[PATCH net-next v1 4/8] net: ieee8021q: add tt_to_pcp()

[Oleksij Rempel <o.rempel@pengutronix.de>]

Add ieee8021q_tt_to_pcp(), the inverse of ieee8021q_pcp_to_tt(): it
encodes an 802.1Q traffic type back to its Priority Code Point, and
completes the pcp/tt helper family.

A driver that reseeds its PCP classifier through ieee8021q_pcp_to_tt()
stores traffic types in the hardware tables, while the dcbnl priority
ops exchange 802.1p priorities across the DSA boundary. Reading such a
table back therefore needs the reverse mapping to restore the 802.1p
value, which no existing helper provides. Because pcp_to_tt only
transposes Best Effort and Background and is the identity elsewhere the
two directions are numerically equal; provide tt_to_pcp as its own named
function so the read path does not decode a traffic type as if it were a
PCP.

The first user is the Realtek rtl8365mb DSA driver, whose dcbnl
get_default_prio and get_dscp_prio paths read back traffic types and
must return 802.1p priorities.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
---
 include/net/ieee8021q.h      |  6 ++++++
 net/core/ieee8021q_helpers.c | 40 ++++++++++++++++++++++++++++++++++++
 2 files changed, 46 insertions(+)

diff --git a/include/net/ieee8021q.h b/include/net/ieee8021q.h
index d24e4036577b..b09572da0289 100644
--- a/include/net/ieee8021q.h
+++ b/include/net/ieee8021q.h
@@ -39,6 +39,7 @@ enum ieee8021q_traffic_type {
 
 int ietf_dscp_to_ieee8021q_tt(u8 dscp);
 int ieee8021q_pcp_to_tt(u8 pcp);
+int ieee8021q_tt_to_pcp(enum ieee8021q_traffic_type tt);
 int ieee8021q_tt_to_tc(enum ieee8021q_traffic_type tt, unsigned int num_queues);
 
 #else
@@ -53,6 +54,11 @@ static inline int ieee8021q_pcp_to_tt(u8 pcp)
 	return -EOPNOTSUPP;
 }
 
+static inline int ieee8021q_tt_to_pcp(enum ieee8021q_traffic_type tt)
+{
+	return -EOPNOTSUPP;
+}
+
 static inline int ieee8021q_tt_to_tc(enum ieee8021q_traffic_type tt,
 				     unsigned int num_queues)
 {
diff --git a/net/core/ieee8021q_helpers.c b/net/core/ieee8021q_helpers.c
index 520d86d592eb..205c11793473 100644
--- a/net/core/ieee8021q_helpers.c
+++ b/net/core/ieee8021q_helpers.c
@@ -267,3 +267,43 @@ int ieee8021q_pcp_to_tt(u8 pcp)
 	return -EINVAL;
 }
 EXPORT_SYMBOL_GPL(ieee8021q_pcp_to_tt);
+
+/**
+ * ieee8021q_tt_to_pcp - Map an IEEE 802.1Q traffic type to a PCP
+ * @tt: IEEE 802.1Q traffic type
+ *
+ * Encode @tt back to its IEEE 802.1Q Priority Code Point per Table I-7
+ * ("Priority Code Point decoding") for the 8P0D encoding, the inverse of
+ * ieee8021q_pcp_to_tt(). It is nearly the identity, except traffic types Best
+ * Effort and Background map to swapped PCP values: Best Effort is the default
+ * (PCP 0) despite being higher priority than Background (PCP 1) (Table I-2:
+ * BE = priority 0, BK = priority 1).
+ *
+ * Return: the PCP value, or a negative value on error.
+ */
+int ieee8021q_tt_to_pcp(enum ieee8021q_traffic_type tt)
+{
+	switch (tt) {
+	case IEEE8021Q_TT_BK:
+		return 1;
+	case IEEE8021Q_TT_BE:
+		return 0;
+	case IEEE8021Q_TT_EE:
+		return 2;
+	case IEEE8021Q_TT_CA:
+		return 3;
+	case IEEE8021Q_TT_VI:
+		return 4;
+	case IEEE8021Q_TT_VO:
+		return 5;
+	case IEEE8021Q_TT_IC:
+		return 6;
+	case IEEE8021Q_TT_NC:
+		return 7;
+	case IEEE8021Q_TT_MAX:
+		break;
+	}
+
+	return -EINVAL;
+}
+EXPORT_SYMBOL_GPL(ieee8021q_tt_to_pcp);
-- 
2.47.3