ixgbe: Add a few safety nets for register writes and descriptor cleanups

There are possible times that a driver may fail to completely initialize,
due to a buggy platform or a buggy kernel.  In those cases, we'd rather
fail gracefully instead of a panic.  Add a few safety checks to some
critical paths to try and prevent a panic in these corner-case situations.
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ixgbe/ixgbe_main.c

@@ -2458,7 +2458,9 @@ static void ixgbe_clean_rx_ring(struct ixgbe_adapter *adapter,
 	rx_ring->next_to_clean = 0;
 	rx_ring->next_to_use = 0;
 
+	if (rx_ring->head)
 		writel(0, adapter->hw.hw_addr + rx_ring->head);
+	if (rx_ring->tail)
 		writel(0, adapter->hw.hw_addr + rx_ring->tail);
 }
 
@@ -2490,7 +2492,9 @@ static void ixgbe_clean_tx_ring(struct ixgbe_adapter *adapter,
 	tx_ring->next_to_use = 0;
 	tx_ring->next_to_clean = 0;
 
+	if (tx_ring->head)
 		writel(0, adapter->hw.hw_addr + tx_ring->head);
+	if (tx_ring->tail)
 		writel(0, adapter->hw.hw_addr + tx_ring->tail);
 }
 
@@ -3327,6 +3331,7 @@ static void ixgbe_free_all_tx_resources(struct ixgbe_adapter *adapter)
 	int i;
 
 	for (i = 0; i < adapter->num_tx_queues; i++)
+		if (adapter->tx_ring[i].desc)
 			ixgbe_free_tx_resources(adapter, &adapter->tx_ring[i]);
 }
 
@@ -3363,6 +3368,7 @@ static void ixgbe_free_all_rx_resources(struct ixgbe_adapter *adapter)
 	int i;
 
 	for (i = 0; i < adapter->num_rx_queues; i++)
+		if (adapter->rx_ring[i].desc)
 			ixgbe_free_rx_resources(adapter, &adapter->rx_ring[i]);
 }