Add+Fix software padding processing in Ethernet Tx path.

Add software padding processing in Ethernet Tx path.

It's found that too-short packets would lead to
switch Tx CRC error, followed by switch output
queue stuck issue. So Ethernet driver should check
if the packet is too short and conduct software
padding when necessary.

If without this patch, switch might encounter output
queue stuck issue.

Change-Id: Ibd94cbf3be0530d2b9ee61477b0362d099c47d8e
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/7109899

Fix software padding processing in Ethernet Tx path.

It's found that 20bytes(LLC), 28bytes, and 36bytes(PPP LCP Configuration
Ack) packets with additional 4bytes special tag for dsa driver would lead
to switch Tx CRC error, followed by switch output queue stuck issue.
Therefore, Ethernet driver check skb->len <= 40(36bytes PPP LCP + 4bytes
special tag) and conduct software padding.

Moreover, skb linearization should be checked again after conduct
software padding.

If without this patch, users might encounter problems when establishing
a PPPoE tunnel.

Change-Id: Id28fa3870dc9e905207d95634fe5cd692f363761
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/9129634

frank-w: changed condition to MTK_MIN_TX_LENGTH

drivers/net/ethernet/mediatek/mtk_eth_soc.c

@@ -1565,6 +1565,14 @@ static int mtk_tx_map(struct sk_buff *skb, struct net_device *dev,
 	int queue = skb_get_queue_mapping(skb);
 	int k = 0;
 
+	if (skb->len <= MTK_MIN_TX_LENGTH) {
+		if (skb_put_padto(skb, MTK_MIN_TX_LENGTH))
+			return -ENOMEM;
+
+		txd_info.last = !skb_is_nonlinear(skb);
+		txd_info.size = skb_headlen(skb);
+	}
+
 	txq = netdev_get_tx_queue(dev, queue);
 	itxd = ring->next_free;
 	itxd_pdma = qdma_to_pdma(ring, itxd);

drivers/net/ethernet/mediatek/mtk_eth_soc.h

@@ -32,6 +32,7 @@
 #define MTK_MAX_RX_LENGTH_2K	2048
 #define MTK_TX_DMA_BUF_LEN	0x3fff
 #define MTK_TX_DMA_BUF_LEN_V2	0xffff
+#define MTK_MIN_TX_LENGTH	60
 #define MTK_QDMA_RING_SIZE	2048
 #define MTK_DMA_SIZE(x)		(SZ_##x)
 #define MTK_FQ_DMA_HEAD		32