mac80211 HT/VHT MCS check change causing Wi-Fi speed drop (~20 Mbps cap) on iwlwifi

[Jayden Fowler <jaydenfowler831@gmail.com>]

Hello,

I think I’ve run into a regression in mac80211 that affects Wi-Fi speeds 
on Intel iwlwifi (and possibly some Realtek cards too).

After testing a bunch of kernels, I noticed something pretty consistent:

Kernels with this commit Wi-Fi gets stuck around ~20 Mbps max
Kernels without it (or after removing a small part of it) speeds go back 
to normal (hundreds of Mbps as expected)

The commit I bisected it to is:

574faa0e936d12718e2cadad11ce1e184d9e5a32
"wifi: mac80211: add HT and VHT basic set verification"

The issue seems to come from the new HT/VHT MCS verification logic. When 
it runs, it sometimes decides the AP’s advertised rates are 
“unsupported” and then forces a downgrade in connection mode/bandwidth.

That downgrade looks like it’s what causes the big speed drop.

When I remove this from `net/mac80211/mlme.c`, Wi-Fi goes back to normal 
speeds:

if (conn->mode >= IEEE80211_CONN_MODE_HT &&
     !ieee80211_verify_sta_ht_mcs_support(sdata, sband,
                                          elems->ht_operation)) {
     conn->mode = IEEE80211_CONN_MODE_LEGACY;
     conn->bw_limit = IEEE80211_CONN_BW_LIMIT_20;
     link_id_info(sdata, link_id,
                  "required MCSes not supported, disabling HT\n");
}

if (conn->mode >= IEEE80211_CONN_MODE_VHT &&
     !ieee80211_verify_sta_vht_mcs_support(sdata, link_id, sband,
elems->vht_operation)) {
     conn->mode = IEEE80211_CONN_MODE_HT;
     conn->bw_limit = min_t(enum ieee80211_conn_bw_limit,
                            conn->bw_limit,
                            IEEE80211_CONN_BW_LIMIT_40);
     link_id_info(sdata, link_id,
                  "required MCSes not supported, disabling VHT\n");
}

This is my wifi card i'm testing on Network controller: Intel 
Corporation Dual Band Wireless-AC 3168NGW [Stone Peak] (rev 10)

Thanks, Jayden Fowler.