can: bittiming: allow TDC{V,O} to be zero and add can_tdc_const::tdc{v,o,f}_min

ISO 11898-1 specifies in section 11.3.3 "Transmitter delay
compensation" that "the configuration range for [the] SSP position
shall be at least 0 to 63 minimum time quanta."

Because SSP = TDCV + TDCO, it means that we should allow both TDCV and
TDCO to hold zero value in order to honor SSP's minimum possible
value.

However, current implementation assigned special meaning to TDCV and
TDCO's zero values:
  * TDCV = 0 -> TDCV is automatically measured by the transceiver.
  * TDCO = 0 -> TDC is off.

In order to allow for those values to really be zero and to maintain
current features, we introduce two new flags:
  * CAN_CTRLMODE_TDC_AUTO indicates that the controller support
    automatic measurement of TDCV.
  * CAN_CTRLMODE_TDC_MANUAL indicates that the controller support
    manual configuration of TDCV. N.B.: current implementation failed
    to provide an option for the driver to indicate that only manual
    mode was supported.

TDC is disabled if both CAN_CTRLMODE_TDC_AUTO and
CAN_CTRLMODE_TDC_MANUAL flags are off, c.f. the helper function
can_tdc_is_enabled() which is also introduced in this patch.

Also, this patch adds three fields: tdcv_min, tdco_min and tdcf_min to
struct can_tdc_const. While we are not convinced that those three
fields could be anything else than zero, we can imagine that some
controllers might specify a lower bound on these. Thus, those minimums
are really added "just in case".

Comments of struct can_tdc and can_tdc_const are updated accordingly.

Finally, the changes are applied to the etas_es58x driver.

Link: https://lore.kernel.org/all/20210918095637.20108-2-mailhol.vincent@wanadoo.fr
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

1 files changed, 4 insertions, 0 deletions

diff --git a/include/linux/can/dev.h b/include/linux/can/dev.h
index 2413253e54c7..6dacbbb41e68 100644
--- a/include/linux/can/dev.h
+++ b/include/linux/can/dev.h
@@ -96,6 +96,10 @@ struct can_priv {
 #endif
 };
 
+static inline bool can_tdc_is_enabled(const struct can_priv *priv)
+{
+	return !!(priv->ctrlmode & CAN_CTRLMODE_TDC_MASK);
+}
 
 /* helper to define static CAN controller features at device creation time */
 static inline void can_set_static_ctrlmode(struct net_device *dev,