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/20220928083354.1062321-1-mkl@pengutronix.de
Link: https://lore.kernel.org/all/15a8084b-9617-2da1-6704-d7e39d60643b@gmail.com
Reported-by: Jacob Kroon <jacob.kroon@gmail.com>
Tested-by: Jacob Kroon <jacob.kroon@gmail.com>
Cc: stable@vger.kernel.org # v5.15
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

1 files changed, 15 insertions, 2 deletions

diff --git a/drivers/net/can/c_can/c_can.h b/drivers/net/can/c_can/c_can.h
index f23a03300a81..029cd8194ed5 100644
--- a/drivers/net/can/c_can/c_can.h
+++ b/drivers/net/can/c_can/c_can.h
@@ -235,9 +235,22 @@ static inline u8 c_can_get_tx_tail(const struct c_can_tx_ring *ring)
 	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 */