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

commit de594488 upstream.

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.
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/net/can/flexcan.c

@@ -632,6 +632,9 @@ static irqreturn_t flexcan_irq(int irq, void *dev_id)
 	if (reg_iflag1 & (1 << FLEXCAN_TX_BUF_ID)) {
 		/* tx_bytes is incremented in flexcan_start_xmit */
 		stats->tx_packets++;
+		/* 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);
 	}