Most info on getty has been moved to Modem-HOWTO with a little info on the use of getty with directly connected terminals now found in Text-Terminal-HOWTO.
A few Linux programs will monitor the modem control lines and indicate if they are positive (1) or negative (0).
irqtune will give serial port interrupts higher
priority to improve performance.hdparm for hard-disk tuning may help some more.
setserial is a program which allows you to tell the device driver
software the I/O address of the serial port, which IRQ is set in the
port's hardware, etc. With appropriate options, it can also probe (at
a given I/O address) for a serial port but you must guess the I/O
address (or it may use whatever address the driver thinks your
/dev/ttySx is at). Setserial does not set either IRQ's nor I/O
addresses in the serial port hardware itself. You must tell setserial
the identical values that have been set in the hardware. It's set in
the hardware either by jumpers or by plug-and-play. Do not just
invent some values that you think would be nice to use. However, if
you know the I/O address but don't know the IRQ you may command
setserial to attempt to determine it.
You can see a list of possible commands to use (but not the one-letter
options such as -v for verbose --which you should normally use when
troubleshooting) by typing setserial with no arguments. Note
that setserial calls an I/O address a "port". If the argument to
setserial is for example just /dev/ttyS1, then you'll see some info
about how that device driver is configured for that port. But this
doesn't tell you if the hardware actually has these values set in it.
If fact, you can run setserial and assign a purely fictitious I/O
address, any IRQ, and whatever uart type you would like to have. Then
the next time you type "setserial ..." it will display these bogus
values without complaint. Note that assignments made by setserial are
lost when the PC is powered down so it is usually run automatically
somewhere each time that Linux is booted.
In order to try to find out if you have a certain piece of serial
hardware you must first know its I/O address (or the device driver
must have an I/O address for it, likely previously set by setserial).
To try to detect the physical hardware use the -v (verbose) and
autoconfig command to setserial. If the resulting message
shows a uart type such as 16550A, then you're OK. If instead it shows
"unknown" for the uart type, then there is likely no serial port
at all at that I/O address. Some cheap serial ports don't identify
themselves correctly so if you see "unknown" you still might have
something there. See the file in which "setserial" is run at
boot-time. Besides auto-probing for uart type, setserial can
auto-probe for IRQ's but this doesn't always work right either.
There should be a file somewhere that runs setserial early at
boot-time before any process uses the serial port. If it's not run at
boot-time then your Linux system will automatically configure only
ttyS{0-3} using the default IRQs of 4 and 3 (with the default IRQ
conflicts). In 1998 it was (temporarily ?) changed to only
ttyS{0-1}. So if you have more than 2 serial ports, or want to
have control over how the ports are configured you should configure
using setserial. In fact, your distribution may have set things up so
that the setserial program runs automatically at boot-time. It's
claimed that Redhat 6.0 failed to provide for this.
The file that runs setserial at boot-time is likely somewhere in the
/etc directory-tree. You might use "locate" to find a file named:
rc.serial, or 0setserial (Debian), etc. If no such file exists you
may need to create it and make sure that it gets run at boot-time. If
such a file is supplied, it should contain a number of commented-out
examples. By uncommenting some of these and/or modifying them, you
should be able to set things up correctly. Make sure that you are
using a valid path for setserial, and a valid device name. You
could do a test by executing this file manually (just type its name as
the super-user) to see if it works right. Testing like this is a lot
faster than doing repeated reboots to get it right. Of course you can
also test a single setserial command by just typing it on the
command line.
The file most commonly used to run setserial at boot-time is
/etc/rc.d/rc.serial. The Debian distribution uses
/etc/rc.boot/0setserial. Another file some have used is
/etc/rc.d/rc.local but it's not a good idea to use this since
it may not be run early enough. It's been reported that other
processes may try to open the serial port before rc.local runs
resulting in serial communication failure.
By default, both ttyS0 and ttyS2 share IRQ 4, while ttyS0 and ttyS3 share IRQ 3. But sharing serial interrupts is not permitted unless you have kernel 2.2 or better. If you don't have this modern kernel but only have two serial ports ttyS0 and ttyS1 you're still OK since IRQ sharing conflicts don't exist for non-existent devices.
But if you do have more than 2 serial ports, then for kernels < 2.2 such sharing may be dangerous if the two devices with the same IRQ are being used at the same time. If you add an internal modem and retain ttyS0 and ttyS1, then you should attempt to find an unused IRQ and set it both on your modem card (or serial port) and then use setserial to assign it to your device driver. If IRQ 5 is not being used for a sound card, this may be one you can use for a modem. To set the IRQ in hardware you may need to use isapnp, a PnP BIOS or patch Linux to make it PnP. To help you determine which spare IRQ's you might have, type "man setserial" and search for say: "IRQ 11".
isapnp is a program to configure Plug-and-Play (PnP) devices
on the ISA bus including internal modems. It comes in a package
called "isapnptools" and includes another program, "pnpdump" which
finds all your ISA PnP devices and shows you options for configuring
them in a format which may be added to the PnP configuration file:
/etc/isapnp.conf. The isapnp command may be put into a startup file
so that it runs each time you start the computer and thus will
configure ISA PnP devices. It is able to do this even if your BIOS
doesn't support PnP. See Plug-and-Play-HOWTO.