netdev - [RFC PATCH net-next 11/15] net/sched: taprio: warn about missing size table

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Message-Id: <20230128010719.2182346-12-vladimir.oltean@nxp.com>
Date:   Sat, 28 Jan 2023 03:07:15 +0200
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     netdev@...r.kernel.org
Cc:     Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        Kurt Kanzenbach <kurt@...utronix.de>
Subject: [RFC PATCH net-next 11/15] net/sched: taprio: warn about missing size table

Vinicius intended taprio to take the L1 overhead into account when
estimating packet transmission time through user input, specifically
through the qdisc size table (man tc-stab).

Something like this:

tc qdisc replace dev $eth root stab overhead 24 taprio \
	num_tc 8 \
	map 0 1 2 3 4 5 6 7 \
	queues 1@0 1@1 1@2 1@3 1@4 1@5 1@6 1@7 \
	base-time 0 \
	sched-entry S 0x7e 9000000 \
	sched-entry S 0x82 1000000 \
	max-sdu 0 0 0 0 0 0 0 200 \
	flags 0x0 clockid CLOCK_TAI

Without the overhead being specified, transmission times will be
underestimated and will cause late transmissions.

We can't make it mandatory, but we can warn the user with a netlink
extack.

Link: https://patchwork.kernel.org/project/netdevbpf/patch/20220505160357.298794-1-vladimir.oltean@nxp.com/
Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>
---
 net/sched/sch_taprio.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
index 8ec3c0e1f741..d50b2ffe32f6 100644
--- a/net/sched/sch_taprio.c
+++ b/net/sched/sch_taprio.c
@@ -1671,6 +1671,7 @@ static int taprio_new_flags(const struct nlattr *attr, u32 old,
 static int taprio_change(struct Qdisc *sch, struct nlattr *opt,
 			 struct netlink_ext_ack *extack)
 {
+	struct qdisc_size_table *stab = rtnl_dereference(sch->stab);
 	struct nlattr *tb[TCA_TAPRIO_ATTR_MAX + 1] = { };
 	struct sched_gate_list *oper, *admin, *new_admin;
 	struct taprio_sched *q = qdisc_priv(sch);
@@ -1823,6 +1824,10 @@ static int taprio_change(struct Qdisc *sch, struct nlattr *opt,
 	new_admin = NULL;
 	err = 0;
 
+	if (!stab)
+		NL_SET_ERR_MSG_MOD(extack,
+				   "Size table not specified, frame length estimations may be inaccurate");
+
 unlock:
 	spin_unlock_bh(qdisc_lock(sch));
 
-- 
2.34.1