mac80211: "No basic rates" regression on 2.4GHz with MT7922 — fix in 6.17.9 missing from 6.18.7

[n8behavior]

Summary

Kernel 6.18.7-76061807 (and Ubuntu HWE 6.17.0-20) emit "No basic rates, using min rate instead" from net/mac80211/mlme.c when connecting to any 2.4GHz AP. This does not occur on 5GHz, and does not occur on System76 kernel 6.17.9-76061709 — indicating a fix exists in 6.17.9 that was not forward-ported.

Impact

• Degraded 2.4GHz throughput — ~5x slower than the fixed 6.17.9 kernel
• Radio death after WNM roaming — if a Unifi AP sends a WNM Disassociation Imminent frame while the buggy rate tables are active, the radio goes completely deaf (no scan results, no APs visible — not even neighbors or phone hotspots)
• Inconsistent boot behavior — if NetworkManager auto-connects to a 5GHz AP first, the bug never triggers; if it picks 2.4GHz, it does

Hardware

• Machine: Thelio Major (thelio-major-r5)
• WiFi: MediaTek MT7922 802.11ax PCIe (0a:00.0, PCI ID 14c3:0616)
• Driver: mt7921e (srcversion 579FF3B9B2F0D659D1EE01B — identical across all tested kernels)
• Firmware: WIFI_MT7922_patch_mcu_1_1_hdr.bin, WIFI_RAM_CODE_MT7922_1.bin (Build Time: 20241106163228a)
• APs: Ubiquiti Unifi (multiple, WNM/BSS Transition enabled)
• OS: Ubuntu 24.04.4 LTS (Noble)

Reproduction

Deterministic on 6.18.7-76061807:

# Triggers bug (2.4GHz):
nmcli connection modify <ssid> 802-11-wireless.band bg
nmcli device wifi connect <ssid>
sudo dmesg | grep 'basic rate'
# Output: "wlp10s0: No basic rates, using min rate instead" (twice — once pre-auth, once post-AssocResp)

# No bug (5GHz):
nmcli connection modify <ssid> 802-11-wireless.band a
nmcli device wifi connect <ssid>
sudo dmesg | grep 'basic rate'
# No output

The difference correlates with the AP's capability field:

• 2.4GHz AP (capab=0x1011) → bug triggered
• 5GHz AP (capab=0x1111) → clean

Three-kernel comparison

Kernel	Source	"No basic rates" on 2.4GHz	"No basic rates" on 5GHz
6.17.9-76061709	System76 PPA	No	No
6.17.0-20-generic	Ubuntu HWE	Yes	Not tested
6.18.7-76061807	System76 PPA	Yes	No

The bug is in upstream mac80211 (present in Ubuntu HWE too), but System76's 6.17.9 kernel carries a fix that avoids it. That fix was not carried forward to 6.18.7.

dmesg comparison

6.18.7 on 2.4GHz (broken):

wlp10s0: No basic rates, using min rate instead
wlp10s0: authenticate with d0:21:f9:42:5f:9d (local address=a8:3b:76:1e:04:71)
wlp10s0: send auth to d0:21:f9:42:5f:9d (try 1/3)
wlp10s0: authenticated
wlp10s0: associate with d0:21:f9:42:5f:9d (try 1/3)
wlp10s0: RX AssocResp from d0:21:f9:42:5f:9d (capab=0x1011 status=0 aid=10)
wlp10s0: No basic rates, using min rate instead
wlp10s0: associated

6.18.7 on 5GHz (clean):

wlp10s0: authenticate with d0:21:f9:42:5f:9e (local address=a8:3b:76:1e:04:71)
wlp10s0: send auth to d0:21:f9:42:5f:9e (try 1/3)
wlp10s0: authenticated
wlp10s0: associate with d0:21:f9:42:5f:9e (try 1/3)
wlp10s0: RX AssocResp from d0:21:f9:42:5f:9e (capab=0x1111 status=0 aid=11)
wlp10s0: associated

Workaround

nmcli connection modify <ssid> 802-11-wireless.band a

Forces 5GHz only, avoiding the 2.4GHz rate parsing bug.

Request

Please identify the fix present in the 6.17.9-76061709 kernel tree that resolves the 2.4GHz basic rate parsing and forward-port it to the next kernel release.

[n8behavior]

Upstream bug filed: https://bugzilla.kernel.org/show_bug.cgi?id=221356

Additional findings since opening this issue:

• Reproduced on a second machine: System76 Pangolin with a different MT7922 variant (PCI 14c3:7922, AzureWave) running Ubuntu HWE 6.17.0-20-generic. Bug triggered organically during AP roaming to 2.4 GHz.

• The ieee80211_get_rates() and ieee80211_mgd_setup_link_sta() source in mlme.c appears identical across all three tested kernels (6.17.9, 6.17.0-20, 6.18.7). The fix in 6.17.9 may be outside mac80211 itself.

• Workaround: nmcli connection modify <ssid> 802-11-wireless.band a (force 5 GHz)