[PATCH net-next v2 04/10] net: ieee8021q: add tt_to_pcp()

[Oleksij Rempel]

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@xxxxxxxxxxxxxx>
Reviewed-by: Linus Walleij <linusw@xxxxxxxxxx>
---
Changes in v2:
- look the PCP up in the pcp_to_tt() table instead of a switch, so the
  two helpers cannot disagree (Luiz)
- check at build time that the table has one entry per traffic type
- kernel-doc: cite the encoding table, Table 6-2 (6.9.3), not the
  decoding Table I-7, and name the IEEE 802.1Q-2022 edition
- kernel-doc: refer to the enum doc for the Best Effort/Background
  order, and document each return value, including the stub's
- keep Linus's Reviewed-by: the mapping is unchanged
---
 include/net/ieee8021q.h      |  6 ++++++
 net/core/ieee8021q_helpers.c | 26 ++++++++++++++++++++++++++
 2 files changed, 32 insertions(+)

diff --git a/include/net/ieee8021q.h b/include/net/ieee8021q.h
index c6c8c94ec7fb39603f631d50928f2354bfcd8f81..c0cb6a8004da451fe2be835b9ec1f6fbef1779d5 100644
--- a/include/net/ieee8021q.h
+++ b/include/net/ieee8021q.h
@@ -42,6 +42,7 @@ 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
@@ -56,6 +57,11 @@ 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 8fd3ea87c7bcb6d7f34a4cf5969530633edd6abb..d9033398b0ec6320c4e0e2fa3f0913d96937d72e 100644
--- a/net/core/ieee8021q_helpers.c
+++ b/net/core/ieee8021q_helpers.c
@@ -245,6 +245,8 @@ static const u8 ieee8021q_pcp_to_tt_map[] = {
 
 static_assert(ARRAY_SIZE(ieee8021q_pcp_to_tt_map) ==
 	      (VLAN_PRIO_MASK >> VLAN_PRIO_SHIFT) + 1);
+/* ieee8021q_tt_to_pcp() needs one entry per traffic type. */
+static_assert(ARRAY_SIZE(ieee8021q_pcp_to_tt_map) == IEEE8021Q_TT_MAX);
 
 /**
  * ieee8021q_pcp_to_tt - Map an IEEE 802.1Q PCP to a traffic type
@@ -269,3 +271,27 @@ int ieee8021q_pcp_to_tt(u8 pcp)
 	return ieee8021q_pcp_to_tt_map[pcp];
 }
 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 to its priority per Table I-2 (IEEE 802.1Q-2022, I.4). Then
+ * encode the priority to its PCP per the 8P0D row of Table 6-2 (6.9.3). This
+ * is the inverse of ieee8021q_pcp_to_tt(). The enum values of Best Effort and
+ * Background are not their priorities, see &enum ieee8021q_traffic_type.
+ *
+ * Return: the PCP (0..7), or -EINVAL if @tt is not a traffic type. Without
+ * CONFIG_NET_IEEE8021Q_HELPERS, the stub returns -EOPNOTSUPP.
+ */
+int ieee8021q_tt_to_pcp(enum ieee8021q_traffic_type tt)
+{
+	int pcp;
+
+	for (pcp = 0; pcp < ARRAY_SIZE(ieee8021q_pcp_to_tt_map); pcp++)
+		if (ieee8021q_pcp_to_tt_map[pcp] == tt)
+			return pcp;
+
+	return -EINVAL;
+}
+EXPORT_SYMBOL_GPL(ieee8021q_tt_to_pcp);

-- 
2.47.3