netdev - Re: [PATCH] can: j1939: convert PGN structure to a table

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Message-ID: <20201022122453.GA31522@x1.vandijck-laurijssen.be>
Date:   Thu, 22 Oct 2020 14:24:53 +0200
From:   Kurt Van Dijck <dev.kurt@...dijck-laurijssen.be>
To:     Marc Kleine-Budde <mkl@...gutronix.de>
Cc:     yegorslists@...glemail.com, linux-can@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH] can: j1939: convert PGN structure to a table

On Thu, 22 Oct 2020 12:33:45 +0200, Marc Kleine-Budde wrote:
> On 10/22/20 12:29 PM, yegorslists@...glemail.com wrote:
> > From: Yegor Yefremov <yegorslists@...glemail.com>
> > 
> > Use table markup to show the PGN structure.
> > 
> > Signed-off-by: Yegor Yefremov <yegorslists@...glemail.com>
> > ---
> >  Documentation/networking/j1939.rst | 12 ++++++++----
> >  1 file changed, 8 insertions(+), 4 deletions(-)
> > 
> > diff --git a/Documentation/networking/j1939.rst b/Documentation/networking/j1939.rst
> > index faf2eb5c5052..f3fb9d880910 100644
> > --- a/Documentation/networking/j1939.rst
> > +++ b/Documentation/networking/j1939.rst
> > @@ -71,10 +71,14 @@ PGN
> >  
> >  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
> > +  ----------------------------------------------------------------
> > +  25            24              23 ... 16        15 ... 8
> 
> ICan you add a row description that indicated that these numbers are. They are
> probably bit positions within the CAN-ID?

This is true for up to 99.9%, depending on who you ask.
this maps indeed to the bit positions in the CAN-ID, as in J1939-21.
The trouble is that PGN's are also communicated as such in the payload,
e.g. in the TP and ETP (see J1939-81 if i remember correctly).
Since only PGN is written there, without SA, the bit position relative
to the CAN-ID are ... making things look fuzzy.

So I the best I can propose is to add a 2nd row :-)

> 
> > +  ============  ==============  ===============  =================
> > +  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
> > 
> 
> Marc
> 
> -- 
> Pengutronix e.K.                 | Marc Kleine-Budde           |
> Embedded Linux                   | https://www.pengutronix.de  |
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>