mac80211-mcs-patch

Kernel patch that fixes WiFi stuck at 54 Mbps when your card has fewer spatial streams than the AP's basic MCS set requires.

Update: Kernel version 7.0.13 and above already fixed this problem. See this commit for details.

The Problem

Some access points (e.g. Xfinity XB8, other 4x4 MIMO routers) include 3 or 4 spatial stream MCS indexes in their basic HT-MCS set. Linux's mac80211 checks if your card supports all basic MCS rates — if it doesn't (e.g. a 2x2 card like MediaTek MT7922), HT is disabled entirely and you're stuck at 802.11a/g rates (54 Mbps max).

$ dmesg | grep HT
wlan0: required MCSes not supported, disabling HT

The card works fine using its own 2-stream rates for data — mac80211 is just being overly strict about an AP misconfiguration you can't control.

The Fix

A one-line patch to ieee80211_verify_sta_ht_mcs_support() in net/mac80211/mlme.c that skips the basic MCS set validation.

Before: 54 Mbps, no HT After: ~960 Mbps, HE-MCS 9, 80MHz, 2 NSS

Install (Arch Linux)

Requires linux-headers and base-devel:

git clone https://github.com/WoodyWoodster/mac80211-mcs-patch.git
cd mac80211-mcs-patch
sudo ./install.sh

This copies the script and patch to /usr/local/share/mac80211-mcs-patch/, installs a pacman hook for auto-rebuild on kernel updates, and builds the module for your current kernel.

Load it now (drops WiFi for ~2 seconds):

sudo bash -c 'rmmod mt7921e mt7921_common mt792x_lib mt76_connac_lib mt76 mac80211 && modprobe mac80211 && modprobe mt7921e'

Or just reboot.

Install (Other Distros)

git clone https://github.com/WoodyWoodster/mac80211-mcs-patch.git
cd mac80211-mcs-patch
sudo ./rebuild-mac80211.sh

The rebuild script downloads the matching kernel source, applies the patch, builds, and installs the module. You'll need to re-run it after kernel updates (or set up your own hook).

Do I Need This?

dmesg | grep "required MCSes not supported"

If that shows disabling HT, this patch is for you.

Tested With

• Card: MediaTek MT7922 (mt7921e) — 2x2 WiFi 6E
• AP: Xfinity XB8 — 4x4 MIMO
• Kernel: 6.19.x

Should help any 1x1/2x2 card paired with a 3x3/4x4 AP that sets aggressive basic MCS rates. The patch is in generic mac80211, not driver-specific.

Files

skip-basic-mcs-check.patch  # The kernel patch
rebuild-mac80211.sh          # Downloads source, patches, builds, installs
install.sh                   # Arch Linux installer (copies files + pacman hook)
91-mac80211-patch.hook       # Pacman hook for auto-rebuild on kernel updates

License

MIT