mt7921e: MT7902 (Filogic 310) fails to authenticate on 2.4GHz, works fine on 5GHz

Summary

On a MediaTek MT7902 (Filogic 310) PCIe wifi card, mt7921e reliably fails to
authenticate with any 2.4GHz access point (send auth x3 → authentication ... timed out, zero response from the AP), regardless of signal strength,
SSID, or security type. The exact same laptop associates successfully within
milliseconds against the exact same physical AP on 5GHz.

System

• Kernel: 7.1.5-arch1-2 (Arch Linux, built 2026-07-28)
• Distro: Arch Linux (fresh install, 2026-08-06)
• Driver: mt7921e (in-tree)
• linux-firmware-mediatek: tested both 20260622-1 and 20260519-1 (identical
  MT7902 firmware blob in both — build time unchanged, see below — so this is
  not a firmware-version regression)

Hardware

02:00.0 Network controller [0280]: MEDIATEK Corp. MT7902 802.11ax PCIe Wireless Network Adapter [Filogic 310] [14c3:7902]
	Subsystem: AzureWave Device [1a3b:5520]
	Kernel driver in use: mt7921e

• ASIC revision: 79020000
• WM Firmware Version string reported by driver: ____000000 (build time
  20251212032127) — this malformed-looking version string is consistent
  across both firmware package versions tested.
• Laptop: ASUS Vivobook, AMD platform

Symptom

Any attempt to associate with a 2.4GHz AP fails identically: the driver sends
3 authentication frames and gets no response at all from the AP, then times
out. This happens against multiple different APs/SSIDs (including different
routers, one at 7% signal and one at 100% signal, both weak and full-signal
targets fail the same way), so it is not signal-strength or a specific
router's fault.

The exact same AP, switched to 5GHz on the same channel plan, associates
correctly:

wlp2s0: authenticate with 50:bb:b5:23:01:d4 (local address=a8:e2:91:08:16:8c)
wlp2s0: send auth to 50:bb:b5:23:01:d4 (try 1/3)
wlp2s0: send auth to 50:bb:b5:23:01:d4 (try 2/3)
wlp2s0: authenticated
wlp2s0: associate with 50:bb:b5:23:01:d4 (try 1/3)
wlp2s0: RX AssocResp from 50:bb:b5:23:01:d4 (capab=0x11 status=0 aid=2)
wlp2s0: associated

vs. the 2.4GHz failure against the same BSSID minutes earlier:

wlp2s0: authenticate with 50:bb:b5:23:01:d4 (local address=a8:e2:91:08:16:8c)
wlp2s0: send auth to 50:bb:b5:23:01:d4 (try 1/3)
wlp2s0: send auth to 50:bb:b5:23:01:d4 (try 2/3)
wlp2s0: send auth to 50:bb:b5:23:01:d4 (try 3/3)
wlp2s0: authentication with 50:bb:b5:23:01:d4 timed out

Once on 5GHz, the connection is fully functional (DHCP lease obtained, <10ms
ping RTT to 1.1.1.1, no further drops observed).

What I've ruled out

• Signal strength: fails identically from 7% to 100% signal.
• PCIe ASPM: set options mt7921e disable_aspm=1, confirmed
  disabling ASPM L1 in dmesg on reload — no change to 2.4GHz behavior.
• Regulatory domain: was unset (country 00, forcing PASSIVE-SCAN on all
  channels) due to /etc/conf.d/wireless-regdom not actually being applied by
  any service on a systemd-only install. Fixed live with iw reg set SE — no
  change to 2.4GHz behavior.
• Stale driver/firmware state: full rfkill block/unblock cycle and
  modprobe -r mt7921e && modprobe mt7921e reload — no change.
• Firmware version: downgraded linux-firmware-mediatek from 20260622-1
  to 20260519-1 — the MT7902 firmware blob's reported build time
  (20251212032127) was byte-identical before and after, so no behavioral
  change (expected, given the file didn't actually change).
• Missing driver support: checked wireless.git (main) and
  wireless-next.git (for-next) — MT7902 PCIe/MCU/DMA-layout/IRQ-quirk
  support landed 2026-03-24 and 2026-06-09, both already included in this
  kernel build (2026-07-28). No newer MT7902-related commits exist in
  for-next beyond what's already merged.

Additional context

The same laptop/AP combination reportedly associated fine on 5GHz a few days
prior under CachyOS (different kernel build/config, not directly compared
here). Not a controlled comparison, but noted in case it's a useful data
point for triage.

Reproduction

1. mt7921e-based MT7902 card, any 2.4GHz-only AP or a dual-band AP with the
   client forced to associate on 2.4GHz.
2. nmcli connection up <profile> or iw manual auth — same result either
   way, the failure is at the mac80211/driver/firmware auth-frame level,
   below NetworkManager/wpa_supplicant.
3. Observe dmesg/journalctl -k: 3x send auth, then
   authentication ... timed out, with the AP never responding.
4. Switch the same AP to 5GHz (or connect to a different, 5GHz-only AP): auth
   and association complete normally within one attempt.

Happy to provide mt76/mac80211 debugfs output, a packet capture, or test
patches if useful — just let me know what's needed.

Start with the mt7921e driver and the mac80211/firmware authentication path, using the reproduced 2.4GHz failure and successful 5GHz connection as the comparison. Collect the offered mt76/mac80211 debugfs output, dmesg or journalctl -k logs, and a packet capture; done means identifying and fixing the cause of missing 2.4GHz authentication responses without regressing 5GHz.