net: bgmac: Pad packets to a minimum size

In preparation for enabling Broadcom tags with b53, pad packets to a
minimum size of 64 bytes (sans FCS) in order for the Broadcom switch to
accept ingressing frames. Without this, we would typically be able to
DHCP, but not resolve with ARP because packets are too small and get
rejected by the switch.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/broadcom/bgmac.c

@@ -15,6 +15,7 @@
 #include <linux/bcm47xx_nvram.h>
 #include <linux/phy.h>
 #include <linux/phy_fixed.h>
+#include <net/dsa.h>
 #include "bgmac.h"
 
 static bool bgmac_wait_value(struct bgmac *bgmac, u16 reg, u32 mask,
@@ -127,6 +128,8 @@ bgmac_dma_tx_add_buf(struct bgmac *bgmac, struct bgmac_dma_ring *ring,
 	dma_desc->ctl1 = cpu_to_le32(ctl1);
 }
 
+#define ENET_BRCM_TAG_LEN	4
+
 static netdev_tx_t bgmac_dma_tx_add(struct bgmac *bgmac,
 				    struct bgmac_dma_ring *ring,
 				    struct sk_buff *skb)
@@ -139,6 +142,18 @@ static netdev_tx_t bgmac_dma_tx_add(struct bgmac *bgmac,
 	u32 flags;
 	int i;
 
+	/* The Ethernet switch we are interfaced with needs packets to be at
+	 * least 64 bytes (including FCS) otherwise they will be discarded when
+	 * they enter the switch port logic. When Broadcom tags are enabled, we
+	 * need to make sure that packets are at least 68 bytes
+	 * (including FCS and tag) because the length verification is done after
+	 * the Broadcom tag is stripped off the ingress packet.
+	 */
+	if (netdev_uses_dsa(net_dev)) {
+		if (skb_put_padto(skb, ETH_ZLEN + ENET_BRCM_TAG_LEN))
+			goto err_stats;
+	}
+
 	if (skb->len > BGMAC_DESC_CTL1_LEN) {
 		netdev_err(bgmac->net_dev, "Too long skb (%d)\n", skb->len);
 		goto err_drop;
@@ -225,6 +240,7 @@ err_dma_head:
 
 err_drop:
 	dev_kfree_skb(skb);
+err_stats:
 	net_dev->stats.tx_dropped++;
 	net_dev->stats.tx_errors++;
 	return NETDEV_TX_OK;