Linux Kernel Architecture

Chapter 6: Device Drivers

struct block_device *i_bdev;

struct cdev *i_cdev;

};

...

};

❑ To uniquely identify that device associated with a device file, the kernel stores the file type

(block- or character-oriented) ini_modeand the major and minor numbers ini_rdev.Thesetwo

items of information are combined in the kernel into a single variable of typedev_t.

The definition of this data type is by no means set in stone but may be modified if

kernel developers deem it necessary. Forthis reason, only the two helper functions

imajorandiminor— which expect a pointer to aninodeinstance as

parameter — should be used to extract the major and minor number fromi_rdev.

❑ i_fopis a collection of file operations such as open, read, and write used by the virtual filesystem

to work with the block device (the exact definition of the structure is given in Chapter 8).

❑ Depending on whether the inode represents a block or a character device,i_bdevori_cdev

point to more specific information, which is discussed further below.

6.3.2 Standard File Operations

When a device file is opened, the various filesystem implementations invoke theinit_special_inode

function to create the inode for a block or character device file.^6

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;

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);

}

The underlying filesystem must return the major and minor numbers of a device in addition to the device

type (block or character) passed inmode. The inode is supplied with different file operations depending

on device type.

6.3.3 Standard Operations for Character Devices

The situation with character devices is initially very unclear because only a single file operation is made

available.

(^6) For the sake of simplicity, I omit the creation of inodes for sockets and fifos as these are not relevant in this context.