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Message-Id: <20201104155730.25196-1-yegorslists@googlemail.com>
Date: Wed, 4 Nov 2020 16:57:30 +0100
From: yegorslists@...glemail.com
To: linux-can@...r.kernel.org
Cc: mkl@...gutronix.de, netdev@...r.kernel.org,
dev.kurt@...dijck-laurijssen.be,
Yegor Yefremov <yegorslists@...glemail.com>
Subject: [PATCH] can: j1939: add tables for the CAN identifier and its fields
From: Yegor Yefremov <yegorslists@...glemail.com>
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@...glemail.com>
---
Documentation/networking/j1939.rst | 46 +++++++++++++++++++++++++++---
1 file changed, 42 insertions(+), 4 deletions(-)
diff --git a/Documentation/networking/j1939.rst b/Documentation/networking/j1939.rst
index 0a4b73b03b99..19d9878d7194 100644
--- a/Documentation/networking/j1939.rst
+++ b/Documentation/networking/j1939.rst
@@ -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
--
2.17.0
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