Parport — The Linux Kernel 5.18.5-gnu-1 documentation

# Parport

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

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

    # insmod parport_pc 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`), Sun `bpp`, Amiga, Atari, and MFC3 hardware is supported.

PCI parallel I/O card support comes from `parport_pc`. Base I/O addresses should not be specified for supported PCI cards since they are automatically detected.

### modprobe

If you use modprobe , you will find it useful to add lines as below to a configuration file in /etc/modprobe.d/ directory:

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

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

Note that these are example lines only! You shouldn’t in general need to specify any options to `parport_pc` in order to be able to use a parallel port.

### Parport probe [optional]

In 2.2 kernels there was a module called `parport_probe`, which was used for collecting IEEE 1284 device ID information. This has now been enhanced and now lives with the IEEE 1284 support. When a parallel port is detected, the devices that are connected to it are analysed, and information is logged like this:

    parport0: Printer, BJC-210 (Canon)

The probe information is available from files in `/proc/sys/dev/parport/`.

## 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,nofifo

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/sys/dev/parport`. In there will be a directory entry for each parallel port for which parport is configured. In each of those directories are a collection of files describing that parallel port.

The `/proc/sys/dev/parport` directory tree looks like:

    parport
    |-- default
    |   |-- spintime
    |   `-- timeslice
    |-- parport0
    |   |-- autoprobe
    |   |-- autoprobe0
    |   |-- autoprobe1
    |   |-- autoprobe2
    |   |-- autoprobe3
    |   |-- devices
    |   |   |-- active
    |   |   `-- lp
    |   |       `-- timeslice
    |   |-- base-addr
    |   |-- irq
    |   |-- dma
    |   |-- modes
    |   `-- spintime
    `-- parport1
    |-- autoprobe
    |-- autoprobe0
    |-- autoprobe1
    |-- autoprobe2
    |-- autoprobe3
    |-- devices
    |   |-- active
    |   `-- ppa
    |       `-- timeslice
    |-- base-addr
    |-- irq
    |-- dma
    |-- modes
    `-- spintime

File | Contents
--- | ---
`devices/active` | 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). The string “none” means that there are no device drivers using that port.
`base-addr` | Parallel port’s base address, or addresses if the port has more than one in which case they are separated with tabs. These values might not have any sensible meaning for some ports.
`irq` | Parallel port’s IRQ, or -1 if none is being used.
`dma` | Parallel port’s DMA channel, or -1 if none is being used.
`modes` | Parallel port’s hardware modes, comma-separated, meaning: PCSPP PC-style SPP registers are available. TRISTATE Port is bidirectional. COMPAT Hardware acceleration for printers is available and will be used. EPP Hardware acceleration for EPP protocol is available and will be used. ECP Hardware acceleration for ECP protocol is available and will be used. DMA DMA is available and will be used. Note that the current implementation will only take advantage of COMPAT and ECP modes if it has an IRQ line to use.
`autoprobe` | Any IEEE-1284 device ID information that has been acquired from the (non-IEEE 1284.3) device.
`autoprobe[0-3]` | IEEE 1284 device ID information retrieved from daisy-chain devices that conform to IEEE 1284.3.
`spintime` | The number of microseconds to busy-loop while waiting for the peripheral to respond. You might find that adjusting this improves performance, depending on your peripherals. This is a port-wide setting, i.e. it applies to all devices on a particular port.
`timeslice` | The number of milliseconds that a device driver is allowed to keep a port claimed for. This is advisory, and driver can ignore it if it must.
`default/*` | The defaults for spintime and timeslice. When a new port is registered, it picks up the default spintime. When a new device is registered, it picks up the default timeslice.

## 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 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 support 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.

## Reporting printer problems with parport

If you are having problems printing, please go through these steps to try to narrow down where the problem area is.

When reporting problems with parport, really you need to give all of the messages that `parport_pc` spits out when it initialises. There are several code paths:

- polling

- interrupt-driven, protocol in software

- interrupt-driven, protocol in hardware using PIO

- interrupt-driven, protocol in hardware using DMA

The kernel messages that `parport_pc` logs give an indication of which code path is being used. (They could be a lot better actually..)

For normal printer protocol, having IEEE 1284 modes enabled or not should not make a difference.

To turn off the ‘protocol in hardware’ code paths, disable `CONFIG_PARPORT_PC_FIFO`. Note that when they are enabled they are not necessarily used; it depends on whether the hardware is available, enabled by the BIOS, and detected by the driver.

So, to start with, disable `CONFIG_PARPORT_PC_FIFO`, and load `parport_pc` with `irq=none`. See if printing works then. It really should, because this is the simplest code path.

If that works fine, try with `io=0x378 irq=7` (adjust for your hardware), to make it use interrupt-driven in-software protocol.

If that works fine, then one of the hardware modes isn’t working right. Enable `CONFIG_FIFO` (no, it isn’t a module option, and yes, it should be), set the port to ECP mode in the BIOS and note the DMA channel, and try with:

    io=0x378 irq=7 dma=none (for PIO)
    io=0x378 irq=7 dma=3 (for DMA)

philb@gnu.org tim@cyberelk.net