net: macb: credit unsent frames to BQL in macb_tx_error_task()

The frames dropped on a TX error are never returned to BQL. Each error
shrinks the BQL budget for good until the queue stops with an empty
ring.

netdev_tx_reset_queue() is not safe here as the task runs without
tx_ptr_lock, so macb_start_xmit() may still be queueing.

Fixes: e6a5321 ("net: cadence: macb: Implement BQL")
Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de>

Author: nbuchwitz

drivers/net/ethernet/cadence/macb_main.c

@@ -1357,10 +1357,8 @@ static void macb_tx_error_task(struct work_struct *work)
 					    skb->data);
 				bp->dev->stats.tx_packets++;
 				queue->stats.tx_packets++;
-				packets++;
 				bp->dev->stats.tx_bytes += skb->len;
 				queue->stats.tx_bytes += skb->len;
-				bytes += skb->len;
 			}
 		} else {
 			/* "Buffers exhausted mid-frame" errors may only happen
@@ -1374,6 +1372,12 @@ static void macb_tx_error_task(struct work_struct *work)
 			desc->ctrl = ctrl | MACB_BIT(TX_USED);
 		}
 
+		/* BQL has to see every frame leaving the ring, sent or not */
+		if (skb) {
+			packets++;
+			bytes += skb->len;
+		}
+
 		macb_tx_unmap(bp, tx_skb, 0);
 	}