How to create a device in /dev automatically upon loading of the kernel module for a device driver?

Question

I am attempting to develop Linux device drivers and as my first attempt I am trying to develop a char device driver that has the following file options,

struct file_operations fops{  
.open=open_fun,  
.release=release_fun,  
.write=write_fun,  
.read=read_fun,  
};  

When I load the driver using insmod, I see that /proc/devices lists the driver under char devices but I can't find it in /dev. A Google search suggested use of mknod to create a deivce in /dev and associate it with the driver's major and minor. However, an attempt to do so resulted in "Permission denied" error even when done as a super user.

What should I do to make the device appear in /dev when the kernel module is loaded? I tried both the older (register_chrdev) and the newer version (cdev_init & cdev_add) of registering the device but none of them works.

Thanks,
Mir

Answer 1

• Include the header file linux/device.h and linux/kdev_t.h

• static struct class c_dev;

• static struct dev_t dev;

Add the below API 's inside __init fuction of the driver

• cl = class_create(THIS_MODULE ,"x");

where x - Name to be displayed inside /sys/class/ when driver is loaded.

• Use device_create () kernel api with device_create(cl, NULL, dev, NULL, "d");

where d - device file to be created under /dev.

where dev is variable for the first device number that is initialized during the usage of alloc_chrdev_region API for dynamic allocation of major number for the driver

For Further reference please go through the link http://opensourceforu.com/2011/04/character-device-files-creation-operations/

Answer 2

You may have to create some udev rules to tell the system what device node(s) you need it to create.

Answer 3

I could see entries within /dev after creating node by running following command at console.

sudo mknod -m 0666 /dev/msio c 22 0

The user was not root, so I had to use sudo. My entry name was msio, a character device with major and minor number 22, 0 respectively.

I will let you know if this can be achieved programatically.

Answer 4

CONFIG_DEVTMPFS is quite nice if you can use that with your distro. You can have the kernel automount that for you at boot (CONFIG_DEVTMPFS_MOUNT) or mount it manually (mount -t devtmpfs none /dev).

Answer 5

First you have to use : sudo mknod /dev/devicename c 81 0

It creates device file in /dev but you have to give it read/write permission. to do so,

sudo chmod 777 /dev/devicename

Done !!!