can: flexcan: put TX mailbox into TX_INACTIVE mode after tx-complete

After sending a RTR frame the TX mailbox becomes a RX_EMPTY mailbox. To avoid
side effects when the RX-FIFO is full, this patch puts the TX mailbox into
TX_INACTIVE mode in the transmission complete interrupt handler. This, of
course, leaves a race window between the actual completion of the transmission
and the handling of tx-complete interrupt. However this is the best we can do
without busy polling the tx complete interrupt.

Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/flexcan.c

@@ -765,6 +765,9 @@ static irqreturn_t flexcan_irq(int irq, void *dev_id)
 		stats->tx_bytes += can_get_echo_skb(dev, 0);
 		stats->tx_packets++;
 		can_led_event(dev, CAN_LED_EVENT_TX);
+		/* after sending a RTR frame mailbox is in RX mode */
+		flexcan_write(FLEXCAN_MB_CODE_TX_INACTIVE,
+			      &regs->cantxfg[FLEXCAN_TX_BUF_ID].can_ctrl);
 		flexcan_write((1 << FLEXCAN_TX_BUF_ID), &regs->iflag1);
 		netif_wake_queue(dev);
 	}