Subject: [PATCH net-next v2 1/5] ipv4: igmp: get rid of IGMPV3_{QQIC,MRC} and simplify calculation

From: Ujjal Roy <[email-masked]>
Date: Mon, 30 Mar 2026 19:16:07 +0000

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.

Signed-off-by: Ujjal Roy <[email-masked]>
---
 include/linux/igmp.h      | 78 +++++++++++++++++++++++++++++++++++----
 net/bridge/br_multicast.c |  2 +-
 net/ipv4/igmp.c           |  6 +--
 3 files changed, 74 insertions(+), 12 deletions(-)

diff --git a/include/linux/igmp.h b/include/linux/igmp.h
index 073b30a9b850..3c12c0a63492 100644
--- a/include/linux/igmp.h
+++ b/include/linux/igmp.h
@@ -92,15 +92,77 @@ 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)
+ *
+ * If Max Resp Code >= 128, Max Resp Code represents a floating-point
+ * value as follows:
+ * If QQIC >= 128, QQIC represents a floating-point value 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 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 QQI/MRT 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)
+{
+	/* RFC3376, relevant sections:
+	 *  - 4.1.1. Maximum Response Code
+	 *  - 4.1.7. QQIC (Querier's Query Interval Code)
+	 */
+	if (code < IGMPV3_EXP_MIN_THRESHOLD) {
+		return (unsigned long)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 */
+static inline unsigned long igmpv3_mrt(const struct igmpv3_query *ih3)
+{
+	/* RFC3376, relevant sections:
+	 *  - 4.1.1. Maximum Response Code
+	 *  - 8.3. Query Response Interval
+	 */
+	return igmpv3_exp_field_decode(ih3->code);
+}
+
+/* Calculate Querier's Query Interval from Querier's Query Interval Code */
+static inline unsigned long igmpv3_qqi(const struct igmpv3_query *ih3)
+{
+	/* 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)
+	 */
+	return igmpv3_exp_field_decode(ih3->qqic);
+}
 
 static inline int ip_mc_may_pull(struct sk_buff *skb, unsigned int len)
 {
diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
index 881d866d687a..9fec76e887bc 100644
--- a/net/bridge/br_multicast.c
+++ b/net/bridge/br_multicast.c
@@ -3518,7 +3518,7 @@ static void br_ip4_multicast_query(struct net_bridge_mcast *brmctx,
 			goto out;
 
 		max_delay = ih3->code ?
-			    IGMPV3_MRC(ih3->code) * (HZ / IGMP_TIMER_SCALE) : 1;
+			    igmpv3_mrt(ih3) * (HZ / IGMP_TIMER_SCALE) : 1;
 	} else {
 		goto out;
 	}
diff --git a/net/ipv4/igmp.c b/net/ipv4/igmp.c
index a674fb44ec25..8c6102737096 100644
--- a/net/ipv4/igmp.c
+++ b/net/ipv4/igmp.c
@@ -991,7 +991,7 @@ static bool igmp_heard_query(struct in_device *in_dev, struct sk_buff *skb,
 		 * different encoding. We use the v3 encoding as more likely
 		 * to be intended in a v3 query.
 		 */
-		max_delay = IGMPV3_MRC(ih3->code)*(HZ/IGMP_TIMER_SCALE);
+		max_delay = igmpv3_mrt(ih3)*(HZ/IGMP_TIMER_SCALE);
 		if (!max_delay)
 			max_delay = 1;	/* can't mod w/ 0 */
 	} else { /* v3 */
@@ -1006,7 +1006,7 @@ static bool igmp_heard_query(struct in_device *in_dev, struct sk_buff *skb,
 			ih3 = igmpv3_query_hdr(skb);
 		}
 
-		max_delay = IGMPV3_MRC(ih3->code)*(HZ/IGMP_TIMER_SCALE);
+		max_delay = igmpv3_mrt(ih3)*(HZ/IGMP_TIMER_SCALE);
 		if (!max_delay)
 			max_delay = 1;	/* can't mod w/ 0 */
 		WRITE_ONCE(in_dev->mr_maxdelay, max_delay);
@@ -1016,7 +1016,7 @@ static bool igmp_heard_query(struct in_device *in_dev, struct sk_buff *skb,
 		 * configured value.
 		 */
 		in_dev->mr_qrv = ih3->qrv ?: READ_ONCE(net->ipv4.sysctl_igmp_qrv);
-		in_dev->mr_qi = IGMPV3_QQIC(ih3->qqic)*HZ ?: IGMP_QUERY_INTERVAL;
+		in_dev->mr_qi = igmpv3_qqi(ih3)*HZ ?: IGMP_QUERY_INTERVAL;
 
 		/* RFC3376, 8.3. Query Response Interval:
 		 * The number of seconds represented by the [Query Response
-- 
2.43.0