From: Vincent Mailhol Back in 2021, support for CAN TDC was added to the kernel in series [1] and in iproute2 in series [2]. However, the documentation was never updated. Add a new sub-section under CAN-FD driver support to document how to configure the TDC using the "ip tool". [1] add the netlink interface for CAN-FD Transmitter Delay Compensation (TDC) Link: https://lore.kernel.org/all/20210918095637.20108-1-mailhol.vincent@wanadoo.fr/ [2] iplink_can: cleaning, fixes and adding TDC support Link: https://lore.kernel.org/all/20211103164428.692722-1-mailhol.vincent@wanadoo.fr/ Signed-off-by: Vincent Mailhol Link: https://patch.msgid.link/20251012-can-fd-doc-v1-2-86cc7d130026@kernel.org Signed-off-by: Marc Kleine-Budde --- Documentation/networking/can.rst | 60 ++++++++++++++++++++++++++++++++ 1 file changed, 60 insertions(+) diff --git a/Documentation/networking/can.rst b/Documentation/networking/can.rst index ccd321d29a8a..402fefae0c2f 100644 --- a/Documentation/networking/can.rst +++ b/Documentation/networking/can.rst @@ -1464,6 +1464,66 @@ Example when 'fd-non-iso on' is added on this switchable CAN FD adapter:: can state ERROR-ACTIVE (berr-counter tx 0 rx 0) restart-ms 0 +Transmitter Delay Compensation +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +At high bit rates, the propagation delay from the TX pin to the RX pin of +the transceiver might become greater than the actual bit time causing +measurement errors: the RX pin would still be measuring the previous bit. + +The Transmitter Delay Compensation (thereafter, TDC) resolves this problem +by introducing a Secondary Sample Point (SSP) equal to the distance, in +minimum time quantum, from the start of the bit time on the TX pin to the +actual measurement on the RX pin. The SSP is calculated as the sum of two +configurable values: the TDC Value (TDCV) and the TDC offset (TDCO). + +TDC, if supported by the device, can be configured together with CAN-FD +using the ip tool's "tdc-mode" argument as follow:: + +- **omitted**: when no "tdc-mode" option is provided, the kernel will + automatically decide whether TDC should be turned on, in which case it + will calculate a default TDCO and use the TDCV as measured by the + device. This is the recommended method to use TDC. + +- **"tdc-mode off"**: TDC is explicitly disabled. + +- **"tdc-mode auto"**: the user must provide the "tdco" argument. The TDCV + will be automatically calculated by the device. This option is only + available if the device supports the TDC-AUTO CAN controller mode. + +- **"tdc-mode manual"**: the user must provide both the "tdco" and "tdcv" + arguments. This option is only available if the device supports the + TDC-MANUAL CAN controller mode. + +Note that some devices may offer an additional parameter: "tdcf" (TDC Filter +window). If supported by your device, this can be added as an optional +argument to either "tdc-mode auto" or "tdc-mode manual". + +Example configuring a 500 kbit/s arbitration bitrate, a 5 Mbit/s data +bitrate, a TDCO of 15 minimum time quantum and a TDCV automatically measured +by the device:: + + $ ip link set can0 up type can bitrate 500000 \ + fd on dbitrate 4000000 \ + tdc-mode auto tdco 15 + $ ip -details link show can0 + 5: can0: mtu 72 qdisc pfifo_fast state UP \ + mode DEFAULT group default qlen 10 + link/can promiscuity 0 allmulti 0 minmtu 72 maxmtu 72 + can state ERROR-ACTIVE restart-ms 0 + bitrate 500000 sample-point 0.875 + tq 12 prop-seg 69 phase-seg1 70 phase-seg2 20 sjw 10 brp 1 + ES582.1/ES584.1: tseg1 2..256 tseg2 2..128 sjw 1..128 brp 1..512 \ + brp_inc 1 + dbitrate 4000000 dsample-point 0.750 + dtq 12 dprop-seg 7 dphase-seg1 7 dphase-seg2 5 dsjw 2 dbrp 1 + tdco 15 tdcf 0 + ES582.1/ES584.1: dtseg1 2..32 dtseg2 1..16 dsjw 1..8 dbrp 1..32 \ + dbrp_inc 1 + tdco 0..127 tdcf 0..127 + clock 80000000 + + Supported CAN Hardware ---------------------- -- 2.51.0