[PATCH 2/2] can: add Transmitter Delay Compensation (TDC) documentation

[Vincent Mailhol <mailhol@kernel.org>]

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

-- 
2.49.1