can: c_can: don't cache TX messages for C_CAN cores

As Jacob noticed, the optimization introduced in 387da6bc7a82 ("can:
c_can: cache frames to operate as a true FIFO") doesn't properly work
on C_CAN, but on D_CAN IP cores. The exact reasons are still unknown.

For now disable caching if CAN frames in the TX path for C_CAN cores.

Fixes: 387da6bc7a82 ("can: c_can: cache frames to operate as a true FIFO")
Link: https://lore.kernel.org/all/[e-mail]
Link: https://lore.kernel.org/all/[e-mail]
Reported-by: Jacob Kroon <[e-mail]>
Tested-by: Jacob Kroon <[e-mail]>
Cc: [e-mail] # v5.15
Signed-off-by: Marc Kleine-Budde <[e-mail]>

drivers/net/can/c_can/c_can.h

···
 	return ring->tail & (ring->obj_num - 1);
 }
 
-static inline u8 c_can_get_tx_free(const struct c_can_tx_ring *ring)
+static inline u8 c_can_get_tx_free(const struct c_can_priv *priv,
+				   const struct c_can_tx_ring *ring)
 {
-	return ring->obj_num - (ring->head - ring->tail);
+	u8 head = c_can_get_tx_head(ring);
+	u8 tail = c_can_get_tx_tail(ring);
+
+	if (priv->type == BOSCH_D_CAN)
+		return ring->obj_num - (ring->head - ring->tail);
+
+	/* This is not a FIFO. C/D_CAN sends out the buffers
+	 * prioritized. The lowest buffer number wins.
+	 */
+	if (head < tail)
+		return 0;
+
+	return ring->obj_num - head;
 }
 
 #endif /* C_CAN_H */

drivers/net/can/c_can/c_can_main.c

···
 static bool c_can_tx_busy(const struct c_can_priv *priv,
 			  const struct c_can_tx_ring *tx_ring)
 {
-	if (c_can_get_tx_free(tx_ring) > 0)
+	if (c_can_get_tx_free(priv, tx_ring) > 0)
 		return false;
 
 	netif_stop_queue(priv->dev);
···
 	/* Memory barrier before checking tx_free (head and tail) */
 	smp_mb();
 
-	if (c_can_get_tx_free(tx_ring) == 0) {
+	if (c_can_get_tx_free(priv, tx_ring) == 0) {
 		netdev_dbg(priv->dev,
 			   "Stopping tx-queue (tx_head=0x%08x, tx_tail=0x%08x, len=%d).\n",
 			   tx_ring->head, tx_ring->tail,
···
 
 	idx = c_can_get_tx_head(tx_ring);
 	tx_ring->head++;
-	if (c_can_get_tx_free(tx_ring) == 0)
+	if (c_can_get_tx_free(priv, tx_ring) == 0)
 		netif_stop_queue(dev);
 
 	if (idx < c_can_get_tx_tail(tx_ring))
···
 		return;
 
 	tx_ring->tail += pkts;
-	if (c_can_get_tx_free(tx_ring)) {
+	if (c_can_get_tx_free(priv, tx_ring)) {
 		/* Make sure that anybody stopping the queue after
 		 * this sees the new tx_ring->tail.
 		 */
···
 	stats->tx_packets += pkts;
 
 	tail = c_can_get_tx_tail(tx_ring);
-
-	if (tail == 0) {
+	if (priv->type == BOSCH_D_CAN && tail == 0) {
 		u8 head = c_can_get_tx_head(tx_ring);
 
 		/* Start transmission for all cached messages */