Add HT MCS table and bitrate lookup helper

Introduce a static table for 802.11n HT MCS indexes 0 to 31, covering
20 MHz bandwidth with both 0.8 µs and 0.4 µs guard intervals. This table
defines the modulation and coding scheme parameters used for HT mode.

Provide a helper function to compute bitrate in Kbps based on the
currently selected MCS index and guard interval from the bitrate mask.

This update lays the groundwork for features such as transmission delay
simulation and reporting bitrate via station information

Author: horseface1110

vwifi.c

@@ -200,6 +200,8 @@ struct vwifi_vif {
 
     /* Transmit power */
     s32 tx_power;
+
+    struct cfg80211_bitrate_mask bitrate_mask;
 };
 
 static int station = 2;
@@ -214,6 +216,27 @@ static struct vwifi_context *vwifi = NULL;
 
 static struct sock *nl_sk = NULL;
 
+struct ht_mcs_entry {
+    int mcs_index;
+    int bitrate_20mhz_gi08;  // in kbps
+    int bitrate_20mhz_gi04;  // in kbps
+};
+
+static const struct ht_mcs_entry ht_mcs_table[32] = {
+    {0, 6500, 7200},      {1, 13000, 14400},    {2, 19500, 21700},
+    {3, 26000, 28900},    {4, 39000, 43300},    {5, 52000, 57800},
+    {6, 58500, 65000},    {7, 65000, 72200},    {8, 13000, 14400},
+    {9, 26000, 28900},    {10, 39000, 43300},   {11, 52000, 57800},
+    {12, 78000, 86700},   {13, 104000, 115600}, {14, 117000, 130000},
+    {15, 130000, 144400}, {16, 19500, 21700},   {17, 39000, 43300},
+    {18, 58500, 65000},   {19, 78000, 86700},   {20, 117000, 130000},
+    {21, 156000, 173300}, {22, 175500, 195000}, {23, 195000, 216700},
+    {24, 26000, 28900},   {25, 52000, 57800},   {26, 78000, 86700},
+    {27, 104000, 115600}, {28, 156000, 173300}, {29, 208000, 231100},
+    {30, 234000, 260000}, {31, 260000, 288900},
+};
+
+
 static int denylist_check(char *dest, char *source)
 {
     if (!vwifi->denylist || !*(vwifi->denylist))
@@ -702,6 +725,20 @@ static int vwifi_ndo_stop(struct net_device *dev)
     return 0;
 }
 
+static inline int get_ht_bitrate_kbps(const struct vwifi_vif *vif,
+                                      int mcs_index)
+{
+    enum nl80211_txrate_gi gi = vif->bitrate_mask.control[NL80211_BAND_2GHZ].gi;
+
+    if (mcs_index < 0 || mcs_index >= 32)
+        return 0;  // fallback for invalid index
+
+    if (gi == NL80211_TXRATE_FORCE_SGI)
+        return ht_mcs_table[mcs_index].bitrate_20mhz_gi04;
+    else
+        return ht_mcs_table[mcs_index].bitrate_20mhz_gi08;
+}
+
 static struct net_device_stats *vwifi_ndo_get_stats(struct net_device *dev)
 {
     struct vwifi_vif *vif = ndev_get_vwifi_vif(dev);