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
3. 一、devtmpfs概述
4. 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。
5. 2.重要解释
6. 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.
7. 3.menuconfig 中加入devtmpfs支持
8. make menuconfig-->Device Drivers-->Generic Driver Options
9. Maintain a devtmpfs filesystem to mount at /dev
10. Automount devtmpfs at /dev, after the kernel mounted the rootfs
11. 4.df -T显示devtmpfs
12. 文件系统 类型 1K-块 已用 可用 已用% 挂载点
13. /dev/sda1 ext4 31621016 14985712 15029008 50% /
14. none devtmpfs 399552 276 399276 1% /dev
15. none tmpfs 403804 24 403780 1% /dev/shm
16. none tmpfs 403804 108 403696 1% /var/run
17. none tmpfs 403804 0 403804 0% /var/lock
18. none tmpfs 403804 0 403804 0% /lib/init/rw
19. .host:/ vmhgfs 67151668 54038400 13113268 81% /mnt/hgfs
20. /dev/loop0 ext2 16119 8528 6772 56% /mnt/loop
21. 二、devtmpfs文件系统初始化
22. void __init driver_init(void)
23. {
24. /* These are the core pieces */
25. devtmpfs_init();//devtmpfs文件系统初始化
26. devices_init();
27. buses_init();
28. classes_init();
29. firmware_init();
30. hypervisor_init();
31. platform_bus_init();
32. system_bus_init();
33. cpu_dev_init();
34. memory_dev_init();
35. }
36. static struct file_system_type dev_fs_type = {
37. .name = "devtmpfs",
38. .mount = dev_mount,
39. .kill_sb = kill_litter_super,
40. };
41. int __init devtmpfs_init(void)
42. {
43. 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
44. if (err) {
45. printk(KERN_ERR "devtmpfs: unable to register devtmpfs ""type %i\n", err);
46. return err;
47. }
48. thread = kthread_run(devtmpfsd, &err, "kdevtmpfs");//创建并启动一个内核线程devtmpfsd
49. if (!IS_ERR(thread)) {
50. 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
51. } else {
52. err = PTR_ERR(thread);
53. thread = NULL;
54. }
55. if (err) {
56. printk(KERN_ERR "devtmpfs: unable to create devtmpfs %i\n", err);
57. unregister_filesystem(&dev_fs_type);
58. return err;
59. }
60. printk(KERN_INFO "devtmpfs: initialized\n");
61. return 0;
62. }
63. //请求创建设备节点的请求队列req结构
64. static struct req {
65. struct req *next;
66. struct completion done;
67. int err;
68. const char *name;
69. umode_t mode;//0代表删除
70. struct device *dev;
71. } *requests;
72. //内核线程devtmpfsd
73. static int devtmpfsd(void *p)
74. {
75. char options[] = "mode=0755";
76. int *err = p;
77. *err = sys_unshare(CLONE_NEWNS);
78. if (*err)
79. goto out;
80. //挂载devtmpfs文件系统
81. //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
82. *err = sys_mount("devtmpfs", "/", "devtmpfs", MS_SILENT, options);
83. if (*err)
84. goto out;
85. 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 */
86. sys_chroot(".");
87. complete(&setup_done);//允许一个线程告诉另一个线程工作已经完成
88. while (1) {
89. spin_lock(&req_lock);
90. 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
91. struct req *req = requests;//赋值给临时req
92. requests = NULL;//清空
93. spin_unlock(&req_lock);
94. while (req) {//遍历刚才requests的请求链表
95. struct req *next = req->next;
96. req->err = handle(req->name, req->mode, req->dev);//对链表中的每一个请求调用handle函数
97. complete(&req->done);
98. req = next;
99. }
100. spin_lock(&req_lock);
101. }
102. __set_current_state(TASK_INTERRUPTIBLE);//设置为睡眠状态
103. spin_unlock(&req_lock);
104. schedule();//系统切换
105. }
106. return 0;
107. out:
108. complete(&setup_done);
109. return *err;
110. }
111. static int handle(const char *name, umode_t mode, struct device *dev)
112. {
113. if (mode)
114. return handle_create(name, mode, dev);
115. else
116. return handle_remove(name, dev);
117. }
118. static int handle_create(const char *nodename, umode_t mode, struct device *dev)
119. {
120. struct dentry *dentry;
121. struct path path;
122. int err;
123. //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
124. dentry = kern_path_create(AT_FDCWD, nodename, &path, 0);
125. if (dentry == ERR_PTR(-ENOENT)) {
126. create_path(nodename);
127. dentry = kern_path_create(AT_FDCWD, nodename, &path, 0);
128. }
129. if (IS_ERR(dentry))
130. return PTR_ERR(dentry);
131. //创建设备节点
132. err = vfs_mknod(path.dentry->d_inode,dentry, mode, dev->devt);
133. if (!err) {
134. struct iattr newattrs;
135. newattrs.ia_mode = mode;/* fixup possibly umasked mode */
136. newattrs.ia_valid = ATTR_MODE;
137. mutex_lock(&dentry->d_inode->i_mutex);
138. notify_change(dentry, &newattrs);
139. mutex_unlock(&dentry->d_inode->i_mutex);
140. dentry->d_inode->i_private = &thread;/* mark as kernel-created inode */
141. }
142. done_path_create(&path, dentry);//与前边kern_path_create对应，减少path和dentry的计数等
143. return err;
144. }
145. int vfs_mknod(struct inode *dir, struct dentry *dentry, umode_t mode, dev_t dev)
146. {
147. int error = may_create(dir, dentry);//检查是否可以创建设备文件节点
148. if (error)
149. return error;
150. //必须是字符设备或者块设备，且具有创建节点的权限
151. if ((S_ISCHR(mode) || S_ISBLK(mode)) && !capable(CAP_MKNOD))
152. return -EPERM;
153. if (!dir->i_op->mknod)
154. return -EPERM;
155. error = devcgroup_inode_mknod(mode, dev);
156. if (error)
157. return error;
158. error = security_inode_mknod(dir, dentry, mode, dev);
159. if (error)
160. return error;
161. //调用具体文件系统的mknod()函数
162. //mount时调用shmem_fill_super()-->shmem_get_inode()分配inode节点时做出的初始化
163. /*那么在shmem_get_inode中
164. caseS_IFDIR:
165. inc_nlink(inode);
166. inode->i_size= 2 * BOGO_DIRENT_SIZE;
167. inode->i_op= &shmem_dir_inode_operations;
168. inode->i_fop= &simple_dir_operations;
169. 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。
170. staticconst struct inode_operations shmem_dir_inode_operations = {
171. #ifdefCONFIG_TMPFS
172. .create =shmem_create,
173. .lookup =simple_lookup,
174. .link =shmem_link,
175. .unlink =shmem_unlink,
176. .symlink =shmem_symlink,
177. .mkdir =shmem_mkdir,
178. .rmdir =shmem_rmdir,
179. .mknod =shmem_mknod,
180. .rename =shmem_rename,
181. #endif
182. #ifdefCONFIG_TMPFS_POSIX_ACL
183. .setattr =shmem_notify_change,
184. .setxattr =generic_setxattr,
185. .getxattr =generic_getxattr,
186. .listxattr =generic_listxattr,
187. .removexattr =generic_removexattr,
188. .check_acl =generic_check_acl,
189. #endif
190. };
191. */
192. error = dir->i_op->mknod(dir, dentry, mode, dev);//所以这里调用的就是shmem_mknod
193. if (!error)
194. fsnotify_create(dir, dentry);
195. return error;
196. }
197. shmem_mknod(struct inode *dir, struct dentry *dentry, umode_t mode, dev_t dev)
198. {
199. struct inode *inode;
200. int error = -ENOSPC;
201. 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
202. if (inode) {
203. error = security_inode_init_security(inode, dir,&dentry->d_name,shmem_initxattrs, NULL);
204. if (error) {
205. if (error != -EOPNOTSUPP) {
206. iput(inode);
207. return error;
208. }
209. }
210. #ifdef CONFIG_TMPFS_POSIX_ACL
211. error = generic_acl_init(inode, dir);
212. if (error) {
213. iput(inode);
214. return error;
215. }
216. #else
217. error = 0;
218. #endif
219. dir->i_size += BOGO_DIRENT_SIZE;
220. dir->i_ctime = dir->i_mtime = CURRENT_TIME;
221. 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
222. dget(dentry); //递减dentry的计数
223. }
224. return error;
225. }
226. 三、文件系统的mount
227. 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：
228. void __init prepare_namespace(void)
229. {
230. int is_floppy;
231. if (root_delay) {
232. printk(KERN_INFO "Waiting %dsec before mounting root device...\n",
233. root_delay);
234. ssleep(root_delay);
235. }
236. wait_for_device_probe();
237. md_run_setup();
238. /* 把root_device_name变量置为从启动参数“root”中获取的设备文件名。
239. * 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。*/
240. if (saved_root_name[0]) {
241. root_device_name = saved_root_name;
242. if (!strncmp(root_device_name, "mtd", 3) ||
243. !strncmp(root_device_name, "ubi", 3)) {
244. mount_block_root(root_device_name, root_mountflags);
245. goto out;
246. }
247. ROOT_DEV = name_to_dev_t(root_device_name);//转换为设备号/dev/mtdblock2.
248. if (strncmp(root_device_name, "/dev/", 5) == 0)
249. root_device_name += 5;
250. }
251. if (initrd_load())
252. goto out;
253. /* wait for any asynchronous scanning to complete */
254. if ((ROOT_DEV == 0) && root_wait) {
255. printk(KERN_INFO "Waiting for root device %s...\n",
256. saved_root_name);
257. while (driver_probe_done() != 0 ||
258. (ROOT_DEV = name_to_dev_t(saved_root_name)) == 0)
259. msleep(100);
260. async_synchronize_full();
261. }
262. is_floppy = MAJOR(ROOT_DEV) == FLOPPY_MAJOR;
263. if (is_floppy && rd_doload && rd_load_disk(0))
264. ROOT_DEV = Root_RAM0;
265. mount_root();
266. out:
267. devtmpfs_mount("dev");//挂载devtmpfs文件系统
268. 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。 */
269. sys_chroot(".");
270. }
271. int devtmpfs_mount(const char *mntdir)
272. {
273. int err;
274. if (!mount_dev)
275. return 0;
276. if (!thread)
277. return 0;
278. //将devtmpfs文件系统挂载到/dev目录下
279. err = sys_mount("devtmpfs", (char *)mntdir, "devtmpfs", MS_SILENT, NULL);
280. if (err)
281. printk(KERN_INFO "devtmpfs: error mounting %i\n", err);
282. else
283. printk(KERN_INFO "devtmpfs: mounted\n");
284. return err;
285. }
286. 四、devtmpfs创建节点
287. 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。
288. int devtmpfs_create_node(struct device *dev)
289. {
290. const char *tmp = NULL;
291. struct req req;
292. if (!thread)
293. return 0;
294. req.mode = 0;
295. req.name = device_get_devnode(dev, &req.mode, &tmp);//获得设备名
296. if (!req.name)
297. return -ENOMEM;
298. if (req.mode == 0)
299. req.mode = 0600;
300. if (is_blockdev(dev))
301. req.mode |= S_IFBLK;//块设备
302. else
303. req.mode |= S_IFCHR;//字符设备
304. req.dev = dev;
305. init_completion(&req.done);
306. spin_lock(&req_lock);
307. req.next = requests;//请求添加到requests链表
308. requests = &req;
309. spin_unlock(&req_lock);
310. wake_up_process(thread);//唤醒内核线程devtmpfsd添加设备节点
311. wait_for_completion(&req.done);
312. kfree(tmp);
313. return req.err;
314. }
315. const char *device_get_devnode(struct device *dev,umode_t *mode, const char **tmp)
316. {
317. char *s;
318. *tmp = NULL;
319. /* the device type may provide a specific name */
320. if (dev->type && dev->type->devnode)
321. *tmp = dev->type->devnode(dev, mode);
322. if (*tmp)
323. return *tmp;
324. /* the class may provide a specific name */
325. if (dev->class && dev->class->devnode)
326. *tmp = dev->class->devnode(dev, mode);
327. if (*tmp)
328. return *tmp;
329. /* return name without allocation, tmp == NULL */
330. if (strchr(dev_name(dev), '!') == NULL)
331. return dev_name(dev);
332. /* replace '!' in the name with '/' */
333. *tmp = kstrdup(dev_name(dev), GFP_KERNEL);
334. if (!*tmp)
335. return NULL;
336. while ((s = strchr(*tmp, '!')))
337. s[0] = '/';
338. return *tmp;
339. }