can: mcp251xfd: fix TDC setting for low data bit rates

The TDC is currently hardcoded enabled. This means that even for lower
CAN-FD data bitrates (with a DBRP (data bitrate prescaler) > 2) a TDC
is configured. This leads to a bus-off condition.

ISO 11898-1 section 11.3.3 says "Transmitter delay compensation" (TDC)
is only applicable if DBRP is 1 or 2.

To fix the problem, switch the driver to use the TDC calculation
provided by the CAN driver framework (which respects ISO 11898-1
section 11.3.3). This has the positive side effect that userspace can
control TDC as needed.

Demonstration of the feature in action:
| $ ip link set can0 up type can bitrate 125000 dbitrate 500000 fd on
| $ ip -details link show can0
| 3: 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 0 maxmtu 0
|     can <FD> state ERROR-ACTIVE (berr-counter tx 0 rx 0) restart-ms 0
| 	  bitrate 125000 sample-point 0.875
| 	  tq 50 prop-seg 69 phase-seg1 70 phase-seg2 20 sjw 10 brp 2
| 	  mcp251xfd: tseg1 2..256 tseg2 1..128 sjw 1..128 brp 1..256 brp_inc 1
| 	  dbitrate 500000 dsample-point 0.875
| 	  dtq 125 dprop-seg 6 dphase-seg1 7 dphase-seg2 2 dsjw 1 dbrp 5
| 	  mcp251xfd: dtseg1 1..32 dtseg2 1..16 dsjw 1..16 dbrp 1..256 dbrp_inc 1
| 	  tdcv 0..63 tdco 0..63
| 	  clock 40000000 numtxqueues 1 numrxqueues 1 gso_max_size 65536 gso_max_segs 65535 tso_max_size 65536 tso_max_segs 65535 gro_max_size 65536 parentbus spi parentdev spi0.0
| $ ip link set can0 up type can bitrate 1000000 dbitrate 4000000 fd on
| $ ip -details link show can0
| 3: 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 0 maxmtu 0
|     can <FD,TDC-AUTO> state ERROR-ACTIVE (berr-counter tx 0 rx 0) restart-ms 0
| 	  bitrate 1000000 sample-point 0.750
| 	  tq 25 prop-seg 14 phase-seg1 15 phase-seg2 10 sjw 5 brp 1
| 	  mcp251xfd: tseg1 2..256 tseg2 1..128 sjw 1..128 brp 1..256 brp_inc 1
| 	  dbitrate 4000000 dsample-point 0.700
| 	  dtq 25 dprop-seg 3 dphase-seg1 3 dphase-seg2 3 dsjw 1 dbrp 1
| 	  tdco 7
| 	  mcp251xfd: dtseg1 1..32 dtseg2 1..16 dsjw 1..16 dbrp 1..256 dbrp_inc 1
| 	  tdcv 0..63 tdco 0..63
| 	  clock 40000000 numtxqueues 1 numrxqueues 1 gso_max_size 65536 gso_max_segs 65535 tso_max_size 65536 tso_max_segs 65535 gro_max_size 65536 parentbus spi parentdev spi0.0

There has been some confusion about the MCP2518FD using a relative or
absolute TDCO due to the datasheet specifying a range of [-64,63]. I
have a custom board with a 40 MHz clock and an estimated loop delay of
100 to 216 ns. During testing at a data bit rate of 4 Mbit/s I found
that using can_get_relative_tdco() resulted in bus-off errors. The
final TDCO value was 1 which corresponds to a 10% SSP in an absolute
configuration. This behavior is expected if the TDCO value is really
absolute and not relative. Using priv->can.tdc.tdco instead results in
a final TDCO of 8, setting the SSP at exactly 80%. This configuration
works.

The automatic, manual, and off TDC modes were tested at speeds up to,
and including, 8 Mbit/s on real hardware and behave as expected.

Fixes: 55e5b97 ("can: mcp25xxfd: add driver for Microchip MCP25xxFD SPI CAN")
Reported-by: Kelsey Maes <kelsey@vpprocess.com>
Closes: https://lore.kernel.org/all/C2121586-C87F-4B23-A933-845362C29CA1@vpprocess.com
Reviewed-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Kelsey Maes <kelsey@vpprocess.com>
Link: https://patch.msgid.link/20250430161501.79370-1-kelsey@vpprocess.com
[mkl: add comment]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

Author: optlink

drivers/net/can/spi/mcp251xfd/mcp251xfd-core.c

@@ -75,6 +75,24 @@ static const struct can_bittiming_const mcp251xfd_data_bittiming_const = {
 	.brp_inc = 1,
 };
 
+/* The datasheet of the mcp2518fd (DS20006027B) specifies a range of
+ * [-64,63] for TDCO, indicating a relative TDCO.
+ *
+ * Manual tests have shown, that using a relative TDCO configuration
+ * results in bus off, while an absolute configuration works.
+ *
+ * For TDCO use the max value (63) from the data sheet, but 0 as the
+ * minimum.
+ */
+static const struct can_tdc_const mcp251xfd_tdc_const = {
+	.tdcv_min = 0,
+	.tdcv_max = 63,
+	.tdco_min = 0,
+	.tdco_max = 63,
+	.tdcf_min = 0,
+	.tdcf_max = 0,
+};
+
 static const char *__mcp251xfd_get_model_str(enum mcp251xfd_model model)
 {
 	switch (model) {
@@ -510,8 +528,7 @@ static int mcp251xfd_set_bittiming(const struct mcp251xfd_priv *priv)
 {
 	const struct can_bittiming *bt = &priv->can.bittiming;
 	const struct can_bittiming *dbt = &priv->can.data_bittiming;
-	u32 val = 0;
-	s8 tdco;
+	u32 tdcmod, val = 0;
 	int err;
 
 	/* CAN Control Register
@@ -575,11 +592,16 @@ static int mcp251xfd_set_bittiming(const struct mcp251xfd_priv *priv)
 		return err;
 
 	/* Transmitter Delay Compensation */
-	tdco = clamp_t(int, dbt->brp * (dbt->prop_seg + dbt->phase_seg1),
-		       -64, 63);
-	val = FIELD_PREP(MCP251XFD_REG_TDC_TDCMOD_MASK,
-			 MCP251XFD_REG_TDC_TDCMOD_AUTO) |
-		FIELD_PREP(MCP251XFD_REG_TDC_TDCO_MASK, tdco);
+	if (priv->can.ctrlmode & CAN_CTRLMODE_TDC_AUTO)
+		tdcmod = MCP251XFD_REG_TDC_TDCMOD_AUTO;
+	else if (priv->can.ctrlmode & CAN_CTRLMODE_TDC_MANUAL)
+		tdcmod = MCP251XFD_REG_TDC_TDCMOD_MANUAL;
+	else
+		tdcmod = MCP251XFD_REG_TDC_TDCMOD_DISABLED;
+
+	val = FIELD_PREP(MCP251XFD_REG_TDC_TDCMOD_MASK, tdcmod) |
+		FIELD_PREP(MCP251XFD_REG_TDC_TDCV_MASK, priv->can.tdc.tdcv) |
+		FIELD_PREP(MCP251XFD_REG_TDC_TDCO_MASK, priv->can.tdc.tdco);
 
 	return regmap_write(priv->map_reg, MCP251XFD_REG_TDC, val);
 }
@@ -2083,10 +2105,12 @@ static int mcp251xfd_probe(struct spi_device *spi)
 	priv->can.do_get_berr_counter = mcp251xfd_get_berr_counter;
 	priv->can.bittiming_const = &mcp251xfd_bittiming_const;
 	priv->can.data_bittiming_const = &mcp251xfd_data_bittiming_const;
+	priv->can.tdc_const = &mcp251xfd_tdc_const;
 	priv->can.ctrlmode_supported = CAN_CTRLMODE_LOOPBACK |
 		CAN_CTRLMODE_LISTENONLY | CAN_CTRLMODE_BERR_REPORTING |
 		CAN_CTRLMODE_FD | CAN_CTRLMODE_FD_NON_ISO |
-		CAN_CTRLMODE_CC_LEN8_DLC;
+		CAN_CTRLMODE_CC_LEN8_DLC | CAN_CTRLMODE_TDC_AUTO |
+		CAN_CTRLMODE_TDC_MANUAL;
 	set_bit(MCP251XFD_FLAGS_DOWN, priv->flags);
 	priv->ndev = ndev;
 	priv->spi = spi;