Use newly introduced context buffer management to implement
.ndo_change_mtu() as a context swap: allocate new context ->
reconfigure HW -> free old context.

This resists memory pressure well by failing without closing the
interface and it is much faster by avoiding PHY reinit.

AT91 EMAC is handled differently as their buffer management is separate
and they don't do NAPI. We refuse them (-EBUSY) to avoid implementing
context swapping for them.

Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
---
 drivers/net/ethernet/cadence/macb_main.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/cadence/macb_main.c 

b/drivers/net/ethernet/cadence/macb_main.c
index 5792647eb0a6..1de66f442c59 100644

--- a/drivers/net/ethernet/cadence/macb_main.c
+++ b/drivers/net/ethernet/cadence/macb_main.c

@@ -3493,11 +3493,29 @@ static int macb_close(struct net_device 

*netdev)

 static int macb_change_mtu(struct net_device *netdev, int new_mtu)
 {
-	if (netif_running(netdev))
-		return -EBUSY;
+	struct macb *bp = netdev_priv(netdev);
+	bool running = netif_running(netdev);
+	struct macb_context *new_ctx;
+
+	if (running) {
+		/* Context swapping is not supported for AT91. */
+		if (bp->caps & MACB_CAPS_MACB_IS_EMAC)
+			return -EBUSY;
+
+		new_ctx = macb_context_alloc(bp, new_mtu,
+					     bp->configured_rx_ring_size,
+					     bp->configured_tx_ring_size);
+		if (IS_ERR(new_ctx))
+			return PTR_ERR(new_ctx);
+
+		macb_context_swap_start(bp);
+	}

 	WRITE_ONCE(netdev->mtu, new_mtu);

+	if (running)
+		macb_context_swap_end(bp, new_ctx);
+
 	return 0;
 }