Linux Char Driver

Question

Can anyone tell me how a Char Driver is bind to the corresponding physical device?

Also, I would like to know where inside a char driver we are specifying the physical device related information, which can be used by kernel to do the binding.

Thanks !!

AnshuMan Gupta AnshuMan Gupta · Accepted Answer · 2015-10-09T06:43:22

A global array — bdev_map for block and cdev_map for character devices — is used to implement a hash table, which employs the device major number as hash key.

while registering for char driver following calls get in invoked to get major and minor numbers.

int register_chrdev_region(dev_t from, unsigned count, const char *name) int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count, const char *name);

After a device number range has been obtained, the device needs to be activated by adding it to the character device database.

void cdev_init(struct cdev *cdev, const struct file_operations *fops); int cdev_add(struct cdev *p, dev_t dev, unsigned count);

Here on cdev structure initialize with file operation and respected character device.

Whenever a device file is opened, the various filesystem implementations invoke the init_special_inode function to create the inode for a block or character device file.

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;
inode->i_rdev = rdev;
} else if (S_ISBLK(mode)) {
inode->i_fop = &def_blk_fops;
inode->i_rdev = rdev;
}
else
printk(KERN_DEBUG "init_special_inode: bogus i_mode (%o)\n",
mode);
}

now the default_chr_fpos chrdev_open() method will get invoked. which will look up for the inode->rdev device in cdev_map array and will get a instance of cdev structure. with the reference to cdev it will bind the file->f_op to cdev file operation and invoke the open method for character driver.

Linux Char Driver

2 Answers