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can: j1939: add tables for the CAN identifier and its fields

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Use table markup to show the structure of the CAN identifier, PGN, PDU1, and
PDU2 formats. Also add introductory sentence.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Link: https://lore.kernel.org/r/20201104155730.25196-1-yegorslists@googlemail.com
[mkl: removed trailing whitespace]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
This commit is contained in:
Yegor Yefremov 2020-11-04 16:57:30 +01:00 committed by Marc Kleine-Budde
parent 4082c502bf
commit b7d3c0e593
1 changed files with 42 additions and 4 deletions
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46
Documentation/networking/j1939.rst
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@ -69,18 +69,56 @@ J1939 concepts
PGN
---
The J1939 protocol uses the 29-bit CAN identifier with the following structure:
============ ============== ====================
29 bit CAN-ID
--------------------------------------------------
Bit positions within the CAN-ID
--------------------------------------------------
28 ... 26 25 ... 8 7 ... 0
============ ============== ====================
Priority PGN SA (Source Address)
============ ============== ====================
The PGN (Parameter Group Number) is a number to identify a packet. The PGN
is composed as follows:
1 bit : Reserved Bit
1 bit : Data Page
8 bits : PF (PDU Format)
8 bits : PS (PDU Specific)
============ ============== ================= =================
PGN
------------------------------------------------------------------
Bit positions within the CAN-ID
------------------------------------------------------------------
25 24 23 ... 16 15 ... 8
============ ============== ================= =================
R (Reserved) DP (Data Page) PF (PDU Format) PS (PDU Specific)
============ ============== ================= =================
In J1939-21 distinction is made between PDU1 format (where PF < 240) and PDU2
format (where PF >= 240). Furthermore, when using the PDU2 format, the PS-field
contains a so-called Group Extension, which is part of the PGN. When using PDU2
format, the Group Extension is set in the PS-field.
============== ========================
PDU1 Format (specific) (peer to peer)
----------------------------------------
Bit positions within the CAN-ID
----------------------------------------
23 ... 16 15 ... 8
============== ========================
00h ... EFh DA (Destination address)
============== ========================
============== ========================
PDU2 Format (global) (broadcast)
----------------------------------------
Bit positions within the CAN-ID
----------------------------------------
23 ... 16 15 ... 8
============== ========================
F0h ... FFh GE (Group Extenstion)
============== ========================
On the other hand, when using PDU1 format, the PS-field contains a so-called
Destination Address, which is _not_ part of the PGN. When communicating a PGN
from user space to kernel (or vice versa) and PDU2 format is used, the PS-field