Char device initialization

[rohan puri]

On Wed, Nov 9, 2011 at 7:22 PM, Alexandru Juncu <alex.juncu at rosedu.org>wrote:

> On Wed, Nov 9, 2011 at 3:41 PM, Daniel Hilst Selli
> <danielhilst at gmail.com> wrote:
> > I'm trying to create a example char device. The example compiles fine,
> > but when I try to "cat" I got "No such device or address". I have
> > reviewed the code thousend times and can't see what I'm missing
> >
> > Here is the code -> http://pastebin.com/Td03U0fK
> >
> > The read method is not good, I know, but is never called.
> >
> > I use my own running kenrel to test, I know that is danger. I'm building
> > a qemu enviroment to test this better.
> >
> > Here is uname -a:
> > Linux archlinux 3.0-ARCH #1 SMP PREEMPT Fri Oct 7 11:35:34 CEST 2011
> > x86_64 Intel(R) Core(TM) i3 CPU M 370 @ 2.40GHz GenuineIntel GNU/Linux
> >
> > Any idea?
> >
> > Thanks
>
> You tried to 'cat' a /dev/my_device file, right?
> Was that device file created with the mknod command?
>
> --
> Alexandru Juncu
>
> ROSEdu
> http://rosedu.org
>
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Hello Daniel,

I had the look at the code. The issue is with the cdev_add() call in
init_gcdev() function.

int cdev_add(struct cdev *p, dev_t dev, unsigned count) is the prototype

Now the problem was  *instead of passing second argument of type dev_t you
were passing minor number macro.

*Fix : - Do following additions : -

1. static int major; // Declare a global major no var.

2. In init_gcdev() after call to alloc_chrdev_region() get major no and
store in major var.

major = MAJOR(gcdev->dev);

3. Replace cdev_add() call like this : -

 cdev_add(&gcdev->cdev, MKDEV(major, FIRST_MINOR), 1);

Now its running and your read methos is getting called.

Hello Alexandru,

That error was due to improper args passed to cdev_add(). If device file is
not present (no mknod done) error would be "No such file or dir"

Regards,
Rohan Puri
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