Create a device file using udev

saurabhchokshi · 07-07-2009, 07:04 PM

Hello Folks,

I want to create a device file without using mknod command. However, I want to create a device file using udev framework in Linux Kernel.

Could you please help me out to implement the device driver in the probe function that create the device file dynamically in /dev directory?

Thanks,
Saurabh

Uncle_Theodore · 07-07-2009, 07:50 PM

Well, you can read here
http://reactivated.net/writing_udev_rules.html
or, in a shorter version, here
http://www.linuxjournal.com/article/7316

udev also has documentation files, but they are even harder to read...

osor · 07-07-2009, 09:06 PM

It’s unclear if you are in userspace or kernelspace…

In the kernel, you have various options, depending on how low-level you want to get. You can use (in order of decreasing abstractness), device_create(), device_register(), device_add(), or even kobject_add().

See the kernel’s docbook guide entitled “Linux Device Drivers” (try “make htmldocs” from the kernel sourcetree).

saurabhchokshi · 07-08-2009, 11:30 AM

Hi,

I am in Kernel Space.

I have a character device file and I do not want to execute the mknod command to create device file. I want to create device file using kernel module.

In addition to that, I want to create an entry inside sys directory.

For Example, I have chardev character device file and I want to create the chardev file without mknod command. Also, I need to create a file inside /sys/dev/char/chardev using the kernel module of my device driver.

Actually I know that you can use device_create(), device_register(), device_add(), or even kobject_add() but for this API we need to have "struct kobject *kobj", struct pointer of subsystem like bus, devices, dev, module, etc. , struct pointer of struct class, and struct pointer of attributes (this can be define easily).

Whenever, I build the character device driver, I am using register_chrdev() to register the character device based on Major Num and File Operation pointer. However, I do not know how to get the pointer of kobject of any class, subsystem or device.

Could you please suggest me how to do that?

Thanks,
Saurabh Chokshi

osor · 07-10-2009, 04:28 PM

I don’t quite understand your problem with device_create(). We might need some sample code. Here is a simple chardev module which returns "foo\n" whenever read from.

Code:

$ cat > foo.c << EOF
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <asm/uaccess.h>
#include <linux/device.h>

MODULE_LICENSE("GPL");

#define foo "foo"
#define foolen 4

static ssize_t read_foo(struct file *f, char __user *buf,
			size_t len, loff_t *off)
{
	size_t i = min_t(size_t, len, foolen);
	return copy_to_user(buf, foo "\n", i) ? -EFAULT : i;
}

static int major;
static struct class *class_foo;
static struct device *dev_foo;
static struct file_operations f = { .read = read_foo };

int init_module(void)
{
	void *ptr_err;
	if ((major = register_chrdev(0, foo, &f)) < 0)
		return major;

	class_foo = class_create(THIS_MODULE, foo);
	if (IS_ERR(ptr_err = class_foo))
		goto err2;

	dev_foo = device_create(class_foo, NULL, MKDEV(major, 0), NULL, foo);
	if (IS_ERR(ptr_err = dev_foo))
		goto err;

	/* struct kobject *play_with_this = &dev_foo->kobj; */

	return 0;
err:
	class_destroy(class_foo);
err2:
	unregister_chrdev(major, foo);
	return PTR_ERR(ptr_err);
}

void cleanup_module(void)
{
	device_destroy(class_foo, MKDEV(major, 0));
	class_destroy(class_foo);
	return unregister_chrdev(major, foo);
}
EOF
$ cat > Makefile << EOF
obj-m = foo.o
EOF
$ make M=$PWD -C /lib/modules/`uname -r`/build
make: Entering directory `/usr/src/linux-2.6.30'
  LD      /opt/dvds/src/lq/saurabhchokshi/built-in.o
  CC [M]  /opt/dvds/src/lq/saurabhchokshi/foo.o
  Building modules, stage 2.
  MODPOST 1 modules
  CC      /opt/dvds/src/lq/saurabhchokshi/foo.mod.o
  LD [M]  /opt/dvds/src/lq/saurabhchokshi/foo.ko
make: Leaving directory `/usr/src/linux-2.6.30'
$ head /dev/foo
head: cannot open `/dev/foo' for reading: No such file or directory
$ sudo insmod foo.ko
$ ls -l /dev/foo
crw-rw---- 1 root root 248, 0 2009-07-10 17:28 /dev/foo
$ sudo head /dev/foo
foo
foo
foo
foo
foo
foo
foo
foo
foo
foo
$ ls /sys/class/foo/ /sys/dev/char/248\:0/subsystem/
/sys/class/foo/:
foo

/sys/dev/char/248:0/subsystem/:
foo

What more do you want?

saurabhchokshi · 07-11-2009, 02:34 PM

Hi ,

Thanks for that help. However, I have already implemented that two days back.

However, I have to create the symbolic link of the device in the same class.

For Example,

/dev/device1
/dev/device2

/sys/class/myClass/mydevice1
/sys/class/myClass/mydevice2
/sys/class/myClass/mydevice -> mydevice1

I want to create the symbolic link of mydevice1 as mydevice.

I am using sysfs_create_link(&mydevice_class->dev_kobj,&mydevice2->kobj, "mydevice3")

mydevice_class is the class struct object.
mydevice2 is the device struct object after creating device_create().

I got the error while creating the link.

I just want to confirm, am I passing wrong parameter inside the sysfs_create_link function.

Please help me out for this issue.

Thanks,
Saurabh

osor · 07-12-2009, 06:59 PM

Quote:

Originally Posted by saurabhchokshi

However, I have to create the symbolic link of the device in the same class.

I’m having a hard time understanding why you want this. In any case, the code you’ve provided is wrong. You are taking the address of a pointer to struct kobject, whereas the pointer alone is required. E.g., change this

Code:

&mydevice_class->dev_kobj

to this

Code:

mydevice_class->dev_kobj

On the other hand, I do not think this is kobject you are looking for. In fact, I know of no clean method to get at the kobject for a device’s class (i.e., entry in /sys/class).

For example, in the code I posted above, you have the following:

Code:

&dev_foo->kobj /* refers to the path    /sys/devices/virtual/foo/foo */
class_foo->dev_kobj /* is for the path  /sys/dev/char */
dev_foo->class->dev_kobj /* is also for /sys/dev/char */

If you want a hackish, subject-to-change method you might try:

Code:

&((struct kset*)class_foo->p)->kobj /* /sys/class/foo */

The only other way (which I can think of) to obtain a kobject for /sys/class/foo is to do a dirent lookup manually.

syg00 · 07-12-2009, 07:20 PM

According to this use container_of() macro.

saurabhchokshi · 07-12-2009, 11:44 PM

Hi ,

I am doing because I want to point the default driver name called "device" as device1. For that, I want to create a symbolic link to point the default device as device1.

Sorry I was passing the reference of kobj for struct class.

Thanks a lot for your help. It works!!!

Saurabh

osor · 07-13-2009, 05:15 PM

Quote:

Originally Posted by syg00

According to this use container_of() macro.

Although my original was not as succinct as it could have been (this has since been edited), you can’t use container_of() or other similar macros here for a few reasons, the least of which is that the type of the element class_foo->p is incomplete. It is a private struct whose implementation is not supposed to be visible those outside the class creation code. In fact, the entire kset corresponding to /sys/class is static (unlike those for e.g., /sys/kernel or /sys/power), so you can’t even use it from built-in kernel code without modifying the relevant file.

saurabhchokshi · 07-16-2009, 03:41 PM

Hi,

Through device_create(), device can be created in the /dev/ and the device would be registered in the /sys/dev/char/.

Is there anyway to create the symbolic link in the /dev/ directory.

For Example,

/dev/mydevice -> /dev/mydevice0
/dev/mydevice0
/dev/mydevice1

Thanks,
Saurabh Chokshi

kdawgud · 07-20-2009, 11:17 AM

Quote:

Originally Posted by osor

In the kernel, you have various options, depending on how low-level you want to get. You can use (in order of decreasing abstractness), device_create(), device_register(), device_add(), or even kobject_add().

Is there a way to accomplish device file creation without declaring the module GPL? Currently I use a mknod script, but it would be great to have the module do this automatically when loaded. GPL may be an issue, however.

Thanks!

saurabhchokshi · 09-03-2009, 12:10 PM

Hello Folks,

Is there anyway to create the symbolic link in the /dev/ directory.

For Example,

/dev/mydevice -> /dev/mydevice0
/dev/mydevice0
/dev/mydevice1

Thanks,
Saurabh Chokshi

Tinkster · 09-03-2009, 03:49 PM

I split your hi-jack attempt out into its own thread.

And to answer your question: yes, there is.
Several, indeed.
ln -s to begin with.
And then of course via udev-rules (more work, more elegant ;})

Cheers,
Tink

seaquest · 09-03-2009, 04:16 PM

OSOR:

The way you posted to create a device file under /dev works, but in
my case, the default permission to the device file is crw-------, and the file is owned by root.

How can I change the permission to crw-rw-rw- from within the
kernen module code, I mean, after calling class_create() and device_create(), what function call would change the device file permission ?

Many thanks.

--Seaquest