus上的ID
2. reset function可有可無
3. probe function是必要的，用以設定PHY driver所需的任何東西，

可以參考drivers/net/ethernet/freescale/fsl_pq_mdio.c為例的mdio bus driver，以下為我摘要的片段

static struct platform_driver fsl_pq_mdio_driver = {
    .driver = {
        .name = "fsl-pq_mdio",
        .of_match_table = fsl_pq_mdio_match,
    },
    .probe = fsl_pq_mdio_probe,
    .remove = fsl_pq_mdio_remove,
};

static int fsl_pq_mdio_probe(struct platform_device *pdev)
{
    struct device_node *np = pdev->dev.of_node;
    struct mii_bus *new_bus;

    new_bus = mdiobus_alloc_size(sizeof(*priv));
    if (!new_bus)
        return -ENOMEM;

    priv = new_bus->priv;

    new_bus->name = "Freescale PowerQUICC MII Bus",
    new_bus->read = &fsl_pq_mdio_read;
    new_bus->write = &fsl_pq_mdio_write;
    new_bus->reset = &fsl_pq_mdio_reset;

    new_bus->parent = &pdev->dev;
    platform_set_drvdata(pdev, new_bus);
    err = of_mdiobus_register(new_bus, np);

    return 0;
}

/*
 * Read the bus for PHY at addr mii_id, register regnum, and return the value.
 * Clears miimcom first.
 *
 * All PHY operation done on the bus attached to the local interface, which
 * may be different from the generic mdio bus.  This is helpful in programming
 * interfaces like the TBI which, in turn, control interfaces like on-chip
 * SERDES and are always tied to the local mdio pins, which may not be the
 * same as system mdio bus, used for controlling the external PHYs, for eg.
 */
static int fsl_pq_mdio_read(struct mii_bus *bus, int mii_id, int regnum)
{
    struct fsl_pq_mdio_priv *priv = bus->priv;
    struct fsl_pq_mii __iomem *regs = priv->regs;
    unsigned int timeout;
    u16 value;

    /* Set the PHY address and the register address we want to read */
    iowrite32be((mii_id << 8) | regnum, ®s->miimadd);

    /* Clear miimcom, and then initiate a read */
    iowrite32be(0, ®s->miimcom);
    iowrite32be(MII_READ_COMMAND, ®s->miimcom);

    /* Wait for the transaction to finish, normally less than 100us */
    timeout = MII_TIMEOUT;
    while ((ioread32be(®s->miimind) &
           (MIIMIND_NOTVALID | MIIMIND_BUSY)) && timeout) {
        cpu_relax();
        timeout--;
    }

    if (!timeout)
        return -ETIMEDOUT;


    /* Grab the value of the register from miimstat */
    value = ioread32be(®s->miimstat);

    dev_dbg(&bus->dev, "read %04x from address %x/%x\n", value, mii_id, regnum);
    return value;
}

/*
 * Write value to the PHY at mii_id at register regnum, on the bus attached
 * to the local interface, which may be different from the generic mdio bus
 * (tied to a single interface), waiting until the write is done before
 * returning. This is helpful in programming interfaces like the TBI which
 * control interfaces like onchip SERDES and are always tied to the local
 * mdio pins, which may not be the same as system mdio bus, used for
 * controlling the external PHYs, for example.
 */
static int fsl_pq_mdio_write(struct mii_bus *bus, int mii_id, int regnum,
        u16 value)
{
    struct fsl_pq_mdio_priv *priv = bus->priv;
    struct fsl_pq_mii __iomem *regs = priv->regs;
    unsigned int timeout;

    /* Set the PHY address and the register address we want to write */
    iowrite32be((mii_id << 8) | regnum, ®s->miimadd);

    /* Write out the value we want */
    iowrite32be(value, ®s->miimcon);

    /* Wait for the transaction to finish */
    timeout = MII_TIMEOUT;
    while ((ioread32be(®s->miimind) & MIIMIND_BUSY) && timeout) {
        cpu_relax();
        timeout--;
    }

    return timeout ? 0 : -ETIMEDOUT;
}

/* Reset the MIIM registers, and wait for the bus to free */
static int fsl_pq_mdio_reset(struct mii_bus *bus)
{
    struct fsl_pq_mdio_priv *priv = bus->priv;
    struct fsl_pq_mii __iomem *regs = priv->regs;
    unsigned int timeout;

    mutex_lock(&bus->mdio_lock);

    /* Reset the management interface */
    iowrite32be(MIIMCFG_RESET, ®s->miimcfg);

    /* Setup the MII Mgmt clock speed */
    iowrite32be(MIIMCFG_INIT_VALUE, ®s->miimcfg);

    /* Wait until the bus is free */
    timeout = MII_TIMEOUT;
    while ((ioread32be(®s->miimind) & MIIMIND_BUSY) && timeout) {
        cpu_relax();
        timeout--;
    }

    mutex_unlock(&bus->mdio_lock);

    if (!timeout) {
        dev_err(&bus->dev, "timeout waiting for MII bus\n");
        return -EBUSY;
    }

    return 0;
}

(RG)MII/electrical interface considerations

Due to this design decision, a 1.5ns to 2ns delay must be added between the clock line (RXC or TXC) and the data lines to let the PHY (clock sink) have enough setup and hold times to sample the data lines correctly.



我也摘錄一段Qualcomm的EMAC probe部分,
https://github.com/mauronofrio/android_kernel_nubia_msm8953/blob/master/drivers/net/ethernet/qualcomm/emac/emac_phy.c

int emac_phy_config_external(struct platform_device *pdev,
                 struct emac_adapter *adpt)
{
    struct device_node *np = pdev->dev.of_node;
    struct mii_bus *mii_bus;
    int ret;

    /* Create the mii_bus object for talking to the MDIO bus */
    mii_bus = devm_mdiobus_alloc(&pdev->dev);
    adpt->mii_bus = mii_bus;

    if (!mii_bus)
        return -ENOMEM;

    mii_bus->name = "emac-mdio";
    snprintf(mii_bus->id, MII_BUS_ID_SIZE, "%s", pdev->name);
    mii_bus->read = emac_mdio_read;
    mii_bus->write = emac_mdio_write;
    mii_bus->parent = &pdev->dev;
    mii_bus->priv = adpt;

    if (ACPI_COMPANION(&pdev->dev)) {
        u32 phy_addr;

        ret = mdiobus_register(mii_bus);
        if (ret) {
            emac_err(adpt, "could not register mdio bus\n");
            return ret;
        }
        ret = device_property_read_u32(&pdev->dev, "phy-channel",
                           &phy_addr);
        if (ret)
            /* If we can't read a valid phy address, then assume
             * that there is only one phy on this mdio bus.
             */
            adpt->phydev = phy_find_first(mii_bus);
        else
            adpt->phydev = mii_bus->phy_map[phy_addr];
    } else {
        struct device_node *phy_np;

        ret = of_mdiobus_register(mii_bus, np);

        if (ret) {
            emac_err(adpt, "could not register mdio bus\n");
            return ret;
        }

        phy_np = of_parse_phandle(np, "phy-handle", 0);
        adpt->phydev = of_phy_find_device(phy_np);
        of_node_put(phy_np);
    }

    if (!adpt->phydev) {
        emac_err(adpt, "could not find external phy\n");
        mdiobus_unregister(mii_bus);
        return -ENODEV;
    }

    if (!adpt->phydev->phy_id) {
        emac_err(adpt, "External phy is not up\n");
        mdiobus_unregister(mii_bus);
        return -EPROBE_DEFER;
    }

    if (adpt->phydev->drv) {
        emac_dbg(adpt, probe, "attached PHY driver [%s] ",
             adpt->phydev->drv->name);
        emac_dbg(adpt, probe, "(mii_bus:phy_addr=%s, irq=%d)\n",
             dev_name(&adpt->phydev->dev), adpt->phydev->irq);
    }

    /* Set initial link status to false */
    adpt->phydev->link = 0;
    return 0;
}

/**
 * of_mdiobus_register - Register mii_bus and create PHYs from the device tree
 * @mdio: pointer to mii_bus structure
 * @np: pointer to device_node of MDIO bus.
 *
 * This function registers the mii_bus structure and registers a phy_device
 * for each child node of @np.
 */
int of_mdiobus_register(struct mii_bus *mdio, struct device_node *np)
{
    struct device_node *child;
    const __be32 *paddr;
    bool scanphys = false;
    int addr, rc, i;

    /* Mask out all PHYs from auto probing.  Instead the PHYs listed in
     * the device tree are populated after the bus has been registered */
    mdio->phy_mask = ~0;

    /* Clear all the IRQ properties */
    if (mdio->irq)
        for (i=0; iirq[i] = PHY_POLL;

    mdio->dev.of_node = np;

    /* Register the MDIO bus */
    rc = mdiobus_register(mdio);
    if (rc)
        return rc;


    /* Loop over the child nodes and register a phy_device for each one */
    for_each_available_child_of_node(np, child) {
        addr = of_mdio_parse_addr(&mdio->dev, child);
        if (addr < 0) {
            scanphys = true;
            continue;
        }

        rc = of_mdiobus_register_phy(mdio, child, addr);
        if (rc)
            continue;
    }

    if (!scanphys)
        return 0;

    /* auto scan for PHYs with empty reg property */
    for_each_available_child_of_node(np, child) {
        /* Skip PHYs with reg property set */
        paddr = of_get_property(child, "reg", NULL);
        if (paddr)
            continue;

        for (addr = 0; addr < PHY_MAX_ADDR; addr++) {
            /* skip already registered PHYs */
            if (mdio->phy_map[addr])
                continue;

            /* be noisy to encourage people to set reg property */
            dev_info(&mdio->dev, "scan phy %s at address %i\n",
                 child->name, addr);

            rc = of_mdiobus_register_phy(mdio, child, addr);
            if (rc)
                continue;
        }
    }

    return 0;
}

這兩個sample code都是簡略的顯示management bus的部分，主要展示mdio bus device driver的基本寫法，填寫name、read()、write()、reset()後使用of_mdiobus_register()註冊，

參考資料:
• kernel/msm-4.14/Documentation/networking/phy.txt

busybox init flow

Linux在啟動kernel後，便由kernel載入init程式，由init程式完成餘下的啟動過程，簡略片段如下

Kernel 4.14
start_kernel()
  |--> rest_init()
    |--> pid = kernel_thread(kernel_init, NULL, CLONE_FS);
      |--> kernel_init()
       if (!try_to_run_init_process("/sbin/init") ||
          !try_to_run_init_process("/etc/init") ||
          !try_to_run_init_process("/bin/init") ||
          !try_to_run_init_process("/bin/sh"))
          return 0;
       panic("No working init found.  Try passing init= option to kernel. "
            "See Linux Documentation/admin-guide/init.rst for guidance.");

接著就會交由init開始一連串的init過程，busybox的init會去讀取/etc/inittab設定來進行init，如果沒有/etc/inittab，會執行以下預設的inittab內容

::sysinit:/etc/init.d/rcS
::askfirst:/bin/sh
::ctrlaltdel:/sbin/reboot
::shutdown:/sbin/swapoff -a
::shutdown:/bin/umount -a -r
::restart:/sbin/init
tty2::askfirst:/bin/sh
tty3::askfirst:/bin/sh
tty4::askfirst:/bin/sh

其格式為<id>:<runlevels>:<action>:<process>，說明如下

<id>: 與傳統init意思不同, 會把stdin/stdout設為/dev/<id>

<runlevels>: busybox會ignore該欄位

<action>: 有效的action包含sysinit, wait, once, respawn, askfirst, shutdown, restart and ctrlaltdel.
  "sysinit" 第一個被執行的action, init會等待"sysinit"執行完成後, 再執行wait與once
  "askfirst"/"respawn" 接著被執行, 但是askfirst執行前會先印"Please press Enter to activate this console". 

<process>: 要執行的命令

inittab的action執行code flow如下

# busybox v1.32

static pid_t run(const struct init_action *a)
{
  ...
  if (BB_MMU && (a->action_type & ASKFIRST)) {
    static const char press_enter[] ALIGN1 =
#ifdef CUSTOMIZED_BANNER
#include CUSTOMIZED_BANNER
#endif
        "\nPlease press Enter to activate this console. ";
    full_write(STDOUT_FILENO, press_enter, sizeof(press_enter) - 1);
    ...
  }
  ...
}

static void run_actions(int action_type)
{
  struct init_action *a;

  for (a = G.init_action_list; a; a = a->next) {
    if (!(a->action_type & action_type))
      continue;

    if (a->action_type & (SYSINIT | WAIT | ONCE | CTRLALTDEL | SHUTDOWN)) {
        pid_t pid = run(a);
      if (a->action_type & (SYSINIT | WAIT | CTRLALTDEL | SHUTDOWN))
          waitfor(pid);
    }
    if (a->action_type & (RESPAWN | ASKFIRST)) {
      /* Only run stuff with pid == 0. If pid != 0,
       * it is already running
       */
      if (a->pid == 0)
        a->pid = run(a);
    }
  }
}

int init_main(int argc UNUSED_PARAM, char **argv)
{
  ...
  /* Check if we are supposed to be in single user mode */
  if (argv[1]
   && (strcmp(argv[1], "single") == 0 || strcmp(argv[1], "-s") == 0 || LONE_CHAR(argv[1], '1'))
  ) {
    /* ??? shouldn't we set RUNLEVEL="b" here? */
    /* Start a shell on console */
    new_init_action(RESPAWN, bb_default_login_shell, "");
  } else {
    /* Not in single user mode - see what inittab says */

    /* NOTE that if CONFIG_FEATURE_USE_INITTAB is NOT defined,
    * then parse_inittab() simply adds in some default
    * actions (i.e., INIT_SCRIPT and a pair
    * of "askfirst" shells) */
    parse_inittab();
  }

  ...
  /* Now run everything that needs to be run */
  /* First run the sysinit command */
  run_actions(SYSINIT);
  check_delayed_sigs(&G.zero_ts);
  /* Next run anything that wants to block */
  run_actions(WAIT);
  check_delayed_sigs(&G.zero_ts);
  /* Next run anything to be run only once */
  run_actions(ONCE);
  ...
  while (1) {
    ...
    /* (Re)run the respawn/askfirst stuff */
    run_actions(RESPAWN | ASKFIRST);
    ...
  }
  ...
}

如果要進入signle user mode，就需要傳遞"single"或"-S"給init當參數，要把字串放在kernel parameters "--" 之後

The kernel parses parameters from the kernel command line up to “–”; 
if it doesn’t recognize a parameter and it doesn’t contain a ‘.’, 
the parameter gets passed to init: parameters with ‘=’ go into init’s environment, 
others are passed as command line arguments to init.
Everything after “–” is passed as an argument to init.

如:
qemu-system-arm -M vexpress-a9 -m 512M -kernel ./linux/arch/arm/boot/zImage -dtb \
./linux/arch/arm/boot/dts/vexpress-v2p-ca9.dtb -initrd ./initrd-arm.img \
-nographic -append "console=ttyAMA0 -- single"

busybox並不支援runlevel

參考資料:
• https://en.wikipedia.org/wiki/Init, init
• https://en.wikipedia.org/wiki/Runlevel, runlevel
• https://git.busybox.net/busybox/tree/examples/inittab, busybox example for inittab
• https://www.itread01.com/p/1350587.html, kernel 啟動流程之 【裝置驅動載入】 學習筆記
• https://www.itread01.com/content/1543246633.html, busybox(一)淺析