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>

2 files changed, 51 insertions, 17 deletions

diff --git a/include/linux/can/bittiming.h b/include/linux/can/bittiming.h
index 9de6e9053e34..9e20260611cc 100644
--- a/include/linux/can/bittiming.h
+++ b/include/linux/can/bittiming.h
@@ -19,6 +19,9 @@
 /* Megahertz */
 #define CAN_MHZ 1000000UL
 
+#define CAN_CTRLMODE_TDC_MASK					\
+	(CAN_CTRLMODE_TDC_AUTO | CAN_CTRLMODE_TDC_MANUAL)
+
 /*
  * struct can_tdc - CAN FD Transmission Delay Compensation parameters
  *
@@ -33,29 +36,43 @@
  *
  * This structure contains the parameters to calculate that SSP.
  *
- * @tdcv: Transmitter Delay Compensation Value. Distance, in time
- *	quanta, from when the bit is sent on the TX pin to when it is
- *	received on the RX pin of the transmitter. Possible options:
+ * -+----------- one bit ----------+-- TX pin
+ *  |<--- Sample Point --->|
+ *
+ *                         --+----------- one bit ----------+-- RX pin
+ *  |<-------- TDCV -------->|
+ *                           |<------- TDCO ------->|
+ *  |<----------- Secondary Sample Point ---------->|
+ *
+ * @tdcv: Transmitter Delay Compensation Value. The time needed for
+ *	the signal to propagate, i.e. the distance, in time quanta,
+ *	from the start of the bit on the TX pin to when it is received
+ *	on the RX pin. @tdcv depends on the controller modes:
+ *
+ *	  CAN_CTRLMODE_TDC_AUTO is set: The transceiver dynamically
+ *	  measures @tdcv for each transmitted CAN FD frame and the
+ *	  value provided here should be ignored.
  *
- *	  0: automatic mode. The controller dynamically measures @tdcv
- *	  for each transmitted CAN FD frame.
+ *	  CAN_CTRLMODE_TDC_MANUAL is set: use the fixed provided @tdcv
+ *	  value.
  *
- *	  Other values: manual mode. Use the fixed provided value.
+ *	N.B. CAN_CTRLMODE_TDC_AUTO and CAN_CTRLMODE_TDC_MANUAL are
+ *	mutually exclusive. Only one can be set at a time. If both
+ *	CAN_TDC_CTRLMODE_AUTO and CAN_TDC_CTRLMODE_MANUAL are unset,
+ *	TDC is disabled and all the values of this structure should be
+ *	ignored.
  *
  * @tdco: Transmitter Delay Compensation Offset. Offset value, in time
  *	quanta, defining the distance between the start of the bit
  *	reception on the RX pin of the transceiver and the SSP
  *	position such that SSP = @tdcv + @tdco.
  *
- *	If @tdco is zero, then TDC is disabled and both @tdcv and
- *	@tdcf should be ignored.
- *
  * @tdcf: Transmitter Delay Compensation Filter window. Defines the
- *	minimum value for the SSP position in time quanta. If SSP is
- *	less than @tdcf, then no delay compensations occur and the
- *	normal sampling point is used instead. The feature is enabled
- *	if and only if @tdcv is set to zero (automatic mode) and @tdcf
- *	is configured to a value greater than @tdco.
+ *	minimum value for the SSP position in time quanta. If the SSP
+ *	position is less than @tdcf, then no delay compensations occur
+ *	and the normal sampling point is used instead. The feature is
+ *	enabled if and only if @tdcv is set to zero (automatic mode)
+ *	and @tdcf is configured to a value greater than @tdco.
  */
 struct can_tdc {
 	u32 tdcv;
@@ -67,19 +84,32 @@ struct can_tdc {
  * struct can_tdc_const - CAN hardware-dependent constant for
  *	Transmission Delay Compensation
  *
- * @tdcv_max: Transmitter Delay Compensation Value maximum value.
- *	Should be set to zero if the controller does not support
- *	manual mode for tdcv.
+ * @tdcv_min: Transmitter Delay Compensation Value minimum value. If
+ *	the controller does not support manual mode for tdcv
+ *	(c.f. flag CAN_CTRLMODE_TDC_MANUAL) then this value is
+ *	ignored.
+ * @tdcv_max: Transmitter Delay Compensation Value maximum value. If
+ *	the controller does not support manual mode for tdcv
+ *	(c.f. flag CAN_CTRLMODE_TDC_MANUAL) then this value is
+ *	ignored.
+ *
+ * @tdco_min: Transmitter Delay Compensation Offset minimum value.
  * @tdco_max: Transmitter Delay Compensation Offset maximum value.
  *	Should not be zero. If the controller does not support TDC,
  *	then the pointer to this structure should be NULL.
+ *
+ * @tdcf_min: Transmitter Delay Compensation Filter window minimum
+ *	value. If @tdcf_max is zero, this value is ignored.
  * @tdcf_max: Transmitter Delay Compensation Filter window maximum
  *	value. Should be set to zero if the controller does not
  *	support this feature.
  */
 struct can_tdc_const {
+	u32 tdcv_min;
 	u32 tdcv_max;
+	u32 tdco_min;
 	u32 tdco_max;
+	u32 tdcf_min;
 	u32 tdcf_max;
 };
 
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,