Re: sjw in can_calc_bittiming

[Oliver Hartkopp <socketcan@hartkopp.net>]

Hi Matthias,

On 02.11.21 16:03, Matthias Weißer wrote:

> we recently had a case here where one member of a CAN bus couldn't receive
> frames with data content of only zeros:
> 
> $ cansend can0 123#0000000000000000
> 
> After some investigation we found the root cause to be a slight difference
> (about 1%) in actual bitrates of the two members. The one with showed the
> RX errors had a sjw value of 1 and a lot of time quanta (40) due to the 40MHz
> CAN clock.
> 
> This leads to a build up of phase error (as sjw is not able to compensate for
> enough of the bitrate difference) which at some point leads to a framing
> error due to missing a stuff bit. Playing around with the sample point can
> improve or worsen the behavior.
> 
> We can fix this quite easily by specifying a higher sjw value.

You can specify the sjw value with the ip command (for CAN FD there is 
also a dsjw). And IIRC you can set it to the max. value for your CAN 
controller if you define sjw to be 4.

Best regards,
Oliver

$ ip link help can
Usage: ip link set DEVICE type can
	[ bitrate BITRATE [ sample-point SAMPLE-POINT] ] |
	[ tq TQ prop-seg PROP_SEG phase-seg1 PHASE-SEG1
  	  phase-seg2 PHASE-SEG2 [ sjw SJW ] ]

	[ dbitrate BITRATE [ dsample-point SAMPLE-POINT] ] |
	[ dtq TQ dprop-seg PROP_SEG dphase-seg1 PHASE-SEG1
  	  dphase-seg2 PHASE-SEG2 [ dsjw SJW ] ]

	[ loopback { on | off } ]
	[ listen-only { on | off } ]
	[ triple-sampling { on | off } ]
	[ one-shot { on | off } ]
	[ berr-reporting { on | off } ]
	[ fd { on | off } ]
	[ fd-non-iso { on | off } ]
	[ presume-ack { on | off } ]

	[ restart-ms TIME-MS ]
	[ restart ]

	[ termination { 0..65535 } ]

	Where: BITRATE	:= { 1..1000000 }
		  SAMPLE-POINT	:= { 0.000..0.999 }
		  TQ		:= { NUMBER }
		  PROP-SEG	:= { 1..8 }
		  PHASE-SEG1	:= { 1..8 }
		  PHASE-SEG2	:= { 1..8 }
		  SJW		:= { 1..4 }
		  RESTART-MS	:= { 0 | NUMBER }

> 
> Question is now:
> Wouldn't it make sense to increase sjw in can_calc_bittiming() to something
> like 5% of the total time quanta? This may increase the reliability of the CAN
> network when there are differences in the bitrates of the single members. Are
> there any arguments against such an arbitrary selection of sjw?
> 
> If you agree with such a change I can come up with a proper patch. If I wrote
> totally nonsense please tell me :-)
> 
> Regards
> 
> Matthias
>