[PATCH net] net: stmmac: leave the datapath running for normal-size MTU changes

[James Hilliard]

When both MTUs are at most ETH_DATA_LEN, the receive buffer size and MAC
receive limit do not change. Update the MTU without restarting the
datapath.

This also avoids rebinding a live AF_XDP pool to a temporary RXQ. XDP
rejects jumbo MTUs, so all supported XDP MTU changes take this path.
Jumbo transitions remain on the reopen path until the rollback change.

Fixes: 3470079687448 ("net: ethernet: stmicro: stmmac: permit MTU change with interface up")
Signed-off-by: James Hilliard <james.hilliard1@xxxxxxxxx>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index 3f34d491c959..03cd14d10356 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -6161,7 +6161,12 @@ static int stmmac_change_mtu(struct net_device *dev, int new_mtu)
 	if ((txfifosz < new_mtu) || (new_mtu > BUF_SIZE_16KiB))
 		return -EINVAL;
 
-	if (netif_running(dev)) {
+	/* Normal-size frames use the same buffers and MAC receive limits.
+	 * In particular, do not disturb a live AF_XDP pool: XDP does not
+	 * support jumbo frames, so it never needs the ring replacement below.
+	 */
+	if (netif_running(dev) &&
+	    (dev->mtu > ETH_DATA_LEN || mtu > ETH_DATA_LEN)) {
 		netdev_dbg(priv->dev, "restarting interface to change its MTU\n");
 		/* Try to allocate the new DMA conf with the new mtu */
 		dma_conf = stmmac_setup_dma_desc(priv, mtu);

---
base-commit: a7bfaba4823e3c165bb2004c74eff7c096672bc7
change-id: 20260930-stmmac-mtu-fastpath-68813ead3b90

Best regards,
--  
James Hilliard <james.hilliard1@xxxxxxxxx>