[PATCH net-next v2] net: stmmac: leave the datapath running for normal-size MTU changes

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 still use the existing reopen path.

Found by source review while working on the stmmac MTU/resume recovery
series. The fast path was tested on Allwinner H616 with sun8i EMAC1 and
an AC300 PHY. The AF_XDP scenario was identified by inspection.

Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>
Signed-off-by: James Hilliard <redacted>
---
Changes in v2:
- Target net-next and drop the Fixes tag as requested by Jakub Kicinski.
- Clarify the standalone commit message and how the issue was found.
- Add Nicolai Buchwitz's Reviewed-by.
- No code changes.
- Link to v1: https://patch.msgid.link/20260930-stmmac-mtu-fastpath-v1-1-fd73f378c071@gmail.com (local)
---
 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 56d89a696272..50eaee656cfb 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -6257,7 +6257,12 @@ 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: 05b5019b01fb9443b09ae4ed34af1135a80a84c8
change-id: 20260930-stmmac-mtu-fastpath-68813ead3b90

Best regards,
--  
James Hilliard [off-list ref]