can: add dummy_can driver

During the development of CAN XL, we found the need of creating a
dummy CAN XL driver in order to test the new netlink interface. While
this code was initially intended to be some throwaway, it received
some positive feedback.

Add the dummy_can driver. This driver acts similarly to the vcan
interface in the sense that it will echo back any packet it receives.
The difference is that it exposes a set on bittiming parameters as a
real device would and thus must be configured as if it was a real
physical interface.

The driver comes with a debug mode. If debug message are enabled (for
example by enabling CONFIG_CAN_DEBUG_DEVICES), it will print in the
kernel log all the bittiming values, similar to what a:

ip --details link show can0

would do.

This driver is mostly intended for debugging and testing, but some
developers also may want to look at it as a simple reference
implementation.

Signed-off-by: Vincent Mailhol <[e-mail]>
Signed-off-by: Oliver Hartkopp <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Marc Kleine-Budde <[e-mail]>

drivers/net/can/Kconfig

···
 
 	  If this driver is built as a module, it will be called can327.
 
+config CAN_DUMMY
+	tristate "Dummy CAN"
+	help
+	  A dummy CAN module supporting Classical CAN, CAN FD and CAN XL. It
+	  exposes bittiming values which can be configured through the netlink
+	  interface.
+
+	  The module will simply echo any frame sent to it. If debug messages
+	  are activated, it prints all the CAN bittiming information in the
+	  kernel log. Aside from that it does nothing.
+
+	  This is convenient for testing the CAN netlink interface. Most of the
+	  users will never need this. If unsure, say NO.
+
+	  To compile this driver as a module, choose M here: the module will be
+	  called dummy-can.
+
 config CAN_FLEXCAN
 	tristate "Support for Freescale FLEXCAN based chips"
 	depends on OF || COLDFIRE || COMPILE_TEST

drivers/net/can/Makefile

···
 obj-$(CONFIG_CAN_CC770)		+= cc770/
 obj-$(CONFIG_CAN_C_CAN)		+= c_can/
 obj-$(CONFIG_CAN_CTUCANFD)	+= ctucanfd/
+obj-$(CONFIG_CAN_DUMMY)		+= dummy_can.o
 obj-$(CONFIG_CAN_FLEXCAN)	+= flexcan/
 obj-$(CONFIG_CAN_GRCAN)		+= grcan.o
 obj-$(CONFIG_CAN_IFI_CANFD)	+= ifi_canfd/

drivers/net/can/dummy_can.c

···
+// SPDX-License-Identifier: GPL-2.0
+
+/* Copyright (c) 2025 Vincent Mailhol <[e-mail]> */
+
+#include <linux/array_size.h>
+#include <linux/errno.h>
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/netdevice.h>
+#include <linux/units.h>
+#include <linux/string_choices.h>
+
+#include <linux/can.h>
+#include <linux/can/bittiming.h>
+#include <linux/can/dev.h>
+#include <linux/can/skb.h>
+
+struct dummy_can {
+	struct can_priv can;
+	struct net_device *dev;
+};
+
+static struct dummy_can *dummy_can;
+
+static const struct can_bittiming_const dummy_can_bittiming_const = {
+	.name = "dummy_can CC",
+	.tseg1_min = 2,
+	.tseg1_max = 256,
+	.tseg2_min = 2,
+	.tseg2_max = 128,
+	.sjw_max = 128,
+	.brp_min = 1,
+	.brp_max = 512,
+	.brp_inc = 1
+};
+
+static const struct can_bittiming_const dummy_can_fd_databittiming_const = {
+	.name = "dummy_can FD",
+	.tseg1_min = 2,
+	.tseg1_max = 256,
+	.tseg2_min = 2,
+	.tseg2_max = 128,
+	.sjw_max = 128,
+	.brp_min = 1,
+	.brp_max = 512,
+	.brp_inc = 1
+};
+
+static const struct can_tdc_const dummy_can_fd_tdc_const = {
+	.tdcv_min = 0,
+	.tdcv_max = 0, /* Manual mode not supported. */
+	.tdco_min = 0,
+	.tdco_max = 127,
+	.tdcf_min = 0,
+	.tdcf_max = 127
+};
+
+static const struct can_bittiming_const dummy_can_xl_databittiming_const = {
+	.name = "dummy_can XL",
+	.tseg1_min = 2,
+	.tseg1_max = 256,
+	.tseg2_min = 2,
+	.tseg2_max = 128,
+	.sjw_max = 128,
+	.brp_min = 1,
+	.brp_max = 512,
+	.brp_inc = 1
+};
+
+static const struct can_tdc_const dummy_can_xl_tdc_const = {
+	.tdcv_min = 0,
+	.tdcv_max = 0, /* Manual mode not supported. */
+	.tdco_min = 0,
+	.tdco_max = 127,
+	.tdcf_min = 0,
+	.tdcf_max = 127
+};
+
+static const struct can_pwm_const dummy_can_pwm_const = {
+	.pwms_min = 1,
+	.pwms_max = 8,
+	.pwml_min = 2,
+	.pwml_max = 24,
+	.pwmo_min = 0,
+	.pwmo_max = 16,
+};
+
+static void dummy_can_print_bittiming(struct net_device *dev,
+				      struct can_bittiming *bt)
+{
+	netdev_dbg(dev, "\tbitrate: %u\n", bt->bitrate);
+	netdev_dbg(dev, "\tsample_point: %u\n", bt->sample_point);
+	netdev_dbg(dev, "\ttq: %u\n", bt->tq);
+	netdev_dbg(dev, "\tprop_seg: %u\n", bt->prop_seg);
+	netdev_dbg(dev, "\tphase_seg1: %u\n", bt->phase_seg1);
+	netdev_dbg(dev, "\tphase_seg2: %u\n", bt->phase_seg2);
+	netdev_dbg(dev, "\tsjw: %u\n", bt->sjw);
+	netdev_dbg(dev, "\tbrp: %u\n", bt->brp);
+}
+
+static void dummy_can_print_tdc(struct net_device *dev, struct can_tdc *tdc)
+{
+	netdev_dbg(dev, "\t\ttdcv: %u\n", tdc->tdcv);
+	netdev_dbg(dev, "\t\ttdco: %u\n", tdc->tdco);
+	netdev_dbg(dev, "\t\ttdcf: %u\n", tdc->tdcf);
+}
+
+static void dummy_can_print_pwm(struct net_device *dev, struct can_pwm *pwm,
+				struct can_bittiming *dbt)
+{
+	netdev_dbg(dev, "\t\tpwms: %u\n", pwm->pwms);
+	netdev_dbg(dev, "\t\tpwml: %u\n", pwm->pwml);
+	netdev_dbg(dev, "\t\tpwmo: %u\n", pwm->pwmo);
+}
+
+static void dummy_can_print_ctrlmode(struct net_device *dev)
+{
+	struct dummy_can *priv = netdev_priv(dev);
+	struct can_priv *can_priv = &priv->can;
+	unsigned long supported = can_priv->ctrlmode_supported;
+	u32 enabled = can_priv->ctrlmode;
+
+	netdev_dbg(dev, "Control modes:\n");
+	netdev_dbg(dev, "\tsupported: 0x%08x\n", (u32)supported);
+	netdev_dbg(dev, "\tenabled: 0x%08x\n", enabled);
+
+	if (supported) {
+		int idx;
+
+		netdev_dbg(dev, "\tlist:");
+		for_each_set_bit(idx, &supported, BITS_PER_TYPE(u32))
+			netdev_dbg(dev, "\t\t%s: %s\n",
+				   can_get_ctrlmode_str(BIT(idx)),
+				   enabled & BIT(idx) ? "on" : "off");
+	}
+}
+
+static void dummy_can_print_bittiming_info(struct net_device *dev)
+{
+	struct dummy_can *priv = netdev_priv(dev);
+	struct can_priv *can_priv = &priv->can;
+
+	netdev_dbg(dev, "Clock frequency: %u\n", can_priv->clock.freq);
+	netdev_dbg(dev, "Maximum bitrate: %u\n", can_priv->bitrate_max);
+	netdev_dbg(dev, "MTU: %u\n", dev->mtu);
+	netdev_dbg(dev, "\n");
+
+	dummy_can_print_ctrlmode(dev);
+	netdev_dbg(dev, "\n");
+
+	netdev_dbg(dev, "Classical CAN nominal bittiming:\n");
+	dummy_can_print_bittiming(dev, &can_priv->bittiming);
+	netdev_dbg(dev, "\n");
+
+	if (can_priv->ctrlmode & CAN_CTRLMODE_FD) {
+		netdev_dbg(dev, "CAN FD databittiming:\n");
+		dummy_can_print_bittiming(dev, &can_priv->fd.data_bittiming);
+		if (can_fd_tdc_is_enabled(can_priv)) {
+			netdev_dbg(dev, "\tCAN FD TDC:\n");
+			dummy_can_print_tdc(dev, &can_priv->fd.tdc);
+		}
+	}
+	netdev_dbg(dev, "\n");
+
+	if (can_priv->ctrlmode & CAN_CTRLMODE_XL) {
+		netdev_dbg(dev, "CAN XL databittiming:\n");
+		dummy_can_print_bittiming(dev, &can_priv->xl.data_bittiming);
+		if (can_xl_tdc_is_enabled(can_priv)) {
+			netdev_dbg(dev, "\tCAN XL TDC:\n");
+			dummy_can_print_tdc(dev, &can_priv->xl.tdc);
+		}
+		if (can_priv->ctrlmode & CAN_CTRLMODE_XL_TMS) {
+			netdev_dbg(dev, "\tCAN XL PWM:\n");
+			dummy_can_print_pwm(dev, &can_priv->xl.pwm,
+					    &can_priv->xl.data_bittiming);
+		}
+	}
+	netdev_dbg(dev, "\n");
+}
+
+static int dummy_can_netdev_open(struct net_device *dev)
+{
+	int ret;
+	struct can_priv *priv = netdev_priv(dev);
+
+	dummy_can_print_bittiming_info(dev);
+	netdev_dbg(dev, "error-signalling is %s\n",
+		   str_enabled_disabled(!can_dev_in_xl_only_mode(priv)));
+
+	ret = open_candev(dev);
+	if (ret)
+		return ret;
+	netif_start_queue(dev);
+	netdev_dbg(dev, "dummy-can is up\n");
+
+	return 0;
+}
+
+static int dummy_can_netdev_close(struct net_device *dev)
+{
+	netif_stop_queue(dev);
+	close_candev(dev);
+	netdev_dbg(dev, "dummy-can is down\n");
+
+	return 0;
+}
+
+static netdev_tx_t dummy_can_start_xmit(struct sk_buff *skb,
+					struct net_device *dev)
+{
+	if (can_dev_dropped_skb(dev, skb))
+		return NETDEV_TX_OK;
+
+	can_put_echo_skb(skb, dev, 0, 0);
+	dev->stats.tx_packets++;
+	dev->stats.tx_bytes += can_get_echo_skb(dev, 0, NULL);
+
+	return NETDEV_TX_OK;
+}
+
+static const struct net_device_ops dummy_can_netdev_ops = {
+	.ndo_open = dummy_can_netdev_open,
+	.ndo_stop = dummy_can_netdev_close,
+	.ndo_start_xmit = dummy_can_start_xmit,
+};
+
+static const struct ethtool_ops dummy_can_ethtool_ops = {
+	.get_ts_info = ethtool_op_get_ts_info,
+};
+
+static int __init dummy_can_init(void)
+{
+	struct net_device *dev;
+	struct dummy_can *priv;
+	int ret;
+
+	dev = alloc_candev(sizeof(*priv), 1);
+	if (!dev)
+		return -ENOMEM;
+
+	dev->netdev_ops = &dummy_can_netdev_ops;
+	dev->ethtool_ops = &dummy_can_ethtool_ops;
+	priv = netdev_priv(dev);
+	priv->can.bittiming_const = &dummy_can_bittiming_const;
+	priv->can.bitrate_max = 20 * MEGA /* BPS */;
+	priv->can.clock.freq = 160 * MEGA /* Hz */;
+	priv->can.fd.data_bittiming_const = &dummy_can_fd_databittiming_const;
+	priv->can.fd.tdc_const = &dummy_can_fd_tdc_const;
+	priv->can.xl.data_bittiming_const = &dummy_can_xl_databittiming_const;
+	priv->can.xl.tdc_const = &dummy_can_xl_tdc_const;
+	priv->can.xl.pwm_const = &dummy_can_pwm_const;
+	priv->can.ctrlmode_supported = CAN_CTRLMODE_LISTENONLY |
+		CAN_CTRLMODE_FD | CAN_CTRLMODE_TDC_AUTO |
+		CAN_CTRLMODE_RESTRICTED | CAN_CTRLMODE_XL |
+		CAN_CTRLMODE_XL_TDC_AUTO | CAN_CTRLMODE_XL_TMS;
+	priv->dev = dev;
+
+	ret = register_candev(priv->dev);
+	if (ret) {
+		free_candev(priv->dev);
+		return ret;
+	}
+
+	dummy_can = priv;
+	netdev_dbg(dev, "dummy-can ready\n");
+
+	return 0;
+}
+
+static void __exit dummy_can_exit(void)
+{
+	struct net_device *dev = dummy_can->dev;
+
+	netdev_dbg(dev, "dummy-can bye bye\n");
+	unregister_candev(dev);
+	free_candev(dev);
+}
+
+module_init(dummy_can_init);
+module_exit(dummy_can_exit);
+
+MODULE_DESCRIPTION("A dummy CAN driver, mainly to test the netlink interface");
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Vincent Mailhol <[e-mail]>");