The `parport' code provides parallel-port support under Linux.  This
includes the ability to share one port between multiple device
drivers.

You can pass parameters to the parport code to override its automatic
detection of your hardware.  This is particularly useful if you want
to use IRQs, since in general these can't be autoprobed successfully.
By default IRQs are not used even if they _can_ be probed.  This is
because there are a lot of people using the same IRQ for their
parallel port and a sound card or network card.

The parport code is split into two parts: generic (which deals with
port-sharing) and architecture-dependent (which deals with actually
using the port).


Parport as modules
==================

If you load the parport code as a module, say

	# insmod parport.o

to load the generic parport code.  You then must load the
architecture-dependent code with (for example):

	# insmod parport_pc.o io=0x3bc,0x378,0x278 irq=none,7,auto

to tell the parport code that you want three PC-style ports, one at
0x3bc with no IRQ, one at 0x378 using IRQ 7, and one at 0x278 with an
auto-detected IRQ.  Currently, PC-style (parport_pc) and Sun Ultra/AX
(parport_ax) hardware is supported; more is in the works.


KMod
----

If you use kmod, you will find it useful to edit /etc/conf.modules.
Here is an example of the lines that need to be added:

	alias parport_lowlevel parport_pc
	options parport_pc io=0x378,0x278 irq=7,auto

KMod will then automatically load parport_pc (with the options
"io=0x378,0x278 irq=7,auto") whenever a parallel port device driver
(such as lp) is loaded.


Parport probe [optional]
-------------

Once the architecture-dependent part of the parport code is loaded
into the kernel, you can insert the parport_probe module with:

	# insmod parport_probe.o

This will perform an IEEE1284 probe of any attached devices and log a
message similar to:

	parport0: Printer, BJC-210 (Canon)

(If you are using kmod and have configured parport_probe as a module,
this will just happen.)

The probe information is available in /proc/parport/?/autoprobe.


Parport linked into the kernel statically
=========================================

If you compile the parport code into the kernel, then you can use
kernel boot parameters to get the same effect.  Add something like the
following to your LILO command line:

	parport=0x3bc parport=0x378,7 parport=0x278,auto

You can have many `parport=...' statements, one for each port you want
to add.  Adding `parport=0' to the kernel command-line will disable
parport support entirely.  Adding `parport=auto' to the kernel
command-line will make parport use any IRQ lines or DMA channels that
it auto-detects.


Files in /proc
==============

If you have configured the /proc filesystem into your kernel, you will
see a new directory entry: /proc/parport.  In there will be a
directory entry for each parallel port for which parport is
configured.  In each of those directories are four files describing
that parallel port.  For example:

File:				Contents:

/proc/parport/0/devices		A list of the device drivers using
				that port.  A "+" will appear by the
				name of the device currently using the
				port (it might not appear against any).

/proc/parport/0/hardware	Parallel port's base address, IRQ line
				and DMA channel.

/proc/parport/0/irq		The IRQ that parport is using for that
				port.  This is in a separate file to
                                allow you to alter it by writing a new
				value in (IRQ number or "none").

/proc/parport/0/autoprobe	Any IEEE-1284 device ID information
				that has been acquired.


Device drivers
==============

Once the parport code is initialised, you can attach device drivers to
specific ports.  Normally this happens automatically; if the lp driver
is loaded it will create one lp device for each port found.  You can
override this, though, by using parameters either when you load the lp
driver:

	# insmod lp.o parport=0,2

or on the LILO command line:

	lp=parport0 lp=parport2

Both the above examples would inform lp that you want /dev/lp0 to be
the first parallel port, and /dev/lp1 to be the _third_ parallel port,
with no lp device associated with the second port (parport1).  Note
that this is different to the way older kernels worked; there used to
be a static association between the I/O port address and the device
name, so /dev/lp0 was always the port at 0x3bc.  This is no longer the
case - if you only have one port, it will default to being /dev/lp0,
regardless of base address.

Also:

 * If you selected the IEEE-1284 autoprobe at compile time, you can say
   `lp=auto' on the kernel command line, and lp will create devices
   only for those ports that seem to have printers attached.

 * If you give PLIP the `timid' parameter, either with `plip=timid' on
   the command line, or with `insmod plip timid=1' when using modules,
   it will avoid any ports that seem to be in use by other devices.

 * IRQ autoprobing works only for a few port types at the moment.

--
Philip.Blundell@pobox.com
tim@cyberelk.demon.co.uk