linux-kernel - [PATCH RFC 3/5] can: populate the minimum and maximum MTU values

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Message-Id: <20250918-can-fix-mtu-v1-3-471edb942295@kernel.org>
Date: Thu, 18 Sep 2025 21:59:13 +0900
From: Vincent Mailhol <mailhol@...nel.org>
To: Oliver Hartkopp <socketcan@...tkopp.net>, 
 Marc Kleine-Budde <mkl@...gutronix.de>
Cc: linux-can@...r.kernel.org, linux-kernel@...r.kernel.org, 
 Vincent Mailhol <mailhol@...nel.org>
Subject: [PATCH RFC 3/5] 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 will set 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@...nel.org>
---
With this, when adding the CAN XL netlink interface, all we have to do
is to add one if branch to can_set_default_mtu() like this:

	void can_set_default_mtu(struct net_device *dev)
	{
		struct can_priv *priv = netdev_priv(dev);

		if (priv->ctrlmode & CAN_CTRLMODE_XL) {
			dev->mtu = CANXL_MTU;
			dev->min_mtu = CANXL_MIN_MTU;
			dev->max_mtu = CANXL_MAX_MTU;
		} else 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;
		}
	}

and we will be done!
---
 drivers/net/can/dev/dev.c     | 21 ++++++++++++++++++---
 drivers/net/can/dev/netlink.c |  9 ++++-----
 include/linux/can/dev.h       |  1 +
 3 files changed, 23 insertions(+), 8 deletions(-)

diff --git a/drivers/net/can/dev/dev.c b/drivers/net/can/dev/dev.c
index a0ae659beedcd1fa0979662c12614ae7846032c0..69c00720e9956cad986ea86fbba49285d76f51db 100644
--- a/drivers/net/can/dev/dev.c
+++ b/drivers/net/can/dev/dev.c
@@ -239,11 +239,12 @@ EXPORT_SYMBOL_GPL(can_bus_off);
 void can_setup(struct net_device *dev)
 {
 	dev->type = ARPHRD_CAN;
-	dev->mtu = CAN_MTU;
 	dev->hard_header_len = 0;
 	dev->addr_len = 0;
 	dev->tx_queue_len = 10;
 
+	can_set_default_mtu(dev);
+
 	/* New-style flags. */
 	dev->flags = IFF_NOARP;
 	dev->features = NETIF_F_HW_CSUM;
@@ -309,6 +310,21 @@ 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 +377,7 @@ 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)
-		dev->mtu = CANFD_MTU;
+	can_set_default_mtu(dev);
 
 	return 0;
 }
diff --git a/drivers/net/can/dev/netlink.c b/drivers/net/can/dev/netlink.c
index d9f6ab3efb9767409c318b714f19df8a30e51137..248f607e3864ffbda6f0b8daf4e2484179cf9cd5 100644
--- a/drivers/net/can/dev/netlink.c
+++ b/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 */
-		if (priv->ctrlmode & CAN_CTRLMODE_FD) {
-			dev->mtu = CANFD_MTU;
-		} else {
-			dev->mtu = CAN_MTU;
+		/* Wipe potential leftovers from previous CAN FD config */
+		if (!(priv->ctrlmode & CAN_CTRLMODE_FD)) {
 			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
diff --git a/include/linux/can/dev.h b/include/linux/can/dev.h
index 5dc58360c2d74a1711d4e02d28fe52ae20b146e0..3354f70ed2c684d7d482549560d4cb5838cbebd5 100644
--- a/include/linux/can/dev.h
+++ b/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);

-- 
2.49.1