Linux内核sysfs机制分析

第一部分为sysfs操作：该部分涉及目录创建、文件创建、文件打开、文件读取。
1.目录创建
目录创建函数为sysfs_create_dir_ns()。

/**
 * sysfs_create_dir_ns - create a directory for an object with a namespace tag
 * @kobj: object we're creating directory for
 * @ns: the namespace tag to use
 */
int sysfs_create_dir_ns(struct kobject *kobj, const void *ns)
{
	struct kernfs_node *parent, *kn;

	BUG_ON(!kobj);
//  确定父目录，输入kobj无parent时，指定parent为sysfs的root，表现为/sys/下面的目录
	if (kobj->parent)
		parent = kobj->parent->sd;
	else
		parent = sysfs_root_kn;

	if (!parent)
		return -ENOENT;

	kn = kernfs_create_dir_ns(parent, kobject_name(kobj),
				  S_IRWXU | S_IRUGO | S_IXUGO, kobj, ns);

	kobj->sd = kn;
	return 0;
}

/**
 * kernfs_create_dir_ns - create a directory
 * @parent: parent in which to create a new directory
 * @name: name of the new directory
 * @mode: mode of the new directory
 * @priv: opaque data associated with the new directory
 * @ns: optional namespace tag of the directory
 *
 * Returns the created node on success, ERR_PTR() value on failure.
 */
struct kernfs_node *kernfs_create_dir_ns(struct kernfs_node *parent,
					 const char *name, umode_t mode,
					 void *priv, const void *ns)
{
	struct kernfs_node *kn;
	int rc;

	/* allocate */
	kn = kernfs_new_node(parent, name, mode | S_IFDIR, KERNFS_DIR);
//  调用kmem_cache_zalloc()完成了目录结点的创建，，flags = KERNFS_DIR;
	if (!kn)
		return ERR_PTR(-ENOMEM);

	kn->dir.root = parent->dir.root;
	kn->ns = ns;
	kn->priv = priv;

	/* link in */
	rc = kernfs_add_one(kn);
// 将新创建的节点通过kernfs_link_sibling(红黑树插入)加入到sysfs中
	if (!rc)
		return kn;

	kernfs_put(kn);
	return ERR_PTR(rc);
}

2.文件创建
sysfs里的文件创建函数为sysfs_create_file()。

static inline int __must_check sysfs_create_file(struct kobject *kobj,
						 const struct attribute *attr)
{
	return sysfs_create_file_ns(kobj, attr, NULL);
}

/**
 * sysfs_create_file_ns - create an attribute file for an object with custom ns
 * @kobj: object we're creating for
 * @attr: attribute descriptor
 * @ns: namespace the new file should belong to
 */
int sysfs_create_file_ns(struct kobject *kobj, const struct attribute *attr,
			 const void *ns)
{
	BUG_ON(!kobj || !kobj->sd || !attr);

	return sysfs_add_file_mode_ns(kobj->sd, attr, false, attr->mode, ns);
}

int sysfs_add_file_mode_ns(struct kernfs_node *parent,
			   const struct attribute *attr, bool is_bin,
			   umode_t mode, const void *ns)
{
	struct lock_class_key *key = NULL;
	const struct kernfs_ops *ops;
	struct kernfs_node *kn;
	loff_t size;

	if (!is_bin) {
		struct kobject *kobj = parent->priv;
		const struct sysfs_ops *sysfs_ops = kobj->ktype->sysfs_ops;  // 得到了kobj对应的sysfs_ops，
		if (sysfs_ops->show && sysfs_ops->store) {
			if (mode & SYSFS_PREALLOC)
				ops = &sysfs_prealloc_kfops_rw;
			else
				ops = &sysfs_file_kfops_rw;
		} else if (sysfs_ops->show) {
			if (mode & SYSFS_PREALLOC)
				ops = &sysfs_prealloc_kfops_ro;
			else
				ops = &sysfs_file_kfops_ro;
		} else if (sysfs_ops->store) {
			if (mode & SYSFS_PREALLOC)
				ops = &sysfs_prealloc_kfops_wo;
			else
				ops = &sysfs_file_kfops_wo;
		} else
			ops = &sysfs_file_kfops_empty;

		size = PAGE_SIZE;
	} else {
		struct bin_attribute *battr = (void *)attr;

		if (battr->mmap)
			ops = &sysfs_bin_kfops_mmap;
		else if (battr->read && battr->write)
			ops = &sysfs_bin_kfops_rw;
		else if (battr->read)
			ops = &sysfs_bin_kfops_ro;
		else if (battr->write)
			ops = &sysfs_bin_kfops_wo;
		else
			ops = &sysfs_file_kfops_empty;

		size = battr->size;
	}

#ifdef CONFIG_DEBUG_LOCK_ALLOC
	if (!attr->ignore_lockdep)
		key = attr->key ?: (struct lock_class_key *)&attr->skey;
#endif
	kn = __kernfs_create_file(parent, attr->name, mode & 0777, size, ops,
				  (void *)attr, ns, key);
// __kernfs_create_file()与前面目录文件的创建（kernfs_create_dir_ns()）非常相似，不同的是目录使用kernfs_node.dir，而文件使用kernfs_node.attr；其实dir和attr属于一个union（参看kernfs_node定义）。
	return 0;
}

/**
 * __kernfs_create_file - kernfs internal function to create a file
 * @parent: directory to create the file in
 * @