can: populate the minimum and maximum MTU values

By populating:

  net_device->min_mtu

and

  net_device->max_mtu

the net core infrastructure will automatically:

  1. validate that the user's inputs are in range.

  2. report those min and max MTU values through the netlink
     interface.

Add can_set_default_mtu() which sets the default mtu value as well as
the minimum and maximum values. The logic for the default mtu value
remains unchanged:

  - CANFD_MTU if the device has a static CAN_CTRLMODE_FD.

  - CAN_MTU otherwise.

Call can_set_default_mtu() each time the CAN_CTRLMODE_FD is modified.
This will guarantee that the MTU value is always consistent with the
control mode flags.

With this, the checks done in can_change_mtu() become fully redundant
and will be removed in an upcoming change and it is now possible to
confirm the minimum and maximum MTU values on a physical CAN interface
by doing:

  $ ip --details link show can0

The virtual interfaces (vcan and vxcan) are not impacted by this
change.

Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
Link: https://patch.msgid.link/20250923-can-fix-mtu-v3-3-581bde113f52@kernel.org
[mkl: squashed https://patch.msgid.link/20250924143644.17622-2-mailhol@kernel.org]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/dev/dev.c

@@ -240,6 +240,8 @@ void can_setup(struct net_device *dev)
 {
 	dev->type = ARPHRD_CAN;
 	dev->mtu = CAN_MTU;
+	dev->min_mtu = CAN_MTU;
+	dev->max_mtu = CAN_MTU;
 	dev->hard_header_len = 0;
 	dev->addr_len = 0;
 	dev->tx_queue_len = 10;
@@ -309,6 +311,21 @@ void free_candev(struct net_device *dev)
 }
 EXPORT_SYMBOL_GPL(free_candev);
 
+void can_set_default_mtu(struct net_device *dev)
+{
+	struct can_priv *priv = netdev_priv(dev);
+
+	if (priv->ctrlmode & CAN_CTRLMODE_FD) {
+		dev->mtu = CANFD_MTU;
+		dev->min_mtu = CANFD_MTU;
+		dev->max_mtu = CANFD_MTU;
+	} else {
+		dev->mtu = CAN_MTU;
+		dev->min_mtu = CAN_MTU;
+		dev->max_mtu = CAN_MTU;
+	}
+}
+
 /* changing MTU and control mode for CAN/CANFD devices */
 int can_change_mtu(struct net_device *dev, int new_mtu)
 {
@@ -361,8 +378,7 @@ int can_set_static_ctrlmode(struct net_device *dev, u32 static_mode)
 	priv->ctrlmode = static_mode;
 
 	/* override MTU which was set by default in can_setup()? */
-	if (static_mode & CAN_CTRLMODE_FD)
+	can_set_default_mtu(dev);
-		dev->mtu = CANFD_MTU;
 
 	return 0;
 }

drivers/net/can/dev/netlink.c

@@ -223,17 +223,16 @@ static int can_changelink(struct net_device *dev, struct nlattr *tb[],
 		priv->ctrlmode &= ~cm->mask;
 		priv->ctrlmode |= maskedflags;
 
-		/* CAN_CTRLMODE_FD can only be set when driver supports FD */
+		/* Wipe potential leftovers from previous CAN FD config */
-		if (priv->ctrlmode & CAN_CTRLMODE_FD) {
+		if (!(priv->ctrlmode & CAN_CTRLMODE_FD)) {
-			dev->mtu = CANFD_MTU;
-		} else {
-			dev->mtu = CAN_MTU;
 			memset(&priv->fd.data_bittiming, 0,
 			       sizeof(priv->fd.data_bittiming));
 			priv->ctrlmode &= ~CAN_CTRLMODE_FD_TDC_MASK;
 			memset(&priv->fd.tdc, 0, sizeof(priv->fd.tdc));
 		}
 
+		can_set_default_mtu(dev);
+
 		fd_tdc_flag_provided = cm->mask & CAN_CTRLMODE_FD_TDC_MASK;
 		/* CAN_CTRLMODE_TDC_{AUTO,MANUAL} are mutually
 		 * exclusive: make sure to turn the other one off

include/linux/can/dev.h

@@ -166,6 +166,7 @@ struct can_priv *safe_candev_priv(struct net_device *dev);
 
 int open_candev(struct net_device *dev);
 void close_candev(struct net_device *dev);
+void can_set_default_mtu(struct net_device *dev);
 int can_change_mtu(struct net_device *dev, int new_mtu);
 int __must_check can_set_static_ctrlmode(struct net_device *dev,
 					 u32 static_mode);