[PATCH v4 net-next] net: ftmac100: support mtu > 1500

[Sergei Antonov <saproj@gmail.com>]

The ftmac100 controller considers some packets FTL (frame
too long) and drops them. An example of a dropped packet:
6 bytes - dst MAC
6 bytes - src MAC
2 bytes - EtherType IPv4 (0800)
1504 bytes - IPv4 packet

Do the following to let the driver receive these packets.
Set FTMAC100_MACCR_RX_FTL when mtu>1500 in the MAC Control Register.
For received packets marked with FTMAC100_RXDES0_FTL check if packet
length (with FCS excluded) is within expected limits, that is not
greater than netdev->mtu + 14 (Ethernet headers). Otherwise trigger
an error.

Fixes: 8d77c036b57c ("net: add Faraday FTMAC100 10/100 Ethernet driver")
Signed-off-by: Sergei Antonov <saproj@gmail.com>
Suggested-by: Vladimir Oltean <olteanv@gmail.com>
---
 drivers/net/ethernet/faraday/ftmac100.c | 29 ++++++++++++++++++++++---
 1 file changed, 26 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/faraday/ftmac100.c b/drivers/net/ethernet/faraday/ftmac100.c
index d95d78230828..f89b53845f21 100644
--- a/drivers/net/ethernet/faraday/ftmac100.c
+++ b/drivers/net/ethernet/faraday/ftmac100.c
@@ -159,6 +159,7 @@ static void ftmac100_set_mac(struct ftmac100 *priv, const unsigned char *mac)
 static int ftmac100_start_hw(struct ftmac100 *priv)
 {
 	struct net_device *netdev = priv->netdev;
+	unsigned int maccr;
 
 	if (ftmac100_reset(priv))
 		return -EIO;
@@ -175,7 +176,20 @@ static int ftmac100_start_hw(struct ftmac100 *priv)
 
 	ftmac100_set_mac(priv, netdev->dev_addr);
 
-	iowrite32(MACCR_ENABLE_ALL, priv->base + FTMAC100_OFFSET_MACCR);
+	maccr = MACCR_ENABLE_ALL;
+
+	/* We have to set FTMAC100_MACCR_RX_FTL in case MTU > 1500
+	 * and do extra length check in ftmac100_rx_packet_error().
+	 * Otherwise the controller silently drops these packets.
+	 *
+	 * When the MTU of the interface is standard 1500, rely on
+	 * the controller's functionality to drop too long packets
+	 * and save some CPU time.
+	 */
+	if (netdev->mtu > 1500)
+		maccr |= FTMAC100_MACCR_RX_FTL;
+
+	iowrite32(maccr, priv->base + FTMAC100_OFFSET_MACCR);
 	return 0;
 }
 
@@ -337,9 +351,18 @@ static bool ftmac100_rx_packet_error(struct ftmac100 *priv,
 		error = true;
 	}
 
-	if (unlikely(ftmac100_rxdes_frame_too_long(rxdes))) {
+	/* If the frame-too-long flag FTMAC100_RXDES0_FTL is set, check
+	 * if ftmac100_rxdes_frame_length(rxdes) exceeds the currently
+	 * set MTU plus ETH_HLEN.
+	 * The controller would set FTMAC100_RXDES0_FTL for all incoming
+	 * frames longer than 1518 (includeing FCS) in the presense of
+	 * FTMAC100_MACCR_RX_FTL in the MAC Control Register.
+	 */
+	if (unlikely(ftmac100_rxdes_frame_too_long(rxdes) &&
+		     ftmac100_rxdes_frame_length(rxdes) > netdev->mtu + ETH_HLEN)) {
 		if (net_ratelimit())
-			netdev_info(netdev, "rx frame too long\n");
+			netdev_info(netdev, "rx frame too long (%u)\n",
+				    ftmac100_rxdes_frame_length(rxdes));
 
 		netdev->stats.rx_length_errors++;
 		error = true;
-- 
2.34.1