net: systemport: rewrite bcm_sysport_rx_refill

Currently, bcm_sysport_desc_rx() calls bcm_sysport_rx_refill() at the end of Rx
packet processing loop, after the current Rx packet has already been passed to
napi_gro_receive(). However, bcm_sysport_rx_refill() might fail to allocate a new
Rx skb, thus leaving a hole on the Rx queue where no valid Rx buffer exists.

To eliminate this situation:

1. Rewrite bcm_sysport_rx_refill() to retain the current Rx skb on the
Rx queue if a new replacement Rx skb can't be allocated and DMA-mapped.
In this case, the data on the current Rx skb is effectively dropped.

2. Modify bcm_sysport_desc_rx() to call bcm_sysport_rx_refill() at the
top of Rx packet processing loop, so that the new replacement Rx skb is
already in place before the current Rx skb is processed.

This is loosely inspired from d6707bec5986 ("net: bcmgenet: rewrite
bcmgenet_rx_refill()")

Signed-off-by: Florian Fainelli <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/ethernet/broadcom/bcmsysport.c

···
 	dma_unmap_addr_set(cb, dma_addr, 0);
 }
 
-static int bcm_sysport_rx_refill(struct bcm_sysport_priv *priv,
-				 struct bcm_sysport_cb *cb)
+static struct sk_buff *bcm_sysport_rx_refill(struct bcm_sysport_priv *priv,
+					     struct bcm_sysport_cb *cb)
 {
 	struct device *kdev = &priv->pdev->dev;
 	struct net_device *ndev = priv->netdev;
+	struct sk_buff *skb, *rx_skb;
 	dma_addr_t mapping;
-	int ret;
 
-	cb->skb = netdev_alloc_skb(priv->netdev, RX_BUF_LENGTH);
-	if (!cb->skb) {
+	/* Allocate a new SKB for a new packet */
+	skb = netdev_alloc_skb(priv->netdev, RX_BUF_LENGTH);
+	if (!skb) {
+		priv->mib.alloc_rx_buff_failed++;
 		netif_err(priv, rx_err, ndev, "SKB alloc failed\n");
-		return -ENOMEM;
+		return NULL;
 	}
 
-	mapping = dma_map_single(kdev, cb->skb->data,
+	mapping = dma_map_single(kdev, skb->data,
 				 RX_BUF_LENGTH, DMA_FROM_DEVICE);
-	ret = dma_mapping_error(kdev, mapping);
-	if (ret) {
+	if (dma_mapping_error(kdev, mapping)) {
 		priv->mib.rx_dma_failed++;
-		bcm_sysport_free_cb(cb);
+		dev_kfree_skb_any(skb);
 		netif_err(priv, rx_err, ndev, "DMA mapping failure\n");
-		return ret;
+		return NULL;
 	}
 
+	/* Grab the current SKB on the ring */
+	rx_skb = cb->skb;
+	if (likely(rx_skb))
+		dma_unmap_single(kdev, dma_unmap_addr(cb, dma_addr),
+				 RX_BUF_LENGTH, DMA_FROM_DEVICE);
+
+	/* Put the new SKB on the ring */
+	cb->skb = skb;
 	dma_unmap_addr_set(cb, dma_addr, mapping);
 	dma_desc_set_addr(priv, cb->bd_addr, mapping);
 
 	netif_dbg(priv, rx_status, ndev, "RX refill\n");
 
-	return 0;
+	/* Return the current SKB to the caller */
+	return rx_skb;
 }
 
 static int bcm_sysport_alloc_rx_bufs(struct bcm_sysport_priv *priv)
 {
 	struct bcm_sysport_cb *cb;
-	int ret = 0;
+	struct sk_buff *skb;
 	unsigned int i;
 
 	for (i = 0; i < priv->num_rx_bds; i++) {
 		cb = &priv->rx_cbs[i];
-		if (cb->skb)
-			continue;
-
-		ret = bcm_sysport_rx_refill(priv, cb);
-		if (ret)
-			break;
+		skb = bcm_sysport_rx_refill(priv, cb);
+		if (skb)
+			dev_kfree_skb(skb);
+		if (!cb->skb)
+			return -ENOMEM;
 	}
 
-	return ret;
+	return 0;
 }
 
 /* Poll the hardware for up to budget packets to process */
 static unsigned int bcm_sysport_desc_rx(struct bcm_sysport_priv *priv,
 					unsigned int budget)
 {
-	struct device *kdev = &priv->pdev->dev;
 	struct net_device *ndev = priv->netdev;
 	unsigned int processed = 0, to_process;
 	struct bcm_sysport_cb *cb;
···
 	unsigned int p_index;
 	u16 len, status;
 	struct bcm_rsb *rsb;
-	int ret;
 
 	/* Determine how much we should process since last call */
 	p_index = rdma_readl(priv, RDMA_PROD_INDEX);
···
 
 	while ((processed < to_process) && (processed < budget)) {
 		cb = &priv->rx_cbs[priv->rx_read_ptr];
-		skb = cb->skb;
+		skb = bcm_sysport_rx_refill(priv, cb);
 
-		processed++;
-		priv->rx_read_ptr++;
-
-		if (priv->rx_read_ptr == priv->num_rx_bds)
-			priv->rx_read_ptr = 0;
 
 		/* We do not have a backing SKB, so we do not a corresponding
 		 * DMA mapping for this incoming packet since
···
 			netif_err(priv, rx_err, ndev, "out of memory!\n");
 			ndev->stats.rx_dropped++;
 			ndev->stats.rx_errors++;
-			goto refill;
+			goto next;
 		}
-
-		dma_unmap_single(kdev, dma_unmap_addr(cb, dma_addr),
-				 RX_BUF_LENGTH, DMA_FROM_DEVICE);
 
 		/* Extract the Receive Status Block prepended */
 		rsb = (struct bcm_rsb *)skb->data;
···
 			netif_err(priv, rx_status, ndev, "fragmented packet!\n");
 			ndev->stats.rx_dropped++;
 			ndev->stats.rx_errors++;
-			bcm_sysport_free_cb(cb);
-			goto refill;
+			dev_kfree_skb_any(skb);
+			goto next;
 		}
 
 		if (unlikely(status & (RX_STATUS_ERR | RX_STATUS_OVFLOW))) {
···
 				ndev->stats.rx_over_errors++;
 			ndev->stats.rx_dropped++;
 			ndev->stats.rx_errors++;
-			bcm_sysport_free_cb(cb);
-			goto refill;
+			dev_kfree_skb_any(skb);
+			goto next;
 		}
 
 		skb_put(skb, len);
···
 		ndev->stats.rx_bytes += len;
 
 		napi_gro_receive(&priv->napi, skb);
-refill:
-		ret = bcm_sysport_rx_refill(priv, cb);
-		if (ret)
-			priv->mib.alloc_rx_buff_failed++;
+next:
+		processed++;
+		priv->rx_read_ptr++;
+
+		if (priv->rx_read_ptr == priv->num_rx_bds)
+			priv->rx_read_ptr = 0;
 	}
 
 	return processed;