net: octeon: mgmt: Repair filling of RX ring

The removal of mips_swiotlb_ops exposed a problem in octeon_mgmt Ethernet
driver. mips_swiotlb_ops had an mb() after most of the operations and the
removal of the ops had broken the receive functionality of the driver.
My code inspection has shown no other places except
octeon_mgmt_rx_fill_ring() where an explicit barrier would be obviously
missing. The latter function however has to make sure that "ringing the
bell" doesn't happen before RX ring entry is really written.

The patch has been successfully tested on Octeon II.

Fixes: a999933db9 ("MIPS: remove mips_swiotlb_ops")
Cc: stable@vger.kernel.org
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/cavium/octeon/octeon_mgmt.c

@@ -234,6 +234,11 @@ static void octeon_mgmt_rx_fill_ring(struct net_device *netdev)
 
 		/* Put it in the ring.  */
 		p->rx_ring[p->rx_next_fill] = re.d64;
+		/* Make sure there is no reorder of filling the ring and ringing
+		 * the bell
+		 */
+		wmb();
+
 		dma_sync_single_for_device(p->dev, p->rx_ring_handle,
 					   ring_size_to_bytes(OCTEON_MGMT_RX_RING_SIZE),
 					   DMA_BIDIRECTIONAL);