[PATCH v3 net] net: ftmac100: support frames with DSA tag

[Sergei Antonov <saproj@gmail.com>]

Fixes the problem when frames coming from DSA were discarded.
DSA tag might make frame size >1518. Such frames are discarded
by the controller when FTMAC100_MACCR_RX_FTL is not set in the
MAC Control Register, see datasheet [1].

Set FTMAC100_MACCR_RX_FTL 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. In the presence of DSA netdev->mtu is 1504 to accommodate
for VLAN tag.

[1]
https://bitbucket.org/Kasreyn/mkrom-uc7112lx/src/master/documents/FIC8120_DS_v1.2.pdf

Fixes: 8d77c036b57c ("net: add Faraday FTMAC100 10/100 Ethernet driver")
Signed-off-by: Sergei Antonov <saproj@gmail.com>
Suggested-by: Andrew Lunn <andrew@lunn.ch>
---

v2 -> v3:
* Corrected the explanation of the problem: datasheet is correct.
* Rewrote the code to use the currently set mtu to handle DSA frames.

v1 -> v2:
* Typos in description fixed.

 drivers/net/ethernet/faraday/ftmac100.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/faraday/ftmac100.c b/drivers/net/ethernet/faraday/ftmac100.c
index d95d78230828..9187331e83dc 100644
--- a/drivers/net/ethernet/faraday/ftmac100.c
+++ b/drivers/net/ethernet/faraday/ftmac100.c
@@ -154,6 +154,7 @@ static void ftmac100_set_mac(struct ftmac100 *priv, const unsigned char *mac)
 				 FTMAC100_MACCR_CRC_APD	| \
 				 FTMAC100_MACCR_FULLDUP	| \
 				 FTMAC100_MACCR_RX_RUNT	| \
+				 FTMAC100_MACCR_RX_FTL	| \
 				 FTMAC100_MACCR_RX_BROADPKT)
 
 static int ftmac100_start_hw(struct ftmac100 *priv)
@@ -337,9 +338,18 @@ 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