Does register_chrdev function make device file under /dev ?

[Mandeep Sandhu]

On Tue, Mar 11, 2014 at 1:31 PM, Kim Chan <ckim at etri.re.kr> wrote:
>
> Yeah, I know mknod command. I just wanted to know if the register_chrdev
> makes the file or not.

It doesn't. Also note that this is the old way of registering a char
device. New code should use cdev_add() etc.

> To my understanding, device file is needed when a user program wants to
> access the device and we don't need to generate the device file for us to
> use the device in kernel. Is my understanding correct?

Yes, device nodes are needed for userspace code to access a device
(the good ol "devices are also files" Unix paradigm! :)).

I'm not sure how an in-kernel access will work (maybe via directly
getting the corresponding cdec structure).

>
> For example, busybox (a user program) starts shell on tty2-4 by default. (as
> the assumed default inittab below)
> tty2::askfirst:-/bin/sh
> tty3::askfirst:-/bin/sh
> tty4::askfirst:-/bin/sh
> I have had some mknod commands before in /etc/init.d/rcS to make tty2~tty4
> device files, but when I remove them for test, I can see busybox (almost
> sure it's coming from busybox) complaining that it cannot find the tty2~tty4
> device files.

Correct. You will have to create those nodes manually.

>
> I am trying to open a shell on my LCD (is it going to be tty2 ? I don't
> know) and I'm not sure if I have to make vcs1 vcs2 vcs3 files. And I don't
> know how to switch to LCD shell. during the boot, the texts come out on the
> LCD but I cannot see the texts (printf) from busybox. and cannot see the
> shell on LCD. When I press the keyboad, it looks like it's connected to
> tty1(I can see using prints on uart window). Can anybody tell me some
> direction?

I think you need to point getty or a shell to the correct tty before
you can see o/p on the screen. Do you know which device file you're
supposed to create for the LCD? If so, just use that in your inittab.
Something like:

::askfirst:-/bin/sh

HTH,
-mandeep


> ________________________________
> From : "Kernel" <bu.kernel at gmail.com>
> Sent : 2014-03-11 14:44:09 ( +09:00 )
> To : Kim Chan <ckim at etri.re.kr>
> Cc : kernelnewbies at kernelnewbies.org <kernelnewbies at kernelnewbies.org>
> Subject : Re: Does register_chrdev function make device file under /dev ?
>
>
>
> On Mar 10, 2014, at 10:25 PM, 김찬 <ckim at etri.re.kr> wrote:
>
> Hi,
>
> This should be a basic question.
> Does the regisetr_chrdev function make the device file under /dev directory?
> I am running linux on our embedded system and inside vcs_init,
> register_chrdev is called as below.
>
> int __init vcs_init(void)
> {
>         unsigned int i;
>         if (register_chrdev(VCS_MAJOR, "vcs", &vcs_fops))
>                 panic("unable to get major %d for vcs device", VCS_MAJOR);
>
> I can see the function is returning ok but I don't see any vcs* file under
> /dev.
> Does it only register the device on /sys or /proc directory and not under
> /dev?
> Thanks in advance.
>
> Chan
>
>
>
> You have to create a device file using 'mknod'
>
> Ex: mknod -m 666 /dev/vcs c <major no> <minor no>
>
>
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