CAN FD, data phase bitrate is equal to arbitration phase (RPI4 + MCP2518FD)

[JuraszekL]

Hello team! I am testing the FD driver for the first time, and i am a bit confused about the results. To make the thing simple:

• my hardware config is RPI4 with with CAN FD shield.

  $ uname -a
  Linux rpilite 6.1.21-v7l+ #1642 SMP Mon Apr  3 17:22:30 BST 2023 armv7l GNU/Linux

• both interfaces are connected together.

• I've used the command from the official website:

  $ sudo ip link set can0 up type can bitrate 500000 sample-point 0.75 dbitrate 4000000 dsample-point 0.8 fd on fd-non-iso off
  $ sudo ip link set can1 up type can bitrate 500000 sample-point 0.75 dbitrate 4000000 dsample-point 0.8 fd on fd-non-iso off

  that gives me this result:

  $ ip -details link show can0
  4: can0: <NOARP,UP,LOWER_UP,ECHO> mtu 72 qdisc pfifo_fast state UP mode DEFAULT group default qlen 10
  link/can  promiscuity 0 minmtu 0 maxmtu 0 
  can <FD> state ERROR-ACTIVE (berr-counter tx 0 rx 0) restart-ms 0 
        bitrate 500000 sample-point 0.750 
        tq 25 prop-seg 29 phase-seg1 30 phase-seg2 20 sjw 1
        mcp251xfd: tseg1 2..256 tseg2 1..128 sjw 1..128 brp 1..256 brp-inc 1
        dbitrate 4000000 dsample-point 0.800 
        dtq 25 dprop-seg 3 dphase-seg1 4 dphase-seg2 2 dsjw 1
        mcp251xfd: dtseg1 1..32 dtseg2 1..16 dsjw 1..16 dbrp 1..256 dbrp-inc 1
        clock 40000000 numtxqueues 1 numrxqueues 1 gso_max_size 65536 gso_max_segs 65535 

• i can send and receive CAN FD frames:

  $ cansend can0 123##0112233445566778899101011111111223344556677899665544343

  $ candump can1
  can1  123  [32]  11 22 33 44 55 66 77 88 99 10 10 11 11 11 11 22 33 44 55 66 77 89 96 65 54 43 43 00 00 00 00 00

• bitrate of the bus doesn't change during the data phase:

As You can see, max. bit width is 2us, and that gives 1/0,000002s = 500000b/s, which is fine for arbitration phase, but not for the data phase. Ignore the fact that Saleae doesn't recognize the data fields properly - the CAN FD analyzer is not implemented yet.

The MCP2518FD CAN controller is compliant with ISO 11898-1:2015.

Am i missing something? Please let me know what i'm doing wrong.

BR. Łukasz

[hartkopp]

Everything is fine in your setup.

The BRS (bit rate setting) bit is part of every CAN FD frame.

In your example $ cansend can0 123##0112233445566778899101011111111223344556677899665544343 The BRS bit is not set. The struct canfd_frame.flags (directly behind the two ##'s) is zero.

Try $ cansend can0 123##5112233445566778899101011111111223344556677899665544343

That should make it.

Btw. fd-non-iso off is obsolete, as this setting is off by default.

[JuraszekL]

There couldn't be a better example of how to use the BRS bit than my issue. Thank You so much!

[hartkopp]

Thanks!