[PATCH net v2 2/5] net: ethernet: mtk_eth_soc: work around issue with sending small fragments

[Felix Fietkau <nbd@nbd.name>]

When lots of frames are sent with a number of very small fragments, an
internal FIFO can overflow, causing the DMA engine to lock up lock up and
transmit attempts time out.

Fix this on MT7986 by increasing the reserved FIFO space.
Fix this on older chips by detecting the presence of small fragments and use
skb_gso_segment + skb_linearize to deal with them.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
---
v2: add proper fix for MT7986, limit workaround to NETSYS v1

 drivers/net/ethernet/mediatek/mtk_eth_soc.c | 38 +++++++++++++++++++--
 drivers/net/ethernet/mediatek/mtk_eth_soc.h |  2 +-
 2 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/mediatek/mtk_eth_soc.c b/drivers/net/ethernet/mediatek/mtk_eth_soc.c
index e3de9a53b2d9..8245cddccb5a 100644
--- a/drivers/net/ethernet/mediatek/mtk_eth_soc.c
+++ b/drivers/net/ethernet/mediatek/mtk_eth_soc.c
@@ -1441,12 +1441,28 @@ static void mtk_wake_queue(struct mtk_eth *eth)
 	}
 }
 
+static bool mtk_skb_has_small_frag(struct sk_buff *skb)
+{
+	int min_size = 16;
+	int i;
+
+	if (skb_headlen(skb) < min_size)
+		return true;
+
+	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)
+		if (skb_frag_size(&skb_shinfo(skb)->frags[i]) < min_size)
+			return true;
+
+	return false;
+}
+
 static netdev_tx_t mtk_start_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct mtk_mac *mac = netdev_priv(dev);
 	struct mtk_eth *eth = mac->hw;
 	struct mtk_tx_ring *ring = &eth->tx_ring;
 	struct net_device_stats *stats = &dev->stats;
+	struct sk_buff *segs, *next;
 	bool gso = false;
 	int tx_num;
 
@@ -1468,6 +1484,18 @@ static netdev_tx_t mtk_start_xmit(struct sk_buff *skb, struct net_device *dev)
 		return NETDEV_TX_BUSY;
 	}
 
+	if (!MTK_HAS_CAPS(eth->soc->caps, MTK_NETSYS_V2) &&
+	    skb_is_gso(skb) && mtk_skb_has_small_frag(skb)) {
+		segs = skb_gso_segment(skb, dev->features & ~NETIF_F_ALL_TSO);
+		if (IS_ERR(segs))
+			goto drop;
+
+		if (segs) {
+			consume_skb(skb);
+			skb = segs;
+		}
+	}
+
 	/* TSO: fill MSS info in tcp checksum field */
 	if (skb_is_gso(skb)) {
 		if (skb_cow_head(skb, 0)) {
@@ -1483,8 +1511,14 @@ static netdev_tx_t mtk_start_xmit(struct sk_buff *skb, struct net_device *dev)
 		}
 	}
 
-	if (mtk_tx_map(skb, dev, tx_num, ring, gso) < 0)
-		goto drop;
+	if (!MTK_HAS_CAPS(eth->soc->caps, MTK_NETSYS_V2))
+		skb_list_walk_safe(skb, skb, next) {
+			if ((mtk_skb_has_small_frag(skb) && skb_linearize(skb)) ||
+			    mtk_tx_map(skb, dev, tx_num, ring, gso) < 0) {
+				stats->tx_dropped++;
+				dev_kfree_skb_any(skb);
+			}
+		}
 
 	if (unlikely(atomic_read(&ring->free_count) <= ring->thresh))
 		netif_tx_stop_all_queues(dev);
diff --git a/drivers/net/ethernet/mediatek/mtk_eth_soc.h b/drivers/net/ethernet/mediatek/mtk_eth_soc.h
index 628ed72911bd..ce8044e8b3f7 100644
--- a/drivers/net/ethernet/mediatek/mtk_eth_soc.h
+++ b/drivers/net/ethernet/mediatek/mtk_eth_soc.h
@@ -246,7 +246,7 @@
 #define MTK_CHK_DDONE_EN	BIT(28)
 #define MTK_DMAD_WR_WDONE	BIT(26)
 #define MTK_WCOMP_EN		BIT(24)
-#define MTK_RESV_BUF		(0x40 << 16)
+#define MTK_RESV_BUF		(0x80 << 16)
 #define MTK_MUTLI_CNT		(0x4 << 12)
 #define MTK_LEAKY_BUCKET_EN	BIT(11)
 
-- 
2.38.1