Character Device Basics (Slideshow)¶

Major and Minor Numbers: User’s View ¶

$ ls -l /dev/{mem,null}
crw-r-----. 1 root kmem 1, 1 Jan 18 13:29 /dev/mem
crw-rw-rw-. 1 root root 1, 3 Jan 18 13:29 /dev/null

See how /dev/mem has major=1, minor=1
Refer to /dev/null: drivers/char/mem.c
It’s the numbers that count, not the names (/dev/mem)
mknod blah c 1 3 ⟶ /dev/null

Major and Minor Numbers: Kernel Space ¶

dev_t: an opaque thing (unsigned int)
Holds major and minor numbers

#include <linux/cdev.h>

dev_t majmin = MKDEV(1,3);
unsigned major = MAJOR(majmin);
unsigned minor = MINOR(majmin);

History: Hardcoded Device Numbers (1)¶

Before going public with a character device,

Reserve its major number
… together with a range of minor numbers

#include <linux/fs.h>

int register_chrdev_region(dev_t, unsigned count, const char* drivername);
void unregister_chrdev_region(dev_t, unsigned count);

History: Hardcoded Device Numbers (2)¶

Reserve numbers for the mem driver¶

err = register_chrdev_region(MKDEV(1, 1), 17, "mem");
if (err) ...

This reserves the range (1,1) to (1,17)

Free numbers¶

unregister_chrdev_region(MKDEV(1,3), 17);

Character Device (1)¶

Once we have the device numbers guaranteed, a device is created.

#include <linux/cdev.h>
#include <linux/fs.h> /*struct file_operations*/

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

cdev_init() initializes a struct cdev
cdev_add() goes live with it (dev and count semantics as in register_chrdev_region())
cdev_del() is the inverse of cdev_add()

Character Device (2)¶

#include <linux/cdev.h>
#include <linux/fs.h>

struct file_operations my_fops = { ... };
struct cdev my_cdev;

cdev_init(&my_cdev, &my_fops);
err = cdev_add(&my_cdev, MKDEV(1, 1), 17);
if (err) ...

And User Space?¶

So what?

cdev_add() goes live with the device
We only see pairs of (major,minor) in the driver code
major is the driver’s “ID”
minor is the “functionality” within that driver
So how does a user reach the device ``(1,3)``?

Answer

# mknod /some/path/to/devicefile c 1 3