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, 12 insertions, 5 deletions

diff --git a/drivers/net/can/dev/bittiming.c b/drivers/net/can/dev/bittiming.c
index b1b5a82f0829..9dda44c0ae9d 100644
--- a/drivers/net/can/dev/bittiming.c
+++ b/drivers/net/can/dev/bittiming.c
@@ -182,9 +182,12 @@ void can_calc_tdco(struct net_device *dev)
 	struct can_tdc *tdc = &priv->tdc;
 	const struct can_tdc_const *tdc_const = priv->tdc_const;
 
-	if (!tdc_const)
+	if (!tdc_const ||
+	    !(priv->ctrlmode_supported & CAN_CTRLMODE_TDC_AUTO))
 		return;
 
+	priv->ctrlmode &= ~CAN_CTRLMODE_TDC_MASK;
+
 	/* As specified in ISO 11898-1 section 11.3.3 "Transmitter
 	 * delay compensation" (TDC) is only applicable if data BRP is
 	 * one or two.
@@ -193,9 +196,10 @@ void can_calc_tdco(struct net_device *dev)
 		/* Reuse "normal" sample point and convert it to time quanta */
 		u32 sample_point_in_tq = can_bit_time(dbt) * dbt->sample_point / 1000;
 
+		if (sample_point_in_tq < tdc_const->tdco_min)
+			return;
 		tdc->tdco = min(sample_point_in_tq, tdc_const->tdco_max);
-	} else {
-		tdc->tdco = 0;
+		priv->ctrlmode |= CAN_CTRLMODE_TDC_AUTO;
 	}
 }
 #endif /* CONFIG_CAN_CALC_BITTIMING */
diff --git a/drivers/net/can/usb/etas_es58x/es58x_fd.c b/drivers/net/can/usb/etas_es58x/es58x_fd.c
index af042aa55f59..4f0cae29f4d8 100644
--- a/drivers/net/can/usb/etas_es58x/es58x_fd.c
+++ b/drivers/net/can/usb/etas_es58x/es58x_fd.c
@@ -428,7 +428,7 @@ static int es58x_fd_enable_channel(struct es58x_priv *priv)
 		es58x_fd_convert_bittiming(&tx_conf_msg.data_bittiming,
 					   &priv->can.data_bittiming);
 
-		if (priv->can.tdc.tdco) {
+		if (can_tdc_is_enabled(&priv->can)) {
 			tx_conf_msg.tdc_enabled = 1;
 			tx_conf_msg.tdco = cpu_to_le16(priv->can.tdc.tdco);
 			tx_conf_msg.tdcf = cpu_to_le16(priv->can.tdc.tdcf);
@@ -505,8 +505,11 @@ static const struct can_bittiming_const es58x_fd_data_bittiming_const = {
  * Register" from Microchip.
  */
 static const struct can_tdc_const es58x_tdc_const = {
+	.tdcv_min = 0,
 	.tdcv_max = 0, /* Manual mode not supported. */
+	.tdco_min = 0,
 	.tdco_max = 127,
+	.tdcf_min = 0,
 	.tdcf_max = 127
 };
 
@@ -523,7 +526,7 @@ const struct es58x_parameters es58x_fd_param = {
 	.clock = {.freq = 80 * CAN_MHZ},
 	.ctrlmode_supported = CAN_CTRLMODE_LOOPBACK | CAN_CTRLMODE_LISTENONLY |
 	    CAN_CTRLMODE_3_SAMPLES | CAN_CTRLMODE_FD | CAN_CTRLMODE_FD_NON_ISO |
-	    CAN_CTRLMODE_CC_LEN8_DLC,
+	    CAN_CTRLMODE_CC_LEN8_DLC | CAN_CTRLMODE_TDC_AUTO,
 	.tx_start_of_frame = 0xCEFA,	/* FACE in little endian */
 	.rx_start_of_frame = 0xFECA,	/* CAFE in little endian */
 	.tx_urb_cmd_max_len = ES58X_FD_TX_URB_CMD_MAX_LEN,