linux-kernel - Re: How do I choose an arbitrary minor number for my tty device?

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Date:	Thu, 18 Nov 2010 12:02:30 -0800
From:	Greg KH <gregkh@...e.de>
To:	Timur Tabi <timur@...escale.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Scott Wood <scottwood@...escale.com>,
	Stuart Yoder <stuart.yoder@...escale.com>
Subject: Re: How do I choose an arbitrary minor number for my tty device?

On Thu, Nov 18, 2010 at 01:35:33PM -0600, Timur Tabi wrote:
> Greg KH wrote:
> >> I just booted a Linux kernel with the driver I just emailed you, and there's no
> >> > /dev/serial/ directory.  The only directories under /dev/ are 'shm' and 'pts',
> >> > both of which are empty.
> 
> > Then plug in a serial port device and see what happens.  You didn't hook
> > everything up in your driver correctly it seems, do your devices show up
> > under /sys/class/tty?
> 
> If I delete the call to device_create() in ehv_bc_init() (so that it creates the
> TTY devices only, and not the character devices), I get this:
> 
> # ls -l /sys/class/tty/ttyEHV*
> lrwxrwxrwx    1 root     root            0 Jan  1 00:04 /sys/class/tty/ttyEHV0
> -> ../../devices/virtual/tty/ttyEHV0
> lrwxrwxrwx    1 root     root            0 Jan  1 00:04 /sys/class/tty/ttyEHV1
> -> ../../devices/virtual/tty/ttyEHV1
> lrwxrwxrwx    1 root     root            0 Jan  1 00:04 /sys/class/tty/ttyEHV2
> -> ../../devices/virtual/tty/ttyEHV2
> 
> >> > I'm also running a Fedora 13 x86 system, just to see if I need a full modern OS
> >> > to see these files.  Again, there is no /dev/serial/, even though I have serial
> >> > ports.
> > Dynamic ones like a usb to serial device?
> 
> No, not dynamic ones.
> 
> >> > Also not that since I'm not registering the byte channels as serial devices, I
> >> > wouldn't expect anything in /dev/serial/ to reference them.
> >> > 
> >> > What does my driver need to do in order for these /dev/xxxx/ entries to contain
> >> > that information?
> 
> > See the udev rules for details.
> 
> udev rules still need some way for the driver to tell user-space that
> /dev/ttyEHV0 is associated with byte channel handle 73.  I still don't know what
> mechanism my driver is supposed to use to make that information available to
> user space.
> 
> I could fake it by doing this:
> 
> for (i = 0; i < num_byte_channels; i++) {
> 	bc->handle = get_the_byte_channel_handle(i);
> 	ehv_bc_driver->name_base = bc->handle - i;
> 	tty_register_device(ehv_bc_driver, i, NULL);

Why are you not setting up a parent device of your tty device?  That
should be essencial.

> }
> 
> This actually works and does what I want, but I seriously doubt it's acceptable.
>  When I do this, I get:
> 
> # ls -l /dev/ttyEH*
> crw-rw----    1 root     uucp     253,   0 Jan  1 00:00 /dev/ttyEHV73
> crw-rw----    1 root     uucp     253,   1 Jan  1 00:00 /dev/ttyEHV76
> crw-rw----    1 root     uucp     253,   2 Jan  1 00:00 /dev/ttyEHV79

Does that work for you?  Looks fine to me :)

But again, you really should get the driver model portion right...

thanks,

greg k-h
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