[PATCH net 1/4] net: ethernet: cortina: Fix MTU max setting

[Linus Walleij]

The RX max frame size is over 10000 for the Gemini ethernet,
but the TX max frame size is actually just 2047 (0x7ff after
checking the datasheet). Reflect this in what we offer to Linux,
cap the MTU at the TX max frame minus ethernet headers.

Use the BIT() macro for related bit flags so these TX settings
are consistent.

Fixes: 4d5ae32f5e1e ("net: ethernet: Add a driver for Gemini gigabit ethernet")
Signed-off-by: Linus Walleij <linus.walleij@xxxxxxxxxx>
---
 drivers/net/ethernet/cortina/gemini.c | 12 +++++++++---
 drivers/net/ethernet/cortina/gemini.h |  8 ++++----
 2 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/drivers/net/ethernet/cortina/gemini.c b/drivers/net/ethernet/cortina/gemini.c
index 5423fe26b4ef..e12d14359133 100644
--- a/drivers/net/ethernet/cortina/gemini.c
+++ b/drivers/net/ethernet/cortina/gemini.c
@@ -1151,6 +1151,11 @@ static int gmac_map_tx_bufs(struct net_device *netdev, struct sk_buff *skb,
 	if (skb->protocol == htons(ETH_P_8021Q))
 		mtu += VLAN_HLEN;
 
+	if (mtu > MTU_SIZE_BIT_MASK) {
+		netdev_err(netdev, "%s: MTU too big, max size 2047 bytes, capped\n", __func__);
+		mtu = MTU_SIZE_BIT_MASK;
+	}
+
 	word1 = skb->len;
 	word3 = SOF_BIT;
 
@@ -2464,11 +2469,12 @@ static int gemini_ethernet_port_probe(struct platform_device *pdev)
 
 	netdev->hw_features = GMAC_OFFLOAD_FEATURES;
 	netdev->features |= GMAC_OFFLOAD_FEATURES | NETIF_F_GRO;
-	/* We can handle jumbo frames up to 10236 bytes so, let's accept
-	 * payloads of 10236 bytes minus VLAN and ethernet header
+	/* We can receive jumbo frames up to 10236 bytes but only
+	 * transmit 2047 bytes so, let's accept payloads of 2047
+	 * bytes minus VLAN and ethernet header
 	 */
 	netdev->min_mtu = ETH_MIN_MTU;
-	netdev->max_mtu = 10236 - VLAN_ETH_HLEN;
+	netdev->max_mtu = MTU_SIZE_BIT_MASK - VLAN_ETH_HLEN;
 
 	port->freeq_refill = 0;
 	netif_napi_add(netdev, &port->napi, gmac_napi_poll);
diff --git a/drivers/net/ethernet/cortina/gemini.h b/drivers/net/ethernet/cortina/gemini.h
index 9fdf77d5eb37..d7101bfcb4a0 100644
--- a/drivers/net/ethernet/cortina/gemini.h
+++ b/drivers/net/ethernet/cortina/gemini.h
@@ -499,10 +499,10 @@ union gmac_txdesc_3 {
 };
 
 #define SOF_EOF_BIT_MASK	0x3fffffff
-#define SOF_BIT			0x80000000
-#define EOF_BIT			0x40000000
-#define EOFIE_BIT		BIT(29)
-#define MTU_SIZE_BIT_MASK	0x1fff
+#define SOF_BIT			BIT(31) /* Start of Frame */
+#define EOF_BIT			BIT(30) /* End of Frame */
+#define EOFIE_BIT		BIT(29) /* End of Frame Interrupt Enable */
+#define MTU_SIZE_BIT_MASK	0x7ff /* Max MTU 2047 bytes */
 
 /* GMAC Tx Descriptor */
 struct gmac_txdesc {

-- 
2.34.1