net: fec: make rx skb handling more robust

Allocate, and then map the receive skb before writing any data to the
ring descriptor or storing the skb.  When freeing the receive ring
entries, unmap and free the skb, and then clear the stored skb pointer.

This means we have ring data and skb pointer in one of two states:
either both fully setup, or nothing setup.

This simplifies the cleanup, as we can use just the skb pointer to
indicate whether the descriptor is setup, and thus avoids potentially
calling dma_unmap_single() on a DMA error value.
Acked-by: NFugang Duan <B38611@freescale.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/freescale/fec_main.c

@@ -2066,12 +2066,12 @@ static void fec_enet_free_buffers(struct net_device *ndev)
 	bdp = fep->rx_bd_base;
 	for (i = 0; i < fep->rx_ring_size; i++) {
 		skb = fep->rx_skbuff[i];
-
-		if (bdp->cbd_bufaddr)
+		fep->rx_skbuff[i] = NULL;
+		if (skb) {
 			dma_unmap_single(&fep->pdev->dev, bdp->cbd_bufaddr,
 					FEC_ENET_RX_FRSIZE, DMA_FROM_DEVICE);
-		if (skb)
 			dev_kfree_skb(skb);
+		}
 		bdp = fec_enet_get_nextdesc(bdp, fep);
 	}
 
@@ -2089,21 +2089,26 @@ static int fec_enet_alloc_buffers(struct net_device *ndev)
 
 	bdp = fep->rx_bd_base;
 	for (i = 0; i < fep->rx_ring_size; i++) {
+		dma_addr_t addr;
+
 		skb = netdev_alloc_skb(ndev, FEC_ENET_RX_FRSIZE);
 		if (!skb) {
 			fec_enet_free_buffers(ndev);
 			return -ENOMEM;
 		}
-		fep->rx_skbuff[i] = skb;
 
-		bdp->cbd_bufaddr = dma_map_single(&fep->pdev->dev, skb->data,
+		addr = dma_map_single(&fep->pdev->dev, skb->data,
 				FEC_ENET_RX_FRSIZE, DMA_FROM_DEVICE);
-		if (dma_mapping_error(&fep->pdev->dev, bdp->cbd_bufaddr)) {
+		if (dma_mapping_error(&fep->pdev->dev, addr)) {
+			dev_kfree_skb(skb);
 			fec_enet_free_buffers(ndev);
 			if (net_ratelimit())
 				netdev_err(ndev, "Rx DMA memory map failed\n");
 			return -ENOMEM;
 		}
+
+		fep->rx_skbuff[i] = skb;
+		bdp->cbd_bufaddr = addr;
 		bdp->cbd_sc = BD_ENET_RX_EMPTY;
 
 		if (fep->bufdesc_ex) {