mlxsw: Adjust MTU value to hardware check

Ethernet frame consists of - Ethernet header, payload, FCS. The MTU value
which is used by user is the size of the payload, which means that when
user sets MTU to X, the total frame size will be larger due to the addition
of the Ethernet header and FCS.

Spectrum ASICs take into account Ethernet header and FCS as part of packet
size for MTU check. Adjust MTU value when user sets MTU, to configure the
MTU size which is required by hardware. The Tx header length which was used
by the driver is not relevant for such calculation, take into account
Ethernet header (with VLAN extension) and FCS.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/f3203c2477bb8ed18b1e79642fa3e3713e1e55bb.1718275854.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/mellanox/mlxsw/port.h

@@ -7,6 +7,7 @@
 #include <linux/types.h>
 
 #define MLXSW_PORT_MAX_MTU		(10 * 1024)
+#define MLXSW_PORT_ETH_FRAME_HDR	(ETH_HLEN + VLAN_HLEN + ETH_FCS_LEN)
 
 #define MLXSW_PORT_DEFAULT_VID		1
 

drivers/net/ethernet/mellanox/mlxsw/spectrum.c

@@ -425,7 +425,7 @@ static int mlxsw_sp_port_mtu_set(struct mlxsw_sp_port *mlxsw_sp_port, u16 mtu)
 	struct mlxsw_sp *mlxsw_sp = mlxsw_sp_port->mlxsw_sp;
 	char pmtu_pl[MLXSW_REG_PMTU_LEN];
 
-	mtu += MLXSW_TXHDR_LEN + ETH_HLEN;
+	mtu += MLXSW_PORT_ETH_FRAME_HDR;
 	if (mtu > mlxsw_sp_port->max_mtu)
 		return -EINVAL;