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: 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>

diff --git a/Documentation/networking/can.rst b/Documentation/networking/can.rst
index ccd321d29a8aafd6f4668404f5280e7c2f2d3f80..194e305ae9733dca96a035d89a1bb8202d9ce564 100644 (file)
--- a/Documentation/networking/can.rst
+++ b/Documentation/networking/can.rst
@@ -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
 ----------------------