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

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Message-ID: <4CE58005.7040308@freescale.com>
Date:	Thu, 18 Nov 2010 13:35:33 -0600
From:	Timur Tabi <timur@...escale.com>
To:	Greg KH <gregkh@...e.de>
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?

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);
}

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

-- 
Timur Tabi
Linux kernel developer at Freescale

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