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>The Linux 2.4 Parallel Port Subsystem</TH
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>The solutions</H1
><P
> The <TT
CLASS="LITERAL"
>parport</TT
> code in Linux 2.2 was designed to
meet these problems of architectural differences in parallel
ports, of port-sharing between devices with pass-through ports,
and of lack of support for IEEE 1284 transfer modes.
</P
><P
> There are two layers to the <TT
CLASS="LITERAL"
>parport</TT
>
subsystem, only one of which deals directly with the hardware.
The other layer deals with sharing and IEEE 1284 transfer modes.
In this way, parallel support for a particular architecture comes
in the form of a module which registers itself with the generic
sharing layer.
</P
><P
> The sharing model provided by the <TT
CLASS="LITERAL"
>parport</TT
>
subsystem is one of exclusive access. A device driver, such as
the printer driver, must ask the <TT
CLASS="LITERAL"
>parport</TT
>
layer for access to the port, and can only use the port once
access has been granted. When it has finished a
<SPAN
CLASS="QUOTE"
>"transaction"</SPAN
>, it can tell the
<TT
CLASS="LITERAL"
>parport</TT
> layer that it may release the port
for other device drivers to use.
</P
><P
> Devices with pass-through ports all manage to share a parallel
port with other devices in generally the same way. The device has
a latch for each of the pins on its pass-through port. The normal
state of affairs is pass-through mode, with the device copying the
signal lines between its host port and its pass-through port.
When the device sees a special signal from the host port, it
latches the pass-through port so that devices further downstream
don't get confused by the pass-through device's conversation with
the host parallel port: the device connected to the pass-through
port (and any devices connected in turn to it) are effectively cut
off from the computer. When the pass-through device has completed
its transaction with the computer, it enables the pass-through
port again.
</P
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> This technique relies on certain <SPAN
CLASS="QUOTE"
>"special signals"</SPAN
>
being invisible to devices that aren't watching for them. This
tends to mean only changing the data signals and leaving the
control signals alone. IEEE 1284.3 documents a standard protocol
for daisy-chaining devices together with parallel ports.
</P
><P
> Support for standard transfer modes are provided as operations
that can be performed on a port, along with operations for setting
the data lines, or the control lines, or reading the status lines.
These operations appear to the device driver as function pointers;
more later.
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