net/mlx4_en: Fix selftest for small MTUs

Set the minimal MTU threshold for running loopback selftest.
MTU should be big enough to include packet payload, NET_IP_ALIGN,
Ethernet headers and preamble length.

Fixes: e7c1c2c4 ("mlx4_en: Added self diagnostics test implementation")
Signed-off-by: NEugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: NTariq Toukan <tariqt@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx4/en_selftest.c

@@ -185,7 +185,7 @@ void mlx4_en_ex_selftest(struct net_device *dev, u32 *flags, u64 *buf)
 		if (priv->mdev->dev->caps.flags &
 					MLX4_DEV_CAP_FLAG_UC_LOOPBACK) {
 			buf[3] = mlx4_en_test_registers(priv);
-			if (priv->port_up)
+			if (priv->port_up && dev->mtu >= MLX4_SELFTEST_LB_MIN_MTU)
 				buf[4] = mlx4_en_test_loopback(priv);
 		}
 

drivers/net/ethernet/mellanox/mlx4/mlx4_en.h

@@ -153,6 +153,9 @@
 #define SMALL_PACKET_SIZE      (256 - NET_IP_ALIGN)
 #define HEADER_COPY_SIZE       (128 - NET_IP_ALIGN)
 #define MLX4_LOOPBACK_TEST_PAYLOAD (HEADER_COPY_SIZE - ETH_HLEN)
+#define PREAMBLE_LEN           8
+#define MLX4_SELFTEST_LB_MIN_MTU (MLX4_LOOPBACK_TEST_PAYLOAD + NET_IP_ALIGN + \
+				  ETH_HLEN + PREAMBLE_LEN)
 
 #define MLX4_EN_MIN_MTU		46
 /* VLAN_HLEN is added twice,to support skb vlan tagged with multiple