MT7921 on BlankOn: SSH Kept Dropping Until I Turned Off Wi-Fi Power Save

My desktop, the same BlankOn Sinambung box I hardened last week, has one job most evenings: sit there and accept SSH. For a while it did not. Connections would open, then hang, then die with Connection timed out or Connection closed by 192.168.1.49 port 22. Not every time. Just enough to be annoying, and never when I was watching closely.

I assumed the driver at first. The chipset is a MediaTek MT7921K (Wi-Fi 6E), driven by mt7921e, and that driver has a reputation. Turned out the driver was fine. The problem was the default power management sitting on top of it.

Confirming it was the link, not the box

The PC stayed up the whole time. systemctl status sleep.target was clean, the machine answered ARP, and uptime never reset. So it was not a suspend problem. To prove the link was flapping, I opened TCP port 22 thirty times in a row from another host:

import socket, time
ok = fail = 0
for _ in range(30):
    s = socket.socket()
    s.settimeout(1.0)
    try:
        s.connect(('192.168.1.49', 22)); ok += 1
    except Exception:
        fail += 1
    finally:
        s.close()
    time.sleep(0.15)
print(f'OK={ok} FAIL={fail}')

OK=4 FAIL=26. That is not an overloaded sshd. That is a radio dropping packets. Same port, same key, same everything, and three quarters of the handshakes never finished.

For comparison, the same probe against a wired host on the same switch returned OK=30 FAIL=0 without breaking a sweat.

What was actually wrong

lspci -k named the culprit:

03:00.0 Network controller: MEDIATEK Corp. MT7922 802.11ax PCI Express Wireless Network Adapter
    Subsystem: AzureWave MT7922
    Kernel driver in use: mt7921e
    Kernel modules: mt7921e

Then the tell, once iw was installed (apt install iw, it is not on the base image):

iw dev wlp4s0 get power_save
# Power save: on

Power save was on by default, because NetworkManager ships with wifi.powersave set to its default value, and the MT7921 firmware’s power save is aggressive. When the radio dozes between frames it takes a moment to wake, and under a burst of small TCP packets (exactly what an interactive SSH session is) those wake-ups show up as latency and lost handshakes. It is fine for browsing. It is terrible for keeping a shell open.

nmcli confirmed the connection was using the default:

802-11-wireless.powersave: 0 (default)

0 here means “use the driver’s default”, not “off”. The driver’s default was on.

The fix

Two changes. First, make it permanent so a reconnect does not undo it:

sudo tee /etc/NetworkManager/conf.d/wifi-powersave.conf >/dev/null <<'EOF'
[connection]
wifi.powersave = 2
EOF

wifi.powersave = 2 is “disable”, as opposed to 3 (enable) or 0 (default). Then apply it now instead of waiting for a reconnect:

sudo apt install iw          # if it is missing
sudo iw dev wlp4s0 set power_save off
sudo nmcli connection reload
sudo systemctl restart NetworkManager

If you are on the Wi-Fi link you are about to restart, run this from a wired session or a console, because NetworkManager will bounce the interface and drop your connection mid-command.

Verify:

iw dev wlp4s0 get power_save
# Power save: off

Then re-ran the probe. OK=30 FAIL=0. Same link, same night, same distance from the router. The only difference was power save.

After

SSH holds. Long sessions stop dropping. dmesg no longer fills with wlp4s0: deauth entries every few minutes.

The one thing worth naming: this is not an MT7921 bug. It is a sensible default for a laptop that runs on battery, applied to a desktop that never leaves the wall. On a stationary machine the radio has no reason to nap, and once you tell NetworkManager that, the chipset behaves.

Config state

• /etc/NetworkManager/conf.d/wifi-powersave.conf with wifi.powersave = 2
• iw installed
• Connection KENARI_5G on 5 GHz, Power save: off

Rollback is deleting the conf file and reloading NetworkManager. I am not doing that.