[PATCH net v4 06/15] net: stmmac: leave the datapath running for normal-size MTU changes

[James Hilliard <james.hilliard1@gmail.com>]

Changing an MTU at or below ETH_DATA_LEN does not change the receive
buffer size or the MAC receive limit when the previous MTU was also in
that range. Do not release and reopen the datapath for those changes.

Besides avoiding unnecessary hardware resets and their failure paths,
this keeps live AF_XDP pool bindings intact. Preparing replacement rings
before stopping the old rings otherwise binds the pool to a temporary
RXQ and consumes fill-ring entries while its current RXQ is still
active. XDP already rejects jumbo MTUs, so all supported live XDP MTU
changes can use this path without preparing replacement rings.

Keep jumbo transitions on the existing reinitialization path for now.
The later ownership and rollback changes address that path separately.

Fixes: 3470079687448 ("net: ethernet: stmicro: stmmac: permit MTU change with interface up")
Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
---
 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 8b0bc7c46df9..bdebc5bfa2b8 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -6166,7 +6166,12 @@ 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);

-- 
2.53.0