1 files changed, 87 insertions, 0 deletions
diff --git a/include/linux/igmp.h b/include/linux/igmp.h
index 0e095c9f234e..3a2d35a9f307 100644
--- a/include/linux/igmp.h
+++ b/include/linux/igmp.h
@@ -109,6 +109,93 @@ struct ip_mc_list {
 
 /* IGMPv3 floating-point exponential field min threshold */
 #define IGMPV3_EXP_MIN_THRESHOLD	128
+/* IGMPv3 FP max threshold (mant = 0xF, exp = 7) -> 31744 */
+#define IGMPV3_EXP_MAX_THRESHOLD	31744
+
+/* V3 exponential field encoding */
+
+/* IGMPv3 MRC/QQIC 8-bit exponential field encode
+ *
+ * RFC3376, 4.1.1 & 4.1.7. defines only the decoding formula:
+ *     MRT/QQI = (mant | 0x10) << (exp + 3)
+ *
+ * but does NOT define the encoding procedure. To derive exponent:
+ *
+ * For any value of mantissa and exponent, the decoding formula
+ * indicates that the "hidden bit" (0x10) is shifted 4 bits left
+ * to sit above the 4-bit mantissa. The RFC again shifts this
+ * entire block left by (exp + 3) to reconstruct the value.
+ * So, 'hidden bit' is the MSB which is shifted by (4 + exp + 3).
+ *
+ * Total left shift of the 'hidden bit' = 4 + (exp + 3) = exp + 7.
+ * This is the MSB at the 0-based bit position: (exp + 7).
+ * Since fls() is 1-based, fls(value) - 1 = exp + 7.
+ *
+ * Therefore:
+ *     exp  = fls(value) - 8
+ *     mant = (value >> (exp + 3)) & 0x0F
+ *
+ * Final encoding formula:
+ *     0x80 | (exp << 4) | mant
+ *
+ * Example (value = 3200):
+ *  0               1
+ *  0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ * |0 0 0 0 1 1 0 0 1 0 0 0 0 0 0 0| (value = 3200)
+ * |        ^-^-mant^ ^..(exp+3)..^| exp = 4, mant = 9
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ *
+ * Encoded:
+ *   0x80 | (4 << 4) | 9 = 0xC9
+ */
+static inline u8 igmpv3_exp_field_encode(unsigned long value)
+{
+	u8 mc_exp, mc_man;
+
+	/* MRC/QQIC < 128 is literal */
+	if (value < IGMPV3_EXP_MIN_THRESHOLD)
+		return value;
+
+	/* Saturate at max representable (mant = 0xF, exp = 7) -> 31744 */
+	if (value >= IGMPV3_EXP_MAX_THRESHOLD)
+		return 0xFF;
+
+	mc_exp  = fls(value) - 8;
+	mc_man = (value >> (mc_exp + 3)) & 0x0F;
+
+	return 0x80 | (mc_exp << 4) | mc_man;
+}
+
+/* Calculate Maximum Response Code from Max Resp Time
+ *
+ * RFC3376, relevant sections:
+ *  - 4.1.1. Maximum Response Code
+ *  - 8.3. Query Response Interval
+ *
+ * MRC represents the encoded form of Max Resp Time (MRT); once
+ * decoded, the resulting value is in units of 0.1 seconds (100 ms).
+ */
+static inline u8 igmpv3_mrc(unsigned long mrt)
+{
+	return igmpv3_exp_field_encode(mrt);
+}
+
+/* Calculate Querier's Query Interval Code from Querier's Query Interval
+ *
+ * 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)
+ *
+ * QQIC represents the encoded form of Querier's Query Interval (QQI);
+ * once decoded, the resulting value is in units of seconds.
+ */
+static inline u8 igmpv3_qqic(unsigned long qi)
+{
+	return igmpv3_exp_field_encode(qi);
+}
 
 /* V3 exponential field decoding */