The logic for the tdco calculation is to just reuse the normal sample
point: tdco = sp. Because the sample point is expressed in tenth of
percent and the tdco is expressed in time quanta, a conversion is
needed.

At the end,
     ssp = tdcv + tdco
         = tdcv + sp.

Another popular method is to set tdco to the middle of the bit:
     tdc->tdco = can_bit_time(dbt) / 2
During benchmark tests, we could not find a clear advantages for one
of the two methods.

The tdco calculation is triggered each time the data_bittiming is
changed so that users relying on automated calculation can use the
netlink interface the exact same way without need of new parameters.
For example, a command such as:
	ip link set canX type can bitrate 500000 dbitrate 4000000 fd on
would trigger the calculation.

The user using CONFIG_CAN_CALC_BITTIMING who does not want automated
calculation needs to manually set tdco to zero.
For example with:
	ip link set canX type can tdco 0 bitrate 500000 dbitrate 4000000 fd on
(if the tdco parameter is provided in a previous command, it will be
overwritten).

If tdcv is set to zero (default), it is automatically calculated by
the transiver for each frame. As such, there is no code in the kernel
to calculate it.

tdcf has no automated calculation functions because we could not
figure out a formula for this parameter.

Link: https://lore.kernel.org/r/20210224002008.4158-6-mailhol.vincent@wanadoo.frSigned-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>