[PATCH 6.18 204/227] net: fec: account for VLAN header in frame length calculations

Greg Kroah-Hartman
2026-01-28T16:02:32Z

From: Clemens Gruber <mail@clemensgruber.at>

commit ca1bb3fedf26a08ed31974131bc0064d4fe33649 upstream.

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@kernel.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@clemensgruber.at>
Reviewed-by: Wei Fang <wei.fang@nxp.com>
Link: https://patch.msgid.link/20260121083751.66997-1-mail@clemensgruber.at
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/net/ethernet/freescale/fec_main.c | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)

--- a/drivers/net/ethernet/freescale/fec_main.c
+++ b/drivers/net/ethernet/freescale/fec_main.c
@@ -1151,7 +1151,7 @@ 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);
@@ -1286,12 +1286,13 @@ 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);
@@ -4052,7 +4053,7 @@ static int fec_change_mtu(struct net_dev
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;
@@ -4609,7 +4610,7 @@ 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)