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can: add Transmitter Delay Compensation (TDC) documentation

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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 <mailhol@kernel.org>
Link: https://patch.msgid.link/20251013-can-fd-doc-v2-2-5d53bdc8f2ad@kernel.org
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
This commit is contained in:
Vincent Mailhol 2025-10-13 19:10:23 +09:00 committed by Marc Kleine-Budde
parent c282993ccd
commit b5746b3e8e
1 changed files with 64 additions and 0 deletions
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64
Documentation/networking/can.rst
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@ -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
----------------------