mirror of
https://github.com/team-infusion-developers/android_kernel_samsung_msm8976.git
synced 2024-11-01 10:33:27 +00:00
1da177e4c3
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
215 lines
8 KiB
Text
215 lines
8 KiB
Text
PLIP: The Parallel Line Internet Protocol Device
|
|
|
|
Donald Becker (becker@super.org)
|
|
I.D.A. Supercomputing Research Center, Bowie MD 20715
|
|
|
|
At some point T. Thorn will probably contribute text,
|
|
Tommy Thorn (tthorn@daimi.aau.dk)
|
|
|
|
PLIP Introduction
|
|
-----------------
|
|
|
|
This document describes the parallel port packet pusher for Net/LGX.
|
|
This device interface allows a point-to-point connection between two
|
|
parallel ports to appear as a IP network interface.
|
|
|
|
What is PLIP?
|
|
=============
|
|
|
|
PLIP is Parallel Line IP, that is, the transportation of IP packages
|
|
over a parallel port. In the case of a PC, the obvious choice is the
|
|
printer port. PLIP is a non-standard, but [can use] uses the standard
|
|
LapLink null-printer cable [can also work in turbo mode, with a PLIP
|
|
cable]. [The protocol used to pack IP packages, is a simple one
|
|
initiated by Crynwr.]
|
|
|
|
Advantages of PLIP
|
|
==================
|
|
|
|
It's cheap, it's available everywhere, and it's easy.
|
|
|
|
The PLIP cable is all that's needed to connect two Linux boxes, and it
|
|
can be built for very few bucks.
|
|
|
|
Connecting two Linux boxes takes only a second's decision and a few
|
|
minutes' work, no need to search for a [supported] netcard. This might
|
|
even be especially important in the case of notebooks, where netcards
|
|
are not easily available.
|
|
|
|
Not requiring a netcard also means that apart from connecting the
|
|
cables, everything else is software configuration [which in principle
|
|
could be made very easy.]
|
|
|
|
Disadvantages of PLIP
|
|
=====================
|
|
|
|
Doesn't work over a modem, like SLIP and PPP. Limited range, 15 m.
|
|
Can only be used to connect three (?) Linux boxes. Doesn't connect to
|
|
an existing Ethernet. Isn't standard (not even de facto standard, like
|
|
SLIP).
|
|
|
|
Performance
|
|
===========
|
|
|
|
PLIP easily outperforms Ethernet cards....(ups, I was dreaming, but
|
|
it *is* getting late. EOB)
|
|
|
|
PLIP driver details
|
|
-------------------
|
|
|
|
The Linux PLIP driver is an implementation of the original Crynwr protocol,
|
|
that uses the parallel port subsystem of the kernel in order to properly
|
|
share parallel ports between PLIP and other services.
|
|
|
|
IRQs and trigger timeouts
|
|
=========================
|
|
|
|
When a parallel port used for a PLIP driver has an IRQ configured to it, the
|
|
PLIP driver is signaled whenever data is sent to it via the cable, such that
|
|
when no data is available, the driver isn't being used.
|
|
|
|
However, on some machines it is hard, if not impossible, to configure an IRQ
|
|
to a certain parallel port, mainly because it is used by some other device.
|
|
On these machines, the PLIP driver can be used in IRQ-less mode, where
|
|
the PLIP driver would constantly poll the parallel port for data waiting,
|
|
and if such data is available, process it. This mode is less efficient than
|
|
the IRQ mode, because the driver has to check the parallel port many times
|
|
per second, even when no data at all is sent. Some rough measurements
|
|
indicate that there isn't a noticeable performance drop when using IRQ-less
|
|
mode as compared to IRQ mode as far as the data transfer speed is involved.
|
|
There is a performance drop on the machine hosting the driver.
|
|
|
|
When the PLIP driver is used in IRQ mode, the timeout used for triggering a
|
|
data transfer (the maximal time the PLIP driver would allow the other side
|
|
before announcing a timeout, when trying to handshake a transfer of some
|
|
data) is, by default, 500usec. As IRQ delivery is more or less immediate,
|
|
this timeout is quite sufficient.
|
|
|
|
When in IRQ-less mode, the PLIP driver polls the parallel port HZ times
|
|
per second (where HZ is typically 100 on most platforms, and 1024 on an
|
|
Alpha, as of this writing). Between two such polls, there are 10^6/HZ usecs.
|
|
On an i386, for example, 10^6/100 = 10000usec. It is easy to see that it is
|
|
quite possible for the trigger timeout to expire between two such polls, as
|
|
the timeout is only 500usec long. As a result, it is required to change the
|
|
trigger timeout on the *other* side of a PLIP connection, to about
|
|
10^6/HZ usecs. If both sides of a PLIP connection are used in IRQ-less mode,
|
|
this timeout is required on both sides.
|
|
|
|
It appears that in practice, the trigger timeout can be shorter than in the
|
|
above calculation. It isn't an important issue, unless the wire is faulty,
|
|
in which case a long timeout would stall the machine when, for whatever
|
|
reason, bits are dropped.
|
|
|
|
A utility that can perform this change in Linux is plipconfig, which is part
|
|
of the net-tools package (its location can be found in the
|
|
Documentation/Changes file). An example command would be
|
|
'plipconfig plipX trigger 10000', where plipX is the appropriate
|
|
PLIP device.
|
|
|
|
PLIP hardware interconnection
|
|
-----------------------------
|
|
|
|
PLIP uses several different data transfer methods. The first (and the
|
|
only one implemented in the early version of the code) uses a standard
|
|
printer "null" cable to transfer data four bits at a time using
|
|
data bit outputs connected to status bit inputs.
|
|
|
|
The second data transfer method relies on both machines having
|
|
bi-directional parallel ports, rather than output-only ``printer''
|
|
ports. This allows byte-wide transfers and avoids reconstructing
|
|
nibbles into bytes, leading to much faster transfers.
|
|
|
|
Parallel Transfer Mode 0 Cable
|
|
==============================
|
|
|
|
The cable for the first transfer mode is a standard
|
|
printer "null" cable which transfers data four bits at a time using
|
|
data bit outputs of the first port (machine T) connected to the
|
|
status bit inputs of the second port (machine R). There are five
|
|
status inputs, and they are used as four data inputs and a clock (data
|
|
strobe) input, arranged so that the data input bits appear as contiguous
|
|
bits with standard status register implementation.
|
|
|
|
A cable that implements this protocol is available commercially as a
|
|
"Null Printer" or "Turbo Laplink" cable. It can be constructed with
|
|
two DB-25 male connectors symmetrically connected as follows:
|
|
|
|
STROBE output 1*
|
|
D0->ERROR 2 - 15 15 - 2
|
|
D1->SLCT 3 - 13 13 - 3
|
|
D2->PAPOUT 4 - 12 12 - 4
|
|
D3->ACK 5 - 10 10 - 5
|
|
D4->BUSY 6 - 11 11 - 6
|
|
D5,D6,D7 are 7*, 8*, 9*
|
|
AUTOFD output 14*
|
|
INIT output 16*
|
|
SLCTIN 17 - 17
|
|
extra grounds are 18*,19*,20*,21*,22*,23*,24*
|
|
GROUND 25 - 25
|
|
* Do not connect these pins on either end
|
|
|
|
If the cable you are using has a metallic shield it should be
|
|
connected to the metallic DB-25 shell at one end only.
|
|
|
|
Parallel Transfer Mode 1
|
|
========================
|
|
|
|
The second data transfer method relies on both machines having
|
|
bi-directional parallel ports, rather than output-only ``printer''
|
|
ports. This allows byte-wide transfers, and avoids reconstructing
|
|
nibbles into bytes. This cable should not be used on unidirectional
|
|
``printer'' (as opposed to ``parallel'') ports or when the machine
|
|
isn't configured for PLIP, as it will result in output driver
|
|
conflicts and the (unlikely) possibility of damage.
|
|
|
|
The cable for this transfer mode should be constructed as follows:
|
|
|
|
STROBE->BUSY 1 - 11
|
|
D0->D0 2 - 2
|
|
D1->D1 3 - 3
|
|
D2->D2 4 - 4
|
|
D3->D3 5 - 5
|
|
D4->D4 6 - 6
|
|
D5->D5 7 - 7
|
|
D6->D6 8 - 8
|
|
D7->D7 9 - 9
|
|
INIT -> ACK 16 - 10
|
|
AUTOFD->PAPOUT 14 - 12
|
|
SLCT->SLCTIN 13 - 17
|
|
GND->ERROR 18 - 15
|
|
extra grounds are 19*,20*,21*,22*,23*,24*
|
|
GROUND 25 - 25
|
|
* Do not connect these pins on either end
|
|
|
|
Once again, if the cable you are using has a metallic shield it should
|
|
be connected to the metallic DB-25 shell at one end only.
|
|
|
|
PLIP Mode 0 transfer protocol
|
|
=============================
|
|
|
|
The PLIP driver is compatible with the "Crynwr" parallel port transfer
|
|
standard in Mode 0. That standard specifies the following protocol:
|
|
|
|
send header nibble '0x8'
|
|
count-low octet
|
|
count-high octet
|
|
... data octets
|
|
checksum octet
|
|
|
|
Each octet is sent as
|
|
<wait for rx. '0x1?'> <send 0x10+(octet&0x0F)>
|
|
<wait for rx. '0x0?'> <send 0x00+((octet>>4)&0x0F)>
|
|
|
|
To start a transfer the transmitting machine outputs a nibble 0x08.
|
|
That raises the ACK line, triggering an interrupt in the receiving
|
|
machine. The receiving machine disables interrupts and raises its own ACK
|
|
line.
|
|
|
|
Restated:
|
|
|
|
(OUT is bit 0-4, OUT.j is bit j from OUT. IN likewise)
|
|
Send_Byte:
|
|
OUT := low nibble, OUT.4 := 1
|
|
WAIT FOR IN.4 = 1
|
|
OUT := high nibble, OUT.4 := 0
|
|
WAIT FOR IN.4 = 0
|