netdev - [PATCH net-next mlxsw v1 02/10] mlxsw: spectrum

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Date:   Mon, 16 Dec 2019 17:01:42 +0000
From:   Petr Machata <petrm@...lanox.com>
To:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     Petr Machata <petrm@...lanox.com>,
        David Miller <davem@...emloft.net>,
        Roopa Prabhu <roopa@...ulusnetworks.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Roman Mashak <mrv@...atatu.com>,
        Ido Schimmel <idosch@...lanox.com>,
        Jiri Pirko <jiri@...nulli.us>, Jiri Pirko <jiri@...lanox.com>
Subject: [PATCH net-next mlxsw v1 02/10] mlxsw: spectrum_qdisc: Clarify a
 comment

Expand the comment at mlxsw_sp_qdisc_prio_graft() to make the problem that
this function is trying to handle clearer.

Signed-off-by: Petr Machata <petrm@...lanox.com>
Reviewed-by: Ido Schimmel <idosch@...lanox.com>
Acked-by: Jiri Pirko <jiri@...lanox.com>
---

Notes:
    v2 (internal):
    - s/coment/comment in the commit message.
    
    v1 (internal):
    - Expand the explanation with an explicit example.

 .../ethernet/mellanox/mlxsw/spectrum_qdisc.c  | 31 ++++++++++++++-----
 1 file changed, 24 insertions(+), 7 deletions(-)

diff --git a/drivers/net/ethernet/mellanox/mlxsw/spectrum_qdisc.c b/drivers/net/ethernet/mellanox/mlxsw/spectrum_qdisc.c
index 68cc6737d45c..135fef6c54b1 100644
--- a/drivers/net/ethernet/mellanox/mlxsw/spectrum_qdisc.c
+++ b/drivers/net/ethernet/mellanox/mlxsw/spectrum_qdisc.c
@@ -631,10 +631,30 @@ static struct mlxsw_sp_qdisc_ops mlxsw_sp_qdisc_ops_prio = {
 	.clean_stats = mlxsw_sp_setup_tc_qdisc_prio_clean_stats,
 };
 
-/* Grafting is not supported in mlxsw. It will result in un-offloading of the
- * grafted qdisc as well as the qdisc in the qdisc new location.
- * (However, if the graft is to the location where the qdisc is already at, it
- * will be ignored completely and won't cause un-offloading).
+/* Linux allows linking of Qdiscs to arbitrary classes (so long as the resulting
+ * graph is free of cycles). These operations do not change the parent handle
+ * though, which means it can be incomplete (if there is more than one class
+ * where the Qdisc in question is grafted) or outright wrong (if the Qdisc was
+ * linked to a different class and then removed from the original class).
+ *
+ * E.g. consider this sequence of operations:
+ *
+ *  # tc qdisc add dev swp1 root handle 1: prio
+ *  # tc qdisc add dev swp1 parent 1:3 handle 13: red limit 1000000 avpkt 10000
+ *  RED: set bandwidth to 10Mbit
+ *  # tc qdisc link dev swp1 handle 13: parent 1:2
+ *
+ * At this point, both 1:2 and 1:3 have the same RED Qdisc instance as their
+ * child. But RED will still only claim that 1:3 is its parent. If it's removed
+ * from that band, its only parent will be 1:2, but it will continue to claim
+ * that it is in fact 1:3.
+ *
+ * The notification for child Qdisc replace (e.g. TC_RED_REPLACE) comes before
+ * the notification for parent graft (e.g. TC_PRIO_GRAFT). We take the replace
+ * notification to offload the child Qdisc, based on its parent handle, and use
+ * the graft operation to validate that the class where the child is actually
+ * grafted corresponds to the parent handle. If the two don't match, we
+ * unoffload the child.
  */
 static int
 mlxsw_sp_qdisc_prio_graft(struct mlxsw_sp_port *mlxsw_sp_port,
@@ -644,9 +664,6 @@ mlxsw_sp_qdisc_prio_graft(struct mlxsw_sp_port *mlxsw_sp_port,
 	int tclass_num = MLXSW_SP_PRIO_BAND_TO_TCLASS(p->band);
 	struct mlxsw_sp_qdisc *old_qdisc;
 
-	/* Check if the grafted qdisc is already in its "new" location. If so -
-	 * nothing needs to be done.
-	 */
 	if (p->band < IEEE_8021QAZ_MAX_TCS &&
 	    mlxsw_sp_port->tclass_qdiscs[tclass_num].handle == p->child_handle)
 		return 0;
-- 
2.20.1