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#7216·linux

mt7921u: Failed to start radar detection: -16 (Device or resource busy) on Raspberry Pi 5

Author: michaliskonCreated Jan 29, 2026Updated Sep 13, 2026

Describe the bug

The mt7921u driver consistently fails to initiate DFS Radar Detection (CAC) on the Raspberry Pi 5. Attempts to start hostapd on DFS channels (e.g., Channel 132) result in an immediate -16 (Device or resource busy) error. This happens even after ruling out power management and firmware issues.

Steps to reproduce the behaviour

  1. Connect a MediaTek MT7921U USB adapter
  2. Set regulatory domain
  3. Configure hostapd for a DFS channel (e.g., 52, 100, 132)
  4. Run hostapd -dd and observe the transition to DFS state

Device (s)

Raspberry Pi 5

System

OS: Raspberry Pi reference 2025-12-04 Generated using pi-gen, https://github.com/RPi-Distro/pi-gen, 4997bf4e4e49bc3305eb182a4a08bd023529da04, stage2

Firmware version:
ab8a9dde

Kernel: 6.12.62+rpt-rpi-2712 #1 SMP PREEMPT Debian 1:6.12.62-1+rpt1 (2025-12-18) aarch64 GNU/Linux

Firmware: MediaTek Nov 6 2024 (20241106151045) Hardware: MediaTek MT7921U USB WiFi Dongle (I used a Brostrend AXE3000)

Logs

pi@raspberrypi:~ $ cat simple_hostapd.conf interface=wlan0 driver=nl80211 ssid=test_ssid hw_mode=a channel=132 country_code=DK ieee80211d=1 ieee80211h=1 ieee80211n=1 ieee80211ac=1 vht_oper_chwidth=0 vht_oper_centr_freq_seg0_idx=0 wmm_enabled=1

pi@raspberrypi:~ $ sudo hostapd -dd ~/simple_hostapd.conf random: getrandom() support available Configuration file: /home/pi/simple_hostapd.conf nl80211: TDLS supported nl80211: TDLS external setup nl80211: Supported cipher 00-0f-ac:1 nl80211: Supported cipher 00-0f-ac:5 nl80211: Supported cipher 00-0f-ac:2 nl80211: Supported cipher 00-0f-ac:4 nl80211: Supported cipher 00-0f-ac:10 nl80211: Supported cipher 00-0f-ac:8 nl80211: Supported cipher 00-0f-ac:9 nl80211: Supported cipher 00-0f-ac:6 nl80211: Supported cipher 00-0f-ac:13 nl80211: Supported cipher 00-0f-ac:11 nl80211: Supported cipher 00-0f-ac:12 nl80211: Using driver-based off-channel TX nl80211: Driver-advertised extended capabilities (default) - hexdump(len=8): 00 00 00 00 00 00 00 40 nl80211: Driver-advertised extended capabilities mask (default) - hexdump(len=8): 00 00 00 00 00 00 00 40 nl80211: Use separate P2P group interface (driver advertised support) nl80211: Enable multi-channel concurrent (driver advertised support) nl80211: key_mgmt=0x1ff0f enc=0xfef auth=0x7 flags=0x24003d10d35bfbe0 rrm_flags=0x30 probe_resp_offloads=0x0 max_stations=0 max_remain_on_chan=5000 max_scan_ssids=4 nl80211: interface wlan0 in phy phy0 nl80211: Set mode ifindex 3 iftype 3 (AP)

nl80211: Failed to set interface 3 to mode 3: -16 (Device or resource busy)

nl80211: Try mode change after setting interface down nl80211: Set mode ifindex 3 iftype 3 (AP) nl80211: Mode change succeeded while interface is down nl80211: Setup AP(wlan0) - device_ap_sme=0 use_monitor=0 nl80211: Subscribe to mgmt frames with AP handle 0x555669e44720 nl80211: Register frame type=0xb0 (WLAN_FC_STYPE_AUTH) nl_handle=0x555669e44720 match= multicast=0 nl80211: Register frame type=0x0 (WLAN_FC_STYPE_ASSOC_REQ) nl_handle=0x555669e44720 match= multicast=0 nl80211: Register frame type=0x20 (WLAN_FC_STYPE_REASSOC_REQ) nl_handle=0x555669e44720 match= multicast=0 nl80211: Register frame type=0xa0 (WLAN_FC_STYPE_DISASSOC) nl_handle=0x555669e44720 match= multicast=0 nl80211: Register frame type=0xc0 (WLAN_FC_STYPE_DEAUTH) nl_handle=0x555669e44720 match= multicast=0 nl80211: Register frame type=0x40 (WLAN_FC_STYPE_PROBE_REQ) nl_handle=0x555669e44720 match= multicast=0 nl80211: Register frame type=0xd0 (WLAN_FC_STYPE_ACTION) nl_handle=0x555669e44720 match=04 multicast=0 nl80211: Register frame type=0xd0 (WLAN_FC_STYPE_ACTION) nl_handle=0x555669e44720 match=0501 multicast=0 nl80211: Register frame type=0xd0 (WLAN_FC_STYPE_ACTION) nl_handle=0x555669e44720 match=0503 multicast=0 nl80211: Register frame type=0xd0 (WLAN_FC_STYPE_ACTION) nl_handle=0x555669e44720 match=0504 multicast=0 nl80211: Register frame type=0xd0 (WLAN_FC_STYPE_ACTION) nl_handle=0x555669e44720 match=06 multicast=0 nl80211: Register frame type=0xd0 (WLAN_FC_STYPE_ACTION) nl_handle=0x555669e44720 match=08 multicast=0 nl80211: Register frame type=0xd0 (WLAN_FC_STYPE_ACTION) nl_handle=0x555669e44720 match=09 multicast=0 nl80211: Register frame type=0xd0 (WLAN_FC_STYPE_ACTION) nl_handle=0x555669e44720 match=0a multicast=0 nl80211: Register frame type=0xd0 (WLAN_FC_STYPE_ACTION) nl_handle=0x555669e44720 match=11 multicast=0 nl80211: Register frame type=0xd0 (WLAN_FC_STYPE_ACTION) nl_handle=0x555669e44720 match=12 multicast=0 nl80211: Register frame type=0xd0 (WLAN_FC_STYPE_ACTION) nl_handle=0x555669e44720 match=7f multicast=0 rfkill: initial event: idx=0 type=1 op=0 soft=0 hard=0 nl80211: Add own interface ifindex 3 (ifidx_reason -1) nl80211: if_indices[16]: 3(-1) nl80211: Do not open EAPOL RX socket - using control port for RX phy: phy0 BSS count 1, BSSID mask 00:00:00:00:00:00 (0 bits) wlan0: interface state UNINITIALIZED->COUNTRY_UPDATE Previous country code DK, new country code DK nl80211: Regulatory information - country=DK (DFS-ETSI) nl80211: 2400-2483 @ 40 MHz 20 mBm nl80211: 5150-5250 @ 80 MHz 23 mBm (no outdoor) nl80211: 5250-5350 @ 80 MHz 20 mBm (no outdoor) (DFS) nl80211: 5470-5725 @ 160 MHz 26 mBm (DFS) nl80211: 5725-5875 @ 80 MHz 13 mBm nl80211: 5945-6425 @ 320 MHz 23 mBm (no outdoor) nl80211: 57000-66000 @ 2160 MHz 40 mBm nl80211: Added 802.11b mode based on 802.11g information nl80211: Mode IEEE 802.11g: 2412 2417 2422 2427 2432 2437 2442 2447 2452 2457 2462 2467 2472 2484[DISABLED][NO_IR] nl80211: Mode IEEE 802.11a: 5180 5200 5220 5240 5260[RADAR] 5280[RADAR] 5300[RADAR] 5320[RADAR] 5500[RADAR] 5520[RADAR] 5540[RADAR] 5560[RADAR] 5580[RADAR] 5600[RADAR] 5620[RADAR] 5640[RADAR] 5660[RADAR] 5680[RADAR] nl80211: Mode IEEE 802.11a: 5955 5975 5995 6015 6035 6055 6075 6095 6115 6135 6155 6175 6195 6215 6235 6255 6275 6295 6315 6335 6355 6375 6395 6415 6435[DISABLED] 6455[DISABLED] 6475[DISABLED] 6495[DISABLED] 6515[DISABLED] nl80211: Mode IEEE 802.11b: 2412 2417 2422 2427 2432 2437 2442 2447 2452 2457 2462 2467 2472 2484[DISABLED][NO_IR] Allowed channel: mode=1 chan=1 freq=2412 MHz max_tx_power=20 dBm Allowed channel: mode=1 chan=2 freq=2417 MHz max_tx_power=20 dBm Allowed channel: mode=1 chan=3 freq=2422 MHz max_tx_power=20 dBm Allowed channel: mode=1 chan=4 freq=2427 MHz max_tx_power=20 dBm Allowed channel: mode=1 chan=5 freq=2432 MHz max_tx_power=20 dBm Allowed channel: mode=1 chan=6 freq=2437 MHz max_tx_power=20 dBm Allowed channel: mode=1 chan=7 freq=2442 MHz max_tx_power=20 dBm Allowed channel: mode=1 chan=8 freq=2447 MHz max_tx_power=20 dBm Allowed channel: mode=1 chan=9 freq=2452 MHz max_tx_power=20 dBm Allowed channel: mode=1 chan=10 freq=2457 MHz max_tx_power=20 dBm Allowed channel: mode=1 chan=11 freq=2462 MHz max_tx_power=20 dBm Allowed channel: mode=1 chan=12 freq=2467 MHz max_tx_power=20 dBm Allowed channel: mode=1 chan=13 freq=2472 MHz max_tx_power=20 dBm Allowed channel: mode=2 chan=36 freq=5180 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=40 freq=5200 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=44 freq=5220 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=48 freq=5240 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=52 freq=5260 MHz max_tx_power=20 dBm (DFS state = usable) Allowed channel: mode=2 chan=56 freq=5280 MHz max_tx_power=20 dBm (DFS state = usable) Allowed channel: mode=2 chan=60 freq=5300 MHz max_tx_power=20 dBm (DFS state = usable) Allowed channel: mode=2 chan=64 freq=5320 MHz max_tx_power=20 dBm (DFS state = usable) Allowed channel: mode=2 chan=100 freq=5500 MHz max_tx_power=26 dBm (DFS state = usable) Allowed channel: mode=2 chan=104 freq=5520 MHz max_tx_power=26 dBm (DFS state = usable) Allowed channel: mode=2 chan=108 freq=5540 MHz max_tx_power=26 dBm (DFS state = usable) Allowed channel: mode=2 chan=112 freq=5560 MHz max_tx_power=26 dBm (DFS state = usable) Allowed channel: mode=2 chan=116 freq=5580 MHz max_tx_power=26 dBm (DFS state = usable) Allowed channel: mode=2 chan=120 freq=5600 MHz max_tx_power=26 dBm (DFS state = usable) Allowed channel: mode=2 chan=124 freq=5620 MHz max_tx_power=26 dBm (DFS state = usable) Allowed channel: mode=2 chan=128 freq=5640 MHz max_tx_power=26 dBm (DFS state = usable) Allowed channel: mode=2 chan=132 freq=5660 MHz max_tx_power=26 dBm (DFS state = usable) Allowed channel: mode=2 chan=136 freq=5680 MHz max_tx_power=26 dBm (DFS state = usable) Allowed channel: mode=2 chan=140 freq=5700 MHz max_tx_power=26 dBm (DFS state = usable) Allowed channel: mode=2 chan=144 freq=5720 MHz max_tx_power=0 dBm (DFS state = usable) Allowed channel: mode=2 chan=149 freq=5745 MHz max_tx_power=13 dBm Allowed channel: mode=2 chan=153 freq=5765 MHz max_tx_power=13 dBm Allowed channel: mode=2 chan=157 freq=5785 MHz max_tx_power=13 dBm Allowed channel: mode=2 chan=161 freq=5805 MHz max_tx_power=13 dBm Allowed channel: mode=2 chan=165 freq=5825 MHz max_tx_power=13 dBm Allowed channel: mode=2 chan=169 freq=5845 MHz max_tx_power=13 dBm Allowed channel: mode=2 chan=173 freq=5865 MHz max_tx_power=13 dBm Allowed channel: mode=2 chan=1 freq=5955 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=5 freq=5975 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=9 freq=5995 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=13 freq=6015 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=17 freq=6035 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=21 freq=6055 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=25 freq=6075 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=29 freq=6095 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=33 freq=6115 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=37 freq=6135 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=41 freq=6155 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=45 freq=6175 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=49 freq=6195 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=53 freq=6215 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=57 freq=6235 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=61 freq=6255 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=65 freq=6275 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=69 freq=6295 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=73 freq=6315 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=77 freq=6335 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=81 freq=6355 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=85 freq=6375 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=89 freq=6395 MHz max_tx_power=23 dBm Allowed channel: mode=2 chan=93 freq=6415 MHz max_tx_power=23 dBm Allowed channel: mode=0 chan=1 freq=2412 MHz max_tx_power=20 dBm Allowed channel: mode=0 chan=2 freq=2417 MHz max_tx_power=20 dBm Allowed channel: mode=0 chan=3 freq=2422 MHz max_tx_power=20 dBm Allowed channel: mode=0 chan=4 freq=2427 MHz max_tx_power=20 dBm Allowed channel: mode=0 chan=5 freq=2432 MHz max_tx_power=20 dBm Allowed channel: mode=0 chan=6 freq=2437 MHz max_tx_power=20 dBm Allowed channel: mode=0 chan=7 freq=2442 MHz max_tx_power=20 dBm Allowed channel: mode=0 chan=8 freq=2447 MHz max_tx_power=20 dBm Allowed channel: mode=0 chan=9 freq=2452 MHz max_tx_power=20 dBm Allowed channel: mode=0 chan=10 freq=2457 MHz max_tx_power=20 dBm Allowed channel: mode=0 chan=11 freq=2462 MHz max_tx_power=20 dBm Allowed channel: mode=0 chan=12 freq=2467 MHz max_tx_power=20 dBm Allowed channel: mode=0 chan=13 freq=2472 MHz max_tx_power=20 dBm hw vht capab: 0x339071b2, conf vht capab: 0x0 DFS support is enabled Completing interface initialization Mode: IEEE 802.11a Channel: 132 Frequency: 5660 MHz DFS 1 channels required radar detection DFS all channels available, (SKIP CAC): no DFS 0 chans unavailable - choose other channel: no wlan0: interface state COUNTRY_UPDATE->DFS DFS start CAC on 5660 MHz wlan0: DFS-CAC-START freq=5660 chan=132 sec_chan=0, width=0, seg0=0, seg1=0, cac_time=60s nl80211: Start radar detection (CAC) 5660 MHz (ht_enabled=1, vht_enabled=1, he_enabled=0, bandwidth=20 MHz, cf1=5660 MHz, cf2=0 MHz)

  • freq=5660
  • he_enabled=0
  • vht_enabled=1
  • ht_enabled=1
  • bandwidth=20
  • channel_width=1
  • center_freq1=5660
  • center_freq2=0 nl80211: Failed to start radar detection: -16 (Device or resource busy) DFS start_dfs_cac() failed, -1 Interface initialization failed wlan0: interface state DFS->DISABLED wlan0: AP-DISABLED wlan0: Unable to setup interface. hostapd_interface_deinit_free(0x555669e2d970) hostapd_interface_deinit_free: num_bss=1 conf->num_bss=1 hostapd_interface_deinit(0x555669e2d970) wlan0: interface state DISABLED->DISABLED hostapd_bss_deinit: deinit bss wlan0 wlan0: Deauthenticate all stations nl80211: send_mlme - da=ff:ff:ff:ff:ff:ff noack=0 freq=0 no_cck=0 offchanok=0 wait_time=0 no_encrypt=0 fc=0xc0 (WLAN_FC_STYPE_DEAUTH) nlmode=3 nl80211: send_mlme - Use bss->freq=0 nl80211: send_mlme -> send_frame_cmd nl80211: CMD_FRAME freq=0 wait=0 no_cck=0 no_ack=0 offchanok=0 CMD_FRAME - hexdump(len=26): c0 00 00 00 ff ff ff ff ff ff 74 19 f8 16 ac ac 74 19 f8 16 ac ac 00 00 03 00 nl80211: Frame command failed: ret=-22 (Invalid argument) (freq=0 wait=0) wlan0: AP-DISABLED hostapd_cleanup(hapd=0x555669e2f140 (wlan0)) wlan0: CTRL-EVENT-TERMINATING hostapd_free_hapd_data: Interface wlan0 wasn't started hostapd_interface_deinit_free: driver=0x55565221e588 drv_priv=0x555669e44d30 -> hapd_deinit nl80211: deinit ifname=wlan0 disabled_11b_rates=0 nl80211: Remove monitor interface: refcount=0 nl80211: Remove beacon (ifindex=3) netlink: Operstate: ifindex=3 linkmode=0 (kernel-control), operstate=6 (IF_OPER_UP) nl80211: Set mode ifindex 3 iftype 2 (STATION) nl80211: Failed to set interface 3 to mode 2: -16 (Device or resource busy) nl80211: Try mode change after setting interface down nl80211: Set mode ifindex 3 iftype 2 (STATION) nl80211: Mode change succeeded while interface is down nl80211: Teardown AP(wlan0) - device_ap_sme=0 use_monitor=0 nl80211: Unsubscribe mgmt frames handle 0x8888dddee16ccfa9 (AP teardown) hostapd_interface_free(0x555669e2d970) hostapd_interface_free: free hapd 0x555669e2f140 hostapd_cleanup_iface(0x555669e2d970) hostapd_cleanup_iface_partial(0x555669e2d970) hostapd_cleanup_iface: free iface=0x555669e2d970 pi@raspberrypi:~ $

Additional context

No response

Source: raspberrypi/linux

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