Certain i.MX SoCs, such as i.MX8QM and i.MX8QXP, feature enhanced
FEC hardware that supports Ethernet Jumbo frames with packet sizes
up to 16K bytes.

When Jumbo frames are supported, the TX FIFO may not be large enough
to hold an entire frame. To handle this, the FIFO is configured to
operate in cut-through mode when the frame size exceeds
(PKT_MAXBUF_SIZE - ETH_HLEN - ETH_FCS_LEN), which allows transmission
to begin once the FIFO reaches a certain threshold.

Signed-off-by: Shenwei Wang <shenwei.wang@nxp.com>
---
 drivers/net/ethernet/freescale/fec.h      |  3 +++
 drivers/net/ethernet/freescale/fec_main.c | 25 +++++++++++++++++++----
 2 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ethernet/freescale/fec.h b/drivers/net/ethernet/freescale/fec.h
index 0127cfa5529f..41e0d85d15da 100644
--- a/drivers/net/ethernet/freescale/fec.h
+++ b/drivers/net/ethernet/freescale/fec.h
@@ -514,6 +514,9 @@ struct bufdesc_ex {
  */
 #define FEC_QUIRK_HAS_MDIO_C45		BIT(24)
 
+/* Jumbo Frame support */
+#define FEC_QUIRK_JUMBO_FRAME		BIT(25)
+
 struct bufdesc_prop {
 	int qid;
 	/* Address of Rx and Tx buffers */
diff --git a/drivers/net/ethernet/freescale/fec_main.c b/drivers/net/ethernet/freescale/fec_main.c
index a2aa08afa4bd..329320395285 100644
--- a/drivers/net/ethernet/freescale/fec_main.c
+++ b/drivers/net/ethernet/freescale/fec_main.c
@@ -167,7 +167,8 @@ static const struct fec_devinfo fec_imx8qm_info = {
 		  FEC_QUIRK_ERR007885 | FEC_QUIRK_BUG_CAPTURE |
 		  FEC_QUIRK_HAS_RACC | FEC_QUIRK_HAS_COALESCE |
 		  FEC_QUIRK_CLEAR_SETUP_MII | FEC_QUIRK_HAS_MULTI_QUEUES |
-		  FEC_QUIRK_DELAYED_CLKS_SUPPORT | FEC_QUIRK_HAS_MDIO_C45,
+		  FEC_QUIRK_DELAYED_CLKS_SUPPORT | FEC_QUIRK_HAS_MDIO_C45 |
+		  FEC_QUIRK_JUMBO_FRAME,
 };
 
 static const struct fec_devinfo fec_s32v234_info = {
@@ -233,6 +234,7 @@ MODULE_PARM_DESC(macaddr, "FEC Ethernet MAC address");
  * 2048 byte skbufs are allocated. However, alignment requirements
  * varies between FEC variants. Worst case is 64, so round down by 64.
  */
+#define MAX_JUMBO_BUF_SIZE	(round_down(16384 - FEC_DRV_RESERVE_SPACE - 64, 64))
 #define PKT_MAXBUF_SIZE		(round_down(2048 - 64, 64))
 #define PKT_MINBUF_SIZE		64
 
@@ -1279,8 +1281,18 @@ fec_restart(struct net_device *ndev)
 	if (fep->quirks & FEC_QUIRK_ENET_MAC) {
 		/* enable ENET endian swap */
 		ecntl |= FEC_ECR_BYTESWP;
-		/* enable ENET store and forward mode */
-		writel(FEC_TXWMRK_STRFWD, fep->hwp + FEC_X_WMRK);
+
+		/* When Jumbo Frame is enabled, the FIFO may not be large enough
+		 * to hold an entire frame. In such cases, if the MTU exceeds
+		 * (PKT_MAXBUF_SIZE - ETH_HLEN - ETH_FCS_LEN), configure the interface
+		 * to operate in cut-through mode, triggered by the FIFO threshold.
+		 * Otherwise, enable the ENET store-and-forward mode.
+		 */
+		if ((fep->quirks & FEC_QUIRK_JUMBO_FRAME) &&
+		    (ndev->mtu > (PKT_MAXBUF_SIZE - ETH_HLEN - ETH_FCS_LEN)))
+			writel(0xF, fep->hwp + FEC_X_WMRK);
+		else
+			writel(FEC_TXWMRK_STRFWD, fep->hwp + FEC_X_WMRK);
 	}
 
 	if (fep->bufdesc_ex)
@@ -4581,7 +4593,12 @@ fec_probe(struct platform_device *pdev)
 
 	fep->pagepool_order = 0;
 	fep->rx_frame_size = FEC_ENET_RX_FRSIZE;
-	fep->max_buf_size = PKT_MAXBUF_SIZE;
+
+	if (fep->quirks & FEC_QUIRK_JUMBO_FRAME)
+		fep->max_buf_size = MAX_JUMBO_BUF_SIZE;
+	else
+		fep->max_buf_size = PKT_MAXBUF_SIZE;
+
 	ndev->max_mtu = fep->max_buf_size - ETH_HLEN - ETH_FCS_LEN;
 
 	ret = register_netdev(ndev);
-- 
2.43.0