[canxl v3 09/17] can: calc_bittiming: add PWM calculation

Oliver Hartkopp
2025-11-20T18:45:39Z

Perform the PWM calculation according to CiA recommendations.

Note that for databitrates greater than 5 MBPS, tqmin is less than
CAN_PWM_NS_MAX (which is defined to 200 nano seconds), consequently,
the result of the division:

DIV_ROUND_UP(xl_ns, CAN_PWM_NS_MAX)

is one and thus the for loop automatically stops on the first
iteration giving a single PWM symbol per bit as expected. Because of
that, there is no actual need for a separate conditional branch for
when the databitrate is greater than 5 MBPS.

Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
---
drivers/net/can/dev/calc_bittiming.c | 36 ++++++++++++++++++++++++++++
include/linux/can/bittiming.h | 10 ++++++++
2 files changed, 46 insertions(+)

diff --git a/drivers/net/can/dev/calc_bittiming.c b/drivers/net/can/dev/calc_bittiming.c
index 394d6974f481..268ec6fa7c49 100644
--- a/drivers/net/can/dev/calc_bittiming.c
+++ b/drivers/net/can/dev/calc_bittiming.c
@@ -1,9 +1,10 @@
// SPDX-License-Identifier: GPL-2.0-only
/* Copyright (C) 2005 Marc Kleine-Budde, Pengutronix
* Copyright (C) 2006 Andrey Volkov, Varma Electronics
* Copyright (C) 2008-2009 Wolfgang Grandegger <wg@grandegger.com>
+ * Copyright (C) 2021-2025 Vincent Mailhol <mailhol@kernel.org>
*/

#include <linux/units.h>
#include <linux/can/dev.h>

@@ -196,5 +197,40 @@ void can_calc_tdco(struct can_tdc *tdc, const struct can_tdc_const *tdc_const,
return;
tdc->tdco = min(sample_point_in_tc, tdc_const->tdco_max);
*ctrlmode |= tdc_auto;
}
}
+
+int can_calc_pwm(struct net_device *dev, struct netlink_ext_ack *extack)
+{
+ struct can_priv *priv = netdev_priv(dev);
+ const struct can_pwm_const *pwm_const = priv->xl.pwm_const;
+ struct can_pwm *pwm = &priv->xl.pwm;
+ u32 xl_tqmin = can_bit_time_tqmin(&priv->xl.data_bittiming);
+ u32 xl_ns = can_tqmin_to_ns(xl_tqmin, priv->clock.freq);
+ u32 nom_tqmin = can_bit_time_tqmin(&priv->bittiming);
+ int pwm_per_bit_max = xl_tqmin / (pwm_const->pwms_min + pwm_const->pwml_min);
+ int pwm_per_bit;
+ u32 pwm_tqmin;
+
+ /* For 5 MB/s databitrate or greater, xl_ns < CAN_PWM_NS_MAX
+ * giving us a pwm_per_bit of 1 and the loop immediately breaks
+ */
+ for (pwm_per_bit = DIV_ROUND_UP(xl_ns, CAN_PWM_NS_MAX);
+ pwm_per_bit <= pwm_per_bit_max; pwm_per_bit++)
+ if (xl_tqmin % pwm_per_bit == 0)
+ break;
+
+ if (pwm_per_bit > pwm_per_bit_max) {
+ NL_SET_ERR_MSG_FMT(extack,
+ "Can not divide the XL data phase's bit time: %u tqmin into multiple PWM symbols",
+ xl_tqmin);
+ return -EINVAL;
+ }
+
+ pwm_tqmin = xl_tqmin / pwm_per_bit;
+ pwm->pwms = DIV_ROUND_UP_POW2(pwm_tqmin, 4);
+ pwm->pwml = pwm_tqmin - pwm->pwms;
+ pwm->pwmo = nom_tqmin % pwm_tqmin;
+
+ return 0;
+}
diff --git a/include/linux/can/bittiming.h b/include/linux/can/bittiming.h
index 2504fafc72e4..726d909e87ce 100644
--- a/include/linux/can/bittiming.h
+++ b/include/linux/can/bittiming.h
@@ -178,10 +178,12 @@ int can_calc_bittiming(const struct net_device *dev, struct can_bittiming *bt,
const struct can_bittiming_const *btc, struct netlink_ext_ack *extack);

void can_calc_tdco(struct can_tdc *tdc, const struct can_tdc_const *tdc_const,
const struct can_bittiming *dbt,
u32 tdc_mask, u32 *ctrlmode, u32 ctrlmode_supported);
+
+int can_calc_pwm(struct net_device *dev, struct netlink_ext_ack *extack);
#else /* !CONFIG_CAN_CALC_BITTIMING */
static inline int
can_calc_bittiming(const struct net_device *dev, struct can_bittiming *bt,
const struct can_bittiming_const *btc, struct netlink_ext_ack *extack)
{
@@ -193,10 +195,18 @@ static inline void
can_calc_tdco(struct can_tdc *tdc, const struct can_tdc_const *tdc_const,
const struct can_bittiming *dbt,
u32 tdc_mask, u32 *ctrlmode, u32 ctrlmode_supported)
{
}
+
+static inline int
+can_calc_pwm(struct net_device *dev, struct netlink_ext_ack *extack)
+{
+ NL_SET_ERR_MSG(extack,
+ "bit-timing calculation not available: manually provide PWML and PWMS\n");
+ return -EINVAL;
+}
#endif /* CONFIG_CAN_CALC_BITTIMING */

void can_sjw_set_default(struct can_bittiming *bt);

int can_sjw_check(const struct net_device *dev, const struct can_bittiming *bt,
--
2.47.3