转自：https://biscuitos.github.io/blog/DTS-of_node_put/

Github: of_node_put

Email: BuddyZhang1 buddy.zhang@aliyun.com

目录

• 源码分析

• 实践

• 附录

---

源码分析

of_node_put

of_node_put
|
|---kref_get

函数作用：增加节点的引用计数

平台： ARM64

linux： 3.10/4.18/5.0

#ifdef CONFIG_OF_DYNAMIC
/**
*    of_node_release - release a dynamically allocated node
*    @kref:  kref element of the node to be released
*
*    In of_node_put() this function is passed to kref_put()
*    as the destructor.
*/
static void of_node_release(struct kref *kref)
{
    struct device_node *node = kref_to_device_node(kref);
    struct property *prop = node->properties;

    /* We should never be releasing nodes that haven't been detached. */
    if (!of_node_check_flag(node, OF_DETACHED)) {
        pr_err("ERROR: Bad of_node_put() on %s\n", node->full_name);
        dump_stack();
        kref_init(&node->kref);
        return;
    }

    if (!of_node_check_flag(node, OF_DYNAMIC))
        return;

    while (prop) {
        struct property *next = prop->next;
        kfree(prop->name);
        kfree(prop->value);
        kfree(prop);
        prop = next;

        if (!prop) {
            prop = node->deadprops;
            node->deadprops = NULL;
        }
    }
    kfree(node->full_name);
    kfree(node->data);
    kfree(node);
}

/**
*    of_node_put - Decrement refcount of a node
*    @node:    Node to dec refcount, NULL is supported to
*        simplify writing of callers
*
*/
void of_node_put(struct device_node *node)
{
    if (node)
        kref_put(&node->kref, of_node_release);
}
EXPORT_SYMBOL(of_node_put);
#else /* CONFIG_OF_DYNAMIC */
static inline void of_node_put(struct device_node *node) { }
#endif /* !CONFIG_OF_DYNAMIC */

参数 node 指向节点

如果内核不支持 CONFIG_OF_DYNAMIC，直接返回节点。

---

实践

实践目的是在 DTS 文件中构建一个私有节点，私有节点默认打开，然后通过调用 of_node_put() 函数添加引用计数，函数定义如下：

static inline void of_node_put(struct device_node *node) { }

这个函数经常用用于减少节点的引用计数

本文实践基于 Linux 4.20.8 arm32 平台，开发者可以参考如下文章快速搭建一个 调试环境：

Establish Linux 4.20.8 on ARM32

DTS 文件

由于使用的平台是 ARM32，所以在源码 /arch/arm/boot/dts 目录下创建一个 DTSI 文件，在 root 节点之下创建一个名为 DTS_demo 的子节点，节点默认打开。具体内容如下：

/*
 * DTS Demo Code
 *
 * (C) 2019.01.06 <buddy.zhang@aliyun.com>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 */
/ {
        DTS_demo {
                compatible = "DTS_demo, BiscuitOS";
                status = "okay";
        };
};

创建完毕之后，将其保存并命名为 DTS_demo.dtsi。然后开发者在 Linux 4.20.8 的源 码中，找到 arch/arm/boot/dts/vexpress-v2p-ca9.dts 文件，然后在文件开始地方添 加如下内容：

#include "DTS_demo.dtsi"

编写 DTS 对应驱动

准备好 DTSI 文件之后，开发者编写一个简单的驱动，这个驱动作为 DTS_demo 的设备驱 动，在 DTS 机制遍历时会调用匹配成功的驱动，最终运行驱动里面的代码。在驱动的 probe 函数中，首先获得驱动所对应的节点，通过 platform_device 的 of_node 成员传 递。获得驱动对应的节点之后，通过调用 of_node_put() 函数减少节点的引用计数，驱 动编写如下：

/*
 * DTS: of_node_put
 *
 * static inline struct device_node *of_node_put(struct device_node *node)
 *
 * (C) 2019.01.11 BuddyZhang1 <buddy.zhang@aliyun.com>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 */

/*
 * Private DTS file: DTS_demo.dtsi
 *
 * / {
 *        DTS_demo {
 *                compatible = "DTS_demo, BiscuitOS";
 *                status = "okay";
 *        };
 * };
 *
 * On Core dtsi:
 * 
 * include "DTS_demo.dtsi"
 */

#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/of_platform.h>
#include <linux/module.h>

/* define name for device and driver */
#define DEV_NAME "DTS_demo"

/* probe platform driver */
static int DTS_demo_probe(struct platform_device *pdev)
{
    struct device_node *np = pdev->dev.of_node;

    printk("DTS probe entence...\n");

    if (of_node_check_flag(np, OF_DYNAMIC)) {
        printk("%s is dynamic\n", np->name);
        /* Add reference count for device node */
        of_node_get(np);
        /* Declare reference count for device node */
        of_node_put(np);
    }

    return 0;
}

/* remove platform driver */
static int DTS_demo_remove(struct platform_device *pdev)
{
    return 0;
}

static const struct of_device_id DTS_demo_of_match[] = {
    { .compatible = "DTS_demo, BiscuitOS",  },
    { },
};
MODULE_DEVICE_TABLE(of, DTS_demo_of_match);

/* platform driver information */
static struct platform_driver DTS_demo_driver = {
    .probe  = DTS_demo_probe,
    .remove = DTS_demo_remove,
    .driver = {
        .owner = THIS_MODULE,
        .name = DEV_NAME, /* Same as device name */
        .of_match_table = DTS_demo_of_match,
    },
};
module_platform_driver(DTS_demo_driver);

编写好驱动之后，将其编译进内核。编译内核和 dts，如下命令：

make ARCH=arm BiscuitOS/output/linux-4.20.8/arm-linux-gnueabi/arm-linux-gnueabi/bin/arm-linux-gnueabi- j8
make ARCH=arm BiscuitOS/output/linux-4.20.8/arm-linux-gnueabi/arm-linux-gnueabi/bin/arm-linux-gnueabi- dtbs

启动内核，在启动阶段就会运行驱动的 probe 函数，并打印如下信息：

[    3.565634] DTS demo probe entence
[    3.565649] DTS_demo is dynamic