Avoiding autonomous 5GHz band Wi-Fi steering when a P2P-GO is set to freq=2

Start by reproducing the P2P-GO setup with wpa_supplicant, nl80211, and the p2p_group_add freq=2 command, then compare it with the direct iw setup described in the issue. Trace the Raspberry Pi wireless driver handling for P2P-GO frequency selection and determine why the interface changes bands despite the requested frequency. Done means identifying a driver-side cause or supported prevention mechanism and verifying behavior with iw dev.

Is this the right place for my bug report?

I think that this issue might be related to the BCM2711 nl80211 wireless kernel device driver.

Describe the bug

With a Raspberry Pi supporting the 5GHz Wi-Fi band (e.g., Raspberry Pi 4 Model B), if a Wi-Fi Direct P2P-GO group is created with option 2.4 GHz (freq=2), considering also the presence of some P2P Clients only supporting 2.4 GHz and some supporting Wi-Fi dual bands, the starting P2P-GO band will be 2.4GHz, but after some time the band will autonomously move to 5 GHz.

To reproduce

wpa_supplicant config file (wpa_supplicant.conf):

ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
device_name=DIRECT-test
device_type=6-0050F204-1
p2p_go_intent=15
country=IT

No AP shall be active (P2P-GO and AP cannot run concurrently on such hw),

wpa_supplicant command:

sudo wpa_supplicant -c/etc/wpa_supplicant/wpa_supplicant.conf -iwlan0 -Dnl80211,wext

Start wpa_cli -i p2p-dev-wlan0 and issue:

p2p_group_add freq=2

The result is the following (iw dev):

        Interface p2p-wlan0-0
                ifindex 5
                wdev 0x4
                addr de:a6:32:01:82:03
                type P2P-GO
                channel 1 (2412 MHz), width: 20 MHz, center1: 2412 MHz
                txpower 31.00 dBm
        Unnamed/non-netdev interface
                wdev 0x2
                addr dc:a6:32:01:02:03
                type P2P-device
                txpower 31.00 dBm
        Interface wlan0
                ifindex 3
                wdev 0x1
                addr dc:a6:32:00:9a:f5
                ssid TIM-86817771
                type managed
                channel 104 (5520 MHz), width: 40 MHz, center1: 5510 MHz
                txpower 31.00 dBm

Notice the different channels of p2p-wlan0-0 (2.4 GHz) and wlan0 (5 GHz)

Ensure wpa_supplicant is not interfering:

sudo killall -STOP wpa_supplicant

Wait for several minutes.

The iw dev will show a 5 GHz channel for the p2p-wlan0-0 Interface:

phy#0
        Interface p2p-wlan0-1
                ifindex 5
                wdev 0x4
                addr de:a6:32:01:82:03
                type P2P-GO
                channel 36 (5180 MHz), width: 40 MHz, center1: 5190 MHz
                txpower 31.00 dBm
        Unnamed/non-netdev interface
                wdev 0x2
                addr dc:a6:32:01:02:03
                type P2P-device
                txpower 31.00 dBm
        Interface wlan0
                ifindex 3
                wdev 0x1
                addr dc:a6:32:00:9a:f5
                ssid TIM-86817771
                type managed
                channel 104 (5520 MHz), width: 40 MHz, center1: 5510 MHz
                txpower 31.00 dBm

Expected behaviour

p2p-wlan0-0 channel shall not autonomously move to different frequencies when not driven by wpa_supplicant.

Actual behaviour

Just after creating the P2P-GO group, wpa_supplicant correctly configures the band, in fact the initial P2P-GO virtual interface (p2p-wlan0-0) is set to a 2.4 GHz frequency even if the Wi-Fi interface (wlan0) is set to a frequency in the 5 GHz band. To verify that wpa_supplicant does not interfere, we can send a SIGSTOP signal just after the P2P-GO group is created.

After several minutes, the Broadcom nl80211 wireless device driver autonomously appears to move the P2P-GO frequency from the 2.4 GHz band to the 5 GHz band.

The result is that 5 GHz P2P Devices can still connect the P2P-GO group after a while, while the 2.4 GHz ones will no more receive the group beaconing data.

Considering to find a workaround by modifying the wpa_supplicant sources, which is the correct nl80211 wireless device driver option to prevent this (wrong) default behavior when creating the NL80211_IFTYPE_P2P_GO type interface? Notice that if the P2P-GO interface is created with NL80211_ATTR_FREQ_FIXED property, the results are unchanged.

System
Results of the raspinfo command

• Which model of Raspberry Pi? Pi4 Model B

• Which OS and version (cat /etc/rpi-issue)?
  Generated using pi-gen, https://github.com/RPi-Distro/pi-gen, 9a3a10bf1019ebb2d59053564dc6b90068bad27d, stage4

• Which firmware version (vcgencmd version)?
  Oct 22 2020 13:59:27
  Copyright (c) 2012 Broadcom
  version 74e754ff8947c58d2773253f77f6f68a303188f8 (clean) (release) (start)

• Which kernel version (uname -a)?
  Linux RaspberryPi 5.4.72-v7l+ #1356 SMP Thu Oct 22 13:57:51 BST 2020 armv7l GNU/Linux

Additional context

I'm not sure that this is misleading, but there should be a way to overcome the issue, in fact if I run the following commands (after resuming and stopping wpa_supplicant with sudo killall -CONT wpa_supplicant; sleep 10; sudo killall wpa_supplicant), the frequency of the interface appears to stably remain to 2.4 GHz even if, before issuing them, the wlan0 frequency was in a 5 GHz band:

iw dev wlan0 interface add p2p-wlan0-0 type __p2pgo
ifconfig p2p-wlan0-0 192.168.4.1 netmask 255.255.255.0 up
iw dev p2p-wlan0-0 ap start DIRECT-PP-Toyota 2437 100 1 head 80000000ffffffffffff0a85bc6459460a85bc645946000000000000000000006400110400104449524543542d50502d546f796f746101088c129824b048606c030101
iw dev

The output is the following:

phy#0
        Interface p2p-wlan0-0
                ifindex 6
                wdev 0x5
                addr de:a6:32:01:82:03
                type P2P-GO
                channel 6 (2437 MHz), width: 20 MHz, center1: 2437 MHz
                txpower 31.00 dBm
        Interface wlan0
                ifindex 3
                wdev 0x1
                addr dc:a6:32:00:9a:f5
                type managed
                channel 6 (2437 MHz), width: 20 MHz, center1: 2437 MHz

Such case is anyway different and possibly not appropriate: both p2p-wlan0-0 and wlan0 are stably set to the same fixed 2.4 GHz frequency, while ideally only the P2P-GO group should be set to a 2.4 GHz frequency.