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.

Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
---
 drivers/net/ethernet/cadence/macb_main.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
index 543356554c11..e10791bf1f4d 100644
--- a/drivers/net/ethernet/cadence/macb_main.c
+++ b/drivers/net/ethernet/cadence/macb_main.c
@@ -3438,11 +3438,25 @@ 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) {
+		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;
 }
 

-- 
2.53.0