• devtmpfs文件系统创建设备节点

    1. 分类： LINUX

      原文地址：devtmpfs文件系统创建设备节点 作者：wangbaolin719

    2. http://blog.chinaunix.net/uid-27097876-id-4334356.html
    4. 一、devtmpfs概述
    5. 1.devtmpfs 的功用是在 Linux 核心 启动早期建立一个初步的 /dev，令一般启动程序不用等待 udev，缩短 GNU/Linux 的开机时间。

    7. 2.重要解释
    8. Devtmpfs lets the kernel create a tmpfs very early at kernel initialization, before any driver core device is registered. Every device with a major/minor will have a device node created in this tmpfs instance. After the rootfs is mounted by the kernel, the populated tmpfs is mounted at /dev. In initramfs, it can be moved to the manually mounted root filesystem before /sbin/init is executed.

    10. 3.menuconfig 中加入devtmpfs支持
    11. make menuconfig-->Device Drivers-->Generic Driver Options
    12. Maintain a devtmpfs filesystem to mount at /dev
    13. Automount devtmpfs at /dev, after the kernel mounted the rootfs

    15. 4.df -T显示devtmpfs
    16. 文件系统 类型 1K-块 已用 可用 已用% 挂载点
    17. /dev/sda1 ext4 31621016 14985712 15029008 50% /
    18. none devtmpfs 399552 276 399276 1% /dev
    19. none tmpfs 403804 24 403780 1% /dev/shm
    20. none tmpfs 403804 108 403696 1% /var/run
    21. none tmpfs 403804 0 403804 0% /var/lock
    22. none tmpfs 403804 0 403804 0% /lib/init/rw
    23. .host:/ vmhgfs 67151668 54038400 13113268 81% /mnt/hgfs
    24. /dev/loop0 ext2 16119 8528 6772 56% /mnt/loop

    26. 二、devtmpfs文件系统初始化
    27. void __init driver_init(void)
    28. {
    29.     /* These are the core pieces */
    30.     devtmpfs_init();//devtmpfs文件系统初始化
    31.     devices_init();
    32.     buses_init();
    33.     classes_init();
    34.     firmware_init();
    35.     hypervisor_init();
    36.     platform_bus_init();
    37.     system_bus_init();
    38.     cpu_dev_init();
    39.     memory_dev_init();
    40. }

    42. static struct file_system_type dev_fs_type = {
    43.     .name = "devtmpfs",
    44.     .mount = dev_mount,
    45.     .kill_sb = kill_litter_super,
    46. };
    47.    
    48. int __init devtmpfs_init(void)
    49. {
    50.     int err = register_filesystem(&dev_fs_type);//注册dev_fs_type文件系统，即将dev_fs_type添加到内核全局总链表中file_systems
    51.     if (err) {
    52.         printk(KERN_ERR "devtmpfs: unable to register devtmpfs ""type %i ", err);
    53.         return err;
    54.     }
    55.     
    56.     thread = kthread_run(devtmpfsd, &err, "kdevtmpfs");//创建并启动一个内核线程devtmpfsd
    57.     if (!IS_ERR(thread)) {
    58.         wait_for_completion(&setup_done);//进行一个不可打断的等待,允许一个线程告诉另一个线程工作已经完成
    59.     } else {
    60.         err = PTR_ERR(thread);
    61.         thread = NULL;
    62.     }
    63.     
    64.     if (err) {
    65.         printk(KERN_ERR "devtmpfs: unable to create devtmpfs %i ", err);
    66.         unregister_filesystem(&dev_fs_type);
    67.         return err;
    68.     }
    69.     
    70.     printk(KERN_INFO "devtmpfs: initialized ");
    71.     return 0;
    72. }

    74. //请求创建设备节点的请求队列req结构
    75. static struct req {
    76.     struct req *next;
    77.     struct completion done;
    78.     int err;
    79.     const char *name;
    80.     umode_t mode;//0代表删除
    81.     struct device *dev;
    82. } *requests;

    84. //内核线程devtmpfsd
    85. static int devtmpfsd(void *p)
    86. {
    87.     char options[] = "mode=0755";
    88.     int *err = p;
    89.     
    90.     *err = sys_unshare(CLONE_NEWNS);
    91.     if (*err)
    92.         goto out;
    93.         
    94.     //挂载devtmpfs文件系统
    95.     //devtmpfs是待安装设备的路径名，“/”是安装点路径名，”devtmpfs“表示文件系统类型，MS_SILENT=32768，即0x8000
    96.     *err = sys_mount("devtmpfs", "/", "devtmpfs", MS_SILENT, options);
    97.     if (*err)
    98.         goto out;
    99.     sys_chdir("/.."); //将进程的当前工作目录(pwd)设定为devtmpfs文件系统的根目录/* will traverse into overmounted root */
    100.     sys_chroot(".");
    101.     complete(&setup_done);//允许一个线程告诉另一个线程工作已经完成
    102.     while (1) {
    103.         spin_lock(&req_lock);
    104.         while (requests) {//扫描请求链表，每当要创建一个设备节点时，都需要向requests链表中添加请求
    105.             struct req *req = requests;//赋值给临时req
    106.             requests = NULL;//清空
    107.             spin_unlock(&req_lock);
    108.             while (req) {//遍历刚才requests的请求链表
    109.                 struct req *next = req->next;
    110.                 req->err = handle(req->name, req->mode, req->dev);//对链表中的每一个请求调用handle函数
    111.                 complete(&req->done);
    112.                 req = next;
    113.             }
    114.             spin_lock(&req_lock);
    115.         }
    116.         __set_current_state(TASK_INTERRUPTIBLE);//设置为睡眠状态
    117.         spin_unlock(&req_lock);
    118.         schedule();//系统切换
    119.     }
    120.     return 0;
    121. out:
    122.     complete(&setup_done);
    123.     return *err;
    124. }

    126. static int handle(const char *name, umode_t mode, struct device *dev)
    127. {
    128.     if (mode)
    129.         return handle_create(name, mode, dev);
    130.     else
    131.         return handle_remove(name, dev);
    132. }

    134. static int handle_create(const char *nodename, umode_t mode, struct device *dev)
    135. {
    136.     struct dentry *dentry;
    137.     struct path path;
    138.     int err;
    139.     
    140.     //查找节点名称的路径以及返回节点对应的父目录dentry结构，即在此目录下创建一个设备节点，即是/dev目录对应的dentry结构
    141.     dentry = kern_path_create(AT_FDCWD, nodename, &path, 0);
    142.     if (dentry == ERR_PTR(-ENOENT)) {
    143.         create_path(nodename);
    144.         dentry = kern_path_create(AT_FDCWD, nodename, &path, 0);
    145.     }
    146.     if (IS_ERR(dentry))
    147.         return PTR_ERR(dentry);
    148.     
    149.     //创建设备节点
    150.     err = vfs_mknod(path.dentry->d_inode,dentry, mode, dev->devt);
    151.     if (!err) {
    152.         struct iattr newattrs;
    153.         newattrs.ia_mode = mode;/* fixup possibly umasked mode */
    154.         newattrs.ia_valid = ATTR_MODE;
    155.         mutex_lock(&dentry->d_inode->i_mutex);
    156.         notify_change(dentry, &newattrs);
    157.         mutex_unlock(&dentry->d_inode->i_mutex);
    158.         dentry->d_inode->i_private = &thread;/* mark as kernel-created inode */
    159.     }
    160.     done_path_create(&path, dentry);//与前边kern_path_create对应，减少path和dentry的计数等
    161.     return err;
    162. }

    164. int vfs_mknod(struct inode *dir, struct dentry *dentry, umode_t mode, dev_t dev)
    165. {
    166.     int error = may_create(dir, dentry);//检查是否可以创建设备文件节点
    167.     
    168.     if (error)
    169.         return error;
    170.     
    171.     //必须是字符设备或者块设备，且具有创建节点的权限
    172.     if ((S_ISCHR(mode) || S_ISBLK(mode)) && !capable(CAP_MKNOD))
    173.         return -EPERM;
    174.     
    175.     if (!dir->i_op->mknod)
    176.         return -EPERM;
    177.     
    178.     error = devcgroup_inode_mknod(mode, dev);
    179.     if (error)
    180.         return error;
    181.     
    182.     error = security_inode_mknod(dir, dentry, mode, dev);
    183.     if (error)
    184.         return error;
    185.     
    186.     //调用具体文件系统的mknod()函数
    187.     //mount时调用shmem_fill_super()-->shmem_get_inode()分配inode节点时做出的初始化
    188.     /*那么在shmem_get_inode中
    189.         caseS_IFDIR:
    190.         inc_nlink(inode);
    191.         inode->i_size= 2 * BOGO_DIRENT_SIZE;
    192.         inode->i_op= &shmem_dir_inode_operations;
    193.         inode->i_fop= &simple_dir_operations;
    194.         由于mountpoint是dev这个目录，所以dev对应的inode的i_op就是shmem_dir_inode_operations。
    195.         staticconst struct inode_operations shmem_dir_inode_operations = {
    196.             #ifdefCONFIG_TMPFS
    197.             .create =shmem_create,
    198.             .lookup =simple_lookup,
    199.             .link =shmem_link,
    200.             .unlink =shmem_unlink,
    201.             .symlink =shmem_symlink,
    202.             .mkdir =shmem_mkdir,
    203.             .rmdir =shmem_rmdir,
    204.             .mknod =shmem_mknod,
    205.             .rename =shmem_rename,
    206.             #endif
    207.             #ifdefCONFIG_TMPFS_POSIX_ACL
    208.             .setattr =shmem_notify_change,
    209.             .setxattr =generic_setxattr,
    210.             .getxattr =generic_getxattr,
    211.             .listxattr =generic_listxattr,
    212.             .removexattr =generic_removexattr,
    213.             .check_acl =generic_check_acl,
    214.             #endif
    215.             };
    216.         */
    217.     error = dir->i_op->mknod(dir, dentry, mode, dev);//所以这里调用的就是shmem_mknod
    218.     if (!error)
    219.         fsnotify_create(dir, dentry);
    220.     return error;
    221. }

    223. shmem_mknod(struct inode *dir, struct dentry *dentry, umode_t mode, dev_t dev)
    224. {
    225.     struct inode *inode;
    226.     int error = -ENOSPC;
    227.     
    228.     inode = shmem_get_inode(dir->i_sb, dir, mode, dev, VM_NORESERVE);//获得一个要创建的设备节点的inode，并初始化
    229.     if (inode) {
    230.         error = security_inode_init_security(inode, dir,&dentry->d_name,shmem_initxattrs, NULL);
    231.         if (error) {
    232.             if (error != -EOPNOTSUPP) {
    233.                 iput(inode);
    234.                 return error;
    235.             }
    236.         }
    237. #ifdef CONFIG_TMPFS_POSIX_ACL
    238.         error = generic_acl_init(inode, dir);
    239.         if (error) {
    240.             iput(inode);
    241.             return error;
    242.         }
    243. #else
    244.         error = 0;
    245. #endif
    246.         dir->i_size += BOGO_DIRENT_SIZE;
    247.         dir->i_ctime = dir->i_mtime = CURRENT_TIME;
    248.         d_instantiate(dentry, inode);//与dentry建立关,此时就可以在/dev下看到这个字符设备节点了
    249.         dget(dentry); //递减dentry的计数
    250.     }
    251.     return error;
    252. }

    254. 三、文件系统的mount
    255. 内核主要是通过kernel_init调用prepare_namespace()函数执行安装实际根文件系统的操作：
    256. void __init prepare_namespace(void)
    257. {
    258.     int is_floppy;
    259.   
    260.     if (root_delay) {
    261.         printk(KERN_INFO "Waiting %dsec before mounting root device... ",
    262.                root_delay);
    263.         ssleep(root_delay);
    264.     }
    265.     wait_for_device_probe();
    266.   
    267.     md_run_setup();
    268.     /* 把root_device_name变量置为从启动参数“root”中获取的设备文件名。
    269.   * 同样，把ROOT_DEV变量置为同一设备文件的主设备号和次设备号。*/
    270.     if (saved_root_name[0]) {
    271.         root_device_name = saved_root_name;
    272.         if (!strncmp(root_device_name, "mtd", 3) ||
    273.             !strncmp(root_device_name, "ubi", 3)) {
    274.             mount_block_root(root_device_name, root_mountflags);
    275.             goto out;
    276.         }
    277.         ROOT_DEV = name_to_dev_t(root_device_name);//转换为设备号/dev/mtdblock2.
    278.         if (strncmp(root_device_name, "/dev/", 5) == 0)
    279.             root_device_name += 5;
    280.     }
    281.   
    282.     if (initrd_load())
    283.         goto out;
    284.   
    285.     /* wait for any asynchronous scanning to complete */
    286.     if ((ROOT_DEV == 0) && root_wait) {
    287.         printk(KERN_INFO "Waiting for root device %s... ",
    288.             saved_root_name);
    289.         while (driver_probe_done() != 0 ||
    290.             (ROOT_DEV = name_to_dev_t(saved_root_name)) == 0)
    291.             msleep(100);
    292.         async_synchronize_full();
    293.     }
    294.   
    295.     is_floppy = MAJOR(ROOT_DEV) == FLOPPY_MAJOR;
    296.   
    297.     if (is_floppy && rd_doload && rd_load_disk(0))
    298.         ROOT_DEV = Root_RAM0;
    299.   
    300.     mount_root();
    301. out:
    302.     devtmpfs_mount("dev");//挂载devtmpfs文件系统
    303.     sys_mount(".", "/", NULL, MS_MOVE, NULL); /* 移动rootfs文件系统根目录上的已安装文件系统的安装点。 */
    304.     sys_chroot(".");
    305. }

    307. int devtmpfs_mount(const char *mntdir)
    308. {
    309.     int err;
    310.     
    311.     if (!mount_dev)
    312.         return 0;
    313.     
    314.     if (!thread)
    315.         return 0;
    316.     //将devtmpfs文件系统挂载到/dev目录下
    317.     err = sys_mount("devtmpfs", (char *)mntdir, "devtmpfs", MS_SILENT, NULL);
    318.     if (err)
    319.         printk(KERN_INFO "devtmpfs: error mounting %i ", err);
    320.     else
    321.         printk(KERN_INFO "devtmpfs: mounted ");
    322.     return err;
    323. }


    326. 四、devtmpfs创建节点
    327. 系统在启动过程中，扫描到的设备会通过devtmpfs_create_node()函数来添加设备节点。
    328. int devtmpfs_create_node(struct device *dev)
    329. {
    330.     const char *tmp = NULL;
    331.     struct req req;
    332.     
    333.     if (!thread)
    334.     return 0;
    335.     
    336.     req.mode = 0;
    337.     req.name = device_get_devnode(dev, &req.mode, &tmp);//获得设备名
    338.     if (!req.name)
    339.         return -ENOMEM;
    340.     
    341.     if (req.mode == 0)
    342.         req.mode = 0600;
    343.     if (is_blockdev(dev))
    344.         req.mode |= S_IFBLK;//块设备
    345.     else
    346.         req.mode |= S_IFCHR;//字符设备
    347.     
    348.     req.dev = dev;
    349.     
    350.     init_completion(&req.done);
    351.     
    352.     spin_lock(&req_lock);
    353.     req.next = requests;//请求添加到requests链表
    354.     requests = &req;
    355.     spin_unlock(&req_lock);
    356.     
    357.     wake_up_process(thread);//唤醒内核线程devtmpfsd添加设备节点
    358.     wait_for_completion(&req.done);
    359.     
    360.     kfree(tmp);
    361.     
    362.     return req.err;
    363. }

    365. const char *device_get_devnode(struct device *dev,umode_t *mode, const char **tmp)
    366. {
    367.     char *s;
    368.     
    369.     *tmp = NULL;
    370.     
    371.     /* the device type may provide a specific name */
    372.     if (dev->type && dev->type->devnode)
    373.         *tmp = dev->type->devnode(dev, mode);
    374.     if (*tmp)
    375.         return *tmp;
    376.     
    377.     /* the class may provide a specific name */
    378.     if (dev->class && dev->class->devnode)
    379.         *tmp = dev->class->devnode(dev, mode);
    380.     if (*tmp)
    381.         return *tmp;
    382.     
    383.     /* return name without allocation, tmp == NULL */
    384.     if (strchr(dev_name(dev), '!') == NULL)
    385.         return dev_name(dev);
    386.     
    387.     /* replace '!' in the name with '/' */
    388.     *tmp = kstrdup(dev_name(dev), GFP_KERNEL);
    389.     if (!*tmp)
    390.         return NULL;
    391.     while ((s = strchr(*tmp, '!')))
    392.         s[0] = '/';
    393.     return *tmp;
    394. }
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  • 原文地址：https://www.cnblogs.com/mull/p/4801282.html
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