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This means that even for lower > CAN-FD data bitrates (with a DBRP (data bitrate prescaler) > 2) a TDC > is configured. This leads to a bus-off condition. > > ISO 11898-1 section 11.3.3 says "Transmitter delay compensation" (TDC) > is only applicable if DBRP is 1 or 2. > > To fix the problem, switch the driver to use the TDC calculation > provided by the CAN driver framework (which respects ISO 11898-1 > section 11.3.3). This has the positive side effect that userspace can > control TDC as needed. > > Demonstration of the feature in action: > | $ ip link set can0 up type can bitrate 125000 dbitrate 500000 fd on > | $ ip -details link show can0 > | 3: can0: mtu 72 qdisc pfifo_fast state UP mode DEFAULT group default qlen 10 > | link/can promiscuity 0 allmulti 0 minmtu 0 maxmtu 0 > | can state ERROR-ACTIVE (berr-counter tx 0 rx 0) restart-ms 0 > | bitrate 125000 sample-point 0.875 > | tq 50 prop-seg 69 phase-seg1 70 phase-seg2 20 sjw 10 brp 2 > | mcp251xfd: tseg1 2..256 tseg2 1..128 sjw 1..128 brp 1..256 brp_inc 1 > | dbitrate 500000 dsample-point 0.875 > | dtq 125 dprop-seg 6 dphase-seg1 7 dphase-seg2 2 dsjw 1 dbrp 5 > | mcp251xfd: dtseg1 1..32 dtseg2 1..16 dsjw 1..16 dbrp 1..256 dbrp_inc 1 > | tdcv 0..63 tdco 0..63 > | clock 40000000 numtxqueues 1 numrxqueues 1 gso_max_size 65536 gso_max_segs 65535 tso_max_size 65536 tso_max_segs 65535 gro_max_size 65536 parentbus spi parentdev spi0.0 > | $ ip link set can0 up type can bitrate 1000000 dbitrate 4000000 fd on > | $ ip -details link show can0 > | 3: can0: mtu 72 qdisc pfifo_fast state UP mode DEFAULT group default qlen 10 > | link/can promiscuity 0 allmulti 0 minmtu 0 maxmtu 0 > | can state ERROR-ACTIVE (berr-counter tx 0 rx 0) restart-ms 0 > | bitrate 1000000 sample-point 0.750 > | tq 25 prop-seg 14 phase-seg1 15 phase-seg2 10 sjw 5 brp 1 > | mcp251xfd: tseg1 2..256 tseg2 1..128 sjw 1..128 brp 1..256 brp_inc 1 > | dbitrate 4000000 dsample-point 0.700 > | dtq 25 dprop-seg 3 dphase-seg1 3 dphase-seg2 3 dsjw 1 dbrp 1 > | tdco 7 > | mcp251xfd: dtseg1 1..32 dtseg2 1..16 dsjw 1..16 dbrp 1..256 dbrp_inc 1 > | tdcv 0..63 tdco 0..63 > | clock 40000000 numtxqueues 1 numrxqueues 1 gso_max_size 65536 gso_max_segs 65535 tso_max_size 65536 tso_max_segs 65535 gro_max_size 65536 parentbus spi parentdev spi0.0 > > There has been some confusion about the MCP2518FD using a relative or > absolute TDCO due to the datasheet specifying a range of [-64,63]. I > have a custom board with a 40 MHz clock and an estimated loop delay of > 100 to 216 ns. During testing at a data bit rate of 4 Mbit/s I found > that using can_get_relative_tdco() resulted in bus-off errors. The > final TDCO value was 1 which corresponds to a 10% SSP in an absolute > configuration. This behavior is expected if the TDCO value is really > absolute and not relative. Using priv->can.tdc.tdco instead results in > a final TDCO of 8, setting the SSP at exactly 80%. This configuration > works. > > The automatic, manual, and off TDC modes were tested at speeds up to, > and including, 8 Mbit/s on real hardware and behave as expected. > > Fixes: 55e5b97f003e ("can: mcp25xxfd: add driver for Microchip MCP25xxFD SPI CAN") > Reported-by: Kelsey Maes > Closes: https://lore.kernel.org/all/C2121586-C87F-4B23-A933-845362C29CA1@vpprocess.com > Reviewed-by: Vincent Mailhol > Signed-off-by: Kelsey Maes > Link: https://patch.msgid.link/20250430161501.79370-1-kelsey@vpprocess.com > [mkl: add comment] > Signed-off-by: Marc Kleine-Budde > --- Reviewed-by: Jacob Keller