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Linux 设备模型【1】- devm_kzalloc()

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前言

Linux Kernel devm_* API源码分析
本文通过对devm_kzalloc函数的实现进行解析，帮助理解devm系列api的用法。

devm系列api的主要特别之处：

• 使用devm系列申请到的资源可以由系统自动释放，解放双手

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一、devm_kzalloc & devm_kfree代码

struct devres {
    struct devres_node      node;
    /* -- 3 pointers */
    unsigned long long      data[]; /* guarantee ull alignment */
};
struct devres_node {
    struct list_head        entry;
    dr_release_t            release;
#ifdef CONFIG_DEBUG_DEVRES
    const char          *name;
    size_t              size;
#endif
};

void * devm_kzalloc(struct device *dev, size_t size, gfp_t gfp)
{
    struct devres *dr;
 
    /* use raw alloc_dr for kmalloc caller tracing */
    dr = alloc_dr(devm_kzalloc_release, size, gfp);
    if (unlikely(!dr))
        return NULL;
 
    set_node_dbginfo(&dr->node, "devm_kzalloc_release", size);
    devres_add(dev, dr->data);
    return dr->data;
}

static __always_inline struct devres * alloc_dr(dr_release_t release,
                        size_t size, gfp_t gfp)
{
    size_t tot_size = sizeof(struct devres) + size;
    struct devres *dr;
 
    dr = kmalloc_track_caller(tot_size, gfp);
    if (unlikely(!dr))
        return NULL;
 
    memset(dr, 0, tot_size);
    INIT_LIST_HEAD(&dr->node.entry);
    dr->node.release = release;
    return dr;
}
/*alloc_dr函数负责分配内存,返回的devres就包含分配到的内存,通过devres.data来访问即可. 
devres_add(dev, dr->data);函数将数据与device进行绑定,实际调用add_dr,在这里将资源的节点头添加到device的devres_head链表中*/
/**
 * devres_add - Register device resource
 * @dev: Device to add resource to
 * @res: Resource to register
 *
 * Register devres @res to @dev.  @res should have been allocated
 * using devres_alloc().  On driver detach, the associated release
 * function will be invoked and devres will be freed automatically.
 */
void devres_add(struct device *dev, void *res)
{
	struct devres *dr = container_of(res, struct devres, data);
	unsigned long flags;

	spin_lock_irqsave(&dev->devres_lock, flags);
	add_dr(dev, &dr->node);
	spin_unlock_irqrestore(&dev->devres_lock, flags);
}
static void add_dr(struct device *dev, struct devres_node *node)
{
    devres_log(dev, node, "ADD");
    BUG_ON(!list_empty(&node->entry));
    list_add_tail(&node->entry, &dev->devres_head);
}

/*现在资源与dev的绑定已经完成,下面就找一找是如何释放资源的. 
device_release_driver -> devres_release_all->release_nodes*/
static int release_nodes(struct device *dev, struct list_head *first,
             struct list_head *end, unsigned long flags)
    __releases(&dev->devres_lock)
{
    LIST_HEAD(todo);
    int cnt;
    struct devres *dr, *tmp;
 
    cnt = remove_nodes(dev, first, end, &todo);
 
    spin_unlock_irqrestore(&dev->devres_lock, flags);
 
    /* Release.  Note that both devres and devres_group are
     * handled as devres in the following loop.  This is safe.
     */
    list_for_each_entry_safe_reverse(dr, tmp, &todo, node.entry) {
        devres_log(dev, &dr->node, "REL");
        dr->node.release(dev, dr->data);
        kfree(dr);
    }
 
    return cnt;
}

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二、资源释放

kzalloc()实现了kmalloc()+memset()的功能，而 devm_kzalloc() 是具有资源管理的 kzalloc()。使用资源管理（resource-managed）函数分配的内存，是会与所属设备相关联。当设备从系统中分离或者设备驱动被卸载，该内存会被自动释放。也可以通过devm_kfree()来释放内存。

static inline void *kzalloc(size_t size, gfp_t flags)
{
	return kmalloc(size, flags | __GFP_ZERO);
}
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