:Linux文件系统之sysfs

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一:前言在设备模型中,sysfs文件系统用来表示设备的结构.将设备的层次结构形象的反应到用户空间中.用户空间可以修改sysfs中的文件属性来修改设备的属性值,今天我们就来详细分析一下,sysfs的实现.二:sysfs的初始化和挂载Sysfs文件系统的初始化是在sysfs_init()中完成的,代码如下:int __init sysfs_init(void){   int err = -ENOMEM;//创建一个分配sysfs_dirent的cache       sysfs_dir_cachep = kmem_cache_create("sysfs_dir_cache",                                         sizeof(struct sysfs_dirent),                                         0, 0, NULL);       if (!sysfs_dir_cachep)              goto out;        err = sysfs_inode_init();       if (err)              goto out_err;       //注册sysfs文件系统s       err = register_filesystem(&sysfs_fs_type);       if (!err) {              //挂载sysfs文件系统              sysfs_mount = kern_mount(&sysfs_fs_type);              if (IS_ERR(sysfs_mount)) {                     printk(KERN_ERR "sysfs: could not mount!\n");                     err = PTR_ERR(sysfs_mount);                     sysfs_mount = NULL;                     unregister_filesystem(&sysfs_fs_type);                     goto out_err;              }       } else              goto out_err;out:       return err;out_err:       kmem_cache_destroy(sysfs_dir_cachep);       sysfs_dir_cachep = NULL;       goto out;}每个kobject对应sysfs中的一个目录,kobject的每个属性对应sysfs文件系统中的文件.struct sysfs_dirent就是用来做kobject与dentry的互相转换用的.它们的关系如下图所示:  上图表示的是一个kobject的层次结构.dentry的d_fsdata字段指定该结点所表示的sysfs_dirent.sysfs_dirent.s_parent表示它的父kobject. sysfs_dirent.s_sibling表示它的兄弟结点. sysfs_dirent.s_dir.children表示它所属的子节点.从上图可知.如果要遍历一个结点下面的子结点,只需要找到sysfs_dirent.s_dir.children结点然后按着子节点的s_sibling域遍历即可.当然,有时候也需要从struct sysfs_dirent导出它所属的dentry结点.我们在代码中遇到的时候再进行分析.Sysfs文件系统的file_system_type定义如下:static struct file_system_type sysfs_fs_type = {       .name           = "sysfs",       .get_sb          = sysfs_get_sb,       .kill_sb    = kill_anon_super,};通过前面文件系统的相关分析,我们知道在sys_mount()中最终会调用struct file_system_type的get_sb函数来实现文件系统的挂载.它的代码如下:static int sysfs_get_sb(struct file_system_type *fs_type,       int flags, const char *dev_name, void *data, struct vfsmount *mnt){       return get_sb_single(fs_type, flags, data, sysfs_fill_super, mnt);}get_sb_single()的代码在前面已经涉及到,它对super_block.以及挂载的dentry和inode的赋值是在回调函数sysfs_fill_super, mnt()中完成的.代码如下:static int sysfs_fill_super(struct super_block *sb, void *data, int silent){       struct inode *inode;       struct dentry *root;        sb->s_blocksize = PAGE_CACHE_SIZE;       sb->s_blocksize_bits = PAGE_CACHE_SHIFT;       sb->s_magic = SYSFS_MAGIC;       sb->s_op = &sysfs_ops;       sb->s_time_gran = 1;       sysfs_sb = sb;        /* get root inode, initialize and unlock it */       inode = sysfs_get_inode(&sysfs_root);       if (!inode) {              pr_debug("sysfs: could not get root inode\n");              return -ENOMEM;       }        /* instantiate and link root dentry */       root = d_alloc_root(inode);       if (!root) {              pr_debug("%s: could not get root dentry!\n",__FUNCTION__);              iput(inode);              return -ENOMEM;       }       //将sysfs_root关联到root       root->d_fsdata = &sysfs_root;       sb->s_root = root;       return 0;}在这里要注意几个全局量. sysfs_sb表示sysfs文件系统的super_block. sysfs_root表示sysfs文件系统根目录的struct sysfs_dirent.sysfs_get_inode(&sysfs_root)用来将sysfs_root导出相应的inode.代码如下:struct inode * sysfs_get_inode(struct sysfs_dirent *sd){       struct inode *inode;       //以super_block和sd->s_ino为哈希值,到哈希表中寻找相应的inode.如果不存在,则新建       inode = iget_locked(sysfs_sb, sd->s_ino);       //对新生成的inode进行初始化       if (inode && (inode->i_state & I_NEW))              sysfs_init_inode(sd, inode);        return inode;}首先,它以sysfs文件系统的super_block和struct sysfs_dirent.的s_ino成员的值做为哈希值到哈希表中寻找相应的inode.如果在哈希表中不存在这个inode,那就新建一个,并将它链入到哈希表.之后,调用sysfs_init_inode()对生成的inode进行初始化.显然.在mount的时候是不会生成inode的.必定会进入sysfs_init_inode()函数.代码如下:static void sysfs_init_inode(struct sysfs_dirent *sd, struct inode *inode){       struct bin_attribute *bin_attr;        inode->i_blocks = 0;       inode->i_mapping->a_ops = &sysfs_aops;       inode->i_mapping->backing_dev_info = &sysfs_backing_dev_info;       inode->i_op = &sysfs_inode_operations;       inode->i_ino = sd->s_ino;       lockdep_set_class(&inode->i_mutex, &sysfs_inode_imutex_key);        if (sd->s_iattr) {              /* sysfs_dirent has non-default attributes               * get them for the new inode from persistent copy               * in sysfs_dirent               */              set_inode_attr(inode, sd->s_iattr);       } else              set_default_inode_attr(inode, sd->s_mode);         /* initialize inode according to type */       switch (sysfs_type(sd)) {       case SYSFS_DIR:              inode->i_op = &sysfs_dir_inode_operations;              inode->i_fop = &sysfs_dir_operations;              inode->i_nlink = sysfs_count_nlink(sd);              break;       case SYSFS_KOBJ_ATTR:              inode->i_size = PAGE_SIZE;              inode->i_fop = &sysfs_file_operations;              break;       case SYSFS_KOBJ_BIN_ATTR:              bin_attr = sd->s_bin_attr.bin_attr;              inode->i_size = bin_attr->size;              inode->i_fop = &bin_fops;              break;       case SYSFS_KOBJ_LINK:              inode->i_op = &sysfs_symlink_inode_operations;              break;       default:              BUG();       }        unlock_new_inode(inode);}在这里,我们可以看到sysfs文件系统中的各种操作函数了..在syfs文件系统中,怎么样判断一个目录下是否有这个文件呢?在前面有关文件系统的分析中我们可以看.有关文件的查找实际上都会由inod->i_op-> lookup()函数进行判断.在sysfs中,这个函数对应为sysfs_lookup().代码如下:static struct dentry * sysfs_lookup(struct inode *dir, struct dentry *dentry,                            struct nameidata *nd){       struct dentry *ret = NULL;       //取得父结点对应的sysfs_dirent       struct sysfs_dirent *parent_sd = dentry->d_parent->d_fsdata;       struct sysfs_dirent *sd;       struct inode *inode;        mutex_lock(&sysfs_mutex);       //父结点的sysfs_dirent中是否有相应的子结点       sd = sysfs_find_dirent(parent_sd, dentry->d_name.name);        /* no such entry */       //如果没有.这个结点是不存在的       if (!sd) {              ret = ERR_PTR(-ENOENT);              goto out_unlock;       }        /* attach dentry and inode *///如果有这个结点,为之生成inod结构.       inode = sysfs_get_inode(sd);       if (!inode) {              ret = ERR_PTR(-ENOMEM);              goto out_unlock;       }        /* instantiate and hash dentry */       dentry->d_op = &sysfs_dentry_ops;       //关联dentry与sysfs_dirent       dentry->d_fsdata = sysfs_get(sd);       d_instantiate(dentry, inode);       d_rehash(dentry);  out_unlock:       mutex_unlock(&sysfs_mutex);       return ret;}由此可见,它的判断会转入到相应的sysfs_dirent中进行判断.如果设备模型在创建目录/文件的时候并不会创建dentry或者inode.只会操作sysfs_dirent结构. 如果找到了这个结构,就为这个结构生成inode.并将其关联到denry中.sysfs_find_dirent()如下: struct sysfs_dirent *sysfs_find_dirent(struct sysfs_dirent *parent_sd,                                   const unsigned char *name){       struct sysfs_dirent *sd;        for (sd = parent_sd->s_dir.children; sd; sd = sd->s_sibling)              if (!strcmp(sd->s_name, name))                     return sd;       return NULL;}它用的搜索方法就是我们在之前分析sysfs_dirent结构所讲述的.分析到这里,sysfs的大概轮廓就出现在我们的眼前了.^_^.接下来分析sysfs文件系统中目录的创建过程 三:在sysfs文件系统中创建目录在linux设备模型中,每注册一个kobject.就会为之创建一个目录.具体的流程在分析linux设备模型的时候再给出详细的分析.创建目录的接口为: sysfs_create_dir().代码如下:int sysfs_create_dir(struct kobject * kobj){       struct sysfs_dirent *parent_sd, *sd;       int error = 0;        BUG_ON(!kobj);        //如果kobject没有指定父结点,则将其父结点指定为sysfs的根目录syfs_root       if (kobj->parent)              parent_sd = kobj->parent->sd;       else              parent_sd = &sysfs_root;        //创建目录       error = create_dir(kobj, parent_sd, kobject_name(kobj), &sd);       //kobj->sd 指向对应的sysfs_dirent       if (!error)              kobj->sd = sd;       return error;}在这里,先为结点指定父目录,然后调用create_dir()在父目录下生成结点.代码如下:static int create_dir(struct kobject *kobj, struct sysfs_dirent *parent_sd,                    const char *name, struct sysfs_dirent **p_sd){       //指定目录的模式       umode_t mode = S_IFDIR| S_IRWXU | S_IRUGO | S_IXUGO;       struct sysfs_addrm_cxt acxt;       struct sysfs_dirent *sd;       int rc;        /* allocate */       //分配并初始化一个sysfs_dirent       sd = sysfs_new_dirent(name, mode, SYSFS_DIR);       if (!sd)              return -ENOMEM;       //初始化sd->s_dir.kobj字段       sd->s_dir.kobj = kobj;        /* link in */       //acxt是一个临时变量.它用来存放父结点的相关信息        //设置acxt->parent_sd  为父结点的sysfs_dirent.acxt->parent_inode为父结点的inode       sysfs_addrm_start(&acxt, parent_sd);       //设置sd->s_parent.并按inod值按顺序链入父结点的children链表       rc = sysfs_add_one(&acxt, sd);       sysfs_addrm_finish(&acxt);        if (rc == 0)              *p_sd = sd;       else              sysfs_put(sd);        return rc;}在这里,为子节点生成了对应的sysfs_dirent.设置了它的父结点域,并将其链入到父结点的children链表.这样,在文件系统中查找父目录下面的子结点了. 四:在sysfs中创建一般属性文件Kobject的每一项属性都对应在sysfs文件系统中，kobject对应的目录下的一个文件.文件名称与属性名称相同.创建一般属性的接口为sysfs_create_file().代码如下:int sysfs_create_file(struct kobject * kobj, const struct attribute * attr){       BUG_ON(!kobj || !kobj->sd || !attr);        //kobject->sd: 为kobject表示目录的struct sysfs_dirent结构       return sysfs_add_file(kobj->sd, attr, SYSFS_KOBJ_ATTR); }最终会调用sysfs_add_file().参数attr.是要生成文件的属性值.int sysfs_add_file(struct sysfs_dirent *dir_sd, const struct attribute *attr,                 int type){       //文件对应的属性       umode_t mode = (attr->mode & S_IALLUGO) | S_IFREG;       struct sysfs_addrm_cxt acxt;       struct sysfs_dirent *sd;       int rc;        //创建一个新的sysfs_dirent.对应的名称为attr->name.即属性的名称       sd = sysfs_new_dirent(attr->name, mode, type);       if (!sd)              return -ENOMEM;       //设置属性值       sd->s_attr.attr = (void *)attr;        //将子结点的struct sysfs_dirent结构关链到父结点       sysfs_addrm_start(&acxt, dir_sd);       rc = sysfs_add_one(&acxt, sd);       sysfs_addrm_finish(&acxt);        if (rc)              sysfs_put(sd);        return rc;}这个流程与创建目录的流程大部份相同.不相同的只是创建目录时,它的父目录为上一层结点,创建文件时,它的父目录就是kobject对应的struct sysfs_dirent.这样,在kobject对应的目录下面就可以看到这个文件了.^_^文件建好之后,要怎么样去读写呢?回忆一下,在sysfs文件系统中,inode的初始化:static void sysfs_init_inode(struct sysfs_dirent *sd, struct inode *inode){       ……       …….       case SYSFS_KOBJ_ATTR:       inode->i_size = PAGE_SIZE;       inode->i_fop = &sysfs_file_operations;       ……}sysfs_file_operations的定义如下:const struct file_operations sysfs_file_operations = {       .read              = sysfs_read_file,       .write             = sysfs_write_file,       .llseek            = generic_file_llseek,       .open             = sysfs_open_file,       .release    = sysfs_release,       .poll        = sysfs_poll,}; 文件的操作全部都在这里了,我们从打开文件说起.sysfs_open_file()代码如下:static int sysfs_open_file(struct inode *inode, struct file *file){       struct sysfs_dirent *attr_sd = file->f_path.dentry->d_fsdata;       struct kobject *kobj = attr_sd->s_parent->s_dir.kobj;       struct sysfs_buffer *buffer;       struct sysfs_ops *ops;       int error = -EACCES;        /* need attr_sd for attr and ops, its parent for kobj */       if (!sysfs_get_active_two(attr_sd))              return -ENODEV;        /* every kobject with an attribute needs a ktype assigned */       //将buffer->ops设置为kobj->ktype->sysfs_ops       if (kobj->ktype && kobj->ktype->sysfs_ops)              ops = kobj->ktype->sysfs_ops;       else {              printk(KERN_ERR "missing sysfs attribute operations for "                     "kobject: %s\n", kobject_name(kobj));              WARN_ON(1);              goto err_out;       }        /* File needs write support.        * The inode's perms must say it's ok,         * and we must have a store method.        */       if (file->f_mode & FMODE_WRITE) {              if (!(inode->i_mode & S_IWUGO) || !ops->store)                     goto err_out;       }        /* File needs read support.        * The inode's perms must say it's ok, and we there        * must be a show method for it.        */       if (file->f_mode & FMODE_READ) {              if (!(inode->i_mode & S_IRUGO) || !ops->show)                     goto err_out;       }        /* No error? Great, allocate a buffer for the file, and store it        * it in file->private_data for easy access.        */       error = -ENOMEM;       buffer = kzalloc(sizeof(struct sysfs_buffer), GFP_KERNEL);       if (!buffer)              goto err_out;        mutex_init(&buffer->mutex);       buffer->needs_read_fill = 1;       buffer->ops = ops;       file->private_data = buffer;        /* make sure we have open dirent struct */       //将buffer链至attr_sd->s_attr.open链表上       error = sysfs_get_open_dirent(attr_sd, buffer);       if (error)              goto err_free;        /* open succeeded, put active references */       sysfs_put_active_two(attr_sd);       return 0;  err_free:       kfree(buffer); err_out:       sysfs_put_active_two(attr_sd);       return error;}在这段代码中,需要注意以下几个操作,1:buffer链接在file-> private_data.具buffer还被链接在sysfs_dirent->s_attr.open.这样.VFS通过file.设备模型通过kobject->sd->s_attr.open都能找到这个要操作的 buffer2:buffer->ops被设置为了kobject->ktype->sysfs_ops 文件的写操作入口如下:static ssize_tsysfs_write_file(struct file *file, const char __user *buf, size_t count, loff_t *ppos){       struct sysfs_buffer * buffer = file->private_data;       ssize_t len;        mutex_lock(&buffer->mutex);       //将buf中的内容copy到了buffer->page       len = fill_write_buffer(buffer, buf, count);       //与设备模型的交互       if (len > 0)              len = flush_write_buffer(file->f_path.dentry, buffer, len);       //更新ppos       if (len > 0)              *ppos += len;       mutex_unlock(&buffer->mutex);       return len;}首先,调用fill_write_buffer()将用户空间传值下来的数值copy到buffer->page.然后再调用flush_write_buffer()与设备模型进行交互.Flush_wirte_buffer()代码如下:static intflush_write_buffer(struct dentry * dentry, struct sysfs_buffer * buffer, size_t count){       struct sysfs_dirent *attr_sd = dentry->d_fsdata;       struct kobject *kobj = attr_sd->s_parent->s_dir.kobj;       struct sysfs_ops * ops = buffer->ops;       int rc;        /* need attr_sd for attr and ops, its parent for kobj */       if (!sysfs_get_active_two(attr_sd))              return -ENODEV;        rc = ops->store(kobj, attr_sd->s_attr.attr, buffer->page, count);        sysfs_put_active_two(attr_sd);        return rc;}我们在分析open()操作的时候曾分析到.buffer的ops是kobject->ktype->ops.在这里,它相当于调用了kobject->ktype->ops->store().参数分别为:操作的kobject.文件对应的属性.写入的值和值的长度.Sysfs这样设计,主要是在VFS保持一个统一的接口,因为每一个kobject对应的属性值都不相同,.相应的,操作方法也不一样,这样,在ktype中就区别开来了. 文件的读操作相应接口为sysfs_read_file().代码如下:static ssize_tsysfs_read_file(struct file *file, char __user *buf, size_t count, loff_t *ppos){       struct sysfs_buffer * buffer = file->private_data;       ssize_t retval = 0;        mutex_lock(&buffer->mutex);       //从设备模型中将值取出.并存入buffer->page中       if (buffer->needs_read_fill) {              retval = fill_read_buffer(file->f_path.dentry,buffer);              if (retval)                     goto out;       }       //将buffer->page中的值copy到用户空间的buf       pr_debug("%s: count = %zd, ppos = %lld, buf = %s\n",               __FUNCTION__, count, *ppos, buffer->page);       retval = simple_read_from_buffer(buf, count, ppos, buffer->page,                                    buffer->count);out:       mutex_unlock(&buffer->mutex);       return retval;}读操作的流程刚好和写操作流程相反.它先从设备模型中取值,然后再copy到用户空间.fill_read_buffer的代码如下:static int fill_read_buffer(struct dentry * dentry, struct sysfs_buffer * buffer){       struct sysfs_dirent *attr_sd = dentry->d_fsdata;       struct kobject *kobj = attr_sd->s_parent->s_dir.kobj;       struct sysfs_ops * ops = buffer->ops;       int ret = 0;       ssize_t count;        if (!buffer->page)              buffer->page = (char *) get_zeroed_page(GFP_KERNEL);       if (!buffer->page)              return -ENOMEM;        /* need attr_sd for attr and ops, its parent for kobj */       if (!sysfs_get_active_two(attr_sd))              return -ENODEV;        buffer->event = atomic_read(&attr_sd->s_attr.open->event);       count = ops->show(kobj, attr_sd->s_attr.attr, buffer->page);        sysfs_put_active_two(attr_sd);        /*        * The code works fine with PAGE_SIZE return but it's likely to        * indicate truncated result or overflow in normal use cases.        */       if (count >= (ssize_t)PAGE_SIZE) {              print_symbol("fill_read_buffer: %s returned bad count\n",                     (unsigned long)ops->show);              /* Try to struggle along */              count = PAGE_SIZE - 1;       }       if (count >= 0) {              buffer->needs_read_fill = 0;              buffer->count = count;       } else {              ret = count;       }       return ret;}在这里,我们看到,最终会调用kobject->ktype->ops->show()方法.参数含义同写操作中是一样的. 五:在sysfs中创建二进制属性文件二制制属性通常用于firmware 中.它用来更新firmware 的固件.它的接口为sysfs_create_bin_file()代码如下:int sysfs_create_bin_file(struct kobject * kobj, struct bin_attribute * attr){       BUG_ON(!kobj || !kobj->sd || !attr);        return sysfs_add_file(kobj->sd, &attr->attr, SYSFS_KOBJ_BIN_ATTR);}Sysfs_add_file()这个函数我们在之前已经分析过.在这个地方,可能会引起迷糊.因为在sysfs_add_file()中.有:int sysfs_add_file(struct sysfs_dirent *dir_sd, const struct attribute *attr,                 int type){       ……       sd->s_attr.attr = (void *)attr;       ……}这里为什么是sd->a_attr呢?应该是sd-> s_bin_attr才对吧!仔细观察struct sysfs_dirent的结构,如下:struct sysfs_dirent {       atomic_t         s_count;       atomic_t         s_active;       struct sysfs_dirent  *s_parent;       struct sysfs_dirent  *s_sibling;       const char             *s_name;        union {              struct sysfs_elem_dir            s_dir;              struct sysfs_elem_symlink    s_symlink;              struct sysfs_elem_attr           s_attr;              struct sysfs_elem_bin_attr     s_bin_attr;       };        unsigned int           s_flags;       ino_t                     s_ino;       umode_t                s_mode;       struct iattr             *s_iattr;};注意中间是一个union 结构,实际上只占用一个内存空间.而且s_attr与s_bin_arre的第一个属性都为struct attribute.所以在这里, sd->a_attr与sd-> s_bin_attr;的效果是一样的.内核这样处理,又少用了一个接口.看来作者在设计的时候,花了很多的心思. 二进制的文件读写与普通属性的文件读写方式大部份都一样,所不同的是.二进制文件的读写接口分别是: sysfs_dirent ->s_bin_attr.bin_attr->read和sysfs_dirent ->s_bin_attr.bin_attr->write 六:sysfs文件系统中的链接文件创建链接文件的接口为: sysfs_create_link().代码如下:int sysfs_create_link(struct kobject * kobj, struct kobject * target, const char * name){       struct sysfs_dirent *parent_sd = NULL;       struct sysfs_dirent *target_sd = NULL;       struct sysfs_dirent *sd = NULL;       struct sysfs_addrm_cxt acxt;       int error;        BUG_ON(!name);        if (!kobj)              parent_sd = &sysfs_root;       else              parent_sd = kobj->sd;        error = -EFAULT;       if (!parent_sd)              goto out_put;        /* target->sd can go away beneath us but is protected with        * sysfs_assoc_lock.  Fetch target_sd from it.        */       spin_lock(&sysfs_assoc_lock);       if (target->sd)              target_sd = sysfs_get(target->sd);       spin_unlock(&sysfs_assoc_lock);        error = -ENOENT;       if (!target_sd)              goto out_put;        error = -ENOMEM;       sd = sysfs_new_dirent(name, S_IFLNK|S_IRWXUGO, SYSFS_KOBJ_LINK);       if (!sd)              goto out_put;        sd->s_symlink.target_sd = target_sd;       target_sd = NULL; /* reference is now owned by the symlink */        sysfs_addrm_start(&acxt, parent_sd);       error = sysfs_add_one(&acxt, sd);       sysfs_addrm_finish(&acxt);        if (error)              goto out_put;        return 0;  out_put:       sysfs_put(target_sd);       sysfs_put(sd);       return error;}上面的操作大部份都与普通文件的创建相似,所不同的只是下面这段代码的区别:sd->s_symlink.target_sd = target_sd;就是在sd->s_symlink.target_sd保存到链接目的地的sysfs_dirent.符号链接的操作如下所示:const struct inode_operations sysfs_symlink_inode_operations = {       .readlink = generic_readlink,       .follow_link = sysfs_follow_link,       .put_link = sysfs_put_link,};在通过符号链接查找文件的时候,在VFS中会调用inod->i_op->.readlink()进行操作.它的代码如下:int generic_readlink(struct dentry *dentry, char __user *buffer, int buflen){       struct nameidata nd;       void *cookie;        nd.depth = 0;       cookie = dentry->d_inode->i_op->follow_link(dentry, &nd);       if (!IS_ERR(cookie)) {              int res = vfs_readlink(dentry, buffer, buflen, nd_get_link(&nd));              if (dentry->d_inode->i_op->put_link)                     dentry->d_inode->i_op->put_link(dentry, &nd, cookie);              cookie = ERR_PTR(res);       }       return PTR_ERR(cookie);}它的操作和其它文件系统一样,都是通用follow_link()取得目的地的路径.然后保存到nd->saved_names[]中,然后,调用vfs_readlink()将目标路径copy到buffer中.接着,调用put_link进行事后处理工作. Follow_link()的操作如下示:static void *sysfs_follow_link(struct dentry *dentry, struct nameidata *nd){       int error = -ENOMEM;       unsigned long page = get_zeroed_page(GFP_KERNEL);       if (page)              error = sysfs_getlink(dentry, (char *) page);        nd_set_link(nd, error ? ERR_PTR(error) : (char *)page);       return NULL;}Ne_set_link()是将page中的值copy到nd->saved_name[]中.sysfs_getlink()的代码如下:sysfs_getlink()-àsysfs_get_target_path()static int sysfs_get_target_path(struct sysfs_dirent *parent_sd,                             struct sysfs_dirent *target_sd, char *path){       struct sysfs_dirent *base, *sd;       char *s = path;       int len = 0;        /* go up to the root, stop at the base */       base = parent_sd;       while (base->s_parent) {              sd = target_sd->s_parent;              while (sd->s_parent && base != sd)                     sd = sd->s_parent;               if (base == sd)                     break;               strcpy(s, "../");              s += 3;              base = base->s_parent;       }        /* determine end of target string for reverse fillup */       sd = target_sd;       while (sd->s_parent && sd != base) {              len += strlen(sd->s_name) + 1;              sd = sd->s_parent;       }       /* check limits */       if (len < 2)              return -EINVAL;       len--;       if ((s - path) + len > PATH_MAX)              return -ENAMETOOLONG;       /* reverse fillup of target string from target to base */       sd = target_sd;       while (sd->s_parent && sd != base) {              int slen = strlen(sd->s_name);               len -= slen;              strncpy(s + len, sd->s_name, slen);              if (len)                     s[--len] = '/';               sd = sd->s_parent;       }       return 0;}这段代码的逻辑比较简单.它先是找到目标路径和当前路径相同的父结点,然后再沿着目标结点往相同的父结点向上走,将路径依次从缓存区后面往前面保存.例如: /sys/eric/kernel/test链接到了/sys/sys/device.它先找到两个路径共有的父结点/sys此时缓存区为:/sys然后,沿着/sys/sys/device往/sys移动.路径加从缓存区的后面往前面加.依次为:1: /sys/     /device2:/sys/sys/device这样就找到了目的地的路径. ^_^.后面sysfs_put_link()的操作就不再讲述了,它只是释放掉缓存区. 六:小结在本小节里,我们深入探讨了sysfs文件系统的实现机理.这对于我们理解linux设备模型是很有帮助的.