设备树API

引子

首先看一个例子，也可参考linux设备树语法中的gpio示例。该示例选自openwrt的gpio-button-hotblug驱动。

设备树code：

    gpio-keys-polled {
        compatible = "gpio-keys-polled";
        #address-cells = <1>;
        #size-cells = <0>;
        poll-interval = <20>;
        bat {
            label = "bat";
            gpios = <&gpio0 9 1>;
            linux,code = <0x211>;
        };
        reset {
            label = "reset";
            gpios = <&gpio0 10 1>;
            linux,code = <0x198>;
        };
        mode {
            label = "mode";
            gpios = <&gpio0 14 1>;
            linux,code = <0x100>;
            linux,input-type = <5>;
        };
    }; 

驱动相关code：

#ifdef CONFIG_OF
static struct gpio_keys_platform_data *
gpio_keys_get_devtree_pdata(struct device *dev)
{
    struct device_node *node, *pp;
    struct gpio_keys_platform_data *pdata;
    struct gpio_keys_button *button;
    int error;
    int nbuttons;
    int i = 0;

    node = dev->of_node;
    if (!node)
        return NULL;

    nbuttons = of_get_child_count(node);
    if (nbuttons == 0)
        return NULL;

    pdata = devm_kzalloc(dev, sizeof(*pdata) + nbuttons * (sizeof *button),
        GFP_KERNEL);
    if (!pdata) {
        error = -ENOMEM;
        goto err_out;
    }

    pdata->buttons = (struct gpio_keys_button *)(pdata + 1);
    pdata->nbuttons = nbuttons;

    pdata->rep = !!of_get_property(node, "autorepeat", NULL);
    of_property_read_u32(node, "poll-interval", &pdata->poll_interval);

    for_each_child_of_node(node, pp) {
        enum of_gpio_flags flags;

        if (!of_find_property(pp, "gpios", NULL)) {
            pdata->nbuttons--;
            dev_warn(dev, "Found button without gpios
");
            continue;
        }

        button = &pdata->buttons[i++];

        button->gpio = of_get_gpio_flags(pp, 0, &flags);
        button->active_low = flags & OF_GPIO_ACTIVE_LOW;

        if (of_property_read_u32(pp, "linux,code", &button->code)) {
            dev_err(dev, "Button without keycode: 0x%x
",
                button->gpio);
            error = -EINVAL;
            goto err_out;
        }

        button->desc = of_get_property(pp, "label", NULL);

        if (of_property_read_u32(pp, "linux,input-type", &button->type))
            button->type = EV_KEY;

        button->wakeup = !!of_get_property(pp, "gpio-key,wakeup", NULL);

        if (of_property_read_u32(pp, "debounce-interval",
                    &button->debounce_interval))
            button->debounce_interval = 5;
    }

    if (pdata->nbuttons == 0) {
        error = -EINVAL;
        goto err_out;
    }

    return pdata;

err_out:
    return ERR_PTR(error);
}

static struct of_device_id gpio_keys_of_match[] = {
    { .compatible = "gpio-keys", },
    { },
};
MODULE_DEVICE_TABLE(of, gpio_keys_of_match);

static struct of_device_id gpio_keys_polled_of_match[] = {
    { .compatible = "gpio-keys-polled", },
    { },
};
MODULE_DEVICE_TABLE(of, gpio_keys_polled_of_match);

#else

static inline struct gpio_keys_platform_data *
gpio_keys_get_devtree_pdata(struct device *dev)
{
    return NULL;
}
#endif

static struct platform_driver gpio_keys_driver = {
    .probe  = gpio_keys_probe,
    .remove = gpio_keys_remove,
    .driver = {
        .name   = "gpio-keys",
        .owner  = THIS_MODULE,
        .of_match_table = of_match_ptr(gpio_keys_of_match),
    },
};

static struct platform_driver gpio_keys_polled_driver = {
    .probe  = gpio_keys_polled_probe,
    .remove = gpio_keys_remove,
    .driver = {
        .name   = "gpio-keys-polled",
        .owner  = THIS_MODULE,
        .of_match_table = of_match_ptr(gpio_keys_polled_of_match),
    },
};

static int __init gpio_button_init(void)
{
    int ret;

    ret = platform_driver_register(&gpio_keys_driver);
    if (ret)
        return ret;

    ret = platform_driver_register(&gpio_keys_polled_driver);
    if (ret)
        platform_driver_unregister(&gpio_keys_driver);

    return ret;
}

static void __exit gpio_button_exit(void)
{
    platform_driver_unregister(&gpio_keys_driver);
    platform_driver_unregister(&gpio_keys_polled_driver);
}

module_init(gpio_button_init);
module_exit(gpio_button_exit);

该驱动同时注册了两种设备驱动：gpio_keys_driver和gpio_keys_polled_driver，前者采用中断方式检测按键状态，后者通过轮询方式检测案件状态。

OF API

设备树API通常以of_开头，实现代码位于drivers/of目录下，drivers/of目录下文件如下：

address.c  fdt.c      of_mtd.c  of_pci_irq.c       pdt.c
base.c     irq.c      of_net.c  of_private.h       platform.c
device.c   of_mdio.c  of_pci.c  of_reserved_mem.c  selftest.c

include/linux/目录下头文件：

of_address.h       of_gpio.h          of_mdio.h          of_pdt.h
of_device.h        of.h               of_mtd.h           of_platform.h
of_dma.h           of_iommu.h         of_net.h           of_reserved_mem.h
of_fdt.h           of_irq.h           of_pci.h 

0. 数据结构

of.h为基础头文件，包含相关数据结构的定义。

typedef u32 phandle;
typedef u32 ihandle;

struct property {
    char    *name;
    int length;
    void    *value;
    struct property *next;
    unsigned long _flags;
    unsigned int unique_id;
};
struct device_node {
    const char *name;
    const char *type;
    phandle phandle;
    const char *full_name;

    struct  property *properties;
    struct  property *deadprops;    /* removed properties */
    struct  device_node *parent;
    struct  device_node *child;
    struct  device_node *sibling;
    struct  device_node *next;  /* next device of same type */
    struct  device_node *allnext;   /* next in list of all nodes */
    struct  proc_dir_entry *pde;    /* this node's proc directory */
    struct  kref kref;
    unsigned long _flags;
    void    *data;
#if defined(CONFIG_SPARC)
    const char *path_component_name;
    unsigned int unique_id;
    struct of_irq_controller *irq_trans;
#endif
};

#define MAX_PHANDLE_ARGS 8
struct of_phandle_args {
    struct device_node *np;
    int args_count;
    uint32_t args[MAX_PHANDLE_ARGS];
};
#define of_match_ptr(_ptr)  (_ptr)

1. 寻找节点

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

判断设备结点的compatible 属性是否包含compat指定的字符串。当一个驱动支持2个或多个设备的时候，这些不同.dts文件中设备的compatible 属性都会进入驱动 OF匹配表。因此驱动可以透过Bootloader传递给内核的Device Tree中的真正结点的compatible 属性以确定究竟是哪一种设备，从而根据不同的设备类型进行不同的处理。

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

根据compatible属性，获得设备结点。遍历Device Tree中所有的设备结点，看看哪个结点的类型、compatible属性与本函数的输入参数匹配，大多数情况下，from、type为NULL，则表示遍历所有节点。

2. 读取属性

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);

int of_property_read_u64(const struct device_node *np, const char

*propname, u64 *out_value);

读取设备结点np的属性名为propname，类型为8、16、32、64位整型数组的属性。对于32位处理器来讲，最常用的是of_property_read_u32_array()。

of_property_read_u32_array(np, "arm,data-latency", data, ARRAY_SIZE(data)); 

of_property_read_u32_array(np, propname, out_value, 1); 

int of_property_read_string(struct device_node *np, const char

*propname, const char **out_string);

int of_property_read_string_index(struct device_node *np, const char

    *propname, int index, const char **output);

前者读取字符串属性，后者读取字符串数组属性中的第index个字符串。

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。

3. 内存映射

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。采用Device Tree后，大量的设备驱动通过of_iomap()进行映射，而不再通过传统的ioremap。

4. 解析中断

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

透过Device Tree或者设备的中断号，实际上是从.dts中的interrupts属性解析出中断号。若设备使用了多个中断，index指定中断的索引号。

5. 获取与节点对应的platform_device

struct platform_device *of_find_device_by_node(struct device_node *np);

在拿到device_node的情况下，反向获取对应的platform_device。

在已知platform_device的情况下，想获取device_node，如下：

static int imx_gpio_probe (struct platform_device *op)
{
     struct device_node *dn = op->dev.of_node;
     ...               
}

参考：

1. linux设备树语法

2. ARM Linux 3.x的设备树（Device Tree）