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[操作系统]char_dev.c 添加中文注释


 

char_dev.c里的中文注释,仅代表个人理解,仅供参考。如有错误之处,请指出,谢谢!

 

 1 /* 2  * linux/fs/char_dev.c 3  * 4  * Copyright (C) 1991, 1992 Linus Torvalds 5 */ 6  7 #include <linux/init.h> 8 #include <linux/fs.h> 9 #include <linux/kdev_t.h> 10 #include <linux/slab.h> 11 #include <linux/string.h> 12  13 #include <linux/major.h> 14 #include <linux/errno.h> 15 #include <linux/module.h> 16 #include <linux/smp_lock.h> 17 #include <linux/seq_file.h> 18  19 #include <linux/kobject.h> 20 #include <linux/kobj_map.h> 21 #include <linux/cdev.h> 22 #include <linux/mutex.h> 23 #include <linux/backing-dev.h> 24  25 #ifdef CONFIG_KMOD 26 #include <linux/kmod.h> 27 #endif 28 #include "internal.h" 29  30 /* 31  * capabilities for /dev/mem, /dev/kmem and similar directly mappable character 32  * devices 33  * - permits shared-mmap for read, write and/or exec 34  * - does not permit private mmap in NOMMU mode (can't do COW) 35  * - no readahead or I/O queue unplugging required 36 */ 37  38 struct backing_dev_info directly_mappable_cdev_bdi = { 39   .capabilities = ( 40 #ifdef CONFIG_MMU 41   /* permit private copies of the data to be taken */ 42   BDI_CAP_MAP_COPY | 43   #endif 44   /* permit direct mmap, for read, write or exec */ 45   BDI_CAP_MAP_DIRECT | 46   BDI_CAP_READ_MAP | BDI_CAP_WRITE_MAP | BDI_CAP_EXEC_MAP), 47 }; 48  49 static struct kobj_map *cdev_map; 50  51 static DEFINE_MUTEX(chrdevs_lock); 52  53 static struct char_device_struct { 54   struct char_device_struct *next; 55   unsigned int major; 56   unsigned int baseminor; 57   int minorct; 58   char name[64]; 59   struct file_operations *fops; 60   struct cdev *cdev; 61   /* will die */ 62 } *chrdevs[CHRDEV_MAJOR_HASH_SIZE]; 63  64 /* index in the above */ 65 static inline int major_to_index(int major) 66 { 67   /* [CGW]: 根据主设备号,转换成对应的索引 68    * 即主设备号就是索引号 69   */ 70   return major % CHRDEV_MAJOR_HASH_SIZE; 71 } 72  73 #ifdef CONFIG_PROC_FS 74  75 void chrdev_show(struct seq_file *f, off_t offset) 76 { 77   struct char_device_struct *cd; 78   /* [CGW]: 根据offset (相当于索引),找到对应设备 */ 79   if (offset < CHRDEV_MAJOR_HASH_SIZE) { 80     /* [CGW]: 上锁 */ 81     mutex_lock(&chrdevs_lock); 82     /* [CGW]: 打印该设备项下链表中所有节点的主设备号,和设备名 */ 83     for (cd = chrdevs[offset]; cd; cd = cd->next) 84       seq_printf(f, "%3d %s\n", cd->major, cd->name); 85     /* [CGW]: 解锁 */ 86     mutex_unlock(&chrdevs_lock); 87   } 88 } 89  90 #endif /* CONFIG_PROC_FS */ 91  92 /* 93  * Register a single major with a specified minor range. 94  * 95  * If major == 0 this functions will dynamically allocate a major and return 96  * its number. 97  * 98  * If major > 0 this function will attempt to reserve the passed range of 99  * minors and will return zero on success.100  *101  * Returns a -ve errno on failure.102 */103 104 static struct char_device_struct *105 __register_chrdev_region(unsigned int major, unsigned int baseminor,106             int minorct, const char *name)107 108 {109   struct char_device_struct *cd, **cp;110   int ret = 0;111   int i;112 113   /* [cgw]: 分配一块char_device_struct大小的内存块 */114   cd = kzalloc(sizeof(struct char_device_struct), GFP_KERNEL);115   /* [cgw]: 分配失败 */116   if (cd == NULL)117     return ERR_PTR(-ENOMEM);118   /*[cgw]: 上锁,进入临界区*/119   mutex_lock(&chrdevs_lock);120   /* temporary */121   /*[cgw]: 如果主设备号为0,则从最大的设备号开始,往下查找第一个未被122    *注册的设备123   */124   if (major == 0) {125     /*[cgw]: 从大到小开始查找*/126     for (i = ARRAY_SIZE(chrdevs)-1; i > 0; i--) {127       /* [cgw]: 找到第一个未被注册的设备 */128       if (chrdevs[i] == NULL)129       break;130     }131     /* [cgw]:未找到空位 */132     if (i == 0) {133       ret = -EBUSY;134       goto out;135     }136     /* [cgw]: 以该空位的序号为主设备号 */137     major = i;138     ret = major;139   }140 141   /* [cgw]: 手工分配一个主设备号,和次设备号基址 */142   cd->major = major;143   /* [cgw]: 手工分配次设备号基址 */144   cd->baseminor = baseminor;145   /* [cgw]: 分配minorct个次设备号 */146   cd->minorct = minorct;147   /* [cgw]: 分配设备名 */148   strncpy(cd->name,name, 64);149   /* [cgw]: 找到主设备号在设备列表中的索引 */150   i = major_to_index(major);151   /* [cgw]: 当前分配的设备号比较设备列表,判断该设备号是否152    * 合法153   */154   for (cp = &chrdevs[i]; *cp; cp = &(*cp)->next)155     /* [cgw]: 当前分配的主设备号,小于该主设备号索引对应的,设备列表156      * 中的主设备号,合法157      * 这个里有点不解,从i = major_to_index(major);看出,主设备号和该主158      * 设备号对应的索引是相等的,为什么(*cp)->major > major是合法呢159     */160     if ((*cp)->major > major ||161       /* [cgw]: 当前分配的主设备号,等于该主设备号索引对应的,设备列表162        * 中的主设备号,并且符合以下条件之一,当前分配的次设备号,等于163        * 小于该主设备号索引对应的,设备列表中的次设备号。或者,当前分164        * 配的次设备号,小于该主设备号索引对应的,设备列表中的次设备基165        * 址以后minorct个次设备号166       */167      ((*cp)->major == major &&168      (((*cp)->baseminor >= baseminor) ||169      ((*cp)->baseminor + (*cp)->minorct > baseminor))))170       break;171   /* Check for overlapping minor ranges. */172   /* [cgw]: 当前分配的主设备号,等于该主设备号索引对应的,设备列表173    * 中的主设备号,判断次设备号是否在范围内174   */175   if (*cp && (*cp)->major == major) {176     int old_min = (*cp)->baseminor;177     int old_max = (*cp)->baseminor + (*cp)->minorct - 1;178     int new_min = baseminor;179     int new_max = baseminor + minorct - 1;180 181     /* New driver overlaps from the left. */182     if (new_max >= old_min && new_max <= old_max) {183       ret = -EBUSY;184       goto out;185     }186 187     /* New driver overlaps from the right. */188     if (new_min <= old_max && new_min >= old_min) {189       ret = -EBUSY;190       goto out;191     }192   }193 194   /* [cgw]: 新加入的设备, 添加到该主设备号链表 */195   cd->next = *cp;196   /* [cgw]: 设备列表指针指向新加入设备*/197   *cp = cd;198   /* [cgw]: 解锁,退出临界区*/199   mutex_unlock(&chrdevs_lock);200   return cd;201 out:202 203   /* [cgw]: 解锁,退出临界区*/204   mutex_unlock(&chrdevs_lock);205   /* [cgw]: 释放为新设备创建的内存*/206   kfree(cd);207   return ERR_PTR(ret);208 }209 210 static struct char_device_struct *211 __unregister_chrdev_region(unsigned major, unsigned baseminor, int minorct)212 213 {214   struct char_device_struct *cd = NULL, **cp;215   /* [CGW]: 根据主设备号,找出该主设备号所在列表中的索引*/216   int i = major_to_index(major);217   /* [CGW]: 上锁,进入临界区 */218   mutex_lock(&chrdevs_lock);219   /* [CGW]: 根据索引,找出该主设备号所在列表项 */220   for (cp = &chrdevs[i]; *cp; cp = &(*cp)->next)221     /* [CGW]:主设备号所在列表项中, 在主设备号对应的链表中,查找判断该主设备号,222      * 次设备号基址,次设备号个数是否已经被注册223     */224   if ((*cp)->major == major &&225    (*cp)->baseminor == baseminor &&226    (*cp)->minorct == minorct)227    /* [CGW]: 已经被注册,停止查找 */228    break;229   /* [CGW]: 主设备号所在链表中的节点230    * (注意: 设备列表中,每个设备号都对应一个链表,该链表用于存放此设备号) 231   */232   if (*cp) {233     /* [CGW]: 取出该节点 */234     cd = *cp;235     /* [CGW]: 更新cp,指向下一个节点*/236     *cp = cd->next;237   }238 239   /* [CGW]: 解锁,退出临界区 */240   mutex_unlock(&chrdevs_lock);241   /* [CGW]: 返回该设备(节点) */242   return cd;243 244 }245 246 247 248 249 /**250  * register_chrdev_region() - register a range of device numbers251  * @from: the first in the desired range of device numbers; must include252  *    the major number.253  * @count: the number of consecutive device numbers required254  * @name: the name of the device or driver.255  *256  * Return value is zero on success, a negative error code on failure.257 */258 259 int register_chrdev_region(dev_t from, unsigned count, const char *name)260 261 {262   struct char_device_struct *cd;263   dev_t to = from + count;264   dev_t n, next;265   /* [CGW]: 分配count个连续的设备 */266   for (n = from; n < to; n = next) {267     /* [CGW]: 主设备号+1递增 */268     next = MKDEV(MAJOR(n)+1, 0);269     if (next > to)270       next = to;271 272     /* [CGW]: 根据主、次设备号基址,分配next - n个连续次设备号的设备,273      * 并根据主设备号分配设备名274      * 如果MINOR(n)为0,next-n的值应恒为256,未验证!!!275     */276     cd = __register_chrdev_region(MAJOR(n), MINOR(n),277     next - n, name);278     /* [CGW]: 分配失败 */279     if (IS_ERR(cd))280       goto fail;281   }282   return 0;283 284 fail:285 286   /* [CGW]: 当前分配到了第n个设备就失败了*/287   to = n;288   /* [CGW]: 注销刚刚分配的所有设备 */289   for (n = from; n < to; n = next) {290     next = MKDEV(MAJOR(n)+1, 0);291     /* [CGW]: 对应的内存空间 */292     kfree(__unregister_chrdev_region(MAJOR(n), MINOR(n), next - n));293   }294   295   /* [CGW]: 返回这个分配失败的设备指针 */296   return PTR_ERR(cd);297 }298 299 /**300  * alloc_chrdev_region() - register a range of char device numbers301  * @dev: output parameter for first assigned number302  * @baseminor: first of the requested range of minor numbers303  * @count: the number of minor numbers required304  * @name: the name of the associated device or driver305  *306  * Allocates a range of char device numbers. The major number will be307  * chosen dynamically, and returned (along with the first minor number)308  * in @dev. Returns zero or a negative error code.309 */310 int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count,311             const char *name)312 313 {314   struct char_device_struct *cd;315   /* [CGW]: 自动分配一个设备,因为主设备号为0 316    * 以baseminor为基址,分配count个次设备号317   */318   cd = __register_chrdev_region(0, baseminor, count, name);319   /* [CGW]: 分配失败 */320   if (IS_ERR(cd))321     return PTR_ERR(cd);322   /* [CGW]: 返回设备号 */323   *dev = MKDEV(cd->major, cd->baseminor);324   return 0;325 }326 327 /**328  * register_chrdev() - Register a major number for character devices.329  * @major: major device number or 0 for dynamic allocation330  * @name: name of this range of devices331  * @fops: file operations associated with this devices332  *333  * If @major == 0 this functions will dynamically allocate a major and return334  * its number.335  *336  * If @major > 0 this function will attempt to reserve a device with the given337  * major number and will return zero on success.338  *339  * Returns a -ve errno on failure.340  *341  * The name of this device has nothing to do with the name of the device in342  * /dev. It only helps to keep track of the different owners of devices. If343  * your module name has only one type of devices it's ok to use e.g. the name344  * of the module here.345  *346  * This function registers a range of 256 minor numbers. The first minor number347  * is 0.348 */349 350 int register_chrdev(unsigned int major, const char *name,351           const struct file_operations *fops)352 353 {354   struct char_device_struct *cd;355   struct cdev *cdev;356   char *s;357   int err = -ENOMEM;358 359   /* [cgw]: 分配一个设备,次设备号为0~255 */360   cd = __register_chrdev_region(major, 0, 256, name);361   if (IS_ERR(cd))362     return PTR_ERR(cd);363   /* [cgw]:分配一个cdev结构体 */364   cdev = cdev_alloc();365   if (!cdev)366     goto out2;367   368   cdev->owner = fops->owner;369   cdev->ops = fops;370 371   /* [cgw]: 设置kobject的名字 */372   kobject_set_name(&cdev->kobj, "%s", name);373   /* [cgw]: 把kobject的名字kobj->name中的'/'替换成'!' */374   for (s = strchr(kobject_name(&cdev->kobj),'/'); s; s = strchr(s, '/'))375     *s = '!';376 377   /* [cgw]: 添加一个字符设备到系统*/378   err = cdev_add(cdev, MKDEV(cd->major, 0), 256);379   if (err)380     goto out;381 382   /* [cgw]: 设置char_device_struct中的cdev指针 */383   cd->cdev = cdev;384   385   return major ? 0 : cd->major;386 387 out:388   /* [cgw]: kobect 引用计数-1 */389   kobject_put(&cdev->kobj);390 391 out2:392   /* [cgw]: 释放刚注册的设备 */393   kfree(__unregister_chrdev_region(cd->major, 0, 256));394   return err;395 }396 397 /**398  * unregister_chrdev_region() - return a range of device numbers399  * @from: the first in the range of numbers to unregister400  * @count: the number of device numbers to unregister401  *402  * This function will unregister a range of @count device numbers,403  * starting with @from. The caller should normally be the one who404  * allocated those numbers in the first place...405 */406 407 void unregister_chrdev_region(dev_t from, unsigned count)408 {409   dev_t to = from + count;410   dev_t n, next;411 412   /* [CGW]: 注销所有从from到to的count个设备 */413   for (n = from; n < to; n = next) {414     /* [CGW]: 查找下一设备号*/415     next = MKDEV(MAJOR(n)+1, 0);416     if (next > to)417       next = to;418     /* [CGW]: 注销所有刚才注册的设备 */419     kfree(__unregister_chrdev_region(MAJOR(n), MINOR(n), next - n));420   }421 }422 423 424 int unregister_chrdev(unsigned int major, const char *name)425 {426   struct char_device_struct *cd;427   /* [CGW]: 根据主设备号,找到对应设备列表项 */428   cd = __unregister_chrdev_region(major, 0, 256);429   /* [CGW]: 该设备项有效,并且被注册 */430   if (cd && cd->cdev)431    /* [CGW]: 注销该设备 */432     cdev_del(cd->cdev);433     /* [CGW]: 释放该设备占用的内存空间 */434   kfree(cd);435   436   return 0;437 }438 439 static DEFINE_SPINLOCK(cdev_lock);440 441 static struct kobject *cdev_get(struct cdev *p)442 {443   struct module *owner = p->owner;444   struct kobject *kobj;445   /* [cgw]:cdev_get uses try_module_get to attempt to increment that module's446    * usage count. If that operation succeeds, kobject_get is used to increment the447    * kobject's reference count as well---<Linux Device Drivers>448    * try_module_get(owner)增加owner (THIS_MODULE)引用计数449   */450   /* [cgw]: module使用计数+1 */451   if (owner && !try_module_get(owner))452     return NULL;453   /* [cgw]: kobj引用计数+1 */454   kobj = kobject_get(&p->kobj);455   /* [cgw]: kobj指针返回失败 */456   if (!kobj)457    /* [cgw]: module使用计数-1 */458     module_put(owner);459     460   return kobj;461 }462 463 void cdev_put(struct cdev *p)464 {465   /* [cgw]: cdev指针不为空 */466   if (p) {467     /* [cgw]: 获得模块指针 */468     struct module *owner = p->owner;469     /* [cgw]: kobj引用计数-1 */470     kobject_put(&p->kobj);471     /* [cgw]: module使用计数-1 */472     module_put(owner);473   }474 }475 476 /*477  * Called every time a character special file is opened478 */479 int chrdev_open(struct inode * inode, struct file * filp)480 {481   struct cdev *p;482   struct cdev *new = NULL;483   int ret = 0;484 485   /* [cgw]: 进入临界区 */486   spin_lock(&cdev_lock);487   /* [cgw]: 从inode中得到一个字符设备cdev指针 */488   p = inode->i_cdev;489   /* [cgw]: struct cdev指针为空 */490   if (!p) {491     struct kobject *kobj;492     int idx;493     /* [cgw]: 进入临界区 */494     spin_unlock(&cdev_lock);495     /* [cgw]: 看看cdev_map的probes[inode->i_rdev]链表是否有inode->i_rdev这个设备496      * 并返回这个设备的kobj497     */498     kobj = kobj_lookup(cdev_map, inode->i_rdev, &idx);499     /* [cgw]: kobj为空,错误 */500     if (!kobj)501       return -ENXIO;502     /* [cgw]: 根据返回的kobj,找出包含这个kobj的struct cdev指针 */503     new = container_of(kobj, struct cdev, kobj);504     /* [cgw]: 进入临界区 */505     spin_lock(&cdev_lock);506     /* [cgw]: 从inode中得到一个字符设备cdev指针 */507     p = inode->i_cdev;508     /* [cgw]: struct cdev指针为空 */509     if (!p) {510       /* [cgw]: 把这个struct cdev指针填装到inode->i_cdev */511       inode->i_cdev = p = new;512       /* [cgw]: 记录对应的索引 */513       inode->i_cindex = idx;514       /* [cgw]: 把inode->i_devices插入到p->list */515       list_add(&inode->i_devices, &p->list);516       /* [cgw]: 清除new指针 */517       new = NULL;518       /* [cgw]: 返回cdev中kobj指针为空,错误 */519     } else if (!cdev_get(p))520       ret = -ENXIO;521   /* [cgw]: 返回cdev中kobj指针为空,错误 */522   } else if (!cdev_get(p))523     ret = -ENXIO;524     /* [cgw]: 退出临界区 */525     spin_unlock(&cdev_lock);526     /* [cgw]: 实际上是cdev->kobj引用计数-1,module使用计数-1 */527     cdev_put(new);528     if (ret)529       return ret;530   /* [cgw]: module使用计数+1,并返回cdev->ops指针 */531   filp->f_op = fops_get(p->ops);532   /* [cgw]: filp->f_op指针为空,失败 */533   if (!filp->f_op) {534     /* [cgw]: 实际上是cdev->kobj引用计数-1,module使用计数-1 */535     cdev_put(p);536     return -ENXIO;537   }538   /* [cgw]: 调用filp->f_op->open,打开的是用户驱动程序中定义的539    * file_operations中的open函数540   */541   if (filp->f_op->open) {542     /* [cgw]: 上锁 */543     lock_kernel();544     /* [cgw]: 调用filp->f_op->open */545     ret = filp->f_op->open(inode,filp);546     /* [cgw]: 解锁 */547     unlock_kernel();548   }549   /* [cgw]: 调用filp->f_op->open失败 */550   if (ret)551     /* [cgw]: 实际上是cdev->kobj引用计数-1,module使用计数-1 */552     cdev_put(p);553     554   return ret;555 }556 557 void cd_forget(struct inode *inode)558 {559   /* [cgw]: 进入临界区 */560   spin_lock(&cdev_lock);561   /* [cgw]: 从链表删除一个inode->i_devices节点,562    * 并重新初始化这个链表563   */564   list_del_init(&inode->i_devices);565   /* [cgw]: inode->i_cdev指针清0 */566   inode->i_cdev = NULL;567   /* [cgw]: 退出临界区 */568   spin_unlock(&cdev_lock);569 }570 571 static void cdev_purge(struct cdev *cdev)572 {573   /* [cgw]: 进入临界区 */574   spin_lock(&cdev_lock);575   /* [cgw]: 测试cdev->list这个链表是否为空576    * 577   */578   while (!list_empty(&cdev->list)) {579     struct inode *inode;580     /* [cgw]: 找出包含cdev->list的struct inode结构体的指针 */581     inode = container_of(cdev->list.next, struct inode, i_devices);582     /* [cgw]: 从链表删除一个inode->i_devices节点,583      * 并重新初始化这个链表584     */585     list_del_init(&inode->i_devices);586     /* [cgw]: inode->i_cdev指针清0 */587     inode->i_cdev = NULL;588   }589   /* [cgw]: 退出临界区 */590   spin_unlock(&cdev_lock);591 }592 593 /*594  * Dummy default file-operations: the only thing this does595  * is contain the open that then fills in the correct operations596  * depending on the special file...597 */598 599 const struct file_operations def_chr_fops = {600   .open = chrdev_open,601 };602 static struct kobject *exact_match(dev_t dev, int *part, void *data)603 {604   struct cdev *p = data;605   /* [cgw]: 返回cdev中kobj成员指针 */606   return &p->kobj;607 }608 609 static int exact_lock(dev_t dev, void *data)610 {611   struct cdev *p = data;612   /* [cgw]: data中kobj引用计数+1,并返回kobj指针 */613   return cdev_get(p) ? 0 : -1;614 }615 616 /**617  * cdev_add() - add a char device to the system618  * @p: the cdev structure for the device619  * @dev: the first device number for which this device is responsible620  * @count: the number of consecutive minor numbers corresponding to this621  *     device622  *623  * cdev_add() adds the device represented by @p to the system, making it624  * live immediately. A negative error code is returned on failure.625 */626 627 int cdev_add(struct cdev *p, dev_t dev, unsigned count)628 {629   /* [cgw]: 分配一个dev设备号给p->dev */630   p->dev = dev;631   /* [cgw]: 分配count个连续的次设备号632    * 这里实际是分配count设备,只是次设备号不一样,主设备号都一样633   */634   p->count = count;635   /* [cgw]: 把新加入的设备填装到一个probe结构,并把这个probe插入到636    * 对应probes[MAJOR(dev)]链表,即probes[]中每一个元素都是一个链表637   */638   return kobj_map(cdev_map, dev, count, NULL, exact_match, exact_lock, p);639 }640 641 static void cdev_unmap(dev_t dev, unsigned count)642 {643   /* [cgw]: 从probes[MAJOR(dev)]链表中删除一个节点(probe) */644   kobj_unmap(cdev_map, dev, count);645 }646 647 /**648  * cdev_del() - remove a cdev from the system649  * @p: the cdev structure to be removed650  *651  * cdev_del() removes @p from the system, possibly freeing the structure652  * itself.653 */654 655 void cdev_del(struct cdev *p)656 {657   /* [cgw]: 从probes[MAJOR(p->dev)]链表中删除一个节点(probe) 658    * 659   */660   cdev_unmap(p->dev, p->count);661   /* [cgw]: kobj引用计数-1 */662   kobject_put(&p->kobj);663 }664 665 static void cdev_default_release(struct kobject *kobj)666 {667   /* [cgw]: 找到包含kobj的结构体struct cdev的指针 */668   struct cdev *p = container_of(kobj, struct cdev, kobj);669   /* [cgw]: 从cdev->list链表中删除cdev */670   cdev_purge(p);671 }672 673 static void cdev_dynamic_release(struct kobject *kobj)674 {675   /* [cgw]: 找到包含kobj的结构体struct cdev的指针 */676   struct cdev *p = container_of(kobj, struct cdev, kobj);677   /* [cgw]: 从cdev->list链表中删除cdev */678   cdev_purge(p);679   /* [cgw]: 释放这个cdev的内存空间 */680   kfree(p);681 }682 683 static struct kobj_type ktype_cdev_default = {684   .release = cdev_default_release,685 };686 687 static struct kobj_type ktype_cdev_dynamic = {688   .release = cdev_dynamic_release,689 };690 691 /**692  * cdev_alloc() - allocate a cdev structure693  *694  * Allocates and returns a cdev structure, or NULL on failure.695 */696 struct cdev *cdev_alloc(void)697 {698   /* [cgw]: 分配一个cdev结构体 */699   struct cdev *p = kzalloc(sizeof(struct cdev), GFP_KERNEL);700 701   /* [cgw]: 分配cdev结构体成功 */702   if (p) {703    /* [cgw]: 分配一个kobj.ktype结构体,指向&ktype_cdev_dynamic704     * 为这个驱动制定一个统一的行为,提供释放kobj的方法705     */706     p->kobj.ktype = &ktype_cdev_dynamic;707     /* [cgw]: 初始化链表,把这个cdev插入链表头 */708     INIT_LIST_HEAD(&p->list);709     /* [cgw]: 初始化kobject,每个对象都有一个kobject */710     kobject_init(&p->kobj);711   }712   return p;713 }714 715 /**716  * cdev_init() - initialize a cdev structure717  * @cdev: the structure to initialize718  * @fops: the file_operations for this device719  *720  * Initializes @cdev, remembering @fops, making it ready to add to the721  * system with cdev_add().722 */723 void cdev_init(struct cdev *cdev, const struct file_operations *fops)724 {725   /* [cgw]: cdev结构体清零 */726   memset(cdev, 0, sizeof *cdev);727   /* [cgw]: 初始化链表,把这个cdev插入链表头 */728   INIT_LIST_HEAD(&cdev->list);729   /* [cgw]: 分配一个kobj.ktype结构体,指向&ktype_cdev_default730    * 为这个驱动制定一个默认的统一的行为,提供恢复默认kobj的方法731    * 没有释放kobj内存空间732   */733   cdev->kobj.ktype = &ktype_cdev_default;734   /* [cgw]: 初始化kobject,每个对象都有一个kobject */735   kobject_init(&cdev->kobj);736   /* [cgw]: cdev->ops指向驱动程序中的file_operations结构体 */737   cdev->ops = fops;738 }739 740 741 742 743 static struct kobject *base_probe(dev_t dev, int *part, void *data)744 {745   if (request_module("char-major-%d-%d", MAJOR(dev), MINOR(dev)) > 0)746     /* Make old-style 2.4 aliases work */747     request_module("char-major-%d", MAJOR(dev));748   return NULL;749 }750 751 void __init chrdev_init(void)752 {753   /*[cgw]: 初始化cdev_map变量 */754   cdev_map = kobj_map_init(base_probe, &chrdevs_lock);755 }756 757 /* Let modules do char dev stuff */758 EXPORT_SYMBOL(register_chrdev_region);759 EXPORT_SYMBOL(unregister_chrdev_region);760 EXPORT_SYMBOL(alloc_chrdev_region);761 EXPORT_SYMBOL(cdev_init);762 EXPORT_SYMBOL(cdev_alloc);763 EXPORT_SYMBOL(cdev_del);764 EXPORT_SYMBOL(cdev_add);765 EXPORT_SYMBOL(register_chrdev);766 EXPORT_SYMBOL(unregister_chrdev);767 EXPORT_SYMBOL(directly_mappable_cdev_bdi);