Commit b5746b3e authored by Vincent Mailhol's avatar Vincent Mailhol Committed by Marc Kleine-Budde
Browse files

can: add Transmitter Delay Compensation (TDC) documentation 

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: default avatarVincent Mailhol <mailhol@kernel.org>
Link: https://patch.msgid.link/20251013-can-fd-doc-v2-2-5d53bdc8f2ad@kernel.org


Signed-off-by: default avatarMarc Kleine-Budde <mkl@pengutronix.de>
parent c282993c

Documentation/networking/can.rst

+64 −0
Original line number Diff line number Diff line
@@ -1464,6 +1464,70 @@ Example when 'fd-non-iso on' is added on this switchable CAN FD adapter:: 
   can <FD,FD-NON-ISO> 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: <NOARP,UP,LOWER_UP,ECHO> mtu 72 qdisc pfifo_fast state UP \

             mode DEFAULT group default qlen 10

        link/can  promiscuity 0 allmulti 0 minmtu 72 maxmtu 72

        can <FD,TDC-AUTO> 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

----------------------