linux-kernel - [PATCH RFC 2/3] can: calc_bittiming: add can_calc_sample_point

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Message-Id: <20251102-pwm_sample_point-v1-2-3bbea180f59e@kernel.org>
Date: Sun, 02 Nov 2025 23:01:23 +0100
From: Vincent Mailhol <mailhol@...nel.org>
To: Marc Kleine-Budde <mkl@...gutronix.de>, 
 Oliver Hartkopp <socketcan@...tkopp.net>
Cc: linux-can@...r.kernel.org, linux-kernel@...r.kernel.org, 
 Vincent Mailhol <mailhol@...nel.org>
Subject: [PATCH RFC 2/3] can: calc_bittiming: add
 can_calc_sample_point_nrz()

CAN XL optimal sample point for PWM encoding (when TMS is on) differs
from the NRZ optimal one. There is thus a need to calculate a
different sample point depending whether TMS is on or off.

This is a preparation change: move the sample point calculation from
can_calc_bittiming() into the new can_calc_sample_point_nrz()
function.

In an upcoming change, a function will be added to calculate the
sample point for PWM encoding.

Signed-off-by: Vincent Mailhol <mailhol@...nel.org>
---
 drivers/net/can/dev/calc_bittiming.c | 25 +++++++++++++++----------
 1 file changed, 15 insertions(+), 10 deletions(-)

diff --git a/drivers/net/can/dev/calc_bittiming.c b/drivers/net/can/dev/calc_bittiming.c
index 222117596704..9b2d0e458518 100644
--- a/drivers/net/can/dev/calc_bittiming.c
+++ b/drivers/net/can/dev/calc_bittiming.c
@@ -10,6 +10,18 @@
 
 #define CAN_CALC_MAX_ERROR 50 /* in one-tenth of a percent */
 
+/* CiA recommended sample points for Non Return to Zero encoding. */
+static int can_calc_sample_point_nrz(const struct can_bittiming *bt)
+{
+	if (bt->bitrate > 800 * KILO /* BPS */)
+		return 750;
+
+	if (bt->bitrate > 500 * KILO /* BPS */)
+		return 800;
+
+	return 875;
+}
+
 /* Bit-timing calculation derived from:
  *
  * Code based on LinCAN sources and H8S2638 project
@@ -78,17 +90,10 @@ int can_calc_bittiming(const struct net_device *dev, struct can_bittiming *bt,
 	u64 v64;
 	int err;
 
-	/* Use CiA recommended sample points */
-	if (bt->sample_point) {
+	if (bt->sample_point)
 		sample_point = bt->sample_point;
-	} else {
-		if (bt->bitrate > 800 * KILO /* BPS */)
-			sample_point = 750;
-		else if (bt->bitrate > 500 * KILO /* BPS */)
-			sample_point = 800;
-		else
-			sample_point = 875;
-	}
+	else
+		sample_point = can_calc_sample_point_nrz(bt);
 
 	/* tseg even = round down, odd = round up */
 	for (tseg = (btc->tseg1_max + btc->tseg2_max) * 2 + 1;

-- 
2.51.0