The first parallel port support for Linux came with the line printer driver, lp. The printer driver is a character special device, and (in Linux 2.0) had support for writing, via write, and configuration and statistics reporting via ioctl.
The printer driver could be used on any computer that had an IBM PC-compatible parallel port. Because some architectures have parallel ports that aren't really the same as PC-style ports, other variants of the printer driver were written in order to support Amiga and Atari parallel ports.
When the Iomega Zip drive was released, and a driver written for it, a problem became apparent. The Zip drive is a parallel port device that provides a parallel port of its own---it is designed to sit between a computer and an attached printer, with the printer plugged into the Zip drive, and the Zip drive plugged into the computer.
The problem was that, although printers and Zip drives were both supported, for any given port only one could be used at a time. Only one of the two drivers could be present in the kernel at once. This was because of the fact that both drivers wanted to drive the same hardware---the parallel port. When the printer driver initialised, it would call the check_region function to make sure that the IO region associated with the parallel port was free, and then it would call request_region to allocate it. The Zip drive used the same mechanism. Whichever driver initialised first would gain exclusive control of the parallel port.
The only way around this problem at the time was to make sure that both drivers were available as loadable kernel modules. To use the printer, load the printer driver module; then for the Zip drive, unload the printer driver module and load the Zip driver module.
The net effect was that printing a document that was stored on a Zip drive was a bit of an ordeal, at least if the Zip drive and printer shared a parallel port. A better solution was needed.
Zip drives are not the only devices that presented problems for Linux. There are other devices with pass-through ports, for example parallel port CD-ROM drives. There are also printers that report their status textually rather than using simple error pins: sending a command to the printer can cause it to report the number of pages that it has ever printed, or how much free memory it has, or whether it is running out of toner, and so on. The printer driver didn't originally offer any facility for reading back this information (although Carsten Gross added nibble mode readback support for kernel 2.2).
The IEEE has issued a standards document called IEEE 1284, which documents existing practice for parallel port communications in a variety of modes. Those modes are: "compatibility", reverse nibble, reverse byte, ECP and EPP. Newer devices often use the more advanced modes of transfer (ECP and EPP). In Linux 2.0, the printer driver only supported "compatibility mode" (i.e. normal printer protocol) and reverse nibble mode.