ipv4: igmp: get rid of IGMPV3_{QQIC,MRC} and simplify calculation

Get rid of the IGMPV3_MRC macro and use the igmpv3_mrt() API to
calculate the Max Resp Time from the Maximum Response Code.

Similarly, for IGMPV3_QQIC, use the igmpv3_qqi() API to calculate
the Querier's Query Interval from the QQIC field.

Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Ujjal Roy <royujjal@gmail.com>
Link: https://patch.msgid.link/20260502131907.987-2-royujjal@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 72 insertions, 8 deletions

diff --git a/include/linux/igmp.h b/include/linux/igmp.h
index 073b30a9b850..0e095c9f234e 100644
--- a/include/linux/igmp.h
+++ b/include/linux/igmp.h
@@ -92,15 +92,79 @@ struct ip_mc_list {
 	struct rcu_head		rcu;
 };
 
+/* RFC3376, relevant sections:
+ *  - 4.1.1. Maximum Response Code
+ *  - 4.1.7. QQIC (Querier's Query Interval Code)
+ *
+ * For both MRC and QQIC, values >= 128 use the same floating-point
+ * encoding as follows:
+ *
+ *  0 1 2 3 4 5 6 7
+ * +-+-+-+-+-+-+-+-+
+ * |1| exp | mant  |
+ * +-+-+-+-+-+-+-+-+
+ */
+#define IGMPV3_FP_EXP(value)		(((value) >> 4) & 0x07)
+#define IGMPV3_FP_MAN(value)		((value) & 0x0f)
+
+/* IGMPv3 floating-point exponential field min threshold */
+#define IGMPV3_EXP_MIN_THRESHOLD	128
+
 /* V3 exponential field decoding */
-#define IGMPV3_MASK(value, nb) ((nb)>=32 ? (value) : ((1<<(nb))-1) & (value))
-#define IGMPV3_EXP(thresh, nbmant, nbexp, value) \
-	((value) < (thresh) ? (value) : \
-        ((IGMPV3_MASK(value, nbmant) | (1<<(nbmant))) << \
-         (IGMPV3_MASK((value) >> (nbmant), nbexp) + (nbexp))))
-
-#define IGMPV3_QQIC(value) IGMPV3_EXP(0x80, 4, 3, value)
-#define IGMPV3_MRC(value) IGMPV3_EXP(0x80, 4, 3, value)
+
+/* IGMPv3 MRC/QQIC 8-bit exponential field decode
+ *
+ * RFC3376, 4.1.1 & 4.1.7. defines the decoding formula:
+ *      0 1 2 3 4 5 6 7
+ *     +-+-+-+-+-+-+-+-+
+ *     |1| exp | mant  |
+ *     +-+-+-+-+-+-+-+-+
+ * Max Resp Time = (mant | 0x10) << (exp + 3)
+ * QQI = (mant | 0x10) << (exp + 3)
+ */
+static inline unsigned long igmpv3_exp_field_decode(const u8 code)
+{
+	if (code < IGMPV3_EXP_MIN_THRESHOLD) {
+		return code;
+	} else {
+		unsigned long mc_man, mc_exp;
+
+		mc_exp = IGMPV3_FP_EXP(code);
+		mc_man = IGMPV3_FP_MAN(code);
+
+		return (mc_man | 0x10) << (mc_exp + 3);
+	}
+}
+
+/* Calculate Max Resp Time from Maximum Response Code
+ *
+ * RFC3376, relevant sections:
+ *  - 4.1.1. Maximum Response Code
+ *  - 8.3. Query Response Interval
+ *
+ * After decode, MRC represents the Maximum Response Time (MRT) in
+ * units of 0.1 seconds (100 ms).
+ */
+static inline unsigned long igmpv3_mrt(const struct igmpv3_query *ih3)
+{
+	return igmpv3_exp_field_decode(ih3->code);
+}
+
+/* Calculate Querier's Query Interval from Querier's Query Interval Code
+ *
+ * RFC3376, relevant sections:
+ *  - 4.1.7. QQIC (Querier's Query Interval Code)
+ *  - 8.2. Query Interval
+ *  - 8.12. Older Version Querier Present Timeout
+ *    (the [Query Interval] in the last Query received)
+ *
+ * After decode, QQIC represents the Querier's Query Interval in units
+ * of seconds.
+ */
+static inline unsigned long igmpv3_qqi(const struct igmpv3_query *ih3)
+{
+	return igmpv3_exp_field_decode(ih3->qqic);
+}
 
 static inline int ip_mc_may_pull(struct sk_buff *skb, unsigned int len)
 {