netdev - [PATCH v2] net: fec: account for VLAN header in frame length calculations

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Message-ID: <20260121083751.66997-1-mail@clemensgruber.at>
Date: Wed, 21 Jan 2026 09:37:51 +0100
From: Clemens Gruber <mail@...mensgruber.at>
To: wei.fang@....com,
	andrew+netdev@...n.ch,
	davem@...emloft.net,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com
Cc: shenwei.wang@....com,
	xiaoning.wang@....com,
	imx@...ts.linux.dev,
	netdev@...r.kernel.org,
	Clemens Gruber <mail@...mensgruber.at>,
	stable@...nel.org
Subject: [PATCH v2] net: fec: account for VLAN header in frame length calculations

The MAX_FL (maximum frame length) and related calculations used ETH_HLEN,
which does not account for the 4-byte VLAN tag in tagged frames. This
caused the hardware to reject valid VLAN frames as oversized, resulting
in RX errors and dropped packets.

Use VLAN_ETH_HLEN instead of ETH_HLEN in the MAX_FL register setup,
cut-through mode threshold, buffer allocation, and max_mtu calculation.

Cc: stable@...nel.org # v6.18+
Fixes: 62b5bb7be7bc ("net: fec: update MAX_FL based on the current MTU")
Fixes: d466c16026e9 ("net: fec: enable the Jumbo frame support for i.MX8QM")
Fixes: 59e9bf037d75 ("net: fec: add change_mtu to support dynamic buffer allocation")
Fixes: ec2a1681ed4f ("net: fec: use a member variable for maximum buffer size")
Signed-off-by: Clemens Gruber <mail@...mensgruber.at>
---
Changes in v2:
- Fix the other frame length calculations too
- Improved title and commit message

One line in fec_restart (after the FEC_QUIRK_JUMBO_FRAME check) has now
82 characters but I think folding it would make it less readable.
Let me know if this should be changed.
---
 drivers/net/ethernet/freescale/fec_main.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/drivers/net/ethernet/freescale/fec_main.c b/drivers/net/ethernet/freescale/fec_main.c
index a753265961af..797ef6899657 100644
--- a/drivers/net/ethernet/freescale/fec_main.c
+++ b/drivers/net/ethernet/freescale/fec_main.c
@@ -1150,7 +1150,7 @@ fec_restart(struct net_device *ndev)
 	u32 rcntl = FEC_RCR_MII;
 
 	if (OPT_ARCH_HAS_MAX_FL)
-		rcntl |= (fep->netdev->mtu + ETH_HLEN + ETH_FCS_LEN) << 16;
+		rcntl |= (fep->netdev->mtu + VLAN_ETH_HLEN + ETH_FCS_LEN) << 16;
 
 	if (fep->bufdesc_ex)
 		fec_ptp_save_state(fep);
@@ -1285,12 +1285,13 @@ fec_restart(struct net_device *ndev)
 
 		/* 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.
+		 * (PKT_MAXBUF_SIZE - VLAN_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)))
+		    (ndev->mtu > (PKT_MAXBUF_SIZE - VLAN_ETH_HLEN - ETH_FCS_LEN)))
 			writel(0xF, fep->hwp + FEC_X_WMRK);
 		else
 			writel(FEC_TXWMRK_STRFWD, fep->hwp + FEC_X_WMRK);
@@ -4037,7 +4038,7 @@ static int fec_change_mtu(struct net_device *ndev, int new_mtu)
 	if (netif_running(ndev))
 		return -EBUSY;
 
-	order = get_order(new_mtu + ETH_HLEN + ETH_FCS_LEN
+	order = get_order(new_mtu + VLAN_ETH_HLEN + ETH_FCS_LEN
 			  + FEC_DRV_RESERVE_SPACE);
 	fep->rx_frame_size = (PAGE_SIZE << order) - FEC_DRV_RESERVE_SPACE;
 	fep->pagepool_order = order;
@@ -4588,7 +4589,7 @@ fec_probe(struct platform_device *pdev)
 	else
 		fep->max_buf_size = PKT_MAXBUF_SIZE;
 
-	ndev->max_mtu = fep->max_buf_size - ETH_HLEN - ETH_FCS_LEN;
+	ndev->max_mtu = fep->max_buf_size - VLAN_ETH_HLEN - ETH_FCS_LEN;
 
 	ret = register_netdev(ndev);
 	if (ret)
-- 
2.52.0