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mlxsw: spectrum_qdisc: Clarify a comment

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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@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Petr Machata 2019-12-18 14:55:10 +00:00 committed by David S. Miller
parent 9586a992fb
commit 5bc146c90e
1 changed files with 24 additions and 7 deletions
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31
drivers/net/ethernet/mellanox/mlxsw/spectrum_qdisc.c
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@ -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;