"The (Re-)Synchronization Jump Width (SJW) defines how far a resynchronization may move the Sample Point inside the limits defined by the Phase Buffer Segments to compensate for edge phase errors." [1] In other words this means the SJW parameter controls the tolerance of the CAN controller against the frequency error compared to other CAN controllers. If the user space doesn't provide a SJW parameter, the kernel chooses a default value of 1. This proved to be a good default value for CAN controllers, but not anymore with modern controllers. In the past, there were CAN controllers like the sja1000 with a rather limited range of bit timing parameters. For the standard bit rates this results in the following bit timing parameters: | Bit timing parameters for sja1000 with 8.000000 MHz ref clock | _----+--------------=> tseg1: 1 … 16 | / / _---------=> tseg2: 1 … 8 | | | / _-----=> sjw: 1 … 4 | | | | / _-=> brp: 1 … 64 (inc: 1) | | | | | / | nominal | | | | | real Bitrt nom real SampP | Bitrate TQ[ns] PrS PhS1 PhS2 SJW BRP Bitrate Error SampP SampP Error BTR0 BTR1 | 1000000 125 2 3 2 1 1 1000000 0.0% 75.0% 75.0% 0.0% 0x00 0x14 | 800000 125 3 4 2 1 1 800000 0.0% 80.0% 80.0% 0.0% 0x00 0x16 | 666666 125 4 4 3 1 1 666666 0.0% 80.0% 75.0% 6.2% 0x00 0x27 | 500000 125 6 7 2 1 1 500000 0.0% 87.5% 87.5% 0.0% 0x00 0x1c | 250000 250 6 7 2 1 2 250000 0.0% 87.5% 87.5% 0.0% 0x01 0x1c | 125000 500 6 7 2 1 4 125000 0.0% 87.5% 87.5% 0.0% 0x03 0x1c | 100000 625 6 7 2 1 5 100000 0.0% 87.5% 87.5% 0.0% 0x04 0x1c | 83333 750 6 7 2 1 6 83333 0.0% 87.5% 87.5% 0.0% 0x05 0x1c | 50000 1250 6 7 2 1 10 50000 0.0% 87.5% 87.5% 0.0% 0x09 0x1c | 33333 1875 6 7 2 1 15 33333 0.0% 87.5% 87.5% 0.0% 0x0e 0x1c | 20000 3125 6 7 2 1 25 20000 0.0% 87.5% 87.5% 0.0% 0x18 0x1c | 10000 6250 6 7 2 1 50 10000 0.0% 87.5% 87.5% 0.0% 0x31 0x1c The attentive reader notices that in most cases the SJW is 1, while the Phase Seg2 is 2. Both values are in TQ units, which itself is a duration in nanoseconds. For example the 500 kbit/s configuration: | nominal real Bitrt nom real SampP | Bitrate TQ[ns] PrS PhS1 PhS2 SJW BRP Bitrate Error SampP SampP Error BTR0 BTR1 | 500000 125 6 7 2 1 1 500000 0.0% 87.5% 87.5% 0.0% 0x00 0x1c the TQ is 125ns, the Phase Seg2 is "2" (== 250ns), the SJW is "1" (== 125 ns). Looking at a more modern CAN controller like a mcp2518fd, it has wider bit timing registers. | Bit timing parameters for mcp251xfd with 40.000000 MHz ref clock | _----+--------------=> tseg1: 2 … 256 | / / _---------=> tseg2: 1 … 128 | | | / _-----=> sjw: 1 … 128 | | | | / _-=> brp: 1 … 256 (inc: 1) | | | | | / | nominal | | | | | real Bitrt nom real SampP | Bitrate TQ[ns] PrS PhS1 PhS2 SJW BRP Bitrate Error SampP SampP Error NBTCFG | 500000 25 34 35 10 1 1 500000 0.0% 87.5% 87.5% 0.0% 0x00440900 The TQ is 25ns, the Phase Seg 2 is "10" (== 250ns), the SJW is "1" (== 25ns). As the kernel chooses a default SJW of 1 independent of the TQ, this results in a much smaller SJW and thus much smaller tolerances against frequency errors. To get the same oscillator tolerances on controllers with wide bit timing registers, choose a default SJW value of Phase Seg2 / 2. This results in the following bit timing parameters: | Bit timing parameters for mcp251xfd with 40.000000 MHz ref clock | _----+--------------=> tseg1: 2 … 256 | / / _---------=> tseg2: 1 … 128 | | | / _-----=> sjw: 1 … 128 | | | | / _-=> brp: 1 … 256 (inc: 1) | | | | | / | nominal | | | | | real Bitrt nom real SampP | Bitrate TQ[ns] PrS PhS1 PhS2 SJW BRP Bitrate Error SampP SampP Error NBTCFG | 500000 25 34 35 10 5 1 500000 0.0% 87.5% 87.5% 0.0% 0x00440904 The TQ is 25ns, the Phase Seg 2 is "10" (== 250ns), the SJW is "5" (== 125ns). Which is the same as on the sja1000 controller. [1] http://web.archive.org/http://www.oertel-halle.de/files/cia99paper.pdf Cc: Mark Bath <mark@xxxxxxxxxxxxxxxxxx> Signed-off-by: Marc Kleine-Budde <mkl@xxxxxxxxxxxxxx> --- drivers/net/can/dev/calc_bittiming.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/drivers/net/can/dev/calc_bittiming.c b/drivers/net/can/dev/calc_bittiming.c index b9aa1f7d0b37..6b4ca7af05fa 100644 --- a/drivers/net/can/dev/calc_bittiming.c +++ b/drivers/net/can/dev/calc_bittiming.c @@ -154,9 +154,9 @@ int can_calc_bittiming(const struct net_device *dev, struct can_bittiming *bt, bt->phase_seg1 = tseg1 - bt->prop_seg; bt->phase_seg2 = tseg2; - /* If user space provides no sjw, use 1 as default */ + /* If user space provides no sjw, use sane default of phase_seg2 / 2 */ if (!bt->sjw) - bt->sjw = 1; + bt->sjw = max(1U, bt->phase_seg2 / 2); /* sjw must not be higher than sjw_max, phase_seg1, and phase_seg2 */ bt->sjw = min3(bt->sjw, btc->sjw_max, -- 2.35.1
