mt76: fix tx skb error handling in mt76_dma_tx_queue_skb

When running out of room in the tx queue after calling drv->tx_prepare_skb,
the buffer list will already have been modified on MT7615 and newer drivers.
This can leak a DMA mapping and will show up as swiotlb allocation failures
on x86.

Fix this by moving the queue length check further up. This is less accurate,
since it can overestimate the needed room in the queue on MT7615 and newer,
but the difference is small enough to not matter in practice.
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210216135119.23809-1-nbd@nbd.name

drivers/net/wireless/mediatek/mt76/dma.c

@@ -345,7 +345,6 @@ mt76_dma_tx_queue_skb(struct mt76_dev *dev, struct mt76_queue *q,
 	};
 	struct ieee80211_hw *hw;
 	int len, n = 0, ret = -ENOMEM;
-	struct mt76_queue_entry e;
 	struct mt76_txwi_cache *t;
 	struct sk_buff *iter;
 	dma_addr_t addr;
@@ -387,6 +386,11 @@ mt76_dma_tx_queue_skb(struct mt76_dev *dev, struct mt76_queue *q,
 	}
 	tx_info.nbuf = n;
 
+	if (q->queued + (tx_info.nbuf + 1) / 2 >= q->ndesc - 1) {
+		ret = -ENOMEM;
+		goto unmap;
+	}
+
 	dma_sync_single_for_cpu(dev->dev, t->dma_addr, dev->drv->txwi_size,
 				DMA_TO_DEVICE);
 	ret = dev->drv->tx_prepare_skb(dev, txwi, q->qid, wcid, sta, &tx_info);
@@ -395,11 +399,6 @@ mt76_dma_tx_queue_skb(struct mt76_dev *dev, struct mt76_queue *q,
 	if (ret < 0)
 		goto unmap;
 
-	if (q->queued + (tx_info.nbuf + 1) / 2 >= q->ndesc - 1) {
-		ret = -ENOMEM;
-		goto unmap;
-	}
-
 	return mt76_dma_add_buf(dev, q, tx_info.buf, tx_info.nbuf,
 				tx_info.info, tx_info.skb, t);
 
@@ -419,9 +418,7 @@ mt76_dma_tx_queue_skb(struct mt76_dev *dev, struct mt76_queue *q,
 	}
 #endif
 
-	e.skb = tx_info.skb;
-	e.txwi = t;
-	dev->drv->tx_complete_skb(dev, &e);
+	dev_kfree_skb(tx_info.skb);
 	mt76_put_txwi(dev, t);
 	return ret;
 }