iavf: allow null RX descriptors

In some circumstances, the hardware can hand us a null receive
descriptor, with no data attached but otherwise valid. Unfortunately,
the driver was ill-equipped to handle such an event, and would stop
processing packets at that point.

To fix this, use the Descriptor Done bit instead of the size to
determine whether or not a descriptor is ready to be processed. Add some
checks to allow for unused buffers.
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/iavf/iavf_txrx.c

@@ -1236,6 +1236,9 @@ static void iavf_add_rx_frag(struct iavf_ring *rx_ring,
 	unsigned int truesize = SKB_DATA_ALIGN(size + iavf_rx_offset(rx_ring));
 #endif
 
+	if (!size)
+		return;
+
 	skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags, rx_buffer->page,
 			rx_buffer->page_offset, size, truesize);
 
@@ -1260,6 +1263,9 @@ static struct iavf_rx_buffer *iavf_get_rx_buffer(struct iavf_ring *rx_ring,
 {
 	struct iavf_rx_buffer *rx_buffer;
 
+	if (!size)
+		return NULL;
+
 	rx_buffer = &rx_ring->rx_bi[rx_ring->next_to_clean];
 	prefetchw(rx_buffer->page);
 
@@ -1299,6 +1305,8 @@ static struct sk_buff *iavf_construct_skb(struct iavf_ring *rx_ring,
 	unsigned int headlen;
 	struct sk_buff *skb;
 
+	if (!rx_buffer)
+		return NULL;
 	/* prefetch first cache line of first page */
 	prefetch(va);
 #if L1_CACHE_BYTES < 128
@@ -1363,6 +1371,8 @@ static struct sk_buff *iavf_build_skb(struct iavf_ring *rx_ring,
 #endif
 	struct sk_buff *skb;
 
+	if (!rx_buffer)
+		return NULL;
 	/* prefetch first cache line of first page */
 	prefetch(va);
 #if L1_CACHE_BYTES < 128
@@ -1398,6 +1408,9 @@ static struct sk_buff *iavf_build_skb(struct iavf_ring *rx_ring,
 static void iavf_put_rx_buffer(struct iavf_ring *rx_ring,
 			       struct iavf_rx_buffer *rx_buffer)
 {
+	if (!rx_buffer)
+		return;
+
 	if (iavf_can_reuse_rx_page(rx_buffer)) {
 		/* hand second half of page back to the ring */
 		iavf_reuse_rx_page(rx_ring, rx_buffer);
@@ -1496,11 +1509,12 @@ static int iavf_clean_rx_irq(struct iavf_ring *rx_ring, int budget)
 		 * verified the descriptor has been written back.
 		 */
 		dma_rmb();
+#define IAVF_RXD_DD BIT(IAVF_RX_DESC_STATUS_DD_SHIFT)
+		if (!iavf_test_staterr(rx_desc, IAVF_RXD_DD))
+			break;
 
 		size = (qword & IAVF_RXD_QW1_LENGTH_PBUF_MASK) >>
 		       IAVF_RXD_QW1_LENGTH_PBUF_SHIFT;
-		if (!size)
-			break;
 
 		iavf_trace(clean_rx_irq, rx_ring, rx_desc, skb);
 		rx_buffer = iavf_get_rx_buffer(rx_ring, size);
@@ -1516,7 +1530,8 @@ static int iavf_clean_rx_irq(struct iavf_ring *rx_ring, int budget)
 		/* exit if we failed to retrieve a buffer */
 		if (!skb) {
 			rx_ring->rx_stats.alloc_buff_failed++;
-			rx_buffer->pagecnt_bias++;
+			if (rx_buffer)
+				rx_buffer->pagecnt_bias++;
 			break;
 		}