can: allow to change the device mtu for CAN FD capable devices

The configuration for CAN FD depends on CAN_CTRLMODE_FD enabled in the driver
specific ctrlmode_supported capabilities.

The configuration can be done either with the 'fd { on | off }' option in the
'ip' tool from iproute2 or by setting the CAN netdevice MTU to CAN_MTU (16) or
to CANFD_MTU (72).
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Acked-by: Stephane Grosjean <s.grosjean@peak-system.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/dev.c

@@ -594,6 +594,39 @@ void free_candev(struct net_device *dev)
 }
 EXPORT_SYMBOL_GPL(free_candev);
 
+/*
+ * changing MTU and control mode for CAN/CANFD devices
+ */
+int can_change_mtu(struct net_device *dev, int new_mtu)
+{
+	struct can_priv *priv = netdev_priv(dev);
+
+	/* Do not allow changing the MTU while running */
+	if (dev->flags & IFF_UP)
+		return -EBUSY;
+
+	/* allow change of MTU according to the CANFD ability of the device */
+	switch (new_mtu) {
+	case CAN_MTU:
+		priv->ctrlmode &= ~CAN_CTRLMODE_FD;
+		break;
+
+	case CANFD_MTU:
+		if (!(priv->ctrlmode_supported & CAN_CTRLMODE_FD))
+			return -EINVAL;
+
+		priv->ctrlmode |= CAN_CTRLMODE_FD;
+		break;
+
+	default:
+		return -EINVAL;
+	}
+
+	dev->mtu = new_mtu;
+	return 0;
+}
+EXPORT_SYMBOL_GPL(can_change_mtu);
+
 /*
  * Common open function when the device gets opened.
  *
@@ -693,6 +726,12 @@ static int can_changelink(struct net_device *dev,
 			return -EOPNOTSUPP;
 		priv->ctrlmode &= ~cm->mask;
 		priv->ctrlmode |= cm->flags;
+
+		/* CAN_CTRLMODE_FD can only be set when driver supports FD */
+		if (priv->ctrlmode & CAN_CTRLMODE_FD)
+			dev->mtu = CANFD_MTU;
+		else
+			dev->mtu = CAN_MTU;
 	}
 
 	if (data[IFLA_CAN_RESTART_MS]) {

include/linux/can/dev.h

@@ -113,6 +113,7 @@ struct can_priv *safe_candev_priv(struct net_device *dev);
 
 int open_candev(struct net_device *dev);
 void close_candev(struct net_device *dev);
+int can_change_mtu(struct net_device *dev, int new_mtu);
 
 int register_candev(struct net_device *dev);
 void unregister_candev(struct net_device *dev);

include/uapi/linux/can/netlink.h

@@ -96,6 +96,7 @@ struct can_ctrlmode {
 #define CAN_CTRLMODE_3_SAMPLES		0x04	/* Triple sampling mode */
 #define CAN_CTRLMODE_ONE_SHOT		0x08	/* One-Shot mode */
 #define CAN_CTRLMODE_BERR_REPORTING	0x10	/* Bus-error reporting */
+#define CAN_CTRLMODE_FD			0x20	/* CAN FD mode */
 
 /*
  * CAN device statistics