來源: ChinaUnix博客 日期： 2008.12.18 20:09真是蠻復雜的，我分三步走，力求講得比較清楚。以字符設備為例，相對於塊設備要簡單些。
基於2.6.26的內核
Understand the Linux Kernel 3rd裡面其實都講到了，所以想再深入的話，可以去參考那本書。
一）驅動注冊open函數都干了些什麼？
register_chrdev -> cdev_add -> kobj_map
file: fs/char_dev.c
int register_chrdev(unsigned int major, const char *name,
const struct file_operations *fops)
{
struct char_device_struct *cd;
struct cdev *cdev;
char *s;
int err = -ENOMEM;
cd = __register_chrdev_region(major, 0, 256, name);
if (IS_ERR(cd))
return PTR_ERR(cd);
cdev = cdev_alloc();
if (!cdev)
goto out2;
cdev->owner = fops->owner;
cdev->ops = fops; // 注意，在後面的 chrdev_open會從cdev再得到 fops
...
}
file: fs/char_dev.c
int cdev_add(struct cdev *p, dev_t dev, unsigned count)
{
p->dev = dev;
p->count = count;
return kobj_map(cdev_map, dev, count, NULL, exact_match, exact_lock, p);
}
file: fs/char_dev.c
static struct kobject *exact_match(dev_t dev, int *part, void *data)
{
struct cdev *p = data;
return &p->kobj;
}
file: drivers/base/map.c
int kobj_map(struct kobj_map *domain, dev_t dev, unsigned long range,
struct module *module, kobj_probe_t *probe,
int (*lock)(dev_t, void *), void *data)
{
unsigned n = MAJOR(dev + range - 1) - MAJOR(dev) + 1;
unsigned index = MAJOR(dev);
unsigned i;
struct probe *p;
if (n > 255)
n = 255;
p = kmalloc(sizeof(struct probe) * n, GFP_KERNEL);
if (p == NULL)
return -ENOMEM;
for (i = 0; i owner = module;
p->get = probe; // 此處其實就是exact_match
p->lock = lock;
p->dev = dev;
p->range = range;
p->data = data;
}
mutex_lock(domain->lock);
for (i = 0, p -= n; i probes[index % 255];
while (*s && (*s)->range next;
p->next = *s;
*s = p;
}
mutex_unlock(domain->lock);
return 0;
}
【參考 Understanding The Linux Kernel 13.5. Character Device Drivers 】
The device driver model defines a kobject mapping domain for the character devices, which is represented by a descriptor of type kobj_map and is referenced by the cdev_map global variable. The kobj_map descriptor includes a hash table of 255 entries indexed
by the major number of the intervals. The hash table stores objects of type probe, one for each registered range of major and minor numbers, whose fields are listed in Table 13-9.
When the kobj_map( ) function is invoked, the specified interval of device numbers is added to the hash table. The data field of the corresponding probe object points to the cdev descriptor of the device driver. The value of this field is passed to the get
and lock methods when they are executed. In this case, the get method is implemented by a short function that returns the address of the kobject embedded in the cdev descriptor; the lock method, instead, essentially increases the reference counter in the embedded
kobject.
The kobj_lookup( ) function receives as input parameters a kobject mapping domain and a device number; it searches the hash table and returns the address of the kobject of the owner of the interval including the number, if it was found. When applied to the
mapping domain of the character devices, the function returns the address of the kobject embedded in the cdev descriptor of the device driver that owns the interval of device numbers.
二）從系統調用往內核走，看當初驅動裡注冊的file_operations裡的open函數怎麼被調用的
sys_open -> do_sys_open -> do_filp_open -> nameidata_to_filp -> __dentry_open
問題是 1）__dentry_open如何找到 chrdev_open?
2）最終又是如何調用file_operations裡的在驅動裡面注冊的open函數的呢?
static struct file *__dentry_open(struct dentry *dentry, struct vfsmount *mnt,
int flags, struct file *f,
int (*open)(struct inode *, struct file *))
{
struct inode *inode;
int error;
f->f_flags = flags;
f->f_mode = ((flags+1) & O_ACCMODE) | FMODE_LSEEK |
FMODE_PREAD | FMODE_PWRITE;
inode = dentry->d_inode;
if (f->f_mode & FMODE_WRITE) {
error = __get_file_write_access(inode, mnt);
if (error)
goto cleanup_file;
if (!special_file(inode->i_mode))
file_take_write(f);
}
f->f_mapping = inode->i_mapping;
f->f_path.dentry = dentry;
f->f_path.mnt = mnt;
f->f_pos = 0;
f->f_op = fops_get(inode->i_fop); // 此處獲得 def_chr_fops
file_move(f, &inode->i_sb->s_files);
error = security_dentry_open(f);
if (error)
goto cleanup_all;
if (!open && f->f_op)
open = f->f_op->open; // 此處調用 def_chr_fops裡的open函數，即chrdev_open
...
}
file: fs/char_dev.c
chrdev_open() {
struct cdev *p;
struct cdev *new = NULL;
int ret = 0;
spin_lock(&cdev_lock);
p = inode->i_cdev;
if (!p) {
struct kobject *kobj;
int idx;
spin_unlock(&cdev_lock);
kobj = kobj_lookup(cdev_map, inode->i_rdev, &idx); // 找到cdev對應的kobj對象, 跟kobj_map遙相對應的，反操作
if (!kobj)
return -ENXIO;
new = container_of(kobj, struct cdev, kobj); // 找到cdev
spin_lock(&cdev_lock);
p = inode->i_cdev;
if (!p) {
inode->i_cdev = p = new;
inode->i_cindex = idx;
list_add(&inode->i_devices, &p->list);
new = NULL;
} else if (!cdev_get(p))
ret = -ENXIO;
} else if (!cdev_get(p))
ret = -ENXIO;
spin_unlock(&cdev_lock);
cdev_put(new);
if (ret)
return ret;
filp->f_op = fops_get(p->ops); // 這裡又找回了當初驅動注冊時的 file_operations指針
if (!filp->f_op) {
cdev_put(p);
return -ENXIO;
}
if (filp->f_op->open) {
lock_kernel();
ret = filp->f_op->open(inode,filp); // 此處算真正的調用了file_operations裡的open函數
unlock_kernel();
}
...
}
file: drivers/base/map.c
struct kobject *kobj_lookup(struct kobj_map *domain, dev_t dev, int *index)
{
struct kobject *kobj;
struct probe *p;
unsigned long best = ~0UL;
retry:
mutex_lock(domain->lock);
for (p = domain->probes[MAJOR(dev) % 255]; p; p = p->next) {
struct kobject *(*probe)(dev_t, int *, void *);
struct module *owner;
void *data;
if (p->dev > dev || p->dev + p->range - 1 range - 1 >= best)
break;
if (!try_module_get(p->owner))
continue;
owner = p->owner;
data = p->data;
probe = p->get; // 這裡其實就是 exact_match函數了
best = p->range - 1;
*index = dev - p->dev;
if (p->lock && p->lock(dev, data) lock);
kobj = probe(dev, index, data); // 這裡調用了 exact_match 函數
/* Currently ->owner protects _only_ ->probe() itself. */
module_put(owner);
if (kobj)
return kobj;
goto retry;
}
mutex_unlock(domain->lock);
return NULL;
}
【參考 Understanding The Linux Kernel 13.5.2. Accessing a Character Device Driver】
We mentioned in the earlier section "VFS Handling of Device Files" that the dentry_open( ) function triggered by the open( ) system call service routine customizes the f_op field in the file object of the character device file so that it points to the def_chr_fops
table. This table is almost empty; it only defines the chrdev_open( ) function as the open method of the device file. This method is immediately invoked by dentry_open( ).
三）什麼時候為字符設備設置的def_chr_fops ?
這個跟具體的文件系統有關系的。
現在/dev/下的設備節點都是通過udev動態創建的，udev會去調用mknod（假定是ext2，內核會調用ext2_mknod），
如果是char設備，會把def_chr_fops附給inode->i_fop，而ext2_mknod會調用init_special_inode()，函數
的部分實現如下：
file: fs/ext2/namei.c
static int ext2_mknod (struct inode * dir, struct dentry *dentry, int mode, dev_t rdev)
{
struct inode * inode;
int err;
if (!new_valid_dev(rdev))
return -EINVAL;
inode = ext2_new_inode (dir, mode);
err = PTR_ERR(inode);
if (!IS_ERR(inode)) {
init_special_inode(inode, inode->i_mode, rdev); // 調用 init_special_inode
file: fs/inode.c
void init_special_inode(struct inode *inode, umode_t mode, dev_t rdev)
{
inode->i_mode = mode;
if (S_ISCHR(mode)) {
inode->i_fop = &def_chr_fops; // 這裡為char設備設置的缺省操作
inode->i_rdev = rdev;
}
...
}
file: fs/char_dev.c
const struct file_operations def_chr_fops = {
.open = chrdev_open,
};