Does register_chrdev function make device file under /dev ?

[김찬]

Hi, Mandeep, Thanks for clearing some things for me.

For accessing the device in kernel code, I think maybe we can use the /proc or /sys file system.
(I remember seeing people saying something like that.. correct me if I'm wrong)

> 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
>
The problem is, after the final sys_execve() which executes /sbin/init of the ramdisk, I cannot see any shell coming up so I cannot use getty command.
Using some prints on separate uart print routine, I can see from the kbd_event function the key value is going to current vc which is using tty1. The key value is then handed to put_queue function where the character is somehow copied to tty buffer. Flush_to_ldisc is scheduled for it and it will probably get passed on to the virtual console.
Added to this, there is a notifier_call_chain and the notifier block is registered by vcs_poll_data_get function which I don't know weather is necessary or not. So one of my questions is  if vcs is necessary for my case. (I guess not. There is no other notifier blocks registered)

I gathered without inittab (I don't use inittab which should be ok for most cases) the assumed default inittab
tty2::askfirst:-/bin/sh
tty3::askfirst:-/bin/sh 
tty4::askfirst:-/bin/sh 
is being applied by the busybox so there maybe 3 shells on tty2,tty3, and tty4 running somewhere.
But even if there are shells running I don't know how to switch to those ttys.
I tried using Ctrl-Alt F2 (or F3, F4) using the USB keyboard to switch to tty2,3,4 to see if any shell comes up on the LCD but couldn't see it.
I tried using inittab with no avail before.

I hope somebody could point me on what should be checked. Any hint will be appreciated.
Regards,
Chan

> -----Original Message-----
> From: Mandeep Sandhu [mailto:mandeepsandhu.chd at gmail.com]
> Sent: Tuesday, March 11, 2014 6:01 PM
> To: Chan Kim
> Cc: Kernel; kernelnewbies at kernelnewbies.org
> Subject: Re: Does register_chrdev function make device file under /dev ?
> 
> 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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