mt7925: severe latency + ~21% TX-retries under moderate 5 GHz contention (other radios fine on same AP)

TL;DR — On a ~17% busy 5 GHz channel, MT7925 latency collapses (high TX-retries, 0% loss, stays on-channel), while MT7615 / MT7921 / iPhone on the same AP, channel, and spot are all fine. Same card is flawless on an uncongested channel — the fault is contention-triggered.

⚠️ Not push-button reproducible — it needs real co-channel OBSS airtime (~15–20% here). Instead of a repro script, this is characterized by a controlled same-air comparison + eliminations below.

Environment

• MT7925 (Filogic 360) [14c3:7925], mt7925e, AMD Ryzen 7 8840HS
• NixOS, NetworkManager + wpa_supplicant backend
• Kernel 6.18.38 and 7.1.3 (both affected); firmware WM 20260605
• Regdomain CN → 5 GHz ch149 / 80 MHz, signal −35…−43 dBm

Controlled comparison — same AP / ch149 / same spot

Channel busy ≈ 17.5% (measured from an adjacent MT7615 iw … survey dump, mostly RX of other BSSes).

Same MT7925 on a separate low-traffic 5 GHz channel: normal, no spikes.

Ruled out

• Host/CPU: ping 127.0.0.1 = 0.4 ms during the spikes
• Roaming/scan: 0 scan/ROC events in iw event over a 54%-spike window
• Deep-sleep PM: radio awake ~30/35 s, low_power_wakes +0
• The air: 3 other radios fine on the same channel

To attempt reproduction

Needs ~15–20% OBSS airtime on the operating channel. Rough recipe: a 2nd AP on the same channel + a client pushing steady traffic (iperf/bulk) to reach ~15–20% busy, then from the MT7925 STA:

ping -i 0.02 -c 400 <gateway>                              # latency + spikes
iw dev wlo1 station dump | grep -E "tx packets|tx retries" # retry rate (diff before/after)
iw dev <mt76-on-same-ch> survey dump | grep -A5 'in use'   # confirm channel busy % (2 snapshots)

Suspected: firmware rate-control / channel-access — 21% retries at ~17% busy with −35 dBm is disproportionate; other radios ignore this load.

Start by reproducing the contention case with the provided ping, station dump, and survey dump commands, confirming roughly 15–20% OBSS airtime and comparing MT7925 with the other radios. Then trace the mt7925e driver and firmware rate-control or channel-access behavior implicated by the retry measurements. Done means isolating the driver or firmware cause and documenting a validated fix or next diagnostic.