Re: [PATCH net-next v1 2/8] net: ieee8021q: add pcp_to_tt()

Add a helper that maps an IEEE 802.1Q Priority Code Point (PCP) to the
corresponding 802.1Q traffic type, complementing the existing
ietf_dscp_to_ieee8021q_tt().

It is very nearly an identity map, except that PCP 0 (Best Effort) and
PCP 1 (Background) are swapped: per 802.1Q-2022 (Annex I, Table I-2)
Background is a lower priority than Best Effort even though it carries
the higher PCP value. PCP 2..7 map straight through to the matching
traffic type, and values above 7 are rejected with -EINVAL. This decodes
the 8P0D encoding (Table I-7), where every PCP value is a priority and
none carries drop eligibility.

Drivers that classify ingress traffic by PCP need this to map into a
priority-ordered internal space without misordering Best Effort and
Background. The first consumer, the realtek rtl8365mb DSA driver, lands
later in this series.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
---
 include/net/ieee8021q.h      | 10 +++++++--
 net/core/ieee8021q_helpers.c | 41 ++++++++++++++++++++++++++++++++++++
 2 files changed, 49 insertions(+), 2 deletions(-)

diff --git a/include/net/ieee8021q.h b/include/net/ieee8021q.h
index 8bfe903dd3d0..d24e4036577b 100644
--- a/include/net/ieee8021q.h
+++ b/include/net/ieee8021q.h

@@ -10,8 +10,8 @@
  * enum ieee8021q_traffic_type - 802.1Q traffic type priority values (802.1Q-2022)
  *
  * @IEEE8021Q_TT_BK: Background
- * @IEEE8021Q_TT_BE: Best Effort (default). According to 802.1Q-2022, BE is 0
- * but has higher priority than BK which is 1.
+ * @IEEE8021Q_TT_BE: Best Effort (default). According to 802.1Q-2022 (Annex I,
+ * Table I-2), BE is 0 but has higher priority than BK which is 1.
  * @IEEE8021Q_TT_EE: Excellent Effort
  * @IEEE8021Q_TT_CA: Critical Applications
  * @IEEE8021Q_TT_VI: Video, < 100 ms latency and jitter

@@ -38,6 +38,7 @@ enum ieee8021q_traffic_type {
 #if IS_ENABLED(CONFIG_NET_IEEE8021Q_HELPERS)

 int ietf_dscp_to_ieee8021q_tt(u8 dscp);
+int ieee8021q_pcp_to_tt(u8 pcp);
 int ieee8021q_tt_to_tc(enum ieee8021q_traffic_type tt, unsigned int num_queues);

 #else

@@ -47,6 +48,11 @@ static inline int ietf_dscp_to_ieee8021q_tt(u8 dscp)
        return -EOPNOTSUPP;
 }

+static inline int ieee8021q_pcp_to_tt(u8 pcp)
+{
+       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 bbacdfff9b4c..a5bd5e039b47 100644
--- a/net/core/ieee8021q_helpers.c
+++ b/net/core/ieee8021q_helpers.c

@@ -222,3 +222,44 @@ int ietf_dscp_to_ieee8021q_tt(u8 dscp)
        return SIMPLE_IETF_DSCP_TO_IEEE8021Q_TT(dscp);
 }
 EXPORT_SYMBOL_GPL(ietf_dscp_to_ieee8021q_tt);
+
+/**
+ * ieee8021q_pcp_to_tt - Map an IEEE 802.1Q PCP to a traffic type
+ * @pcp: IEEE 802.1Q Priority Code Point value
+ *
+ * Decode @pcp to its IEEE 802.1Q traffic type per Table I-7 ("Priority Code
+ * Point decoding") for the 8P0D encoding, where all eight PCP values are
+ * priorities and none carries drop eligibility. It is nearly the identity,
+ * except PCP 0 (Best Effort) and PCP 1 (Background) map to swapped traffic
+ * types: Background is lower priority than Best Effort despite the higher PCP
+ * value (Table I-2: BE = priority 0, BK = priority 1).
+ *
+ * The 7P1D/6P2D/5P3D encodings, whose lower PCP values carry drop eligibility,
+ * are the other rows of Table I-7 and are not decoded here.
+ *
+ * Return: the traffic type, or a negative value on error.
+ */
+int ieee8021q_pcp_to_tt(u8 pcp)
+{
+       switch (pcp) {
+       case 0:
+               return IEEE8021Q_TT_BE;
+       case 1:
+               return IEEE8021Q_TT_BK;
+       case 2:
+               return IEEE8021Q_TT_EE;
+       case 3:
+               return IEEE8021Q_TT_CA;
+       case 4:
+               return IEEE8021Q_TT_VI;
+       case 5:
+               return IEEE8021Q_TT_VO;
+       case 6:
+               return IEEE8021Q_TT_IC;
+       case 7:
+               return IEEE8021Q_TT_NC;
+       }
+
+       return -EINVAL;