can: fix handling of unmodifiable configuration options

As described in 'can: m_can: tag current CAN FD controllers as non-ISO'
(6cfda7fbebe) it is possible to define fixed configuration options by
setting the according bit in 'ctrlmode' and clear it in 'ctrlmode_supported'.
This leads to the incovenience that the fixed configuration bits can not be
passed by netlink even when they have the correct values (e.g. non-ISO, FD).

This patch fixes that issue and not only allows fixed set bit values to be set
again but now requires(!) to provide these fixed values at configuration time.
A valid CAN FD configuration consists of a nominal/arbitration bittiming, a
data bittiming and a control mode with CAN_CTRLMODE_FD set - which is now
enforced by a new can_validate() function. This fix additionally removed the
inconsistency that was prohibiting the support of 'CANFD-only' controller
drivers, like the RCar CAN FD.

For this reason a new helper can_set_static_ctrlmode() has been introduced to
provide a proper interface to handle static enabled CAN controller options.

Reported-by: Ramesh Shanmugasundaram <[e-mail]>
Signed-off-by: Oliver Hartkopp <[e-mail]>
Reviewed-by: Ramesh Shanmugasundaram <[e-mail]>
Cc: <[e-mail]> # >= 3.18
Signed-off-by: Marc Kleine-Budde <[e-mail]>

drivers/net/can/dev.c

···
 	/* allow change of MTU according to the CANFD ability of the device */
 	switch (new_mtu) {
 	case CAN_MTU:
+		/* 'CANFD-only' controllers can not switch to CAN_MTU */
+		if (priv->ctrlmode_static & CAN_CTRLMODE_FD)
+			return -EINVAL;
+
 		priv->ctrlmode &= ~CAN_CTRLMODE_FD;
 		break;
 
 	case CANFD_MTU:
-		if (!(priv->ctrlmode_supported & CAN_CTRLMODE_FD))
+		/* check for potential CANFD ability */
+		if (!(priv->ctrlmode_supported & CAN_CTRLMODE_FD) &&
+		    !(priv->ctrlmode_static & CAN_CTRLMODE_FD))
 			return -EINVAL;
 
 		priv->ctrlmode |= CAN_CTRLMODE_FD;
···
 				= { .len = sizeof(struct can_bittiming_const) },
 };
 
+static int can_validate(struct nlattr *tb[], struct nlattr *data[])
+{
+	bool is_can_fd = false;
+
+	/* Make sure that valid CAN FD configurations always consist of
+	 * - nominal/arbitration bittiming
+	 * - data bittiming
+	 * - control mode with CAN_CTRLMODE_FD set
+	 */
+
+	if (data[IFLA_CAN_CTRLMODE]) {
+		struct can_ctrlmode *cm = nla_data(data[IFLA_CAN_CTRLMODE]);
+
+		is_can_fd = cm->flags & cm->mask & CAN_CTRLMODE_FD;
+	}
+
+	if (is_can_fd) {
+		if (!data[IFLA_CAN_BITTIMING] || !data[IFLA_CAN_DATA_BITTIMING])
+			return -EOPNOTSUPP;
+	}
+
+	if (data[IFLA_CAN_DATA_BITTIMING]) {
+		if (!is_can_fd || !data[IFLA_CAN_BITTIMING])
+			return -EOPNOTSUPP;
+	}
+
+	return 0;
+}
+
 static int can_changelink(struct net_device *dev,
 			  struct nlattr *tb[], struct nlattr *data[])
 {
···
 
 	if (data[IFLA_CAN_CTRLMODE]) {
 		struct can_ctrlmode *cm;
+		u32 ctrlstatic;
+		u32 maskedflags;
 
 		/* Do not allow changing controller mode while running */
 		if (dev->flags & IFF_UP)
 			return -EBUSY;
 		cm = nla_data(data[IFLA_CAN_CTRLMODE]);
+		ctrlstatic = priv->ctrlmode_static;
+		maskedflags = cm->flags & cm->mask;
 
-		/* check whether changed bits are allowed to be modified */
-		if (cm->mask & ~priv->ctrlmode_supported)
+		/* check whether provided bits are allowed to be passed */
+		if (cm->mask & ~(priv->ctrlmode_supported | ctrlstatic))
+			return -EOPNOTSUPP;
+
+		/* do not check for static fd-non-iso if 'fd' is disabled */
+		if (!(maskedflags & CAN_CTRLMODE_FD))
+			ctrlstatic &= ~CAN_CTRLMODE_FD_NON_ISO;
+
+		/* make sure static options are provided by configuration */
+		if ((maskedflags & ctrlstatic) != ctrlstatic)
 			return -EOPNOTSUPP;
 
 		/* clear bits to be modified and copy the flag values */
 		priv->ctrlmode &= ~cm->mask;
-		priv->ctrlmode |= (cm->flags & cm->mask);
+		priv->ctrlmode |= maskedflags;
 
 		/* CAN_CTRLMODE_FD can only be set when driver supports FD */
 		if (priv->ctrlmode & CAN_CTRLMODE_FD)
···
 	.maxtype	= IFLA_CAN_MAX,
 	.policy		= can_policy,
 	.setup		= can_setup,
+	.validate	= can_validate,
 	.newlink	= can_newlink,
 	.changelink	= can_changelink,
 	.get_size	= can_get_size,

drivers/net/can/m_can/m_can.c

···
 	priv->can.do_get_berr_counter = m_can_get_berr_counter;
 
 	/* CAN_CTRLMODE_FD_NON_ISO is fixed with M_CAN IP v3.0.1 */
-	priv->can.ctrlmode = CAN_CTRLMODE_FD_NON_ISO;
+	can_set_static_ctrlmode(dev, CAN_CTRLMODE_FD_NON_ISO);
 
 	/* CAN_CTRLMODE_FD_NON_ISO can not be changed with M_CAN IP v3.0.1 */
 	priv->can.ctrlmode_supported = CAN_CTRLMODE_LOOPBACK |

include/linux/can/dev.h

···
 	struct can_clock clock;
 
 	enum can_state state;
-	u32 ctrlmode;
-	u32 ctrlmode_supported;
+
+	/* CAN controller features - see include/uapi/linux/can/netlink.h */
+	u32 ctrlmode;		/* current options setting */
+	u32 ctrlmode_supported;	/* options that can be modified by netlink */
+	u32 ctrlmode_static;	/* static enabled options for driver/hardware */
 
 	int restart_ms;
 	struct timer_list restart_timer;
···
 {
 	/* the CAN specific type of skb is identified by its data length */
 	return skb->len == CANFD_MTU;
+}
+
+/* helper to define static CAN controller features at device creation time */
+static inline void can_set_static_ctrlmode(struct net_device *dev,
+					   u32 static_mode)
+{
+	struct can_priv *priv = netdev_priv(dev);
+
+	/* alloc_candev() succeeded => netdev_priv() is valid at this point */
+	priv->ctrlmode = static_mode;
+	priv->ctrlmode_static = static_mode;
+
+	/* override MTU which was set by default in can_setup()? */
+	if (static_mode & CAN_CTRLMODE_FD)
+		dev->mtu = CANFD_MTU;
 }
 
 /* get data length from can_dlc with sanitized can_dlc */