Raspberry Pi 3 B+: Wi-Fi latency regression on 6.18.39+rpt-rpi-v8; 6.18.33 works

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Issue-Typ
Bug
Klarheit
Größtenteils klar
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Aktiv
Tech-Stack
c, linux, raspberry-pi
Bereich
embedded-iot, networking, operating-systems

Rechercherichtung

Start by reproducing the gateway-ping comparison on the Raspberry Pi 3 B+ with kernels 6.18.39+rpt-rpi-v8 and 6.18.33+rpt-rpi-v8, using the onboard brcmfmac adapter and the reported power-saving settings. Then investigate the kernel changes between those versions; done means identifying and correcting the regression while preserving normal latency on the affected setup.

Vom Indexierungsmodell aus dem Issue-Text verfasst.

Beschreibung

Describe the bug

After upgrading a Raspberry Pi 3 Model B+ from 6.18.33+rpt-rpi-v8 to 6.18.39+rpt-rpi-v8, onboard Wi-Fi exhibits a repeating latency pattern: most gateway pings take approximately 105–113 ms, interspersed with normal 4–8 ms responses.

There is no packet loss, signal strength is excellent, the system is idle, and Wi-Fi power saving is disabled.

Booting the same system into the retained 6.18.33+rpt-rpi-v8 kernel restores normal latency. No firmware, userspace, NetworkManager, access-point, or Wi-Fi profile changes were made between the decisive tests.

Results

6.18.39+rpt-rpi-v8, Wi-Fi only:

10 packets transmitted, 10 received, 0% packet loss
rtt min/avg/max/mdev = 5.360/78.668/112.640/47.331 ms

Representative responses:

107 ms
110 ms
5.36 ms
111 ms
112 ms
8.63 ms
105 ms
113 ms
5.40 ms
110 ms

6.18.33+rpt-rpi-v8, Wi-Fi only:

20 packets transmitted, 20 received, 0% packet loss
rtt min/avg/max/mdev = 2.110/6.990/39.653/7.566 ms

Apart from one 39.7 ms response, replies were normally 4–6 ms.

The working 6.18.33 test was performed on the same 5 GHz AP radio and frequency (5500 MHz) used when the problem was initially observed.

Steps to reproduce the behaviour

  1. Use a Raspberry Pi 3 Model B+ running 64-bit Debian 13 / Raspberry Pi OS Lite.
  2. Install and boot linux-image-6.18.39+rpt-rpi-v8.
  3. Connect using the onboard BCM4345/6 Wi-Fi adapter.
  4. Disable Wi-Fi power saving.
  5. Disconnect Ethernet.
  6. Run ping -c 20 <LAN gateway>.
  7. Observe repeating responses around 105–113 ms with periodic normal responses.
  8. Boot the same system into linux-image-6.18.33+rpt-rpi-v8, without changing the Wi-Fi firmware or configuration.
  9. Repeat the ping test.
  10. Observe normal latency around 4–6 ms.

Device(s)

Raspberry Pi 3 Mod. B+

System

Raspberry Pi reference 2026-04-21
Generated using pi-gen:
4ad56cc850fa60adcc7f07dc15879bc95cc1d281
stage2

Debian GNU/Linux 13.6 (trixie)
aarch64

Affected kernel:

Linux 6.18.39+rpt-rpi-v8
#1 SMP PREEMPT Debian 1:6.18.39-1+rpt1 (2026-07-29)

Working kernel:

Linux 6.18.33+rpt-rpi-v8
#1 SMP PREEMPT Debian 1:6.18.33-1+rpt1 (2026-06-01)

Firmware:

May 21 2026 11:21:57
version 288930ab4712b99596f32732664aaaeb881ef1e0

Relevant packages:

firmware-brcm80211 1:20260519-1~bpo13+1+rpt1
linux-image-6.18.33+rpt-rpi-v8 1:6.18.33-1+rpt1
linux-image-6.18.39+rpt-rpi-v8 1:6.18.39-1+rpt1
network-manager 1.52.1-1+rpt4
wireless-regdb 2026.05.30-1~deb13u1
wpasupplicant 2:2.10-24

Wi-Fi chipset and firmware:

brcmfmac: using brcm/brcmfmac43455-sdio for chip BCM4345/6
brcmfmac: Firmware: BCM4345/6 wl0: Aug 29 2023 01:47:08
version 7.45.265 (28bca26 CY)
FWID 01-b677b91b

Logs

On the working kernel, NetworkManager briefly enables power saving during initialization and then disables it as configured:

brcmfmac: brcmf_cfg80211_set_power_mgmt: power save enabled
brcmfmac: brcmf_cfg80211_set_power_mgmt: power save disabled

Both the global NetworkManager setting and active connection profile specify disabled power saving:

[connection]
wifi.powersave = 2

802-11-wireless.powersave: 2 (disable)

No brcmfmac errors, disconnects, thermal throttling, or packet loss were observed.

Additional context

The upgrade that introduced the problem changed the kernel from 6.18.33 to 6.18.39. It also updated firmware-brcm80211, but the controlled old/new kernel comparison used the same currently installed firmware package, isolating the kernel version as the changed variable.

The latency was reproduced on two different 5 GHz AP radios/channels. Signal strength was reported between 69% and 97%, including approximately -22 dBm near the access point.

This may be related generally to recent Pi 3 brcmfmac regressions, although the symptoms differ from #7337:

https://github.com/raspberrypi/linux/issues/7337

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