[PATCH net 2/7] net: bcm4908_enet: drop a TX frame whose DMA mapping failed

["Kordian Chmielewski (mlody1212)" <kudzo51@gmail.com>]

When dma_map_single() fails, start_xmit() returns NETDEV_TX_BUSY after
it has already stored the skb in the slot. The stack requeues the same
frame and retries it at once, and the skb is left in a slot that never
gets a descriptor. A failed mapping is not something a retry fixes:
free the skb, count it as dropped and return NETDEV_TX_OK. Map the
buffer before the slot is touched.

Fixes: 4feffeadbcb2 ("net: broadcom: bcm4908enet: add BCM4908 controller driver")
Signed-off-by: Kordian Chmielewski (mlody1212) <kudzo51@gmail.com>
---
 drivers/net/ethernet/broadcom/bcm4908_enet.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/broadcom/bcm4908_enet.c b/drivers/net/ethernet/broadcom/bcm4908_enet.c
index 01a3e1d01..727ad1e96 100644
--- a/drivers/net/ethernet/broadcom/bcm4908_enet.c
+++ b/drivers/net/ethernet/broadcom/bcm4908_enet.c
@@ -535,6 +535,7 @@ static netdev_tx_t bcm4908_enet_start_xmit(struct sk_buff *skb, struct net_devic
 	struct bcm4908_enet_dma_ring_slot *slot;
 	struct device *dev = enet->dev;
 	struct bcm4908_enet_dma_ring_bd *buf_desc;
+	dma_addr_t dma_addr;
 	u32 tmp;
 
 	/* Free transmitted skbs */
@@ -554,12 +555,17 @@ static netdev_tx_t bcm4908_enet_start_xmit(struct sk_buff *skb, struct net_devic
 		return NETDEV_TX_BUSY;
 	}
 
+	dma_addr = dma_map_single(dev, skb->data, skb->len, DMA_TO_DEVICE);
+	if (unlikely(dma_mapping_error(dev, dma_addr))) {
+		dev_kfree_skb_any(skb);
+		netdev->stats.tx_dropped++;
+		return NETDEV_TX_OK;
+	}
+
 	slot = &ring->slots[ring->write_idx];
 	slot->skb = skb;
 	slot->len = skb->len;
-	slot->dma_addr = dma_map_single(dev, skb->data, skb->len, DMA_TO_DEVICE);
-	if (unlikely(dma_mapping_error(dev, slot->dma_addr)))
-		return NETDEV_TX_BUSY;
+	slot->dma_addr = dma_addr;
 
 	tmp = skb->len << DMA_CTL_LEN_DESC_BUFLENGTH_SHIFT;
 	tmp |= DMA_CTL_STATUS_OWN;
-- 
2.56.0