mlxsw: spectrum_buffers: Update port headroom configuration

The recommendation for headroom size for 100Gbps port and 100m cable is
101.6KB, reduced accordingly for split ports. The closest higher number
evenly divisible by cell size for both Spectrum-1 and Spectrum-2, and
such that the number of cells can be further divided by maximum split
factor of 4, is 102528 bytes, or 25632 bytes per lane.

Update mlxsw_sp_port_pb_init() to compute the headroom taking into
account this recommended per-lane value and number of lanes actually
dedicated to a given port.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -260,24 +260,24 @@ static int mlxsw_sp_sb_pm_occ_query(struct mlxsw_sp *mlxsw_sp, u8 local_port,
 				     (unsigned long) pm);
 }
 
-static const u16 mlxsw_sp_pbs[] = {
-	[0] = 2 * ETH_FRAME_LEN,
-	[9] = 2 * MLXSW_PORT_MAX_MTU,
-};
-
-#define MLXSW_SP_PBS_LEN ARRAY_SIZE(mlxsw_sp_pbs)
+/* 1/4 of a headroom necessary for 100Gbps port and 100m cable. */
+#define MLXSW_SP_PB_HEADROOM 25632
 #define MLXSW_SP_PB_UNUSED 8
 
 static int mlxsw_sp_port_pb_init(struct mlxsw_sp_port *mlxsw_sp_port)
 {
+	const u32 pbs[] = {
+		[0] = MLXSW_SP_PB_HEADROOM * mlxsw_sp_port->mapping.width,
+		[9] = 2 * MLXSW_PORT_MAX_MTU,
+	};
 	struct mlxsw_sp *mlxsw_sp = mlxsw_sp_port->mlxsw_sp;
 	char pbmc_pl[MLXSW_REG_PBMC_LEN];
 	int i;
 
 	mlxsw_reg_pbmc_pack(pbmc_pl, mlxsw_sp_port->local_port,
 			    0xffff, 0xffff / 2);
-	for (i = 0; i < MLXSW_SP_PBS_LEN; i++) {
-		u16 size = mlxsw_sp_bytes_cells(mlxsw_sp, mlxsw_sp_pbs[i]);
+	for (i = 0; i < ARRAY_SIZE(pbs); i++) {
+		u16 size = mlxsw_sp_bytes_cells(mlxsw_sp, pbs[i]);
 
 		if (i == MLXSW_SP_PB_UNUSED)
 			continue;