mlxsw: spectrum_qdisc: Promote backlog reduction to mlxsw_sp_qdisc_destroy()

When a qdisc is removed, it is necessary to update the backlog value at its
parent--unless the qdisc is at root position. RED, TBF and FIFO all do
that, each separately. Since all of them need to do this, just promote the
operation directly to mlxsw_sp_qdisc_destroy(), instead of deferring it to
individual destructors. Since FIFO dtor thus becomes trivial, remove it.

Add struct mlxsw_sp_qdisc.parent to point at the parent qdisc. This will be
handy later as deeper structures are offloaded. Use the parent qdisc to
find the chain of parents whose backlog value needs to be updated.
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -65,6 +65,7 @@ struct mlxsw_sp_qdisc {
 	} stats_base;
 
 	struct mlxsw_sp_qdisc_ops *ops;
+	struct mlxsw_sp_qdisc *parent;
 };
 
 struct mlxsw_sp_qdisc_state {
@@ -132,6 +133,15 @@ mlxsw_sp_qdisc_find_by_handle(struct mlxsw_sp_port *mlxsw_sp_port, u32 handle)
 	return NULL;
 }
 
+static void
+mlxsw_sp_qdisc_reduce_parent_backlog(struct mlxsw_sp_qdisc *mlxsw_sp_qdisc)
+{
+	struct mlxsw_sp_qdisc *tmp;
+
+	for (tmp = mlxsw_sp_qdisc->parent; tmp; tmp = tmp->parent)
+		tmp->stats_base.backlog -= mlxsw_sp_qdisc->stats_base.backlog;
+}
+
 static int
 mlxsw_sp_qdisc_destroy(struct mlxsw_sp_port *mlxsw_sp_port,
 		       struct mlxsw_sp_qdisc *mlxsw_sp_qdisc)
@@ -153,7 +163,11 @@ mlxsw_sp_qdisc_destroy(struct mlxsw_sp_port *mlxsw_sp_port,
 		err_hdroom = mlxsw_sp_hdroom_configure(mlxsw_sp_port, &hdroom);
 	}
 
-	if (mlxsw_sp_qdisc->ops && mlxsw_sp_qdisc->ops->destroy)
+	if (!mlxsw_sp_qdisc->ops)
+		return 0;
+
+	mlxsw_sp_qdisc_reduce_parent_backlog(mlxsw_sp_qdisc);
+	if (mlxsw_sp_qdisc->ops->destroy)
 		err = mlxsw_sp_qdisc->ops->destroy(mlxsw_sp_port,
 						   mlxsw_sp_qdisc);
 
@@ -417,13 +431,6 @@ static int
 mlxsw_sp_qdisc_red_destroy(struct mlxsw_sp_port *mlxsw_sp_port,
 			   struct mlxsw_sp_qdisc *mlxsw_sp_qdisc)
 {
-	struct mlxsw_sp_qdisc_state *qdisc_state = mlxsw_sp_port->qdisc;
-	struct mlxsw_sp_qdisc *root_qdisc = &qdisc_state->root_qdisc;
-
-	if (root_qdisc != mlxsw_sp_qdisc)
-		root_qdisc->stats_base.backlog -=
-					mlxsw_sp_qdisc->stats_base.backlog;
-
 	return mlxsw_sp_tclass_congestion_disable(mlxsw_sp_port,
 						  mlxsw_sp_qdisc->tclass_num);
 }
@@ -616,13 +623,6 @@ static int
 mlxsw_sp_qdisc_tbf_destroy(struct mlxsw_sp_port *mlxsw_sp_port,
 			   struct mlxsw_sp_qdisc *mlxsw_sp_qdisc)
 {
-	struct mlxsw_sp_qdisc_state *qdisc_state = mlxsw_sp_port->qdisc;
-	struct mlxsw_sp_qdisc *root_qdisc = &qdisc_state->root_qdisc;
-
-	if (root_qdisc != mlxsw_sp_qdisc)
-		root_qdisc->stats_base.backlog -=
-					mlxsw_sp_qdisc->stats_base.backlog;
-
 	return mlxsw_sp_port_ets_maxrate_set(mlxsw_sp_port,
 					     MLXSW_REG_QEEC_HR_SUBGROUP,
 					     mlxsw_sp_qdisc->tclass_num, 0,
@@ -790,19 +790,6 @@ int mlxsw_sp_setup_tc_tbf(struct mlxsw_sp_port *mlxsw_sp_port,
 	}
 }
 
-static int
-mlxsw_sp_qdisc_fifo_destroy(struct mlxsw_sp_port *mlxsw_sp_port,
-			    struct mlxsw_sp_qdisc *mlxsw_sp_qdisc)
-{
-	struct mlxsw_sp_qdisc_state *qdisc_state = mlxsw_sp_port->qdisc;
-	struct mlxsw_sp_qdisc *root_qdisc = &qdisc_state->root_qdisc;
-
-	if (root_qdisc != mlxsw_sp_qdisc)
-		root_qdisc->stats_base.backlog -=
-					mlxsw_sp_qdisc->stats_base.backlog;
-	return 0;
-}
-
 static int
 mlxsw_sp_qdisc_fifo_check_params(struct mlxsw_sp_port *mlxsw_sp_port,
 				 void *params)
@@ -832,7 +819,6 @@ static struct mlxsw_sp_qdisc_ops mlxsw_sp_qdisc_ops_fifo = {
 	.type = MLXSW_SP_QDISC_FIFO,
 	.check_params = mlxsw_sp_qdisc_fifo_check_params,
 	.replace = mlxsw_sp_qdisc_fifo_replace,
-	.destroy = mlxsw_sp_qdisc_fifo_destroy,
 	.get_stats = mlxsw_sp_qdisc_get_fifo_stats,
 	.clean_stats = mlxsw_sp_setup_tc_qdisc_leaf_clean_stats,
 };
@@ -1825,8 +1811,10 @@ int mlxsw_sp_tc_qdisc_init(struct mlxsw_sp_port *mlxsw_sp_port)
 
 	qdisc_state->root_qdisc.prio_bitmap = 0xff;
 	qdisc_state->root_qdisc.tclass_num = MLXSW_SP_PORT_DEFAULT_TCLASS;
-	for (i = 0; i < IEEE_8021QAZ_MAX_TCS; i++)
+	for (i = 0; i < IEEE_8021QAZ_MAX_TCS; i++) {
 		qdisc_state->tclass_qdiscs[i].tclass_num = i;
+		qdisc_state->tclass_qdiscs[i].parent = &qdisc_state->root_qdisc;
+	}
 
 	mlxsw_sp_port->qdisc = qdisc_state;
 	return 0;