From: Oliver Hartkopp <socketcan@hartkopp.net>

The error-signalling (ES) is a mandatory functionality for CAN CC and
CAN FD to report CAN frame format violations by sending an error-frame
signal on the bus.

A so-called 'mixed-mode' is intended to have (XL-tolerant) CAN FD nodes
and CAN XL nodes on one CAN segment, where the FD-controllers can talk
CC/FD and the XL-controllers can talk CC/FD/XL. This mixed-mode
utilizes the error-signalling for sending CC/FD/XL frames.

The CANXL-only mode disables the error-signalling in the CAN XL
controller. This mode does not allow CC/FD frames to be sent but
additionally offers a CAN XL transceiver mode switching (TMS).

Configured with CAN_CTRLMODE_FD and CAN_CTRLMODE_XL this leads to:

FD=0 XL=0 CC-only mode         (ES=1)
FD=1 XL=0 FD/CC mixed-mode     (ES=1)
FD=1 XL=1 XL/FD/CC mixed-mode  (ES=1)
FD=0 XL=1 XL-only mode         (ES=0, TMS optional)

The helper function can_dev_in_xl_only_mode() determines the required
value to disable error signalling in the CAN XL controller.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
---
 include/linux/can/dev.h | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/include/linux/can/dev.h b/include/linux/can/dev.h
index f15879bd818d..52c8be5c160e 100644
--- a/include/linux/can/dev.h
+++ b/include/linux/can/dev.h
@@ -135,6 +135,19 @@ void can_bus_off(struct net_device *dev);
 const char *can_get_state_str(const enum can_state state);
 const char *can_get_ctrlmode_str(u32 ctrlmode);
 
+static inline bool can_dev_in_xl_only_mode(struct can_priv *priv)
+{
+	const u32 mixed_mode = CAN_CTRLMODE_FD | CAN_CTRLMODE_XL;
+
+	/* When CAN XL is enabled but FD is disabled we are running in
+	 * the so-called 'CANXL-only mode' where the error signalling is
+	 * disabled. This helper function determines the required value
+	 * to disable error signalling in the CAN XL controller.
+	 * The so-called CC/FD/XL 'mixed mode' requires error signalling.
+	 */
+	return ((priv->ctrlmode & mixed_mode) == CAN_CTRLMODE_XL);
+}
+
 /* drop skb if it does not contain a valid CAN frame for sending */
 static inline bool can_dev_dropped_skb(struct net_device *dev, struct sk_buff *skb)
 {
@@ -153,6 +166,12 @@ static inline bool can_dev_dropped_skb(struct net_device *dev, struct sk_buff *s
 		goto invalid_skb;
 	}
 
+	if (can_dev_in_xl_only_mode(priv) && !can_is_canxl_skb(skb)) {
+		netdev_info_once(dev,
+				 "Error signaling is disabled, dropping skb\n");
+		goto invalid_skb;
+	}
+
 	return can_dropped_invalid_skb(dev, skb);
 
 invalid_skb:

-- 
2.51.0