"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 that the SJW parameter controls the tolerance of the CAN controller to frequency errors compared to other CAN controllers. If the user space does not provide an SJW parameter, the kernel chooses a default value of 1. This has proven to be a good default value for classic CAN controllers, but no longer for modern CAN-FD 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 will notice that the SJW is 1 in most cases, while the Seg2 phase is 2. Both values are given in TQ units, which in turn 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). Since the kernel chooses a default SJW of 1 regardless of the TQ, this leads to a much smaller SJW and thus much smaller tolerances to frequency errors. To maintain the same oscillator tolerances on controllers with wide bit timing registers, select a default SJW value of Phase Seg2 / 2 unless Phase Seg 1 is less. 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 Link: https://lore.kernel.org/all/20230202110854.2318594-15-mkl@xxxxxxxxxxxxxx Cc: Mark Bath <mark@xxxxxxxxxxxxxxxxxx> Signed-off-by: Marc Kleine-Budde <mkl@xxxxxxxxxxxxxx> --- drivers/net/can/dev/bittiming.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/drivers/net/can/dev/bittiming.c b/drivers/net/can/dev/bittiming.c index 68287b79afe8..55714e08ca3a 100644 --- a/drivers/net/can/dev/bittiming.c +++ b/drivers/net/can/dev/bittiming.c @@ -11,8 +11,8 @@ void can_sjw_set_default(struct can_bittiming *bt) if (bt->sjw) return; - /* If user space provides no sjw, use 1 as default */ - bt->sjw = 1; + /* If user space provides no sjw, use sane default of phase_seg2 / 2 */ + bt->sjw = max(1U, min(bt->phase_seg1, bt->phase_seg2 / 2)); } int can_sjw_check(const struct net_device *dev, const struct can_bittiming *bt, -- 2.39.1
