Most recent kernel where this bug did not occur: 2.6.18
Distribution: Debian
Hardware Environment: Toshiba Satellite SM30-344 laptop
Software Environment: Debian testing/lenny
Problem Description:

I have a Novatel U630 UMTS card.  After I plug it in, I run a "setpin"
script that looks like this:

"""
#!/usr/bin/env perl

use strict;
use warnings;

my $modem = "/dev/ttyS0";
# Substitute xxxx with your PIN.
# You should probably put your pin somewhere else, e.g. on an USB stick,
# an encrypted file system or something else, and read it from there...
# You have been warned!
my $pin = "1234";

$SIG{ALRM} = sub {
        die("timeout: no response from modem $modem\n");
};

open(MODEM, "+<", $modem) or die("can't open modem $modem");
alarm(10);
print(MODEM "AT+CPIN=\"$pin\"\n\r");
while (<MODEM>) {
        if (m/OK/) {
                close(MODEM);
                print("PIN accepted\n");
                exit(0);
        }
        if (m/ERROR/) {
                close(MODEM);
                print("PIN rejected\n");
                exit(1);
        }
}
"""

With the kernels 2.6.21 and 2.6.22 from Debian and with the kernel 2.6.22.1 from ftp.kernel.org I get no response from this and it times out after the given alarm interval.  With the kernel linux-image-2.6.18-4-686 from Debian etch, this works fine and I get "PIN accepted" after less than a second.

So something was broken along the way.