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>
---
Documentation/networking/can.rst | 60 ++++++++++++++++++++++++++++++++++++++++
1 file changed, 60 insertions(+)
diff --git a/Documentation/networking/can.rst b/Documentation/networking/can.rst
index 58c026d51d94..de9e7549859f 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 <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
----------------------
--
2.49.1On Sun, Oct 12, 2025 at 08:23:43PM +0900, Vincent Mailhol wrote: ... > @@ -1464,6 +1464,66 @@ 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 Hi Vincent, I'm unsure why, but make htmldocs reports: .../can.rst:1484: ERROR: Unexpected indentation. [docutils] > + 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. ...
On Sun, Oct 12, 2025 at 08:23:43PM +0900, Vincent Mailhol wrote: > 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. Hi Vincent I also don't see anything in man ip-link, nor ip link help. Maybe you can add this documentation as well? Andrew
Hi Andrew,
On 12/10/2025 at 23:47, Andrew Lunn wrote:
> On Sun, Oct 12, 2025 at 08:23:43PM +0900, Vincent Mailhol wrote:
>> 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.
>
> Hi Vincent
>
> I also don't see anything in man ip-link, nor ip link help. Maybe you
> can add this documentation as well?
The help is indeed not directly visible. But I think this is intended
because can is a sub type. The can is simply listed in man ip-link
under the Link types enumeration.
The can help then be obtain by providing that can type:
$ ip link help can
Usage: ip link set DEVICE type can
[ bitrate BITRATE [ sample-point SAMPLE-POINT] ] |
[ tq TQ prop-seg PROP_SEG phase-seg1 PHASE-SEG1
phase-seg2 PHASE-SEG2 [ sjw SJW ] ]
[ dbitrate BITRATE [ dsample-point SAMPLE-POINT] ] |
[ dtq TQ dprop-seg PROP_SEG dphase-seg1 PHASE-SEG1
dphase-seg2 PHASE-SEG2 [ dsjw SJW ] ]
[ tdcv TDCV tdco TDCO tdcf TDCF ]
[ loopback { on | off } ]
[ listen-only { on | off } ]
[ triple-sampling { on | off } ]
[ one-shot { on | off } ]
[ berr-reporting { on | off } ]
[ fd { on | off } ]
[ fd-non-iso { on | off } ]
[ presume-ack { on | off } ]
[ cc-len8-dlc { on | off } ]
[ tdc-mode { auto | manual | off } ]
[ restart-ms TIME-MS ]
[ restart ]
[ termination { 0..65535 } ]
Where: BITRATE := { NUMBER in bps }
SAMPLE-POINT := { 0.000..0.999 }
TQ := { NUMBER in ns }
PROP-SEG := { NUMBER in tq }
PHASE-SEG1 := { NUMBER in tq }
PHASE-SEG2 := { NUMBER in tq }
SJW := { NUMBER in tq }
TDCV := { NUMBER in tc }
TDCO := { NUMBER in tc }
TDCF := { NUMBER in tc }
RESTART-MS := { 0 | NUMBER in ms }
Does this make sense?
Yours sincerely,
Vincent Mailhol
On Mon, Oct 13, 2025 at 01:01:15AM +0900, Vincent Mailhol wrote: > Hi Andrew, > > On 12/10/2025 at 23:47, Andrew Lunn wrote: > > On Sun, Oct 12, 2025 at 08:23:43PM +0900, Vincent Mailhol wrote: > >> 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. > > > > Hi Vincent > > > > I also don't see anything in man ip-link, nor ip link help. Maybe you > > can add this documentation as well? > > The help is indeed not directly visible. But I think this is intended > because can is a sub type. The can is simply listed in man ip-link > under the Link types enumeration. > > The can help then be obtain by providing that can type: > > $ ip link help can > Usage: ip link set DEVICE type can > [ bitrate BITRATE [ sample-point SAMPLE-POINT] ] | > [ tq TQ prop-seg PROP_SEG phase-seg1 PHASE-SEG1 > phase-seg2 PHASE-SEG2 [ sjw SJW ] ] > [ dbitrate BITRATE [ dsample-point SAMPLE-POINT] ] | > [ dtq TQ dprop-seg PROP_SEG dphase-seg1 PHASE-SEG1 > dphase-seg2 PHASE-SEG2 [ dsjw SJW ] ] > [ tdcv TDCV tdco TDCO tdcf TDCF ] O.K. Great, thanks for pointing this out. Andrew
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