net: vertexcom: mse102x: Fix RX error handling

In case the CMD_RTS got corrupted by interferences, the MSE102x
doesn't allow a retransmission of the command. Instead the Ethernet
frame must be shifted out of the SPI FIFO. Since the actual length is
unknown, assume the maximum possible value.

Fixes: 2f207cb ("net: vertexcom: Add MSE102x SPI support")
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250430133043.7722-5-wahrenst@gmx.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/vertexcom/mse102x.c

@@ -263,7 +263,7 @@ static int mse102x_tx_frame_spi(struct mse102x_net *mse, struct sk_buff *txp,
 }
 
 static int mse102x_rx_frame_spi(struct mse102x_net *mse, u8 *buff,
-				unsigned int frame_len)
+				unsigned int frame_len, bool drop)
 {
 	struct mse102x_net_spi *mses = to_mse102x_spi(mse);
 	struct spi_transfer *xfer = &mses->spi_xfer;
@@ -281,6 +281,9 @@ static int mse102x_rx_frame_spi(struct mse102x_net *mse, u8 *buff,
 		netdev_err(mse->ndev, "%s: spi_sync() failed: %d\n",
 			   __func__, ret);
 		mse->stats.xfer_err++;
+	} else if (drop) {
+		netdev_dbg(mse->ndev, "%s: Drop frame\n", __func__);
+		ret = -EINVAL;
 	} else if (*sof != cpu_to_be16(DET_SOF)) {
 		netdev_dbg(mse->ndev, "%s: SPI start of frame is invalid (0x%04x)\n",
 			   __func__, *sof);
@@ -308,6 +311,7 @@ static void mse102x_rx_pkt_spi(struct mse102x_net *mse)
 	struct sk_buff *skb;
 	unsigned int rxalign;
 	unsigned int rxlen;
+	bool drop = false;
 	__be16 rx = 0;
 	u16 cmd_resp;
 	u8 *rxpkt;
@@ -330,7 +334,8 @@ static void mse102x_rx_pkt_spi(struct mse102x_net *mse)
 			net_dbg_ratelimited("%s: Unexpected response (0x%04x)\n",
 					    __func__, cmd_resp);
 			mse->stats.invalid_rts++;
-			return;
+			drop = true;
+			goto drop;
 		}
 
 		net_dbg_ratelimited("%s: Unexpected response to first CMD\n",
@@ -342,9 +347,16 @@ static void mse102x_rx_pkt_spi(struct mse102x_net *mse)
 		net_dbg_ratelimited("%s: Invalid frame length: %d\n", __func__,
 				    rxlen);
 		mse->stats.invalid_len++;
-		return;
+		drop = true;
 	}
 
+	/* In case of a invalid CMD_RTS, the frame must be consumed anyway.
+	 * So assume the maximum possible frame length.
+	 */
+drop:
+	if (drop)
+		rxlen = VLAN_ETH_FRAME_LEN;
+
 	rxalign = ALIGN(rxlen + DET_SOF_LEN + DET_DFT_LEN, 4);
 	skb = netdev_alloc_skb_ip_align(mse->ndev, rxalign);
 	if (!skb)
@@ -355,7 +367,7 @@ static void mse102x_rx_pkt_spi(struct mse102x_net *mse)
 	 * They are copied, but ignored.
 	 */
 	rxpkt = skb_put(skb, rxlen) - DET_SOF_LEN;
-	if (mse102x_rx_frame_spi(mse, rxpkt, rxlen)) {
+	if (mse102x_rx_frame_spi(mse, rxpkt, rxlen, drop)) {
 		mse->ndev->stats.rx_errors++;
 		dev_kfree_skb(skb);
 		return;