Re: J1939 message length

[Laurent Vaudoit <laurent.vaudoit@gmail.com>]

Kurt Van Dijck <dev.kurt <at> vandijck-laurijssen.be> writes:

Hi Kurt,
> 
> Hey,
> 
> On Mon, Dec 15, 2014 at 08:58:14AM +0000, pfifre.ext <at> orange.com 
wrote:
> > Hi Kurt,
> > 
> > I resolved the issue. The problem appears with long message because, 
according to the j1939
> specifications, the end of message has to be fulfilled with 0xFF if the 
message is not modulo 8.
> > Apparently, the j1939  drivers don't make the padding with 0xFF.
> 
> You're not the first to find an actual ECU complaining on this.
> I assembled a patch earlier this year, see http://lists.kde.org/?l=linux-
can&m=139825572207448&w=2
> 
> > Then I do that in my software and it works fine know.
> 
> I advise to apply that patch, and avoid to calculate packet sizes in 
userspace.

I've seen this message concerning a bug in the DLC can frame when a more 
than 8 bytes J1939 frame is sent.

Looking at the patch, it seems that every j1939 frame will be padded with 
0xFF, even single frame (less than 8 bytes).
I think this is an error as standard SAEJ1939-21 specifies that for example 
PGN request (0xEA00) is 3 byes length, and so can frame dlc must be 3 set 
to 3.

May be the patch should be move on the transport module who manage Data 
transfer protocol?
Is there something i missed?
> 
> Which is the remote ECU that does not accept the packets?
> 
> > 
> > Perhaps, It will be a good idea to add this in the drivers.
> 
> Yep, I'll do.
> 

Best regards,
Laurent
> Kind regards,
> Kurt
> > 
> > Best regards,
> > 
> > Pascal
> > 
> > -----Message d'origine-----
> > De : Kurt Van Dijck [mailto:dev.kurt <at> vandijck-laurijssen.be] 
> > Envoyé : vendredi 12 décembre 2014 12:08
> > À : zze-Open the box FIFRE P ext IMT/OLPS
> > Cc : linux-can <at> vger.kernel.org
> > Objet : Re: J1939 message length
> > 
> > Hey Pascal,
> > 
> > On Thu, Dec 11, 2014 at 10:44:39AM +0000, pascal wrote:
> > > Hi,
> > > 
> > > I've installed the patch for using j1939 socket CAN under linux 
> > > (beagleBone black).
> > > 
> > > It works fine for receiving j1939 message and sending short message 
(8 
> > > bytes max).
> > > 
> > > But when I try to send long message (>8 bytes long), but the message 
> > > has not been sent.
> > 
> > Ok.
> > You provide little info how you detect that <=8 bytes work, and >8 
bytes do not work.
> > 
> > What do you see on the (raw) CAN bus?
> > 
> > Are you aware of the Transport Protocol on J1939 that delivers messages 
>8 bytes?
> > 
> > Kurt
> > > 
> > > I use the sendTo method to send frame.
> > > 
> > > Do I have to do something special to send long message?
> > 
> > no, not necessarily.
> > 
> > > I'm working with a 3.8.13 kernel on beaglebone black.
> > 
> > Kind regards,
> > Kurt
> > 
> > 
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