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>Design goals</H1
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>The problems</H1
><P
> The first parallel port support for Linux came with the line
printer driver, <TT
CLASS="LITERAL"
>lp</TT
>. The printer driver is a
character special device, and (in Linux 2.0) had support for
writing, via <TT
CLASS="FUNCTION"
>write</TT
>, and configuration and
statistics reporting via <TT
CLASS="FUNCTION"
>ioctl</TT
>.
</P
><P
> 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.
</P
><P
> 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.
</P
><P
> 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
<TT
CLASS="FUNCTION"
>check_region</TT
> function to make sure that the
IO region associated with the parallel port was free, and then it
would call <TT
CLASS="FUNCTION"
>request_region</TT
> to allocate it.
The Zip drive used the same mechanism. Whichever driver
initialised first would gain exclusive control of the parallel
port.
</P
><P
> 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.
</P
><P
> 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.
</P
><P
> 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).
</P
><P
> 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: <SPAN
CLASS="QUOTE"
>"compatibility"</SPAN
>,
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 <SPAN
CLASS="QUOTE"
>"compatibility
mode"</SPAN
> (i.e. normal printer protocol) and reverse nibble
mode.
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