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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 device 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.

Rachid K.
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Mir
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    See also: http://stackoverflow.com/questions/5970595/create-a-device-node-in-code/ – Eugene Jan 02 '12 at 08:42
  • Possible duplicate of [How to create a device node from the init\_module code of a Linux kernel module?](https://stackoverflow.com/questions/5970595/how-to-create-a-device-node-from-the-init-module-code-of-a-linux-kernel-module) – Ciro Santilli OurBigBook.com Aug 06 '17 at 09:25

6 Answers6

8

  • 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/

Harish Venkatesan
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3

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

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.

Abhijeet Kasurde
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Shash316
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    Yes, it is possible to create a device node programmatically. You can use `class_create()` and `device_create` for that. Udev system mentioned by @duskwuff will take care of the rest. – Eugene Jan 02 '12 at 08:45
  • @Eugene Thanks. That helps a lot. Thanks everybody else. – Mir Jan 02 '12 at 19:54
  • according to https://www.kernel.org/doc/Documentation/admin-guide/devices.txt character device with major number 22, minor number 0 is taken for Digiboard serial card, First port. – Algoman May 04 '21 at 08:10
2

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).

Jamie Iles
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1

There are two ways to create the device file in /dev

  • Manually creation of a device file using mknod
    $ mknod -m <permission> <name> <device_type> <major> <minor>
    here
    name is the name of device driver,
    device_type is the type of the device (b-> block devices , c-> char devices),
    major & minor are the device numbers,
    permission is optional you can change it after creation, by using chmod.
  • Dynamic creation using .
dev_t dev=0;
static struct class *devicefileClass;
deviceFileClass=class_create(THIS_MODULE,"device_name");
device_create( deviceFileClass , NULL , dev ,"device_name")

**NOTE** class_create , device_create function call should be in your `__init` function. and include `,`.
Rachid K.
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cvam
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0

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 !!!

Akash Patel
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