[PATCH net v2 3/3] net: lan966x: Fix FDMA when MTU is changed

[Horatiu Vultur <horatiu.vultur@microchip.com>]

When MTU is changed, FDMA is required to calculate what is the maximum
size of the frame that it can received. So it can calculate what is the
page order needed to allocate for the received frames.
The first problem was that, when the max MTU was calculated it was
reading the value from dev and not from HW, so in this way it was
missing L2 header + the FCS.
The other problem was that once the skb is created using
__build_skb_around, it would reserve some space for skb_shared_info.
So if we received a frame which size is at the limit of the page order
then the creating will failed because it would not have space to put all
the data.

Fixes: 2ea1cbac267e ("net: lan966x: Update FDMA to change MTU.")
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
---
 drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c | 8 ++++++--
 drivers/net/ethernet/microchip/lan966x/lan966x_main.c | 2 +-
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c b/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c
index a42035cec611c..c235edd2b182a 100644
--- a/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c
+++ b/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c
@@ -668,12 +668,14 @@ static int lan966x_fdma_get_max_mtu(struct lan966x *lan966x)
 	int i;
 
 	for (i = 0; i < lan966x->num_phys_ports; ++i) {
+		struct lan966x_port *port;
 		int mtu;
 
-		if (!lan966x->ports[i])
+		port = lan966x->ports[i];
+		if (!port)
 			continue;
 
-		mtu = lan966x->ports[i]->dev->mtu;
+		mtu = lan_rd(lan966x, DEV_MAC_MAXLEN_CFG(port->chip_port));
 		if (mtu > max_mtu)
 			max_mtu = mtu;
 	}
@@ -733,6 +735,8 @@ int lan966x_fdma_change_mtu(struct lan966x *lan966x)
 
 	max_mtu = lan966x_fdma_get_max_mtu(lan966x);
 	max_mtu += IFH_LEN * sizeof(u32);
+	max_mtu += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
+	max_mtu += VLAN_HLEN * 2;
 
 	if (round_up(max_mtu, PAGE_SIZE) / PAGE_SIZE - 1 ==
 	    lan966x->rx.page_order)
diff --git a/drivers/net/ethernet/microchip/lan966x/lan966x_main.c b/drivers/net/ethernet/microchip/lan966x/lan966x_main.c
index b3070c3fcad0a..20ee5b28f70a5 100644
--- a/drivers/net/ethernet/microchip/lan966x/lan966x_main.c
+++ b/drivers/net/ethernet/microchip/lan966x/lan966x_main.c
@@ -395,7 +395,7 @@ static int lan966x_port_change_mtu(struct net_device *dev, int new_mtu)
 
 	err = lan966x_fdma_change_mtu(lan966x);
 	if (err) {
-		lan_wr(DEV_MAC_MAXLEN_CFG_MAX_LEN_SET(old_mtu),
+		lan_wr(DEV_MAC_MAXLEN_CFG_MAX_LEN_SET(LAN966X_HW_MTU(old_mtu)),
 		       lan966x, DEV_MAC_MAXLEN_CFG(port->chip_port));
 		dev->mtu = old_mtu;
 	}
-- 
2.38.0