docs: networking: convert PLIP.txt to ReST

- add SPDX header;
- mark code blocks and literals as such;
- adjust identation, whitespaces and blank lines where needed;
- add to networking/index.rst.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/networking/PLIP.txt Documentation/networking/plip.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+================================================
 PLIP: The Parallel Line Internet Protocol Device
+================================================
 
 Donald Becker ([e-mail])
 I.D.A. Supercomputing Research Center, Bowie MD 20715
···
 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. 
+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
···
 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''
+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.
 
···
 
 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:
+two DB-25 male connectors symmetrically connected as follows::
 
     STROBE output	1*
     D0->ERROR	2 - 15		15 - 2
···
     SLCTIN	17 - 17
     extra grounds are 18*,19*,20*,21*,22*,23*,24*
     GROUND	25 - 25
-* Do not connect these pins on either end
+
+    * 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.
···
 ========================
 
 The second data transfer method relies on both machines having
-bi-directional parallel ports, rather than output-only ``printer''
+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
+``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:
+The cable for this transfer mode should be constructed as follows::
 
     STROBE->BUSY 1 - 11
     D0->D0	2 - 2
···
     GND->ERROR	18 - 15
     extra grounds are 19*,20*,21*,22*,23*,24*
     GROUND	25 - 25
-* Do not connect these pins on either end
+
+    * 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.
···
 =============================
 
 The PLIP driver is compatible with the "Crynwr" parallel port transfer
-standard in Mode 0.  That standard specifies the following protocol:
+standard in Mode 0.  That standard specifies the following protocol::
 
    send header nibble '0x8'
    count-low octet
···
    ... data octets
    checksum octet
 
-Each octet is sent as
+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. 
+line.
 
-Restated:
+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
+  (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

Documentation/networking/index.rst

···
    packet_mmap
    phonet
    pktgen
+   plip
 
 .. only::  subproject and html
 

drivers/net/plip/Kconfig

···
 	  bits at a time (mode 0) or with special PLIP cables, to be used on
 	  bidirectional parallel ports only, which can transmit 8 bits at a
 	  time (mode 1); you can find the wiring of these cables in
-	  <file:Documentation/networking/PLIP.txt>.  The cables can be up to
+	  <file:Documentation/networking/plip.rst>.  The cables can be up to
 	  15m long.  Mode 0 works also if one of the machines runs DOS/Windows
 	  and has some PLIP software installed, e.g. the Crynwr PLIP packet
 	  driver (<http://oak.oakland.edu/simtel.net/msdos/pktdrvr-pre.html>)